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How to Choose the Best Linear Actuator for your Application

Tuesday, November 24th, 2020

Actuators discussed in this article; ball screw, lead screw, belt, and rack and pinion

Lead Screw [SDP/SI], Rack and Pinion [QTC Gears], Ball Screw [Reliance Automation], Fixed Belt [Bell-Everman]

There is a wide array of actuators available and there are volumes written about every type. However, there’s not much information comparing different types of actuators in real-world settings. So, how do you choose the best type of actuator for your application? Should you use a belt or a lead screw? A rack and pinion or a ball screw? A fixed or moving motor? If you’re building your own actuator, which types are the easiest to manufacture? Which systems are the most (and least) robust?

Every day, Teknic’s servo systems power hundreds of thousands of actuators. We don’t build actuators, but, for our customers to be successful, their actuators must perform well and reliably. For the past thirty years, Teknic has worked alongside our OEM customers, helping them select and design mechanical systems to optimize their machines’ motion performance. We’ve learned quite a bit over the years and we’d like to share that knowledge with the general public.

General Comparisons Between Actuator Types

If all machines had the same motion requirements, there’d be one type of actuator—but there are many different machines with various requirements. Therefore, there are various types of actuators to achieve these requirements. So, how do you know which actuator is right for your application?

Here are the main considerations to factor in when making actuator selections:

  • Price: sometimes performance requirements dictate price, but sometimes performance can be compromised to achieve price targets.
  • The required velocity, acceleration, and duty cycle.
  • The total moving load (including mass, moment, friction, any forces applied while moving, etc.).
  • Accuracy and repeatability requirements.
  • Stroke distance requirements.

Teknic developed the graphs below to compare actuator types using the metrics above. Based on our experience, these graphs indicate where OEMs enjoy consistent success. There are actuators that fall outside of these ranges that are still successful, but these exception cases usually cost more or require unsustainable technical attention in a production environment.

For example, both a fixed belt actuator and a moving belt actuator will effectively run a light-load, short-stroke application accelerating at 0.1 g. The performance difference between these two actuators for this specific application is negligible. A fixed belt actuator is more expensive, so it doesn’t usually make sense to use one in an application like this.

Approximate Price Range

Price range of actuator types

Table 1: Price range of actuator types
Top Velocity Range (in meters per second)

Table 2: Top velocity range of actuator types
Acceleration Range (in Gs)

Table 3: Acceleration range of actuator types
Moving Load Range (in pounds)

Table 4: Moving load range of actuator types
Position error per meter of travel (in microns)

Table 5: Position error per meter of travel of actuator types
Repeatability per meter of travel (in microns)

Table 6: Position error per meter of travel of actuator types
Actuator travel distance (in meters)

Table 7: Travel distance of actuator types

Specific Drive Systems: Use cases, pros and cons, selection considerations, tips and tricks

Ball Screws Question[1]

Figure 1: Ball Screw [Reliance Automation]

Application Usage Overview:

Out of all the actuators highlighted in this article, ball screws can drive the widest range of applications. They work well with both heavy and light loads, run precisely at high and low speeds, and have excellent repeatability and accuracy. Ball screws provide consistent machine-to-machine performance with minimal assembly effort. Even the selection process for ball screws is pretty straightforward.

On the other hand, these benefits come at a cost: money (sometimes a lot of money).

Ball screw Pros and Cons:

ProsCons
  • Broad load capacity range
  • Smooth operation
  • High accuracy/repeatability
  • Good performance across a wide range of velocities
  • Easy to implement
  • Robust components (even over time)
  • Relatively high price
  • Limited availability with long (fast) leads Question[2]
  • Resonance [3] in long stroke or high RPM applications
  • Practical stroke limits Question[4]
  • Audible noise in some configurations
Table 8: Pros and cons of ball screws

Selection considerations:

1. Higher cost does not necessarily mean better performance or value.

  • Ball screws are a prime example. There are quality US and German suppliers offering equivalent product at half the cost of popular brand names. Shop around carefully.

2. The number of starts has an effect on load capacity, price, audible noise, and more.

  • For a given lead, increasing the number of starts Question[5] can sometimes improve overall ball screw specs, such as load capacity. People often think that having more starts has no downside. However, multiple-start ball screws are more expensive, louder, and introduce more opportunities for failure than single-start units.

3. It’s important to understand sealing requirements.

  • Unnecessary sealing (of the nut, bearings, etc.) adds significant friction, which means you have to use more powerful (and costly) motors. On the flip side, units that are not properly sealed for the operating environment fail quickly.

4. Rolled vs. ground screws.

  • Rolled screws are less expensive but historically less accurate than precision ground screws. However, recent process advancements have improved rolled screw accuracy while maintaining a cost advantage over ground screws. As a result, rolled screws are often a compelling value for many applications.

Tips and Tricks using ball screws:

1. Are you using a gearbox or belt stage between the motor and screw? If yes, consider one of two things:

  • Use a higher gear reduction with a faster screw lead (keeping the overall linear distance traveled per motor revolution the same). You’ll experience a number of benefits by shifting the effective “gearing” from the screw to your gearbox or belt stage. With the combination of higher input gear ratio and a longer lead, the system will still maintain the same linear force, but this setup will reduce the reflected inertia, audible noise, and chance of screw whip.
  • If possible, switch to direct-driving the screw. Direct-driving often increases accuracy and reduces backlash, and simultaneously lowers system cost—a larger servo is often less money than the combination of reduction stage and a smaller motor.

2. The coupling between the motor and the screw is critical (but is commonly not given much thought).

  • Teknic recommends jaw-style (“spider”) couplings with a 92 or 98 durometer Shore A spider (see picture below). For reasons beyond the scope of this article, Teknic recommends against Shore D spiders and metallic spiders. Additionally, be sure to use a coupling that uses a clamp-style shaft attachment method, not a set screw.
  • Bellows couplings will work well, but they are more costly and have tighter alignment requirements than jaw-style couplings. If the machine mechanics (motor and screw) have different shaft diameters, the bellows coupling may need to be custom. Jaw couplings come in three separate pieces, so they can be purchased with different bore sizes and easily mixed and matched to suit.
  • Helical couplings have low torsional stiffness that limits total system bandwidth and can cause extra mechanical resonance. Helical couplings are sometimes acceptable in applications using stepper motors, because the spring compliance can smooth out stepper cogging. However, for applications using high-performance servos, helical couplings limit performance and you should avoid using them.
  • Don’t use a solid coupling. Solid couplings require nearly perfect shaft alignment, which is not reasonable when building multiple machines. Many times, using a solid coupling results in long term damage to the motor and screw bearing systems.

Figure 2: Spider couplings [Ruland], bellows coupling [EC21], helical coupling [MRO Tools]

Summary:

Out of all the mechanical drive systems we cover in this article, ball screws have the broadest range of capability. Moreover, ball screws are typically the most successful option for OEMs who want to design their own actuators.

Lead Screws[6]

Figure 3: Lead screws [SDP/SI]

Application Usage Overview:

It is not necessarily easy to properly select lead screws, but when specified correctly, lead screws provide good accuracy and repeatability for the money. In addition, well-designed lead screw systems are durable and offer repeatable results in high-volume manufacturing environments.

Lead screw Pros and Cons:

ProsCons
  • Priced attractively
  • Operates smoothly
  • Is repeatable and has good accuracy for the cost
  • Can eliminate the need for power-off brake if properly specified (will not back-drive)
  • Has good repeatability across multiple actuators
  • Specs that can be difficult to properly select
  • Limited load and speed capability
  • Low efficiency (sometimes)
  • Limited duty cycle (often)
Table 9: Pros and cons of lead screws

Selection considerations:

1. Screws with short leads [7] are inefficient.

  • As you increase the “gearing” of a lead screw (i.e. decrease the linear distance traveled per revolution of the screw), you simultaneously decrease the screw’s efficiency. We commonly see customers select leads that are too fine (too many revolutions per linear distance traveled), where the gain in mechanical leverage ends up as a net loss of force due to loss of efficiency.

2. Select materials with care.

  • Lead screws have a nut that slides along the screw surface. Choosing the optimal combination of nut and screw material is critical because of friction. For example, a brass nut on a steel screw has a dry (unlubricated) coefficient of friction (μ) of 0.41, whereas a polyacetal nut on a Teflon-coated screw has a dry μ of .08. This change of material results in a 220% change in efficiency on a 15mm diameter, 10mm lead screw. Friction is just one of many material considerations (others include ambient temperature, environmental conditions, duty cycle, etc.)

3. Stay within your “PV” limits when using polymer nuts.

  • Polymer nuts have a continuous duty rating (which is below the peak rating) called the Pressure Velocity rating (or PV for short). Pressure between the screw and nut (which equals the driving force divided by the contact area between the screw and nut), along with sufficient relative surface speed will generate heat. A high enough heat will deform and permanently destroy the polymer nut. You should also consider the duty cycle of motion. Often, the screw can run at relatively high speed and/or pressure as long as it’s for brief periods of time (so that the nut can sufficiently cool between subsequent moves). All lead screw manufacturers readily provide this information so that you can choose the appropriate screw for your application.Question

4. Select the correct number of starts.

  • A fast lead (large linear distance of travel per revolution of the screw) has “dead” space between threads—space that can be used for additional, independent threads. These independent threads (referred to as “starts”) increase the force capacity for a given lead, but also increase friction (thus heat) and impact the PV rating. Lead screw manufacturers will assist with this selection, but don’t assume that additional starts are always better.

Figure 3: Lead screw start demo [Thomson Linear]

Tips and Tricks using lead screws:

1. When speed and acceleration are important, use “fast” leads with enough starts to support the required forces.

  • Teknic acquired two off-the-shelf lead screw actuators with identical specs, except for one difference: one actuator uses a single start (1-start), 5 mm lead screw and the other uses a 4-start, 20 mm lead screw. The 5 mm lead actuator accelerates our payload at 0.5 G to 333 mm/sec maximum velocity. The 20 mm lead actuator, using the same motor and amount of electrical power, accelerates the same payload at 1.7 G to 1,333 mm/sec velocity. So, just a thread change enabled a 340% increase in acceleration and a 400% faster linear velocity.

2. Gear more in the reduction stage and less in the screw.

  • If you have a reduction stage between the motor and the screw, increase the reduction ratio and proportionately select a longer (faster) lead. For example, if you have a 2:1 ratio into a 5 mm lead, change to a 4:1 ratio and a 10 mm lead. This change improves system efficiency (from 15% to 50% depending on friction coefficients) and decreases mechanical wear, audible noise, and reflected inertia.

3. The coupling between the motor and the screw is critical (but is commonly not given much thought).

  • Teknic recommends jaw-style (“spider”) couplings with a 92 or 98 durometer Shore A spider (see picture below). For reasons beyond the scope of this article, Teknic recommends against Shore D spiders and metallic spiders. Additionally, be sure to use a coupling that uses a clamp-style shaft attachment method, not a set screw.
  • Bellows couplings will work well, but they are more costly and have tighter alignment requirements than jaw-style couplings. If the machine mechanics (motor and screw) have different shaft diameters, the bellows coupling may need to be custom. Jaw couplings come in three separate pieces, so they can be purchased with different bore sizes and easily mixed and matched to suit.
  • Helical couplings have low torsional stiffness that limits total system bandwidth and can cause extra mechanical resonance. Helical couplings are sometimes acceptable in applications using stepper motors, because the spring compliance can smooth out stepper cogging. However, for applications using high-performance servos, helical couplings limit performance and you should avoid using them.
  • Don’t use a solid coupling. Solid couplings require nearly perfect shaft alignment, which is not reasonable when building multiple machines. Many times, using a solid coupling results in long term damage to the motor and screw bearing systems.

Figure 4: Spider couplings [Ruland], bellows coupling [EC21], helical coupling [MRO Tools]

4. Certain types of nuts can reduce backlash and improve repeatability.

  • If you have a horizontal application where accuracy and repeatability are important, use anti-backlash nuts. While the specifics are beyond the scope of this article, anti-backlash nuts have a force-applying element (which can be rings, springs, axially-compressed fingers, etc.) that removes clearance between a multi-part nut and the screw. Anti-backlash nuts are application specific, so be sure to contact a lead screw manufacturer for assistance with selecting the appropriate nut.

5. In some circumstances, lead screws can eliminate power-off brakes.

  • Lead screws have different efficiency for forward driving (a motor driving the screw and moving the load linearly) and back-driving (a linearly moving load back-spinning the screw—i.e., the load is spinning the motor). A good example of back-driving is gravity acting on a vertical load.The right combination of screw lead and coefficient of friction will lock the nut in place when the load is pushing back against the screw. No amount of force will cause these screws to back-drive because the linear force produces a larger component vector of friction force than the component vector that creates rotation. Some people implement power-off brakes to prevent vertical loads from falling when motor power is off, but brakes add more cost and complexity. On the other hand, many people carefully choose screws with specific parameters that do not allow back-driving (but still meet forward motion requirements). These screws result in lower cost and simpler design. One thing to note is that screws with zero back-driving efficiency are not very forward efficient either, so you have to review all factors.

Summary:

There is a learning curve involved with proper lead screws specification, but lead screws can offer compelling advantages over competing mechanics when specified well for the application (i.e. affordability, precision, range of options, reduced maintenance, etc.). Moreover, lead screws enable easy-to-achieve machine to machine repeatability compared to other low cost mechanical drive technology.

Rack and Pinions

Figure 5: Rack and pinion

Application Usage Overview:

Rack and pinions are common among applications with heavy loads and long strokes because of their load capacity and price. However, system performance and price vary due to differences in geometric properties and there are significant quality differences from manufacturer to manufacturer.

Rack and pinions offer some unique advantages over other linear motion technologies. For example, you can drive multiple pinions (thus multiple independent axes) per rack, or the motor can drive the rack (rather than the pinion), creating a “rack rod” (a rod that pushes like a pneumatic piston) to provide accurate thrust in a small footprint. (As a note, you can accomplish both of these examples with screws by spinning the nut instead of the screw, but that is usually more difficult.)

Standard (straight gear cut) rack and pinion Pros and Cons:

ProsCons
  • Performs consistently regardless of stroke length
  • Handles large force
  • Cost-effective in longer stroke applications
  • Efficient power translation
  • Configurable into unique designs
  • Produces a lot of audible noise
  • Suffers from vibration
  • Suffers from backlash
  • Requires frequent maintenance
  • Gets “dirty” due to exposed grease
  • Often requires additional gear reduction
Table 10: Pros and cons of rack and pinions

Selection considerations:

1. Performance remains consistent even as actuator length changes.

  • Motion performance and motor requirements remain consistent as actuator stroke changes. Other actuators have characteristics (compliance, reflected inertia, resonance, etc.) that suffer as total stroke increases. As a result, rack and pinions are a popular choice for OEMs producing large equipment or equipment of various sizes.

2. Costs scale nicely.

  • The cost for the “first inch of travel” is more expensive for “moving motor” mechanical designs (e.g. rack and pinions) than for “stationary motor” designs. “Moving motor” actuators have the additional burden of the entire motor assembly, e-chain management, flex cabling, etc. However, the cost of adding additional travel to a rack and pinion system is among the lowest of any actuator technology. As a result, if your application requires more than two meters of travel, rack and pinions are often the best value.

3. A rack and pinion is not a good choice for certain environments, such as labs or clean rooms.

  • It’s important to understand the type of environment where you (or your customers) will run the application. A rack and pinion system is not the quietest actuator and the audible pitch can be unpleasant. Additionally, the lubrication on the system isn’t sealed, so the mechanics are not particularly clean.

4. You can mitigate some of the cons—for a price.

  • Helical teeth. Helical cut teeth (both on the pinion and rack) engage at an angle, which smoothes motion and reduces backlash. The downside is reduced efficiency because forward thrust is maximized at a 90 degree tooth angle (i.e. a straight-cut pinion). In addition, helical pinions want to follow the tooth angle (thus drive off the rack), so you need to include proper bearing structures to keep the pinions on the racks. Both factors (efficiency loss and bearing structure upgrades) increase installed costs by $100-$200 an axis, but helical teeth are worthwhile in applications requiring higher accuracy, lower backlash, and/or smoother motion.
  • Figure 6: Helical teeth rack and pinion
  • Roller rack systems. These systems use pinions comprised of bearing-supported rollers instead of toothed gears. The rollers smooth motion, provide pre-loading (to reduce backlash), run up to 10 meters per second, and are audibly quieter than conventional racks. The downside is that roller racks have limited pinion diameter options and cost much more (up to $1000 more than helical designs). It’s reasonable to compare roller racks to ball screws (given their performance and price) even though the design is a rack and pinion. That said, even compared to ball screws, roller racks are more expensive until the stroke is between two and four meters in length. (The exact stroke length where the prices begin to converge depends on roller pinion and ball screw grades.)
    Given these factors, roller racks are only competitive in long stroke, high precision applications – with one exception. The exception is rotary applications. Arcuate roller racks can be cost competitive and offer exceptional stiffness when compared to strain wave reducers (starting in moderate platen diameter sizes, ~250 mm+).
  • Figure 9: Arcuate rack and pinion

Tips and Tricks using rack and pinions:

1. Pinion pitch diameter has a great deal of impact on the design (beyond just leverage).

  • Comparing two pinions of different diameter, but otherwise the same design, a smaller diameter pinion has greater mechanical leverage. Additionally, the gearbox’s backlash translates to less linear distance. The smaller pinion provides more force and superior accuracy compared to the larger diameter pinion. On the other hand, the larger diameter pinion has more teeth engaged, which reduces vibration and decreases wear. An effective compromise is often a smaller pinion with a finer tooth pitch (generally try to target 5 or more teeth engaged). This allows the advantages of smaller pinions, while also mitigating wear and vibration.

2. Routine maintenance is essential.

  • Properly maintained, rack and pinions are good for hundreds of millions of pinion revolutions. Yet, many customers fail to maintain these mechanics (despite simple procedures) and the components degrade after even just a few months. You should consider investing in self-lubricating systems if you do not expect to be able to keep up with the ideal level of maintenance.

3. Use a matched rack and pinion from an actuator company.

  • Teknic used a laser interferometer to measure total pitch error between the first and last tooth on two different 2-meter stroke systems. One rack was hobbed by a machine shop; the other by Atlanta Drives. Both units had identical specs on paper. The generic rack had 15 mils of accumulated error, while the brand-name was under 2 mils. The superior rack cost only $46 more (as of 6/2018)—certainly a worthwhile investment.

4. Consider an integrated rack (a.k.a. rack & rail).

  • Integrated rack and bearing rail designs typically cost more than the separate parts, but many OEMs find this to be the best value. Integrating the rack and rail saves space, reduces manufacturing time, and eliminates alignment issues as well as the need to machine separate mounting surfaces.

Figure 10: Integrated rack

Summary:

Rack and pinions offer flexibility and significant load capacity and are common in heavy, longer stroke applications. They are especially prevalent at OEMs that make large equipment with various stroke configurations. For high-precision applications in the 2 meter or longer range, it is worth considering helical racks and perhaps even roller racks versus conventional straight (spur cut) racks.

Moving Timing Belts

Figure 11: Moving timing belt [Rollon]

Application Usage Overview:

Timing belts are cost-effective, audibly quiet, repeatable, and can accelerate light loads at remarkable rates. Belts have been around for decades, so individual parts are available from many sources, but this creates a two-fold problem. First, when buying parts from various sources, you need to be an expert on combining those components. Second, the quality of “universal” parts varies from manufacturer to manufacturer. So, proper selection and sourcing are equal challenges. That said, belt systems are cost-effective, reliable actuators for applications with moderate load weights and reasonable accuracy expectations.

Moving belts Pros and Cons:

ProsCons
  • Inexpensive (certain models)
  • Capable of high velocity and acceleration with light loads
  • Translates power efficiently
  • Widely available
  • Varies in performance and other factors as stroke changes
  • Varies in performance depending on direction
  • Requires “fussy” setup (e.g., ensuring proper belt tension)
  • Has many factors to consider and specify
  • Has limited load capacity
Table 11: Pros and cons of moving timing belts

Selection considerations:

Belt systems can be low cost and effective. However, there are more challenges and pitfalls associated with designing belt actuators than other types of actuators. A turn-key belt actuator removes most of the challenges and we strongly recommend this route, but if you’re making your own belt actuator, here are factors to consider:

1. Moving belt actuators are particularly sensitive to change in three areas:

  • Belt Width:Timing belts are rated for capacity and not system dynamics. The most narrow belt rated to meet your acceleration spec, carry your specified load weight, and withstand your application’s duty cycle is likely too compliant (stretchy) for quality motion performance. So, apply the “1-2-3 rule-of-width” (more information on this in the “tips and tricks” section for belts).
  • Load Inertia:Many people remember to consider mass, but forget about (or ignore) the load inertia reflected to the motor.Ignoring reflected inertia is unwise in any motion system. So why bring it up here specifically? Belts have high back-driving efficiency and low effective gearing [8], so if you push the load by hand, it has a uniquely light finger-tip feel. This combination lulls too many engineers into ignoring the effect of reflected inertial mismatch.
  • Stroke Length:Actuator performance can vary as stroke length changes. Belt actuators, in particular, are so uniquely sensitive to changes in stroke that they often require different configurations for every stroke length.A belt can only pull a load, it can’t push one. So, an accelerating or decelerating load experiences force from the opposite side. This also means that the length of the “pulling” belt changes depending on where the load is within the stroke of the axis. Moreover, for every unit of increased stroke length, the change in belt length is double that distance. As a result, even moderate changes in actuator stroke may necessitate changes to pitch, width, tooth profile, and electrical power to maintain consistent motion performance.

Figure 12: Diagram of timing belt length

2. Operating environment affects performance.

  • The optimal belt material depends on application temperature, potential contaminants, static discharge ratings, etc. For example, common bearing grease decreases the life expectancy of some non-urethane belts by as much as 75%. You’ll want to check with the belt manufacturer and explain the specifics of your operating environment prior to selecting the belt material.

3. You need to run and then re-tension belt actuators before you put the axis into service.

  • Most belt actuators have a break-in period during which the belt system loses tension, but after this period tension stays consistent. To prevent issues, exercise the actuator through the break-in period and then re-tension the belt prior to actual use (the reasons for tension loss in the first few hours of use are beyond the scope of this article, but suffice it to say almost no belt system is immune to this).

4. Proper tensioning is the key to longevity and performance.

  • Too much tension reduces belt life, bearing life, and smoothness. Too little tension degrades repeatability, decreases belt life [9], and leads to position loss due to ratcheting [10]. Even when you’re within and acceptable tension range, consistent tension from actuator to actuator is the key to repeatable performance.

Tips and Tricks to using moving belts:

1. Apply the “1-2-3” rule of belt width.

  • Identify the manufacturer’s recommended minimum belt width based on your load, torque demands, and duty cycle. While that minimum width won’t catastrophically fail, it’s likely too compliant for ideal dynamic motion control. To get good performance, select a belt twice that width, but don’t triple the width because excessive width brings other downsides (such as greater bearing structure requirements, higher motor torque demands, and loss of smoothness).

2. High reflected inertia is problematic with belt systems. Don’t ignore it.

  • It’s generally accepted that the reflected load to motor inertia ratio should be no greater than 10:1. Teknic’s servo systems have features that allow a greater (sometimes much greater) ratio when the load and motor are tightly coupled.The acceptable ratio of load inertia to motor inertia depends on a lot factors, but the type of actuator is one of them. The challenges with belts are: 1) low mechanical leverage increases reflected inertia, 2) acceleration induces stretch/deflection in the belt and 3) load to motor coupling stiffness varies based on the direction the force is being applied. So, for a given load weight, moving belts often have higher reflected inertia and more dynamic variability than other mechanical drive systems.

3. Use aluminum pulleys.

  • Steel pulleys have almost triple the inertia of equivalently-sized aluminum pulleys. When your payload is light (which is where belts are often the optimal choice), accelerating steel pulleys can require as much or more torque than accelerating the load itself.

4. Bond or positively lock the pulley to the motor shaft.

  • The point at which the motor shaft attaches to the pulley is a common field failure with belt drives. Never use a set screw or key by itself. Use clamp-style units, locking hubs, or adhesive (e.g., Loctite retaining adhesive). For details on making a high-performance bond, read this article.

5. Select the tooth profile with care.

  • Tooth shape impacts smoothness, repeatability, accuracy, and force capacity. The bullet points below are true for most servo applications:
  • Trapezoidal-shaped teeth can transmit significant force and offer low backlash, but trapezoidal-shaped are louder than other tooth shapes and can cause uneven, or “clicky” motion. Moreover, trap-shape teeth wear quickly in high torque/ high speed applications due to stress concentration at the belt-pulley interface.
  • Figure 13 [Pfeifer Industries]
  • Curvilinear teeth (a.k.a. modified round, HTD, or “Gates” belts) are smooth-running teeth developed to 1) alleviate stress concentrations found in trap profiles and 2) improve power handling capacity. The trade-off, however, is that first-generation curvilinear designs have more backlash.
  • Figure 14 [Pfeifer Industries]
  • Modified curvilinear teeth have shallow depth, greater flank angle than other belt designs, and the load is distributed among the engaged teeth and the base between the teeth. This results in high force transmission capability, smooth rotation, and almost no ratcheting. Unfortunately, modified curvilinear belts have limited material options.
  • Figure 15 [Pfeifer Industries]

6. Consider tooth pitch and width trade-offs.

  • Increasing tooth pitch (leaving all else the same) may increase force capacity a small amount, but decreases accuracy significantly. Usually, a slightly wider belt with a finer tooth is best for positioning applications. Regardless of pitch, continue to apply the 1-2-3 rule of belt width selection.

7. Buy a matching pulley for your belt, not just a compatible one.

  • Belts work with pulleys that have matching tooth profiles, but the performance greatly improves when you use a pulley specifically designed for that belt. For example, an HTD pulley will work with a GT-2 belt (it has the same nominal tooth profile), but the pair will be less smooth and less accurate than the GT-2 belt with a matching GT-2 pulley.

8. Select belt materials[11] carefully.

  • Belts that look nearly identical offer different stiffness, accuracy, and longevity based on what’s under the surface (cord type, weave, reinforcement material, etc.).

Summary:

Belts have more elements to specify—especially pertaining to their construction—than any other mechanical drive technology. If you invest the time in learning about your options and couple that with carefully-considered motion goals and quality assembly practices, belts can be high-value actuators.

Fixed belts

Figure 16: Fixed Belt [Bell-Everman]

Application Usage Overview:

Fixed belts are similar to rack and pinions. The belt pulley moves down the fixed belt much like a pinion moves down a fixed rack. A well-designed fixed belt provides better repeatability and fewer tensioning challenges than moving belts. Additionally, a fixed belt is more quiet and accurate than most racks. However, the load capacity and environmental range for most fixed-belt actuators is limited compared to rack systems.

Fixed belts Pros and Cons:

ProsCons
  • High velocity and acceleration potential
  • Efficient power translation
  • Wide availability of parts
  • Higher accuracy and repeatability than moving belts
  • Limited application range
  • Relatively high cost as a complete actuator
  • Difficult specifications for design
  • Limited load capacity
Table 12: Pros and cons of fixed belts

Selection considerations:

1. The use cases are limited.

  • Fixed belt actuators are typically a good design choice in a subset of applications that include most or all of these characteristics:
  • Long stroke (2+ meters)
  • Light loads (under 25 pounds or so)
  • Moderate to high speeds (1+ meters per second)
  • Low noise requirement

2. Go off-the-shelf if you need more range of capability.

  • If your application does not meet all four parameters mentioned above, other actuators are typically a better fit. However, there are off-the-shelf exceptions. One is the ServoBelt™ Linear
    Drive[12], whose design improves load capacity and accuracy and allows for multiple carriages per axis.

3. There is less universal support than for other actuators.

  • Fixed belt systems are not common, so you won’t find a large community of users for support.

Tips and Tricks using fixed belts:

1. Both sides of the belt matter.

  • Like all belt drives, tooth selection is critical. In addition, fixed belts use idler rollers on the back of the belt, so this design is also sensitive to surface materials on the back of the belt. Be sure to select a belt with a backer that is rated for idler rotation.

2. Lock idlers in position.

  • Calibrated spring idlers effectively tension and wrap the belt, but you must lock the idler in position prior to motion. Springs will compress during acceleration, leading to momentary position loss and even permanent position loss if teeth jump. In addition, the compliance of the springs will introduce dynamic issues into the system.

3. Clamp the ends properly.

  • The idlers in a fixed-belt design ensure plenty of teeth in mesh with the drive pulley. However, tooth engagement in the end clamps is often overlooked. The teeth in the end clamps flex during acceleration, so be sure to engage at least 6 teeth at both end clamps.

4. The motor is driving much more than just the payload.

  • Moving motor designs are impacted by motor weight, the effective load of the e-chain (which is position-dependent), the ratcheting effect of an e-chain, the force of flexing cable, etc. This adds extra weight as well as position and velocity-dependent variable drag.There are other actuators that experience this issue, specifically rack and pinions, but if you have a rack system designed for a 1,000 pound load, these effects are usually insignificant. Moving belts, on the other hand, are often built for light loads using no gear reduction, and a small motor. As a result, a motor sized only for the payload won’t have the power (or gearing) to mask dynamic issues brought about by these extra components. So keep space for a motor that’s larger than payload calculations indicate.

Summary:

Fixed belts are effective actuators—when applied properly. OEMs that use these actuators in light-load, long-stroke applications with a motor sized to control all the forces (not just the payload) are generally successful. The unsuccessful companies either 1) undersize the motor with no room for a larger motor, or 2) tweak the system until they get the desired performance, but it’s unrepeatable in a volume manufacturing environment. If you have realistic performance expectations and a motor sized to handle all the forces, fixed belts can be the basis of highly-effective motion axes.

Advice applying to all actuator types

Over the last three decades, Teknic has learned a few things that apply to all types of actuators:

1. Actuators—and their parts—are not commodities.

  • Components sharing similar paper specifications often yield vastly different real-world results. Actuators and mechanical drive parts are not freely interchangeable. So test the exact device you intend to use, on the frame you intend to use, and in the environment where you intend to use it.

2. Have realistic expectations.

  • It is challenging to achieve significantly better performance than normal for a given type of actuator—you will most likely run into a lack of manufacturability or ongoing maintenance hassles [13]. Even if you’ve tweaked a single actuator to perform uniquely well for its class, tolerance stack-up and mechanical wear over time will make your machine-to-machine performance (and individual machine performance over time) much lower than your finely-tuned prototype.

3. Making your own actuator is not cheap or easy.

  • OEMs often make their own actuators, but it’s not a simple endeavor. If you design your own actuator, 1) you will take longer to get the product to market, 2) you’ll invest more time developing an actuator and less time perfecting machine processes, and 3) your real costs are almost always higher than first estimated (and you won’t always save money compared to off-the-shelf solutions). So, unless you’re manufacturing at least hundreds of machines annually, or you have unique constraints, we strongly recommend that you consider integrating an off-the-shelf actuator.

4. Inexpensive individual drive components don’t necessarily mean inexpensive actuators.

  • Individual parts that are less expensive up front may require more costly and extensive manufacturing processes, so the total cost overall may add up to more than you expect. For example, you can purchase a belt and pulley for under $100, but a ball screw and nut (of the same length of travel) might cost $500. However, belt actuators often require additional machining, more manufacturing steps, and a break-in period. This increases costs of turn-key belt actuators so an off-the-shelf ball screw actuator is sometimes less money than an off-the-shelf belt-driven actuator [14]. It’s important to focus more on your application requirements rather than the cost of subcomponents.Here is a graph comparing the price of the individual parts to the price of the finished actuator (prices are much higher for some specialty actuators, but we removed them to show the typical price range).

5. Don’t spend extra money for precision components, unless you’re willing to make the entire machine precise.

  • Often the weakest link related to a machine’s accuracy is not the mechanical drive. Issues such as thermal expansion (and contraction), machining imperfections, frame flex and vibration, motor accuracy, etc. can have significant effects on overall accuracy. Unless you pay careful attention to all the factors that affect accuracy, the extra precision of expensive components may be wasted. For example, we often see the use of precision ground ball screws where much cheaper rolled ball screws would not significantly affect the overall machine accuracy.

6. Be sure to consider the reflected load inertia to the motor.

  • We see some motor models adeptly control thousands of pounds in some applications, yet the very same motor models fail to adequately control a mere 5 pound load in other applications. The difference between success and failure in these applications is the motor’s leverage on the load.The best indicator of leverage is calculating the reflected load inertia back to the motor. Teknic has seen hundreds of painful redesigns and dozens of outright total machine failures due to engineers overlooking this relatively simple calculation at the start of the specification process.

Selecting the best actuator for your application—Our Summary.

Now that we’ve highlighted various actuators, how do you go about making a selection? These are our suggestions:

1. Many factors influence which actuator makes the most sense for you. You’ll want to consider these factors in light of your application and create an inventory of answers to select the most appropriate actuator.

  • Quantify your entire moving load weight. Be sure to include all the moving parts in the actuator (don’t forget the e-chain!).
  • Carefully consider your quantitative motion goals, such as accuracy, repeatability, move time, etc.
  • List your qualitative goals (such as audible noise) and prioritize them.
  • Consider the machine’s working environment. Is it dirty or clean? A quiet laboratory or a noisy factory floor?
  • How much maintenance (or abuse) do you expect from machine operators?
  • Carefully consider your manufacturing capabilities. Is your manufacturing team made up of old-world craftsmen, or laborers who follow basic work instructions, or something in between? The skill sets required to build different types of actuators vary. Make sure there’s a good match between the manufacturing requirements of your actuator and your manufacturing team’s skill set.

2. Now that you have your inventory of needs and capabilities, use the information in this article to consider different actuators. There is a good chance one actuator stands out.

  • If at this point, there are still multiple options, you have a pretty flexible application. In this case, go with the actuator that is the easiest to implement in your machine.

3. With your actuator selected and requirements quantified:

  • If you’re a high-volume OEM machine builder, design your own actuator using a holistic system-wide approach. Consider starting with an off-the-shelf actuator of a similar design so that other disciplines in the design team (such as software) don’t get stuck waiting for your mechanics to be ready.
  • If you manufacture products using automated machinery—even if you use many axes of motion—don’t make your own actuators. Narrow your selections to a few off-the-shelf options and test them to arrive at a final selection. Most manufacturing companies are best served by focusing on making their products and processes better, not coming up the learning curve that actuator companies already understand.
  • If you’re in the market for a few actuators, buy the best one the budget will allow, oversize the motor a bit to be safe, and forget the extensive research. Time is money.

One final point. Your application is a system. Take a balanced approach. A system is only as good as its weakest link. Even the world’s best motors can only do so much with poor mechanics—or even good mechanics that are misapplied. Carefully quantify your requirements and take a holistic approach.

Footnotes

Securing Mechanics to Motor Shafts

Friday, September 18th, 2020

mechanics

One of the most common points of failure in automated machinery is the coupling point where the motor shaft attaches to the mechanics. This is especially true in demanding motion control applications—those that include frequent start/stops, bi-directional control, frequent changes in torque direction, etc. (less common in applications that are constant velocity and one direction, such as a fan).

Most of these failures occur from selecting an improper attachment method for the type of application (although some failures occur from implementing the attachment method incorrectly). Choosing the optimal method to secure your mechanics to the motor shaft for torque transmission can help prevent failures and ensure your machine performs as expected.

Article Summary

In this article, we will:

  • Review common methods for attaching mechanics to stepper motors and servo motors (the most common motors used in motion control applications). These methods include:
  • Clamps or split clamps
  • Adhesives
  • Keyless bushings
  • Keys and keyways
  • Set screws or grub screws
  • Pinning
  • Evaluate the pros and cons of each attachment method
  • Identify ideal use cases for each attachment method
  • Recommend the overall best attachment methods

Although Teknic doesn’t manufacture mechanical stages or coupling components, we do manufacture the motion control products (brushless AC and DC servo motors) that drive these stages. Teknic’s engineers have worked on thousands of different mechanical systems over the last 30 years and are familiar with the coupling methods that work best in difficult, bi-directional servo applications. We’ve found that the ideal mechanical attachment approach for your application is not always obvious, is often different than what has been done traditionally, and will depend on a variety of metrics (including: cost, reliability and ease of use).

I: Clamps

Clamps Overview:

Clamps, also known as split clamp collars, were invented sometime around WWII as a method to address the shortcomings of using set screws (we will address set screws later on in this article). Clamps were designed for use in bombsights and guidance systems, where the main goal was to prevent axial movement. Over time, they found their way into other industries and applications, including motion control.

Clamps are commonly offered in one-piece or two-piece designs (see picture below) and they provide fairly uniform distribution of surface friction on the shaft (rather than just one point of contact like keys or set screws). The uniformly distributed force increases holding strength.

Given their ease of use, low cost, and high holding torque, Teknic recommends using split clamps in all types of servo applications. (If the motor reaches speeds above 6,000 rpm, you may want to spin-balance the component with the clamp because the construction tends to make them slightly imbalanced.)

split clamps

Figure 1: Split clamps [MiSUMi]

A “split hub and clamp” coupling is one where the clamp and hub are two separate components – see the picture below. In this design, the clamp is tightened around the hub which is tightened around the mechanical shaft. The hub (the pulley or pinion) has prongs that slide over the mechanical shaft and then the clamp collar slides over the pulley’s prongs. As the clamp is tightened, it compresses the prongs uniformly around the shaft.

split hub and clamp

Figure 2: Split hub and clamp diagram [OpenBuilds]

In general, two-piece clamps offer higher holding power because the full seating-torque of the screw(s) is applied directly to the clamping force on the shaft. Whereas with a single-piece clamp, some of the screw’s seating torque is needed to close the clamp around the shaft.

Although the spring force of the clamp tends to prevent the screw from backing out, you should put a little Loctite on the screw when installing it. In addition to providing extra fastening security, the lubrication of the liquid Loctite will help reduce any friction while tightening the screw and will allow you to achieve a consistent clamping force.

Pros of Clamps:

  • Easy and quick to install, uninstall, and adjust
  • Two-piece clamps can be assembled without removing any machine components
  • Reliable
  • Will not damage shaft
  • Cost-effective

Cons of Clamps:

  • A little more expensive than some other options
  • Requires some prep prior to assembly (mating surfaces should be cleaned with isopropyl)

Conclusion:

Overall, clamps are the best option for attaching mechanics to shafts given their ease of use, effectiveness, cost, and reliability. Teknic highly recommends using clamps for any motion control application.

II: Adhesives

Adhesives Overview:

Industrial adhesives have become a popular option for attaching hardware to motors. Loctite is a brand of adhesives from Henkel Corporation that includes a number of “retaining compounds” designed to secure cylindrical components. Currently, there are about ten different types of Loctite retaining compound adhesives, all with different properties (rated temperatures, cure times, holding strengths, etc.). The most commonly used for the applications discussed in this article are the 638, 648, and 680 formulations, but you should verify the best formulation for your specific application.

loctite 638

Figure 3: Loctite 638 adhesive [Loctite]

Of all the attachment options mentioned, Loctite is one of the most cost-effective solutions that takes up the least space without sacrificing reliability or holding force. One negative to using adhesives is the longer setup and removal time. That said, this approach is highly recommended – second only to clamps for any type of servo application.

Pros of adhesives:

  • Cost effective (a little adhesive goes a long way)
  • Allows for tightly integrated components and doesn’t require much space
  • Helps fill in all micro-gaps between motor shaft and mechanics (including any surface irregularities), which helps prevent fretting and corrosion

Cons of adhesives:

  • Requires time for chemicals to cure and bond
  • Cure time can range from minutes to days depending on strength required (be sure to properly secure components so no movement can occur while curing)
  • Curing process can often be expedited by using a chemical activator, but this costs more money and may also weaken holding forces (see the example graph below for Loctite 638 cure time with and without activators)
  • Loctite cure time graph

    Figure 4: Loctite 638 cure time graph [Loctite]
  • If bonding surfaces are not cleaned properly, the adhesive may never fully cure
  • Some grades of Loctite require use of curing agent (e.g. UV light)
  • Usually requires some type of heat source for removal – this process can sometimes be messy
  • Presents numerous considerations for cure time, cure strength, operating temperatures, and types of materials to use with (the options of adhesive grades can seem overwhelming)
  • We suggest contacting engineers at Loctite and/or using the resources available to help you choose your product (see below for an example resource). Fully understanding your application requirements, such as environmental conditions and field repair concerns, will make this process much easier

Loctite flowchart

Figure 5: Loctite usage flowchart [Loctite]

Conclusion:

Overall, industrial adhesives, such as Loctite, are one of the least expensive and reliable means for securing mechanics. When implemented correctly, Loctite can create bonds with shear strengths as high as 4,480 psi (e.g., Loctite 648 used for a steel to steel bond).

As a real world example, using Loctite 648 to secure a 3/4 inch wide aluminum timing pulley to a 5/8 inch diameter steel shaft would allow a torque of about 60 N-m of shear strength (that’s more than 8,000 oz-in). This would provide a very large safety factor when used with just about any motor with a 5/8 inch shaft.

Aside from the potentially messy and lengthy setup/removal time, there are no downsides of using adhesives like Loctite for attaching mechanics. Teknic recommends attachment with adhesive second only to clamps (especially if you need the most compact solution).

That said, for a virtually fail-safe connection, you can use a clamp in combination with a retaining compound. The clamp eliminates any worry of disturbing the adhesive while it’s curing (meaning there is no need for special fixtures), and it provides the security of a parallel attachment method.

III: Keyless Bushings

Keyless Bushing Overview:

Another common method of mechanical attachment is a keyless bushing (although they are less common than clamps). It’s a good option if you plan on attaching and removing mechanics frequently or if the concentricity of the load on the shaft is particularly important. Keyless bushings come in a variety of different brands (such as Trantorque and Fairloc) and are typically easy-to-use, self-contained devices.

keyless bushing

Figure 6: Trantorque keyless bushing [Fenner Drives]

The Trantorque design (as seen in the picture above) is the most common design for shafts under 1.5 inches in diameter. A Trantorque is essentially a 3-piece bushing with an inner contracting collar, an outer expanding sleeve, and a single nut that controls both the collar and sleeve (see picture below).

As the nut is tightened, the inner collar will clamp down on the motor shaft while the outer sleeve expands (the inner collar and outer sleeve have opposing tapers, which is why one contracts as the other expands). As you tighten the nut, the outer bushing expands and the inner collar contracts – this combination generates holding forces while maintaining concentricity.

keyless bushing diagram

Figure 7: Keyless bushing diagram

Unfortunately, keyless bushings are also one of the most expensive options and, given their size, often can’t be used to secure loads with a relatively small diameter. For example, you would not be able to secure a 1 inch pitch-diameter timing pulley to a 5/8 inch shaft (something you could do with a clamp or adhesive) because the outer diameter of the bushing itself would be at least 1 inch (i.e., the bore of the pulley would need to be about an inch in diameter). You would be forced to use a larger than optimal pulley. Keyless bushings also tend to have a large rotational moment of inertia, which can be a significant extra load when the load itself is a small diameter and thus relatively low inertia.

These two factors, along with the radial forces the bushing applies to the load, mean that the ratio of the outer diameter (OD) of the load to its bore (inner) diameter (ID) generally has to be fairly large (typically 1.5 to 2.5x).

Pros of keyless bushings:

  • Evenly distributes holding forces along the motor shaft and hub (prevents slippage)
  • Collar expands uniformly as nut is tightened
  • Easily attaches two different sized parts (e.g. a shaft and a larger hub)

Cons of keyless bushings:

  • They are the most expensive option out of all methods listed in this article (excluding the machining costs associated with pinning, discussed below)
  • Some designs are complicated and require more setup time
  • They have relatively large inertias
  • Keyless bushings can’t be used in situations where the load components are only slightly bigger than the motor shaft (adhesives are best for low profile applications)
  • They require extra prep (cleaning mating surfaces)
  • The Trantorque design generally moves a small amount axially while being tightened down

Conclusion:

Teknic rarely recommends using keyless bushings because of their high price point, inertia, and large OD/ID ratio requirement. Clamps and adhesives offer similar, if not more reliable connections at a fraction of the cost. That said, if load concentricity is critical, or the hub components are much larger than the shaft diameter, keyless bushings are a good option.

IV: Key and Keyway

Key and Keyway Overview:

People have used shaft keys and keyways for many years. This method is still commonly used in applications ranging from HVAC fans to pumps. A key/keyway offers a fast and moderately inexpensive way of transmitting torque to the load (see Figure 8 below).

However, for bi-directional applications that start and stop often (which means the torque is bi-directional), the mechanical components will wear over time due to vibration or mechanical rubbing. Wear and fretting will eventually result in mechanical failures. While keys and keyways can work for single direction applications, they aren’t suitable for applications with frequent changes in torque direction.

key and keyway

Figure 8: Key/keyway [Tradelink Services]

Pros of key and keyways:

  • One of the fastest and easiest methods of attachment
  • No tools are needed because there are no set screws or bushings to tighten (although often a set screw is used in conjunction with a key to prevent axial motion)

Cons of key and keyways:

  • A little bit of clearance between the shaft and key is required – this can cause backlash that will affect accuracy and cause failure over time
  • If you press fit components, the shaft and components can be subjected to forces beyond spec
  • The key can eventually wiggle in the keyway which will cause damage and wear
  • If the key or keyway gets deformed from acceleration/deceleration or other shock loads, the system may be very hard to disassemble

key and keyway diagram

Figure 9: Key/keyway diagram [Linear Motion Tips]

Conclusion:

Given the backlash issues, high probability of wear and fretting, and eventual mechanical failure, Teknic never recommends using keys and keyways as the only form of attachment and torque transmission. In unidirectional applications that do not frequently start and stop, mechanical wear is less likely, and engineers can consider the use of a key. A key can also be used as a back-up mechanism (e.g. a clamp as the main source of torque transmission in conjunction with a key acting as a fail-safe backup).

V: Set Screws

Set Screw Overview:

Although set screws have many drawbacks in motion control applications, they are still commonly used to secure mechanical components to a motor. In fact, the idea of a set screw (or grub screw) has been around for a long time – old enough that the first variants of set screws were made from materials like bone and wood.

Many people choose set screws because they are affordable and easy to install. However, set screws are unreliable in motion control applications and they often damage the motor shaft. While set screws may suffice in very low power applications, Teknic never recommends a set screw in any motion control application.

set screw

Figure 10: Set screw [SDP/SI]

Pros of set screws:

  • Cheap
  • Widely available
  • Easy to install

Cons of set screws:

  • Unreliable method of attachment
  • Set screw can loosen due to machine vibrations over time – allows the load to slip and move freely on the motor shaft
  • If you must use a set screw, we recommend using some type of thread locking agent to prevent the screw from backing out and disengaging
  • Set screws will generally gouge or deform the motor shaft. This can cause more slipping when you re-tighten the set screw to a marred shaft
  • Set screws create a slight radial offset of the load and cause non-concentric motion. This hurts machine accuracy/repeatability and can result in mechanical fatigue of components over time

Conclusion:

While set screws have different characteristics that may allow for more or less holding torque (such as different screw point types – see the picture below), the risks involved and their unreliable nature make them a poor choice for demanding motion control applications.

types of set screws

Figure 11: Point types of set screws [Atlantic Fasteners]

Set screws are still a potential fit for applications with tame motion demands (i.e. slow, low power, single direction, etc.) and where slipping is not detrimental to the rest of the machine. However, with so many better options available, Teknic recommends to never use set screws for any type of motion control application.

VI: Pinning

Pinning Overview:

Pinning, like using set screws, is an approach that has been around for a long time and is still used today in applications ranging from firearms to machinery. While the technology and materials have changed over the years, (e.g. pointed pieces of wood are now replaced with coiled metal pins—see figure below), the concept remains the same and offers a near permanent coupling method when done correctly. However, given the machining risks and costs, this method is unreliable and expensive for motion control applications.

pin

Figure 12: Coiled metal pin [Zoro: Spring Pins]

Pros of Pinning:

  • The pins are fairly inexpensive, but the process requires proper tooling and machining technique which can be expensive
  • This method can be reliable for less aggressive, unidirectional applications

pin diameter

Figure 13: Pin hole diameter [Fastener + Fixing Magazine]

Cons of Pinning:

  • Challenging to do accurately and consistently – risk of machining errors and weak points due to misalignment
  • Requires machining the motor shaft
  • Exposes motor to coolant, machined particulate, and potential extreme radial forces
  • Risk of dynamic loading and wear/fretting id the difference between pin and hole size exceeds a certain spec
  • Ideally, if you must use pinning, the load and shaft should be drilled simultaneously (although this can be challenging to do)
  • Different style pins (such as slotted or solid – see below) have different specs for diameter, length, material, required amount of engagement, etc.) If you must use pinning, Teknic generally would recommend a coiled pin

types of pins

Figure 14: Types of pins [American Ring]

Conclusion:

While pinning can be successful in some applications, Teknic never recommends this method for any type of motion control system. There are readily available options that are easier to implement, less expensive, less risky, and that provide more reliable connections.

Conclusion

Given all the factors a design engineer needs to consider, along with the many different options for securing mechanics to shafts, it is easy to understand why so many engineers overlook the importance of this design step.

To summarize, set screws and keys are poor choices for reliable, automated machinery (even though there may be other types of applications where these are appropriate). Pinning and keyless bushings can work, but they have some negatives worth considering (cost, risk of machining). Split clamps and adhesives are cost-effective and reliable solutions. Teknic always recommends split clamps and adhesives for almost any motion control application.

Selecting the Best Power Supply for your Stepper or Servo Motor Application

Wednesday, June 24th, 2020

Three types of power supplies

If you need a DC power supply for your stepper or servo motor application[1][1], you have three types to choose from:

(more…)

An Open-Source Ventilator Project for COVID-19 Patients

Monday, April 20th, 2020

In the midst of the COVID-19 pandemic, medical teams across the country are experiencing critical shortages. As hospitals run out of available ventilators, doctors are turning to anesthesia workstations, BiPAP machines, and CPAP machines to help ventilate patients. As doctors exhaust their supplies of ventilators and quasi-ventilators, they are scrambling to find alternatives.

At first, the solution may seem obvious—produce more ventilators—but by the time manufacturers are able to scale up production to meet current and future demand, it will be too late.

Given that scaling up conventional ventilators will take too long, medical professionals need additional solutions. At some point, the only remaining option will be to manually ventilate patients with Ambu® bags and hope that conventional ventilators become available. Although Ambu bags are readily available, there are not enough trained clinicians (nurses, doctors, respiratory therapists, etc.) to operate these devices for every patient in need.

Ambu bag

Figure 1: Ambu bag

Operating an Ambu bag requires a clinician to actuate the bag about 10-30 times per minute without stopping—an action that quickly becomes tiring. When one clinician fatigues, a new one must take over, and this cycle must continue until the patient recovers or a ventilator becomes available.

Over the last few weeks, Teknic has been working on a project that automates the operation of Ambu bags (i.e. clinicians no longer need to actuate the bag—which frees up valuable resources for other tasks). Below, we delve into the details of the project.

Project Overview

In mid-March, a team including Dr. Stephen Richardson (an anesthesiologist at the University of Minnesota), Jim McGurran (an engineer from MGC Diagnostics), and a small group from the Earl E. Bakken Medical Devices Center conceived the idea of a one-armed robot (more technically, a single-axis linear actuator) to automate human ventilation using an Ambu bag.

When the team had an early working prototype, they contacted Teknic (on a recommendation from Digi-Key, a large electronics distributor) for advice on the project’s motion control requirements. In just over two weeks, our collective team brought the device all the way through concept, prototype, and production.

The machine, internally nicknamed “Ambu-bot” by Teknic, was not designed to replace ventilators. Rather, it was designed to automate the manual ventilation typically performed by medical personnel so that clinicians in over-stressed hospitals can treat other sick patients.

3D model of ventilator

Figure 2: 3D Model of one-armed robot ventilator

We left out sophisticated adjustments and sensors commonly found (and required) in conventional ventilators in order to drastically speed up the design, prototyping, testing, and production to meet the urgent need.

While using this machine, clinicians will be required to monitor patients more closely than patients on a conventional ventilator. However, this device allows a single clinician to simultaneously monitor multiple patients, opposed to one clinician manually ventilating one patient.

The machine does all of the required manual labor consistently and continuously, and a clinician can monitor multiple patients at the same time to ensure that each patient’s vitals (e.g., blood gases) stay in an acceptable range.

Below is a more detailed timeline of the project.

Conception

Dr. Steve Richardson and Jim McGurran started the design. They used a motor they had on hand and completed the first prototype in one day. That motor was powerful enough to spin the actuator and move the piston, but it was unable to compress the Ambu-bag. They reached out to Teknic (on a recommendation from Digi-Key) for advice on the project’s motion control requirements.

Day 1: Initial Prototype Design

Teknic and the team iterated on the existing design. We updated solid models and created a number of conceptual examples. The team set a target to have a functioning prototype within two days and created a list of goals for the actuator. It needed to:

  • Be reliable and capable of running 24/7
  • Have adjustable speed control
  • Have adjustable compression (stroke volume)
  • Be easy to build and have an open-source design
  • Be ready for scaled up production (1,000 per week) in two weeks, and mass production (10,000 per week) within a few weeks after that
  • Be simple in concept and universally applicable
  • Be easily used by medical personnel with minimal training time

It was important for the prototype to work on the very first try. We needed to collect critical test data (electro-mechanical and clinical) and continue to iterate on the design while we incorporated that test data. A failure at this stage would have been a major roadblock on the critical path of the project.

Below is an early prototype of the actuator using some easy-to-manufacture (i.e. easily machined or 3D printed) components to actuate the bag.

Video 1: Teknic created this video to help raise awareness of the ventilator shortage, and demonstrate a potential solution. We didn’t have time to create a polished video, so we created a screen-capture of a 3D solid model taken directly from a Teknic engineer’s computer. Teknic used a CPM-MCVC-3411S-RLN in this prototype.

Day 2: First Prototype Trial

Engineers worked around the clock, and within 24 hours, we had a few different prototypes up and running. The prototype featured in the video below was not pretty (we used a 4×4, some plywood, and some PVC pipe), but it proved to us that we were on the right path.

Video 2: We took this video shortly after the first prototype was functional. The engineers in this video worked to update the motor’s configuration settings, took torque measurements, characterized the linear force needed to compress the Ambu bag, and analyzed the viability of the mechanics.

You may be wondering why we chose not to incorporate more advanced features (e.g., variable positioning, back and forth motion Question) in this application. When the University medical staff contacted Teknic, we suggested that the team should exclude any proprietary ClearPath features from the actuator design, or any features available only on position control motors (e.g., other servo and stepper motors). We suggested this to ensure that a wide variety of motors could be used in the design, especially motors that are easily sourced in high volumes, such as windshield wiper motors.

Although we would have loved to incorporate more sophisticated features into the ventilator, the added complexity would have slowed down the project due to the extra time required to design, test, document, etc. These additional features also would have compromised manufacturers’ ability to source components.

We ruled out features such as reverse direction capability (at variable cam angles), custom speed vs. angle (e.g., to change the i/e ratio), dwells or pauses in motion, etc. Remember, a critical goal of this project was to move from concept to large-scale development in an extremely short window.

Day 4: Functional Test at the University of Minnesota

We iterated on the design multiple times in rapid succession and the doctors at the University began laboratory testing. The video below provides some of the product details and gives a brief overview of the testing process.

Video 3: The University of Minnesota created this video and provided a brief overview of the project, explained the project’s goals, and raised even more awareness over the lack of ventilators.

A question we’ve been asked is, “Why did [the team] start testing with a ClearPath motor if you planned to use wiper motors in high volume (tens of thousands per week)?”

We decided to use Teknic’s ClearPath motors on the initial prototype units for a number of reasons. Because the team asked Teknic to provide the motor and controls expertise, it made sense for us to start with what we had immediate access to.

The ClearPath motors were readily available in any configuration that made sense for the design, and we were confident that we would have the prototype actuators running quickly on the first try. A redesign at Day 4 would have delayed the critical task of collecting laboratory data and we didn’t have time for delays.

The team had a number of other reasons to use Teknic’s ClearPath motor in the initial prototype units:

  1. The model that we selected had enough power even in a direct-drive mechanism and at worst-case loading (i.e. we didn’t need to source gearing, even if it was beneficial for other reasons).
  2. The electronics (drive and controls) are integrated, so the prototype didn’t require any external circuitry.
  3. A servo-controlled motor eliminated any unforeseen issues related to speed variation under load.
  4. The motor could have been switched to bi-directional velocity mode or reciprocating positional mode in seconds, if necessary.
  5. The motor’s firmware reported operational data in real-time (e.g., torque vs. time, actual velocity, temperature, position, bus voltage)
  6. We could have limited the torque to an arbitrary value (for safety or to mock-up the feasibility of using smaller motors).
  7. We had over 1,000 variations in torque/speed characteristics to choose from. All of these combinations had identical electrical/software interfaces, and we could have shipped any variation immediately.
  8. We were able to pre-program the ClearPath-MC series and the application did not require any software during actual operation.

Day 8: An Update on the Project

We continued to iterate on the design and settled on a version that did not use any gearing at all. While this change required an increase in motor torque, it also meant that the eventual manufacturers of this device didn’t need to source or construct gearing. Even this relatively simple change was helpful to achieve our goals.

Video 4: Over the last eight days Teknic received questions from people who wanted to understand when a clinician might use a device like the Ambu-bot and about the robot’s (intentional) simplicity. We created this video to help answer those questions.

The team started building pre-production quantities and Teknic began working on a preliminary design for a new motor drive. This new drive used the same input as the ClearPath system, but drove a less sophisticated motor. This was to further simplify motor production in mass volume from a wide variety of sources.

Day 11: FDA EUA Submission and the Beginning of Manufacturing

Governor Doug Burgum of North Dakota placed an order for 2,000 units in anticipation of the FDA Emergency Use Authorization. Appareo Systems of North Dakota began sourcing components (including Teknic motors), manufacturing parts, and assembling the devices for rapid deployment.

Day 15: Shipping Began and Another Company Joined the Team

On Day 15, Teknic began shipping motors to Appareo Systems for assembly and production. Boston Scientific, a large medical devices manufacturer, officially announced that they would begin manufacturing these devices as well.

Appareo ventilator

Figure 3: Appareo version of the ventilator

Day 20: Teknic Continued Shipping Motors for Production

Team members from the University of Minnesota and Boston Scientific met with the FDA. According to the team at Boston Scientific, “[The FDA group] greatly appreciates the fast response we are delivering with appropriate use of technical standards, quality and design controls, risk management processes and manufacturing practices to ensure device safety and performance.”

Day 28: The FDA Granted EUA Approval

The FDA formally granted Boston Scientific and the University of Minnesota an Emergency Use Authorization for the Ambu-bot under the official name “Coventor Automatic Adult Manual Resuscitator Compressor.” Boston Scientific announced plans to build 3,000 units, and then more as needed.

Video 5: By now many people followed the project and wanted to know what happened with the FDA’s EUA approval process. We launched this video to inform these people of the project’s success.

Prototype Design Files:*

*Please note that these files are out of date as they were our first prototype version. Teknic and other parties are working hard on improvements and enhancements and we plan on posting updated files soon.

To Our Partners:

Teknic would like to give special thanks to all of the team members who participated on this project: