Archive for the ‘Default’ Category

Securing Mechanics to Motor Shafts

Friday, April 24th, 2020

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.)

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.

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.

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)
  • 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

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.

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.

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.

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

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.

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.

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.

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

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

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.

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:

Teknic

Friday, September 20th, 2013

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