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Power Transmission Distributors Association is...

the leading association for the power transmission/motion control industrial distribution channel, bringing together distributors and manufacturers.

Tech Tip: Clutches & Brakes

Enhance your employees’ product and industry knowledge with PTDA’s Tech Tips. This library of online tips, hints and techniques may be used to educate new and current employees on power transmission/motion control (PT/MC) products, technologies and concepts and serve as reference and reminder for more experienced staff. Tech Tips are based on the expertise of PTDA member companies or content from PTDA’s Power Transmission Handbook®, the definitive resource and training tool on PT/MC products. Subscribe to the monthly Tech Tips email series to receive two new tips each month.
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Why use grease lubrication in a backstop?

Backstops typically are in overrunning mode for the vast majority of their lifetime, and only engage on rare occasions. Because of this, the grease lubrication can offer a longer maintenance cycle and lower operating temperatures with the same protection due to the low frequency of engagement and lower speeds typical of most backstopping applications. Grease lubrication is also far less likely to leak through the seals.

What are the most common reasons for sprag/cam clutches to fail?

The two most common reasons that a sprag/cam clutch fails are improper installation methods and lubricant contamination.
1. Improper installment methods:
  • Misalignment – A common issue when installing a sprag/cam clutch is that not enough attention is paid to the shaft and hub alignment. Sprag/cam clutches are very high tolerance components that require very minute installation tolerances for proper installation. It is essential to maintain proper axial alignment and concentricity of components to ensure a long service life. Another common cause of misalignment is when the torque arm is rigidly mounted to the support frame. The torque arm must have a floating mount to allow for minor movement during operation. Without this floating mount, the unit can seize up during overrunning.
  • Improper press force – Because of the high tolerances required for the shaft and hubs used with sprag/cam clutches, it can be difficult sometimes to get the unit to slide into place. It is a common, but improper, practice to use a hammer or some other form of impact to the outer race of the unit to install a clutch on a shaft. This will cause misalignment internally in the clutch that will severely limit the service life. When installing a backstop to a shaft, the only portion of the unit that can receive impact loads are the inner race. Ensure that your impact is equally distributed across the face of the inner race to avoid damage. The images below show what can occur to an inner and outer race due to installation misalignment.
Clutches-10-19_image-1.jpg   Clutches-10-19_Image-2-(1).jpg

2. Lubricant contamination

In applications that are very dirty, dusty or wet, maintaining healthy lubrication is essential to get the longest service life out of the product. Always make sure to follow the lubrication maintenance cycle described in the product’s instruction manual. Grease lubricated clutches have longer maintenance intervals, which limit the chances for contaminants to enter the unit during regular maintenance. Labyrinth seals also help to improve service life by preventing contaminants from entering the sealed product.


Can lubricating oils with extreme pressure additives be used with backstops installed internally to a gearbox?

To fully answer this, we need to break it down into internal and external backstops.

  • External Backstops
    • External backstops are fully contained units and cannot use lubricants with EP additives. Extreme pressure additives help to make a lubricating oil that has less compressibility than standard oils, which is generally beneficial for high speed, high torque gear box applications. However, this lack of compressibility restricts the interference fit between the sprags/cams and the outer and inner races.
  • Internal Backstop
    • Backstops that are installed internally to gearbox reducers can be used with EP additives in mose cases. These unique units are not fully contained, and are specially designed to have proper function while using EP additives in the lubricant. There are few exceptions, so make sure to always check with the manufacturer before using EP additives with a gearbox that has internal backstops.

What three clutch techniques are commonly used in backstop applications and why?

Sprag, wrap-spring, and ramp and roller style clutches are used in backstop applications due to their self-actuated technology that does not require any input signal to engage when reverse rotation occurs. These all provide low to zero backlash backstopping.

Why use a mechanical lock-up style clutch over other technologies?

The design of mechanical lock-up clutches allow for their use in applications that require no-slip engagement and cycling that other clutch technologies cannot meet. They also can be used in backstopping, overrunning and indexing applications that other clutch technologies cannot.

Why do I hear noise in the newly installed friction clutch/brake?  

It is not uncommon in a newly installed friction clutch/brake for there to be a chirp or brief squeal as the unit engages; as the unit burnishes this will normally go away. Never add any lubricant to try to eliminate this noise as it will disrupt the function of the unit.

Cause of Rapid Unit Failure in a Wrap Spring Clutch/Brake

The rectangular plate commonly used in wrap spring clutch/brakes should be considered a torque arm.  Therefore it should be mounted so that there is some play or compliance in the mounting.  Rigidly mounting the plate will result in binding and rapid unit failure.

Lubrication of Overrunning Clutches

When lubricating an overrunning clutch or backstop, there will be multiple possible recommended lubrication choices from the manufacturer.  It is critical that lubrication of an overrunning clutch or backstop not include extreme pressure (EP) additives.  Overrunning clutches use the gripping force of the sprags and the clutch races to transmit torque.  EP additives will disrupt this gripping force.

Use of New Electro-Magnetic Friction Clutches or Brakes

Many electro-magnetic friction designs require a wear-in or burnishing process to achieve full torque upon new installation (or after rebuilding).  This process can commonly take from 50-100 on-off cycles.  Generally, a burnishing process will call for one-second-on and one-second-off cycling.  Manufacturers are able to hold friction surface flatness within a few thousandths of an inch.  Burnishing will wear down any high spots and allow for full friction face engagement.

Noise in Friction Clutches/Brakes

It is not uncommon in newly installed friction clutch/brakes for there to be a chirp or brief squeal as the unit engages; as the unit burnishes, this will normally go away.  Never add any lubricant to try to eliminate this noise as it will disrupt the function of the unit.

The information provided in Tech Tips is not meant to be all-encompassing, but rather to draw attention to and provide information about the particular subjects covered. All suggestions and recommendations contained in Tech Tips are based upon information that is believed to be accurate to the best of the experience and knowledge of PTDA’s contributing members, but are made without guarantee or representation as to results. PTDA and Tech Tip contributors expressly disclaim any warranties or guaranties, express or implied, as to the accuracy or completeness of any information published in Tech Tips, and disclaims and makes no warranty that the information in Tech Tips will fulfill any of your particular purposes or needs. PTDA and Tech Tip contributors disclaim liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on Tech Tips.