Sprag clutch failure can be broken down into three types:
- Overrunning failure
- Loading failure
- Cage failure
Overrunning Failure
The most common reason for overrunning failure is wear between the sprags and the mating surfaces. The sprags require a lubrication film to maintain separation between the sprag and the mating surfaces so that wear is minimized. However, if the lubrication is not sufficient to maintain a thick enough oil film to overcome the microscopic surface roughness of the sprag and the mating surface, then there can be some metal-to-metal contact.
This can happen with lubrication that doesn’t have a sufficient viscosity to work at the application operating temperatures. Additionally, if contamination is present in the application, the contaminants will bridge the oil film gap and can act as sandpaper against the sprag, speeding up the wear.
The wear will result in flat spots on the sprag that appear toward the back of the sprag engagement curve. Methods of correction include using proper lubrication with sufficient viscosity and ensuring the environment remains free from contamination.
Another potential cause for sprag failure in overrunning conditions can come from excessive application vibrations.
The springs in the sprag clutches are intended to ensure contact between the sprags and the mating surfaces. But, if the vibrations are significant enough, the forces generated from the vibrations can overcome the spring force and cause the sprag to hit the back of the sprag in front and possibly damage the cage. This can result in broken sprag toes and marks on the backs of the sprags in front as well as twisted cage separators.
If heavy vibration is unavoidable, the spring force can be increased on the sprag clutches to help overcome the vibration forces, however this may result in increased overrunning wear.
Loading Failure
Sprag loading failure is generally one of two types: Excessive torque or Popping
Excessive Torque:
If the application generates a torque that is above the maximum torque limit, the sprags will want to extend past the maximum point on the engagement curve and roll over. This will result in permanent destruction of the clutch.
This would be indicated by sprags laying down in the overloaded position, cage separators twisted or removed, and sprag toes damaged. To rectify this, the application needs to be designed to ensure that the maximum possible torque required is below the torque limit of the clutch.
GMN sprag clutches have a built-in safety to help prevent sprag rollover in excessive torque applications. At the point of maximum torque transmission, the engagement angle starts to increase, which causes the sprags to lose friction contact and slip instead of rolling over.
Even though this is part of the sprag design, it is still possible for the conditions to cause the sprags to roll over in an excessive torque situation, especially if it is a rapid torque transmission. (The sprags should not be relied upon to slip.)
Popping:
The other type of loading failure is a phenomenon called popping. This happens at the point of transmission when the sprags go from overloading to transmitting torque.
As the sprags start to engage, they elastically deform the mating surface, building up potential energy. Something can happen to cause the contact to not be maintained and that built-up energy acts as a spring that rapidly propels the sprags forward creating a “popping” sound.
This can result in damaged sprag toes, marks on the back of the sprags from the toes of the sprags behind hitting the sprags in front, and twisted cage separators.
The most common cause for popping is improper lubrication, mainly the presence of extra pressure additives. If the sprags are unable to break through the oil film when they engage, that film will cause the sprags to slip and spring forward.
Other potential causes for popping and overloading failure can be improper mating surface tolerances as well as incorrect mating surface material, hardness, and hardness depth.
Cage Failure
Cage failure is almost never due to a problem with the cage. The purpose of the cage is to evenly space out the sprags to enable proper torque transfer. No loading should ever be placed on the cage.
If the application is designed correctly, the cages should never experience conditions that cause it to fail. If cage failure occurs, it is typically the symptom of one of the failures mentioned above.
If the conditions that are causing the cages to fail are unavoidable, it is possible to make alterations to the cage design to help ensure clutch success. This can include making the cage thicker or changing cage materials.
If you are experiencing one of the sprag clutch failure symptoms above, contact us for assistance.
Interested in GMN Sprag Clutches?
Check out our Sprag Clutch Guide resources below for more information.