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Learn how a transmission sprag clutch Works

Learn how a transmission sprag clutch Works

The sprag clutch is one of the transmission components. It is a type of roller bearing that rotates freely in one direction, but locks when rotating in the opposite direction. Slip clutches are often used in automatic shift transmissions, but are also found on motorcycles, helicopters, and tow trucks. Today we will focus on car clutches and we will tell you what their function is.

What is clutch sprag?

The sprag clutch is a rolling-type bearing that uses drag rollers, which are activated by a spring that allows the element to rotate freely around another, until the moment the direction is reversed, since at that point it locks and it stop spinning.

Automatic transmission car manufacturers incorporate the sprag clutch to have a smooth gear change when the vehicle is in motion, especially when under load. For this, a separate clutch is used, which prepares the transmission to make the gear change, preventing it from turning faster than the gear activated at the moment.

During the process, the torque is released from the slip clutch to allow the transmission to rotate in a controlled manner and to complete the gear change the high gear is engaged so that the pin becomes its clutch and the process is repeated for each successive gear change. In the event of motor failure, the sprag clutch keeps the rotor in automatic rotation in order to keep the tires moving; this feature is used by motorcycles to maintain traction on the rear wheel with the road surface during high-speed downshifts.

Sprag clutches are also very important as a stop mechanism in cargo-carrying vehicles and frequently operating in a tilted position. In this application they are used to prevent the vehicle from moving forward as the clutch support rollers engage to bring the motion to a stop. 

How does it work?

The slip clutch is similar to cylindrical roller bearings, but instead has non rotating asymmetrical figure of eight bearings, which allow rotation in only one direction. When the unit turns in the right direction, the rollers slide or rotate, but when it turns in the opposite direction, the blades tilt and cause friction sticking. The mounts are spring loaded, which ensures that they lock and at the same time shift smoothly. When the reaction force is no longer needed, the sprag clutch releases. 

The slip clutch has been used in the oldest automatic transmissions and continues to be used in recent ones for shifts that require synchronized engagement of one clutch with another automatically. This is common in late model transmissions that are electronically controlled, which do splineless clutch to clutch shifts, where it can be engaged in first gear (with the shift lever in “D”) to provide just driving forward, but not an engine brake. This is done to allow the low opening throttle 2-1 downshift to be smooth as the car decelerates, avoiding hard, abrupt jerking as a shift result.

In general, the sprag clutch has three stopping ways: 

  • Indexing. Provides precision to the transmission system. It consists of a link between the prime mover and a slip clutch race, so that the reciprocating motion of the motor is that of indexing on the clutch race. 
  • Recoil. It is the stop that prevents an incline conveyor from backing up when the drive system stops. The outer race of the clutch is fixed to the frame of the machine by a torque arm, this race rotates freely in the transmission´s forward direction, and when the machine is going to reverse, the slip clutch prevents movement. 
  • Backup brake. It is a load brake used on the hoist to prevent the load from being dropped in the event of a mechanical failure. 
We hope that this information is useful to you and allows you to learn more about your vehicle's automatic transmission components. At Sun Transmissions we have a wide variety of parts and accessories for automatic transmissions, which you can easily purchase through our online transmission parts store and receive at home, like transmission valve bodiesautomatic transmission additives, transmission connectors, automatic transmission bushing kits, and much more. For more information and advice on the spare part you need, we invite you to write to us through our online chat, where we will gladly assist you and answer all your questions.
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