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How a One Way Clutch Works in Automotive Transmissions

A One Way Clutch helps to ensure smooth gear shifting by preventing the upcoming gear from rotating faster than the one currently in use.  Torque flows from the clutch into the transmission, keeping it in freewheeling mode until the moment it shifts gears. At that point the clutch locks in place to allow the transfer to occur smoothly.

Said another way, a one way clutch is a “torque gate” that allows torque to flow in one direction but not another. A typical example is the Koyo? series of sprag clutches. These units break down into several components. While you’ll find some variation among manufacturers, the typical one way clutch you encounter in the shop will have these parts:

The inner and outer rings, which hold the various parts of the sprag clutch in place.

The sprags, which join the inner and outer rings together when transferring torque.

The cage, a heat-treated carbon steel plate that spreads the sprags across the unit to ensure that torque is spread evenly across the unit.

The spring, which ensures the sprags engage both inner and outer rings evenly.

The end bearing, which keeps the inner and outer rings centered throughout the process.

The drag clip, which holds the cage against the outer ring. This synchronizes the cage and outer ring so that the sprags engage smoothly.

Diagnosing One Way Clutch Problems

When these clutches fist begin to fail, a common sign is a clicking sound from the transmission. If left alone, this will progress to complete loss of forward speeds. Here’s a real life example of how sprag clutch problems affect daily driving:

A 1984 Chevy half-ton truck with a four speed automatic transmission develops problems while traveling at highway speeds. The engine revs very high, but the vehicle slows down when the driver tries to accelerate. After coming to a complete stop, the vehicle will once again start off, but it continues to experience the same issues.

Examination by a transmission tech reveals that the sprag clutch is worn. It will no longer lock down, thus torque is not transferred into the upcoming gears. Replacement of the defective unit solves the problem and the vehicle’s owner goes on his way.

To sum up: one way clutches perform a valuable role in automatic transmissions.  When they fail, the results can be severe.  But, with proper troubleshooting and service, these problems can be resolved.

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