I know a sprag unit is used in the transmission.

  • What is it and how does it work?
  • Is it used anywhere else or is it strictly a thing used in transmissions?
  • Are there different kinds of sprags?

A sprag is a kind of clutch that will turn in one direction but not in the other.

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There are many designs but the common one is a set of balls on ramps. When going the wrong way the rollers run up the ramp assisted by the spring and jam rotation. When it's rotated the correct way the rollers are constantly being forced down allowing rotation.

The most common place in a transmission to use this is the stator in the torque converter. The stator hold still starting from a stop to redirect fluid and will spin when at cruise speed.

This kind of clutch has many applications. The bendix of a starter contains one and will spin freely for a few seconds when the engine start but the starter is still engaged. Some alternators have one built into the pulley to reduce the spinning momentum of the engine when the RPM drop suddenly. Hellicoptors have it for if the engine fails the blades can still rotate.

There are several versions of sprags. Some use rollers as mentioned. Others use oblonged feet or wedges.

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  • I am looking at it and can't wrap my head about how it works. What do the springs do? Jan 20 '16 at 9:53
  • 1
    @IhavenoideawhatI'mdoing The springs push the rollers holding them against the outer race and the ramp of the inner race. In one direction the ramp gets narrower pushing the roller against the outer race keeping it from turning. In the other direction the ramp gets wider, no longer holding the rollers. This allows turning.
    – vini_i
    Jan 20 '16 at 12:06
  • This helped: youtube.com/watch?v=QjR7dimpSJA Jan 20 '16 at 12:13
  • GM used a sprage in Jetaway ( hydramatic) trans. It looked like a roller bearing but the rollers were oval . Very simple. I had one fail , it was good until until full throttle then it let go with a bang. I stopped and it reset and worked unless too much torque was applied. Feb 16 at 15:39
  • It is more a ratchet than a clutch. Feb 16 at 15:41

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