Every time the engine decelerates, for example during gear changing or engine shutdown, the alternator's inertia creates resistance against the belt. An overrunning alternator pulley (OAP) allows the alternator to "free-wheel" or "overrun" when the belt suddenly slows down. This prevents the belt from slipping and reduces vibration, wear and chirp noises.
Construction features

  • Pulley with appropriate multi-ribbed belt profile.
  • Internal overrunning clutch unit with double bearing support, allowing the alternator rotor to overrun when the engine decelerates or is shut down.
  • Protective cover.
  • In addition to above features some pulley models incorporate a patented internal application-specific torsion spring which isolates torsional vibration. These pulleys with a higher level of functionality are called overrunning alternator decouplers (OAD).


Illustration of the overrunning function of an OAP.

Watch how the torsional vibrations in the drive are being absorbed.

Watch the difference in performance of the drive.

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