Bendix drive

A Bendix drive is a type of engagement mechanism used in starter motors of internal combustion engines.

The device allows the pinion gear of the starter motor to engage or disengage the ring gear (which is attached to the flywheel or flexplate of the engine) automatically when the starter is powered or when the engine fires, respectively.

The pinion then rotates enough to allow the gears to mesh, after which the pinion then continues along the shaft to reach a stop on the end of its allowed travel, at which point the gears are fully meshed.

When the engine starts, backdrive from the ring gear causes the drive pinion to exceed the rotational speed of the starter, at which point the drive pinion is forced back along the helical spline and out of mesh with the ring gear.

When the starter motor powers and drives the pinion to engage with the flywheel, this spring cushions the rotational impact as the gears and mesh and begin turning together.