Freewheel mechanism

In mechanical or automotive engineering a freewheel is a device in a transmission that disengages the drive shaft from the driven shaft in the case that the driven shaft would rotate at a higher rpm than the driveshaft.

This condition would exist in a bicycle going downhill, when the rider would hold his feet still, no longer pushing the pedals. Without a freewheel, with a fixed connection between the rear wheel and the pedals, the rear wheel would drive the pedals.

The same condition exists in an automobile going down hill or any situation where the driver is taking his foot off the gas pedal, closing the throttle; then the wheels would want to drive the engine, possibly at a relative high rpm. Especially in a two stroke engine this is a lethal situation as here the engine depends on the fuel also for lubrication; no fuel = no oil, and the pistons would seize in the cylinders after a very short time, causing extensive engine damage.

The simplest freewheel device would consist of two saw toothed, spring loaded discs, pressing against each other with the toothed sides, somewhat like a ratchet. Rotating in one direction, the saw teeth of the drive disc lock with the teeth of the driven disc, making it rotate at the same speed. If the drive disc would slow down or stop rotating, the teeth of the driven disc would just slip over the drive disc teeth and continue rotating.

A more sophisticated and rugged design has spring loaded steel rollers inside a driven cylinder. Rotating in one direction the rollers lock with the cylinder making it rotate in unison. Rotating slower, or in the other direction, the steel rollers just slip inside the cylinder.

By its nature, a freewheel acts as an automatic clutch, making it possible to change gears in a manual gearbox, either up or downshifting, without depressing the clutch pedal, limiting the use of the clutch to starting from standstill or stopping.

A freewheel would also provide slightly better fuel efficiency and less wear on the clutch, but more wear on the brakes as there is no longer the effect of engine braking.

In agricultural equipment, the same mechanism is called an overrunning clutch. Such devices are typically used on hay balers and other equipment with a high inertial load, particularly when used in conjunction with a tractor without a live Power take-off (PTO). Without a live PTO, a high inertial load can cause the tractor to continue to move forward even when the foot clutch is depressed, creating an unsafe condition. By disconnecting the load from the PTO under these conditions, the overrunning clutch improves safety.

An overdrive is sometimes also (mistakenly) called a freewheel.