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In mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. An overdrive is sometimes mistakenly called a freewheel, but is otherwise unrelated.

The condition of a driven shaft spinning faster than its driveshaft exists in a bicycle going downhill when the rider holds his or her feet still, no longer pushing the pedals. Without a freewheel the rear wheel would drive the pedals round.

An analogous condition exists in an automobile going down hill or any situation where the driver takes his foot off the gas pedal, closing the throttle; the wheels want to drive the engine, possibly at a higher RPM. In a two-stroke engine this is a lethal situation: as the engine depends on fuel for lubrication, a shortage of fuel to the engine would result in a shortage of oil in the cylinders, and the pistons would seize after a very short time causing extensive engine damage. Saab automobiles used a freewheel system in the two stroke models for this reason and maintained it in the V4 and early Saab 99 for better fuel efficiency.

Contents 1 Mechanics 2 Benefits 3 Uses 4 History 5 External link

The simplest freewheel device consists of two saw-toothed, spring-loaded discs pressing against each other with the toothed sides together, 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 slows down or stops rotating, the teeth of the driven disc 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 also produces slightly better fuel efficiency and less wear on the clutch, but leads to more wear on the brakes as there is no longer any ability to perform engine braking.

In agricultural equipment an overrunning clutch is 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.

A freewheel clutch is now used in many motorcycles with an electric starter motor. It is used as a replacement for the Bendix drive used on most auto starters because it reduces the electrical needs of the starting system.

In the older style of bicycle, where the freewheel mechanism is included in the gear assembly, the system is called a freewheel, whereas the newer style, in which the freewheel mechanism is in the hub, is called a freehub.

A freewheel assembly is also widely used on engine starters as a kind of protective device. Starter motors usually need to spin at 3,000 RPM to get the engine to turn over. When the key is turned to the start position for any amount of time after the engine has already turned over, the starter can not spin fast enough to keep up with the flywheel. Because of the extreme gear ratio between starter gear and flywheel (about 15 or 20:1) it would spin the starter armature at dangerously high speeds, causing an explosion when the centrifugal force acting on the copper coils wound in the armature can no longer resist the outward g-forces acting on them. In starters without the freewheel or overrun clutch this would be a major problem because, with the flywheel spinning at about 1,000 RPM at idle, the starter, if engaged with the flywheel, would be forced to spin between 15,000 and 20,000 RPM. The overrun clutch, in the event of the starter being engaged while the engine is running, causes the flywheel to spin the starter gear but allows the starter's armature to spin at its previously set speed, therefore causing no harm from "over running" the starter.

The freewheel was once known as the "Stieber Clutch" named after its German developer Wilhelm Stieber.

• Stieber Clutch Website