Freewheel

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. 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 most bicycles when the rider stops pedaling. In a fixed-gear bicycle, without a freewheel, the rear wheel drives the pedals around. An analogous condition exists in an automobile with a manual transmission going downhill, or any situation where the driver takes their foot off the gas pedal, closing the throttle: the wheels drive the engine, possibly at a higher RPM. In a two-stroke engine, this can be catastrophic—as many two stroke engines depend on a fuel/oil mixture for lubrication, a shortage of fuel to the engine starves oil from the cylinders, and the pistons can soon seize, causing extensive damage. Saab used a freewheel system in their two-stroke models for this reason and maintained it in the Saab 96 V4 and early Saab 99 for better fuel efficiency. 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, producing a characteristic clicking sound proportionate to the speed difference of the driven gear relative to that of the (slower) driving gear. 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. Most bicycle freewheels use an internally step-toothed drum with two or more spring-loaded, hardened steel pawls to transmit the load. More pawls help spread the wear and give greater reliability although, unless the device is made to tolerances not normally found in bicycle components, simultaneous engagement of more than two pawls is rarely achieved. By its nature, a freewheel mechanism 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 manual clutch to starting from standstill or stopping. The Saab freewheel can be engaged or disengaged by the driver by respectively pushing or pulling a lever. This locks or unlocks the main shaft with the freewheel hub. A freewheel also produces slightly better fuel economy on carbureted engines (without fuel turn-off on engine brake) and less wear on the manual clutch, but leads to more wear on the brakes as there is no longer any ability to perform engine braking. This may make freewheel transmissions dangerous for use on trucks and automobiles driven in mountainous regions, as prolonged and continuous application of brakes to limit vehicle speed soon leads to brake-system overheating followed shortly by total failure. 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. Similarly, many unpowered 'push' cylinder lawnmowers use a freewheel to drive the blades: these are geared or chain-driven to rotate at high speed and the freewheel prevents their momentum being transferred in the reverse direction through the drive when the machine is halted.