What is Freewheel Unit? Construction and Operation of the Freewheel Unit.


A freewheel unit is also known as an overrunning clutch, sprang clutch or one-way clutch. It is an essential part of every overdrive. It transmits power in one direction only from the transmission main shaft to the output shaft when the sun gear unlocked and releases the main shaft from driving the output shaft and when the planetary gears are in overdrive.

Freewheel Unit
Freewheel Unit


A freewheel unit consists of a hub and an outer race. The hub has internal splines to connect it to the transmission main shaft. The outer surface of the hub contains twelve cams so designed to hold twelve rollers in a cage between them and the outer race. The outer race is splined with the overdrive output shaft.

Working Principle

When the hub is driven in the clockwise direction, as shown in Figure, the rollers ride up the cams: and by their wedging action, they force the outer race to follow the hub. Thus, the outer race moves in the same direction and at the same speed as the hub. When the hub speed slows down and the outer race is trying to still move faster than the hub, the rollers move down the cams, releasing the outer race from the hub. Thus, the outer race rotates independent of the hub and the entire unit acts as a roller bearing. The main shaft is connected to the hub and the output shaft is connected to the outer race of the freewheel unit. The free-wheel unit can transmit power only from the transmission main shaft to the output shaft.

You can watch the animation video below to understand the freewheel unit better.

animation video of the freewheel unit

Advantages and disadvantages of Freewheel Unit

By its nature, a freewheel mechanism acts as AN automatic clutch, creating it attainable to alter gears during a manual gearbox, either up- or downshifting, while not depressing the pedal, limiting the employment of the manual clutch to ranging from a standstill or stopping. The Saab freewheel may be engaged or disengaged by the driving force by severally pushing or pulling a lever. This locks or unlocks the most shaft with the freewheel hub.

A freewheel additionally produces slightly higher fuel economy on carbureted engines (without fuel turn-off on engine brake) and fewer decline the manual clutch, however, it results in a lot of declines the brakes as there’s now not any ability to perform engine braking. This makes freewheel unit dangerous to be used on cars and trucks driven in mountainous regions, as prolonged and continuous application of brakes to limit vehicle speed shortly results in brake-system heating followed shortly by total failure.

Bicycle freewheel
Bicycle freewheel

Uses of Freewheel Unit

Agricultural instrumentality

In agricultural equipment, AN overrunning clutch is often used on fodder balers and different instrumentality with a high mechanical phenomenon load, significantly once utilized in conjunction with a tractor while not live power take-off (PTO). while not live power takeoff, a high inertial load will cause the tractor to still move forward even once the foot clutch is depressed, making an unsafe condition.

By disconnecting the load from the power takeoff beneath these conditions, the overrunning clutch improves safety. Similarly, several unpowered ‘push’ cylinder lawnmowers use a freewheel to drive the blades: these are double-geared or chain-driven to rotate at high speed and also the freewheel prevents their momentum being transferred within the reverse direction through the drive once the machine is halted.

Engine starters

A freewheel assembly is additionally wide used on engine starters as a form of protecting the device. Starter motors typically need to spin at 3,000 rpm to induce the engine to show over. once the key’s command within the begin position for any quantity of your time when the engine has started, the starter cannot spin quick enough to stay up with the regulator.

As a result of the acute gear quantitative relation between the starter gear and regulator (about fifteen or 20:1), it’d spin the starter coil at hazardously high speeds, inflicting an explosion once the force working on the copper coils wound within the armature will now not resist the outward force working on them. In starters the freewheel or overrun clutch used.

This may be a serious drawback so, with the regulator spinning at concerning 1,000 rpm at idle, the starter, if engaged with the regulator, would be forced to spin between 15,000 and 20,000 RPM. Once the engine has turned over and is running, the overrun clutch releases the starter from the regulator ANd prevents the gears from re-meshing (as in an accidental turning off the ignition key) whereas the engine is running. A freewheel clutch is currently utilized in several motorcycles with an electrical starter. it’s used as a replacement for the Bendix drive used on most car starters as a result of it reduces the electrical wants of the beginning system.

Vehicle transmissions

In addition to the automotive uses listed higher than (i.e. In two-stroke-engine vehicles and early four-stroke vehicles), freewheels were used in some luxury or up-market typical cars (such as Rovers and Cords) from the Thirties into the Sixties. Some engines of the amount additionally cared-for pass oil past the piston rings beneath conditions with a closed throttle and high engine speed, once the slight vacuum within the combustion chamber combined with high-pressure level and a high degree of splash lubrication from the fast-turning crankshaft would cause oil stepping into the combustion chamber.

You can watch the practical video of assembly and Disassembly Of the freewheel unit below.

assembly and Disassembly Of the freewheel unit

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