In mechanical

Mechanical engineering

Mechanical engineering is a discipline of engineering that applies the principles of physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the...

 or automotive engineering

Automotive engineering

Modern automotive engineering, along with aerospace engineering and marine engineering, is a branch of vehicle engineering, incorporating elements of mechanical, electrical, electronic, software and safety engineering as applied to the design, manufacture and operation of motorcycles, automobiles,...

, a freewheel or overrunning clutch is a device in a transmission

Transmission (mechanics)

A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...

 that disengages the driveshaft

Driveshaft

A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...

 from the driven shaft when the driven shaft rotates faster than the driveshaft. An overdrive

Overdrive (mechanics)

Overdrive is a term used to describe a mechanism that allows an automobile to cruise at sustained speed with reduced engine RPM, leading to better fuel economy, lower noise and lower wear...

 is sometimes mistakenly called a freewheel, but is otherwise unrelated.

The condition of a driven shaft spinning faster than its driveshaft exists in most bicycle

Bicycle

A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....

s when the rider holds his or her feet still, no longer pushing the pedals

Bicycle pedal

A bicycle pedal is the part of a bicycle that the rider pushes with their foot to propel the bicycle. It provides the connection between the cyclist's foot or shoe and the crank allowing the leg to turn the bottom bracket spindle and propel the bicycle's wheels...

. In a fixed-gear bicycle

Fixed-gear bicycle

A fixed-gear bicycle is a bicycle that has no freewheel, meaning it cannot coast, as the pedals are always in motion when the bicycle is moving....

, without a freewheel, the rear wheel would drive the pedals around.

An analogous condition exists in an automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 with a manual transmission

Manual transmission

A manual transmission, also known as a manual gearbox or standard transmission is a type of transmission used in motor vehicle applications...

 going down hill or any situation where the driver takes his foot off the gas

Gasoline

Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

 pedal

Automobile pedal

An automobile may have two to four foot pedals. The arrangement is the same for both right- and left-hand traffic. From left to right:* normally operated by the left foot:**clutch pedal, not in the case of automatic transmission...

, closing the throttle

Throttle

A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

; the wheels want to drive the engine, possibly at a higher RPM. In a two-stroke engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 this can be a catastrophic situation: as many two stroke engines depend on a fuel

Fuel

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

/oil

Oil

An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....

  mixture for lubrication

Lubrication

Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...

, a shortage of fuel to the engine would result in a shortage of oil in the cylinder

Cylinder (engine)

A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...

s, and the piston

Piston

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...

s would seize after a very short time causing extensive engine damage. Saab

Saab Automobile

Saab Automobile AB, better known as Saab , is a Swedish car manufacturer owned by Dutch automobile manufacturer Swedish Automobile NV, formerly Spyker Cars NV. It is the exclusive automobile Royal Warrant holder as appointed by the King of Sweden...

 used a freewheel system in their two-stroke models

Saab two-stroke

The first Saab two-stroke engine was based on a DKW design. The SAAB engine, a two-cylinder with 764 cc engine displacement and 25 hp was transversally placed in the 1950 - 1956 Saab 92, giving it a top speed of . With the 1954 model engine output was raised to...

 for this reason and maintained it in the Saab 96

Saab 96

For the modern car, see Saab 9-6The Saab 96 is an automobile made by Saab. It was introduced in 1960 and was produced until January 1980, a run of 20 years. Like the 93 it replaced, the 96 was a development from the old Saab 92 chassis and, on account of its improvements and modernisation, it...

 V4

V4 engine

A V4 engine is a V form engine with four cylinders and three main bearings.-Automobile use:Lancia produced several narrow-angle V4 engines from the 1920s through 1960s for cars like the Lambda, Augusta, Artena, Aprilia, Ardea, Appia, and Fulvia....

 and early Saab 99

Saab 99

- Development :On April 2, 1965, Gudmund's day in Sweden, after several years of planning, the Saab board started Project Gudmund. This was a project to develop a new and larger car to take the manufacturer beyond the market for the smaller Saab 96...

 for better fuel efficiency

Fuel efficiency

Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...

.

Mechanics

The simplest freewheel device consists of two saw-toothed, spring

Spring (device)

A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

-loaded discs pressing against each other with the toothed sides together, somewhat like a ratchet

Ratchet (device)

A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "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

Roller (disambiguation)

Roller may mean:In biology:* Roller, family of brightly coloured near passerine birds in the genera Coracias and Eurystomus* Roller , varieties of domestic pigeons that tumble or roll in flight.In industry:...

 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 pawl

Pawl

Pawl may refer to:* A common component of a ratchet* A part of the adjustable height locking mechanism of an extension ladder* Pawl , a former racing car constructor...

s 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.

Benefits

By its nature, a freewheel mechanism acts as an automatic clutch

Clutch

A clutch is a mechanical device which provides for the transmission of power from one component to another...

, 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 pulling or pushing a lever. This will lock or unlock the main shaft with the freewheel hub.

A freewheel also produces slightly better fuel economy

Fuel economy in automobiles

Fuel usage in automobiles refers to the fuel efficiency relationship between distance traveled by an automobile and the amount of fuel consumed....

 on carbureted engines (without fuel turn-off on engine brake) and less wear on the manual clutch, but leads to more wear on the brake

Brake

A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....

s as there is no longer any ability to perform engine braking

Engine braking

Engine braking is where the retarding forces within an engine are used to slow a vehicle down, as opposed to using an external braking mechanism, for example friction brakes or magnetic brakes....

. This may make freewheel transmissions dangerous for use on truck

Truck

A truck or lorry is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, with the smallest being mechanically similar to an automobile...

s and automobile

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

s driven in mountain

Mountain

Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...

ous regions, as prolonged and continuous application of brake

Brake

A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....

s to limit vehicle speed soon leads to brake-system overheating followed shortly by total failure.

Agricultural equipment

In agricultural equipment an overrunning clutch is typically used on hay balers and other equipment with a high inertia

Inertia

Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...

l load, particularly when used in conjunction with a tractor

Tractor

A tractor is a vehicle specifically designed to deliver a high tractive effort at slow speeds, for the purposes of hauling a trailer or machinery used in agriculture or construction...

 without a live power take-off

Power take-off

A power take-off or power takeoff is a splined driveshaft, usually on a tractor or truck, that can be used to provide power to an attachment or separate machine. It is designed to be easily connected and disconnected...

 (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

Angular momentum

In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

 being transferred in the reverse direction through the drive when the machine is halted.

Engine starters

A freewheel assembly is also widely used on engine starters as a kind of protective device. Starter motors

Automobile self starter

A starter motor is an electric motor for rotating an internal-combustion engine so as to initiate the engine's operation under its own power.- History :...

 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

Gear ratio

The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...

 between starter gear and flywheel (about 15 or 20:1) it would spin the starter armature

Armature

Armature may refer to:* Armature , the kinematic chains used in computer animation to simulate the motions of virtual characters...

 at dangerously high speeds, causing an explosion when the centripetal force

Centripetal force

Centripetal force is a force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path. The mathematical description was derived in 1659 by Dutch physicist Christiaan Huygens...

 acting on the copper coils wound in the armature can no longer resist the outward force

Centrifugal force

Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...

 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

Flywheel

A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...

, would be forced to spin between 15,000 and 20,000 RPM. Once the engine has turned over and is running, the overrun clutch will release the starter from the flywheel and prevent the gears from re-meshing (as in an accidental turning of the ignition key) while the engine is running. A freewheel clutch is now used in many motorcycles with an electric starter motor. It is used as a replacement for the Bendix drive

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 flywheel of the engine automatically when the starter is powered or when the engine fires, respectively...

 used on most auto starters because it reduces the electrical needs of the starting system.

Vehicle transmissions

In addition to the automotive uses listed above (i.e. in two-stroke-engine vehicles and early four-stroke Saabs), freewheels were used in some luxury or up-market conventional cars (such as Rover

Rover

-Leyland companies:* Rover Company , a British motorcycle and car manufacturing company, absorbed into Leyland Motor Corporation in 1967* Austin Rover Group , a mass-market car manufacturing subsidiary of Leyland...

s and Cords

Cord Automobile

Cord was the brand name of a United States automobile, manufactured by the Auburn Automobile Company from 1929 through 1932 and again in 1936 and 1937....

) from the 1930s into the 1960s. The freewheel meant that the engine returned to its idle speed on the overrun, thus greatly reducing noise from both the engine and gearbox. The mechanism could usually be locked to provide engine braking

Engine braking

Engine braking is where the retarding forces within an engine are used to slow a vehicle down, as opposed to using an external braking mechanism, for example friction brakes or magnetic brakes....

 if needed. A freewheel was also used in the original Land Rover

Land Rover Series

The Land Rover Series I, II, and III are off-road vehicles produced by the British manufacturer Land Rover that were inspired by the US-built Willys Jeep...

 vehicle from 1948 to 1951. The freewheel controlled drive from the gearbox to the front axle

Axle

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

, which disengaged on the overrun. This allowed the vehicle to have a permanent 4 wheel drive system by avoiding 'wind-up' forces in the transmission. This system worked, but produced unpredictable handling, especially in slippery conditions or when towing, and was replaced by a conventional selectable 4WD system.

Other car makers fitted a freewheel between engine and gearbox as a form of automatic clutch

Clutch

A clutch is a mechanical device which provides for the transmission of power from one component to another...

. Once the driver released the throttle and the vehicle was on the overrun the freewheel would disengage and a gearchange could be made without the use of the clutch pedal. This feature appeared mainly on large, luxury cars which often had heavy clutches and gearboxes without synchromesh as the freewheel permitted a smoother and quieter change. Citroën

Citroën

Citroën is a major French automobile manufacturer, part of the PSA Peugeot Citroën group.Founded in 1919 by French industrialist André-Gustave Citroën , Citroën was the first mass-production car company outside the USA and pioneered the modern concept of creating a sales and services network that...

 combined a freewheel and a centrifugal clutch

Centrifugal clutch

A centrifugal clutch is a clutch that uses centrifugal force to connect two concentric shafts, with the driving shaft nested inside the driven shaft....

 to make the so-called 'TraffiClutch' where the car could be started, stopped and the lower gears be changed without using the clutch pedal. This was an option on Citroën 2CV

Citroën 2CV

The Citroën 2CV |tax horsepower]]”) was an economy car produced by the French automaker Citroën between 1948 and 1990. It was technologically advanced and innovative, but with uncompromisingly utilitarian unconventional looks, and deceptively simple Bauhaus inspired bodywork, that belied the sheer...

s and its derivatives and, as the name implied, was marketed as a benefit for driving in congested urban areas.

A common use of freewheeling mechanisms is in automatic transmissions. For instance traditional, hydraulic General Motors transmissions such as the Turbo-Hydramatic

Turbo-Hydramatic

Turbo-Hydramatic is the registered tradename of a family of automatic transmissions developed and produced by General Motors. These transmissions mate a three-element torque converter to a Simpson planetary geartrain, providing three forward speeds plus reverse.The Turbo-Hydramatic series was...

 400 provide freewheeling in all gears lower than the selected gear. E.g., if the gear selector on a three-speed transmission is labelled 'drive'(3)-'super'(2)-'low'(1) and the driver has selected 'super', the transmission will freewheel if first gear is engaged, but not in second or third gears; if in 'drive' it will freewheel in first or second; finally, if in low, it will not freewheel in any gear. This allows the driver to select a lower range to achieve engine braking at various speeds, for instance when descending steep hills.

Bicycles

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

Freehub

A freehub is a type of bicycle hub that incorporates a ratcheting mechanism, and the name freehub is a registered trademark of Shimano. A set of sprockets are mounted onto a splined shaft of the freehub to engage the chain...

.

Helicopters

Freewheels are also used in rotorcraft. As a bicycle's wheels need to be able to rotate faster than the pedals, so do a rotorcraft's blades need to be able to spin faster than its drive engines. This is especially important in the event of an engine failure where a freewheel in the main transmission allows the main and tail rotor systems to continue to spin independent of the drive system. This provides for continued flight control and an autorotation

Autorotation

In aviation, autorotation refers to processes in both fixed-wing and rotary-wing aircraft. The term means significantly different things in each context....

 landing.

History

The friction freewheel was a part of the Torpedo bicycle gear hub invented by Ernst Sachs in 1903.

A bicycle freewheel was developed and patented by cycle component manufacturer Villiers Engineering

Villiers Engineering

Villiers Engineering was a manufacturer of motorcycles and cycle parts, and an engineering company based in Villiers Street, Wolverhampton, England....

 of England in 1902.

The freewheel is sometimes known as the "Stieber Clutch" named after its German developer Ortwin Stieber.