Editing Freewheel (section)

==Uses==

===Agricultural equipment===
In agricultural equipment an '''overrunning clutch''' is typically used on [[hay baler]]s and other equipment with a high [[inertia]]l 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' [[lawnmower|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 [[Angular momentum|momentum]] being transferred in the reverse direction through the drive when the machine is halted.

===Engine starters===
A freewheel assembly is also used on engine starters. [[Automobile self starter|Starter motors]] usually need to spin at 3,000 RPM to get the engine to turn over. When the key is held in the start position for any amount of time after the engine has started, 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 [[centripetal force]] acting on the copper coils wound in the armature can no longer resist the [[Centrifugal force|outward force]] acting on them. In starters without the freewheel or overrun clutch or other disengagement device 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. Once the engine has turned over and is running, the overrun clutch releases the starter from the flywheel and prevents 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 on many combustion-engined mowers. It is used as a replacement for the [[Bendix drive|Starter solenoid]] (or the older [[Bendix drive]]) used on most car starters because it reduces the electrical needs of the starting system and gives reduced complexity.{{Citation needed|date=September 2023}}

===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 [[Packard]], [[Rover Company|Rover]] and [[Cord Automobile|Cord]]) from the 1930s into the 1960s. Some engines of the period also tended to pass oil past the [[piston ring]]s under conditions with a closed throttle and high engine speed, when the slight vacuum in the combustion chamber combined with high oil pressure and a high degree of [[splash lubrication]] from the fast-turning crankshaft would lead to oil getting in the combustion chamber. 

The freewheel meant that the engine returned to its idle speed on the overrun, thus greatly reducing noise from both the engine and gearbox and reducing oil consumption. The mechanism could usually be locked to provide [[engine braking]] if needed. A freewheel was also used in the original [[Land Rover Series|Land Rover]] vehicle from 1948 to 1951. The freewheel controlled drive from the gearbox to the front [[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.

During the Second World War, the military Volkswagen vehicles produced by [[Strength Through Joy|KdF]] ([[Volkswagen Kübelwagen|''Kübelwagen'']], [[Volkswagen Schwimmwagen|''Schwimmwagen'']]) were fitted with a [[ZF Friedrichshafen|ZF]] [[limited-slip differential]] system composed of two freewheels, which sent the whole of the engine power to the slower-turning of the two wheels.<ref>{{Citation |place = DE |title = ''ZF-Axial-Selbstsperrdifferential Typ B70 Beschreibung und Wartung'' |publisher = Zahnradfabrik Friedrichshafen AG |date = July 1941}}.</ref>

Other car makers fitted a freewheel between engine and gearbox as a form of automatic [[clutch]]. Once the driver released the throttle and the vehicle was on the overrun, the freewheel disengaged and the driver could change gears without using the clutch pedal. This feature appeared mainly on large, luxury cars with heavy clutches and gearboxes without [[synchromesh]], as the freewheel permitted a smoother and quieter change. [[Citroën]] combined a freewheel and a [[centrifugal clutch]] to make the so-called 'TraffiClutch', which let the driver start, stop, and change the lower gears without using the clutch. This was an option on [[Citroën 2CV]]s and its derivatives and, as the name implied, was marketed as a benefit for driving in congested urban areas. Similarly, the [[Saab 93]] was available with an optional [[Saxomat]] clutch.

A common use of freewheeling mechanisms is in automatic transmissions. For instance traditional, hydraulic General Motors transmissions such as the [[Turbo-Hydramatic]] 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 freewheels if first gear is engaged, but not in second or third gears; if in 'drive' it freewheels in first or second; finally, if in low, it does not freewheel in any gear. This lets the driver select a lower range to provide engine braking at various speeds, for instance when descending a steep hill.

[[Overdrive (mechanics)#In Europe|Overdrive units]] manufactured by [[Laycock de Normanville]] used a freewheel to facilitate a smooth gear change between locked mode (1:1) and overdrive mode without use of the conventional [[clutch pedal]]. The freewheel would lock the outgoing axle to the outgoing axle in the brief transition period between the [[conical clutch]] for locked mode disengaging and the clutch for overdrive mode engaging.<ref>{{cite web |title=How it Works: The Laycock Overdrive System |url=http://www.uniquecarsandparts.com.au/how_it_works_laycock_overdrive.php |access-date=2 January 2016}}</ref>

===Bicycles===
Apart from the usual use of a freewheel where there is a single sprocket at the wheel, and the older style of [[derailleur gears]] where the freewheel mechanism is included in the gear/sprocket assembly and the system is called a ''[[Cogset#Freewheels|freewheel]]'', the newer style in which the freewheel mechanism is in the hub is called a ''[[freehub]]''.

===Helicopters===
Freewheels are also used in [[rotorcraft]]. Just as a bicycle's wheels must be able to rotate faster than the pedals, a rotorcraft's blades must 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 lets the main and tail rotor systems continue to spin independent of the drive system. This provides for continued flight control and an [[Autorotation (helicopter)|autorotation]] landing.