Freewheel (mechanics)

The freewheel (also overrunning ) is a force acting only in one direction of rotation clutch .

function

Pawl freewheel: The inner part (green) can transmit a clockwise torque to the outer part (blue) by means of the positive locking of the pawl (red). If the inner part rotates more slowly than the outer part, the locking pawl, which only grips positively in one loading direction, allows the outer part to be overtaken.

A freewheel can only transmit torque in one direction of rotation. If the direction of rotation is reversed, or if the speed of the part actually to be driven is greater than that of the driving part, the connection is released automatically. An example is the freewheel in the hub of a bicycle . The rear wheel continues to run freely when the pedal drive is applied more slowly (the rear wheel overtakes the drive) or is stopped.

Executions

Freewheels can be equipped with the following components:

• Pinch rollers
• Clamp body
• Pawls
• Claw rings
• Wrap spring

Ratchet freewheels click in freewheel mode, while the other freewheels work silently.

On the steam car developed by Nicholas Cugnot as early as 1769 , the steerable front wheel was driven by the two steam cylinders via a freewheel mechanism. The bicycle freewheel was invented by Ernst Sachs in Schweinfurt in 1889 .

Roller freewheel

Cross-sectional drawing of a freewheel with cylindrical pinch rollers (roller freewheel). Free: freewheeling, engaged: blocking direction

The drawing shows a freewheel with pinch rollers . The inner arrow shows the direction of rotation of the drive. The springs press the pinch rollers lightly between the inner part rotating together with the pinch rollers (called “star” in technical jargon) and the outer ring. When torque is transmitted , radial forces arise, so that the pinch rollers wedge themselves in their receiving spaces. With a suitable selection of the angle of incidence or clamping angle of the clamping wedge that is being formed, the design is absolutely non-slip, even with the best lubrication - for physical reasons - the state of self-locking prevails . The taper angle must be selected so that it is less than or equal to the arctangent of the coefficient of sliding friction µ.

If the taper or clamping angle is chosen to be greater than arctan (µ), the freewheel slips and is unreliable.

If the direction of rotation is reversed or if the external speed is greater than the internal speed, the pinch rollers roll in the direction of the spring, thus releasing the clamping.

Typed clamping roller freewheels are commercially available under the designation needle roller freewheel or sleeve freewheel. They are mainly used in the automotive sector.

Sprag freewheel

Freewheel (CamClutch) with a large number of non-round sprags

When using non-circular clamping bodies instead of clamping rollers, the inner part of the freewheel is no longer called a star , but is simply called a cylindrical ring - inner ring . Clamping occurs by slightly twisting the clamp body. The wedge spaces between the sprags are often missing. Thus, the number of clamping bodies is higher than that of the clamping rollers, which increases the contact surface. This enables higher torques to be transmitted or the shaft diameter to be reduced (see downsizing ). Often these sprag type freewheels also have a ball roller bearing in the same housing.

With sprag type freewheels there are various designs to increase the service life. With the optional use of a sprag lift-off, this coupling is wear-free in freewheeling mode with the inner ring rotating rapidly. Other constructive designs to increase the service life are z. B. sprag lift-off with rapidly rotating outer ring, sprag coatings, polygonal ground freewheel outer rings or designs with hydrodynamic sprag lift-off.

Wrap spring clutch

Spring coil freewheel

The wrap spring clutch consists of a helical spring wound on a shaft or a cylindrical body , which is attached to one side of the drive. The entrainment effect is based on the fact that the frictional forces between the spring coils and the shaft try to " wind up" the spring, the operating principle is based on rope friction . The momentum is sometimes increased automatically. In the opposite direction of rotation, the low basic friction torque increases the spring diameter (but cannot unwind the spring). Sometimes the wrap spring clutch is used as a slip clutch in the actual freewheeling direction of rotation , for example in printers or tape recorders.

Self-synchronizing clutch

→ Main article : Self-synchronizing clutch

A self-synchronizing clutch is a freewheel in which the torque is transmitted via a toothed clutch . The latter is disengaged when overtaking . The coupling takes place with the help of a parallel built-in pawl freewheel and is self-synchronizing. No torque is transmitted while the clutch is engaged, after that only the gear clutch is torque-loaded. This design is suitable for the transmission of high power up to the megawatt range because the pawls of the auxiliary freewheel are only loaded by the switching process.

Typical applications

• Internal combustion engines:
  • Starter of internal combustion engines .
    The starter freewheel protects the starter from excessive speeds of a started internal combustion engine.
  • Starter crank for internal combustion engines without a starter, especially for diesel engines.
  • Freewheel pulley on alternators to reduce torsional vibrations of the crank drive

• Vehicles:
  • Switchable freewheel hubs are sometimes used in vehicles with switchable all-wheel drive ,
  • In the gearboxes of larger automobiles with two-stroke engines such as DKW , Wartburg , Trabant , Saab 92 - 96 , in order to avoid possible engine damage due to lack of lubrication during overrun, but also to facilitate operation (clutch does not need to be depressed). The disadvantage here was the safety risk (engine brake does not work), which is why the freewheel was often provided with an additional locking device that could be activated when required (downhill, slippery terrain).
  • Agricultural machinery (loading wagons, manure spreaders, composting plants, round balers)
  • Bicycle , e.g. B. as a torpedo freewheel hub to prevent the bicycle crank from rotating continuously

• Tool ( ratchet )
• Turnstile at the entrance / exit of shops
• simple revolving doors
• Ducter drives for offset printing machines
• Paper feed from copier machines
• Backstop in conveyor systems and winches
• Backstop in the fishing reel .
• Speed ​​compensation in transport systems
• Safety coupling in mixers
• Pull-up device for high-voltage switches
• Switching element in textile machines
• Overrunning clutch in packaging machines
• Modelling
• sewing machines
• mechanical calculating machines
• Helicopter , in the gearbox of the main rotor , in order to be able to carry out an autorotation landing in the event of an engine failure .

literature

• Herbert Wittel, Dieter Jannasch, Joachim Vossiek, Christian Spura: Roloff / Matek Machine elements: standardization, calculation, design - textbook and table book . 24th edition. Springer Vieweg , 2019, ISBN 978-3-658-26280-8 , pp. 504-507 .
• Freiherr H. von Thüngen: The free run. Special construction and application examples in motor vehicles . In: Automobiltechnische Zeitschrift 59 (1957), No. 1, pp. 1ff,

Individual evidence

1. ↑ Gisbert Lechner, Bernd Bertsche, Harald Naunheimer: Vehicle transmissions : Basics, selection, design and construction. Springer Science + Business Media , 1994, ISBN 3-540-57423-9 , page 8
2. ↑ Sleeve freewheels (commercial product description)
3. ↑ Clamping body freewheels (commercial product description) Archived copy ( Memento of the original from April 20, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 
4. ↑ Locking the freewheel on the Trabant. In: Motor Vehicle Technology 8/1962, pp. 345–346.