Page 189 - The Tribology Handbook

One-way clutches B6

A freewheel or one-way clutch is a device which transmits torque in one direction, and disengages or freewheels in the other
direction. The most familiar example is the bicycle freewheel which enables the cyclist to stop pedalling at will.
Numerous applications occur in small machinery. Two-speed drives can be designed with two motors of different speeds,
coupled to the driven machine by one-way clutches. Indexing mechanisms using one-way clutches can convert
reciprocating motion into unidirectional rotation. Uphill conveyors can be prevented from rolling back using a one-way
clutch as a backstop.
Table 6.1 Types of one-way clutch

Type Description Special characteristics
Ratchet amd pawl Hard steel spring loaded pawls Backlash equal to ratchet tooth
engage with ratchet teeth. spacing. Makes clicking sound
(Springs not shown). and suffers wear during
freewheeling. Low precision, low
cost. Plastic versions with axial
configurations are possible.

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Locking roller Rollers ride up ramps and drive by Low cost because made by
wedging in place. Spring bias to modifying roller bearings
engage. (Springs not shown).

Sprag clutch Profiled elements restrained by a Highest torque capability of ai1
cage. Jam in place to drive. types. High cost. Can eliminate
Spring bias to engage. (Springs wear when freewheeling by
not shown). means of centrifugal lifting of
elements.

Wrap spring Friction between hubs and coil Can be made very small.
spring causes spring to tighten
Potentially low cost but
r"9 n onto hubs and drive in one commercially available units are
direction. In opposite direction expensive.
spring unwraps and slips easily.
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