Unidirectional Clutch (Free-wheel)
This device is also called a free wheel or one-way clutch. Its action is similar to that used on a bicycle. It transmits drive in one direction only. In the past a free-wheel unit was often mounted as a separate unit behind the gearbox. In this configuration it enabled the vehicle to be free wheeled or coasted when the road conditions were favorable for its use. This short of use slightly improved the fuel consumption, but at the expense of extra brake wear. The device also enabled the driver to make a gear change on a manual gearbox without operating the main clutch. Once the vehicle was in motion, release of the accelerator caused the one-way clutch to disconnect the road wheels from the gearbox. This removed the driving load from the gears and so allowed easy movement of the gear change lever. When the driver did not require the free wheel feature, the unit was locked by a gear to provide a fixed-wheel condition. This was necessary to obtain reverse, so provision was made to lock the unit automatically when this gear was selected.Nowadays the unidirectional clutch is used as a part of a automatic transmission and overdrive units, to limit the movement of a particular member to one direction. In these examples, the clutch works as a simple means for either driving or holding one part of an epicyclic train so that it can only move one way. The two main types of unidirectional clutch in use are roller type and sprag type.
Roller Type.
This design uses a number of parallel rollers, sandwiched between the inner member and the inclined cam faces of the cylindrical outer member (Fig. 25.39). The type shown incorporates a series of concertina-shaped strip springs to wedge the rollers between the two faces. The application shown in the figure is a torque converter. In this case the hub of the unidirectional clutch is mounted on the casing so that it controls the direction of movement of the converter stator.
Fig. 25.39. Roller type unidirectional clutch.
Sprag Type.
The main features of this design are illustrated in Fig. 25.40. A number of inclined wedges called sprags are held in a spring cage and positioned between inner and outer cylindrical tracks. The spring is designed to twist the sprags in the wedge direction and keep them in contact with both the tracks.The inner member is rigidly secured to a component casing, and an anticlockwise movement of the outer track releases the wedge action of the sprags so that the outer track moves freely. If the direction of motion of the outer track is reversed, the friction between the sprag and the track greatly increases the wedge action and prevents slip between the members. Thus, in this case the clutch is fixed when the outer track is turned clockwise, and free when moved anticlockwise.
Unidirectional Clutch Faults.
Since the two operating stages of a unidirectional clutch are free and fixed, the two main faults are free when it should be fixed, and vice versa, termed as slip and seizure respectively.
Fig. 25.40. Sprag type unidirectional clutch.
A broken spring is a common cause of slip, and a fractured roller or sprag is responsible for a seizure, by causing the unit to wedge in both directions. Normally, when these problems occur the complete free wheel unit is replaced.