One-way roller clutches: one idea, many applications

Roller clutches transfer a torque in one direction and move freely in the opposite direction. In neutral, they have a low frictional torque, which prevents drive system losses and saves energy. Drawn cup roller clutches are high-precision machine elements consisting of thin-walled, drawn outer rings (drawn cups) with a series of ramps on the inside diameter, plastic cages, springs and needle rollers. They can be used in a wide range of applications as backstops or overrunning clutches. Drawn cup roller clutches have very compact radial dimensions and carry out indexing from the locking direction in the freewheel direction. Roller clutches with bearing supports can also support radial forces.

Due to their low mass and the resulting low moment of inertia of the clamping elements, drawn cup roller clutches from Schaeffler are characterised by their high indexing frequencies. They also have a low overrunning frictional torque. As the needle rollers are individually spring retained, the shaft, needle roller and clamping surface remain in contact with each other at all times, which enables a high indexing accuracy to be achieved.

One-way clutches for e-bikes

Drawn cup roller clutches from Schaeffler are used as reliable one-way clutches for e-bikes, where they transmit high torques from the drive to the output components, for example, from the bottom bracket spindle to the chain ring. In central motor drives for e-bikes, drawn cup roller clutches perform an overrunning or locking function to decouple the central motor from the drive train. This is needed if the cyclist doesn't require the assistance of the drive system, i.e. on a flat stretch of road or if the drive is deliberately switched off.

The HF, HFL and FRAX series of drawn cup roller clutches are particularly popular in bicycle applications. HF one-way clutches have an very compact design but have no bearing supports and so can only transmit torques. HFL series one-way clutches can also support radial forces due to the integral plain or rolling bearings. Both one-way clutch series are suitable for aluminium housings. Roller clutches with a knurled outside surface have the suffix "R' and are suitable for use with plastic housings.

Protecting motorcycles

The FRAX series of roller clutches have a sleeve design with outer rings and integrated clamping ramps manufactured using forming methods, which improves manufacturing process reliability. Compared with drawn cup roller clutches with needle rollers, FRAX series roller clutches are capable of transferring much higher levels of torque. The compact design provides excellent power density, while simplifying assembly and mounting.

The most common application of the FRAX roller clutch is the E-starter one-way clutch for motorcycles. This forms the connection between the electric starter and the crankshaft. The one-way clutch safeguards the starter components from damage when the combustion engine is started and the engine speed exceeds that of the starter.

Roller clutches are also finding applications in automotive passenger cars, where they are used as one-way clutch discs for decoupling from irregularities in the powertrain drive, as well as automatic transmission converters, torque converters and hybrid transmissions.

Design calculation and simulation

In the automotive industry, minimising energy loss during the "engine idle' phase has become increasingly important in reducing vehicle fuel consumption and CO2 emissions. The friction torque from the roller clutch can be minimised by optimal spring loading: if the outer ring rotates, the rollers can even lift off from the inner ring, eliminating roller clutch friction.

To meet specific customer requirements, Schaeffler can develop custom designed roller clutches using finite element analysis (FEA) and its in-house developed design calculation and simulation software for rolling bearings, BEARINX. The roller clutch can be designed to suit the customer's components and specific torque requirements.

For more information about one-way roller clutches, please go to www.schaeffler.co.uk.