# Unidirectional Clutch

Clutch that transmits power in a single direction

Libraries:
Simscape / Driveline / Clutches

## Description

The Unidirectional Clutch block models an ideal one-way clutch based on a coupled ring-hub. It transmits torque when the relative velocity of hub and ring becomes positive or negative, depending on how you set the transmission direction. For model details, see Equations.

### Equations

The figure displays the geometry of the unidirectional clutch. The table summarizes the unidirectional clutch variables.

If locked, the clutch transmits torque from the hub to the ring. If unlocked, the clutch transmits no torque. The clutch does not have a state of being unlocked but engaged (transmitting torque). The relative angular velocity (slip), ω, is the angular velocity of hub, ωH, relative to the velocity of the ring, ωR, that is, ω=ωHωR.

The Unidirectional Clutch block implements an ideal model of a one-way clutch and does not include any internal dynamics, such as the rollers and springs.

Locking Condition

The transmission direction determines the condition for locking.

• If you choose transmission for the positive direction, the clutch locks if ω = ωHωR ≥ 0 and becomes zero from positive values. The slip ω then remains zero, and the clutch transfers torque.

If the driveline dynamics then causes ω < 0 later in the simulation, the clutch abruptly unlocks and stops transferring torque.

• If you choose transmission for the negative direction, the clutch locks if ω = ωHωR ≤ 0 and becomes zero from negative values. The slip ω then remains zero, and the clutch transfers torque.

If the driveline dynamics then causes ω > 0 later in the simulation, the clutch abruptly unlocks and stops transferring torque.

For example, locking and torque transfer in the positive direction as follows.

• When the hub rotates faster than the ring (ω > 0), the small gap prearranged between the hub and the ring is cleared. The spring-loaded rollers wedge between the hub and the ring, causing the two to contact, lock, and rotate together; ω resets to zero. (The rotation convention in the figure is clockwise.)

• If the ring rotates faster than the hub (ω < 0), the rollers are pushed out of contact and the clutch unlocks.

For a clutch schematic, see Equations.

Power Dissipated by the Clutch

The unidirectional clutch is ideal and does not dissipate power. Either the clutch is unlocked and applies no kinetic friction, or it is locked (no slip) and applies only static friction.

## Ports

### Conserving

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Velocity of the hub driveshaft axis.

Velocity of the ring driveshaft axis.

## Parameters

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Relative rotation direction for which the clutch locks and transfers torque.

Clutch state at the start of simulation. The clutch can be in one of two states, locked and unlocked. A locked clutch constrains the base and follower shafts to spin at the same velocity, that is, as a single unit. An unlocked clutch allows the two shafts to spin at different velocities, resulting in slip between the clutch plates.

## Version History

Introduced in R2011a