Unidirectional Clutch

Clutch that transmits power in a single direction

Library

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 Unidirectional Clutch Model.

Ports

H and R are rotational conserving ports representing, respectively, the hub and ring driveshaft axes. The clutch motion is measured as the slip ω = ωHωR, the angular velocity of hub relative to ring.

Dialog Box and Parameters

Transmission direction

From the drop-down list, select the relative rotation direction (sign of ) for which the clutch locks and transfers torque. The default is From hub to ring for positive rotation.

Initial state

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, i.e., as a single unit. An unlocked clutch allows the two shafts to spin at different velocities, resulting in slip between the clutch plates. The default setting is Unlocked. This parameter becomes active when you set Clutch Model to State machine - suitable for variable step solvers.

Unidirectional Clutch Model

The Unidirectional Clutch, if locked, transmits torque from hub to ring. If it is unlocked, it transmits no torque. The clutch does not have a state of being unlocked but still engaged (transmitting torque).

Clutch Geometry, Variable, and State Summary

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

Clutch Variable

SymbolDefinitionSignificance
ωRelative angular velocity (slip)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 at a later time, 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 at a later time, 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 Clutch Geometry, Variable, and State Summary.

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.

Example

The sdl_stepping_mechanismsdl_stepping_mechanism example model uses the Unidirectional Clutch to translate a mass in one direction only (ratcheting).

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