Hydraulic Clutches Information

Hydraulic clutchActing as an on/off switch for the transfer of rotational energy between a motor or engine and its driven counterpart, hydraulic clutches are power transmission components that contain hydraulically actuated mechanisms for connecting two shafts so that they can either be locked together or decoupled. Hydraulic clutches deliver a higher torque per unit volume than other types of clutches and provide fast response times and smooth engagement. Hydraulic clutches use incompressible fluids such as oil to transfer input movement from typically a pedal and master cylinder to the actuating cylinder on the clutch. Engaging a hydraulic clutch transfers power from an engine to devices such as a transmission and drive wheels. Disengaging a hydraulic clutch stops the power transfer, but allows the engine to continue turning. Hydraulic clutches are self-regulating and require less maintenance than other systems. 


Important specifications to consider when selecting a hydraulic clutch include but are not limited to:

  • Torque Rating—The maximum torque rating for the clutch should equal or exceed the application's requirement. 
  • Power—The maximum power rating for the clutch. 
  • Speed—The maximum rotary speed rating. This specification applies only to rotary clutches. 
  • Maximum Pressure—The maximum pressure for a hydraulic clutch. 
  • Shaft Configuration—Clutch may be mounted in-line, parallel, or right angle.
  • Drive/Load Connection:
    • In-line Shafts—The drive and the load have shafts that attach to a through-bore. 
    • Through Shaft—The drive shaft attaches to a bore and the load is driven through the outer diameter. 
    • Shaft-Pulley/Gear/Sprocket—The drive shaft attaches to a bore and the output is a drive component such as a pulley, gear, or sprocket. These clutches are often designed to accept different drive components. 
    • Flange—The clutch is mounted to the object in motion via a flange.

Several hydraulic clutch engagement methods are available, including:

  • Noncontact—Braking action is achieved through a non-contact technology such as a magnetic field, eddy currents, etc. 
  • Friction—Friction between contact surfaces transmits power. This is the most common configuration. 
  • Toothed—Toothed contact surfaces transmit power without slipping or heat generation. Teeth are engaged only when stopped or running at a slow speed (< 20 rpm). 
  • Wrap Spring—A coiled spring wraps downward onto the rotating element. The device is disengaged when the spring is uncoiled via a control tang at its end. 
  • Oil Shear—Braking action is engaged via the viscous action of the shearing of transmission fluid. 
  • Plate Disc—The torque level is controlled by compression springs that force plates together. 
  • Ball Detent—Ball detent is a slip mechanism in which, upon overload, balls ride up out of seats to overcome springs or air pressure engagement. 
  • Roller Detent—Rollers, held in place by springs, wedge between the inner and outer races to engage the clutch. 
  • Pawl Detent—Pawl detent is a slip clutch mechanism in which, upon overload, the pawl overcomes spring or air pressure engagement and rotates out of its detent. 
  • Sprag—Sprags are steel wedges that tip in one direction to wedge between inner and outer races. They can be configured with either the inner or outer race as the input or output. Too much torque makes the sprags tip so much that contact is not maintained. Often, sprag clutches can transmit more torque than other designs of slip or overrunning clutches.


Additional capacities can be added to hydraulic clutches. These features help customize the unit to meet special or unique requirements. Typical features and options include:

  • Adjustable Torque—Adjustable torque is used primarily for slip clutches and torque limiters. Users can adjust the torque at which the clutch disengages or slips. 
  • Zero Backlash—There is no play or backlash during the engagement of the load and no load disengagement during a direction reversal. 
  • Washdown Capable—The housing is rated for washdown cleaning. 
  • Bi-directional—Devices can be set up to rotate in either direction. 
  • Automatic Re-engagement—The clutch re-engages the load when the torque drops to an acceptable level. 
  • Slip Indication—Slip indication can move a pin radially when an overload occurs, or send an electrical signal to the drive motor. 
  • Feedback—Feedback provides an electrical or electronic signal for monitoring parameters such as position, speed, torque, lockup, or slip status. 

Hydraulic clutches are a predominant feature in mechanical power transmission systems today. Highly visible in the automotive and trucking industries, hydraulic clutch actuation is selected for its simplicity and efficiency in moving disengagement force from the operator’s position to the clutch location. Power take off clutches (PTO) are used with heavy equipment such as dump trucks, snowplows, and tractors to use engine power for applications other than the driveline. Some hydraulic clutches are designed for use with web tension control, automation, or robotics systems. Other devices are designed for use with conveyor drives and pump motor drives. Hydraulic clutches make sense to use in many applications by providing self-adjustment, less actuation effort, and self-lubrication by design.

Related Information

CR4 Community—A Self-contained Marine Hydraulic Clutch System with Integrated PTO

Image credit:

jbj Techniques Limited


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