Electric clutches, also known as electromagnetic clutches, operate a clutch engagement system electrically but rely on mechanical systems to transmit torque. Consequently, they are sometimes referred to as electro-mechanical clutches. When the clutch is actuated, current flows through an electromagnet producing a magnetic field. The rotor portion of the clutch becomes magnetized and the armature is pulled toward the rotor. Depending on the clutch setup, this motion either engages or disengages the clutch and a frictional force is generated at contact allowing torque to be transferred from the motor to the driven components. Activation energy in electric clutches manifests as heat in the electromagnetic actuator when the clutch is engaged. This creates a risk of overheating clutch components. As a result, care must be taken that the operating temperature of the clutch is limited to the temperature rating of the insulation of the electromagnet and other heat susceptible components.

Specifications

Important specifications to consider while selecting an electric 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.
• Operating Voltage: The input voltage range for an electrically-operated 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.

Types

There are several types of magnetic field generation techniques that could be used in the clutch.

• Permanent Magnet: Permanent magnets can be used in different ways. Some are used to provide engagement or disengagement actuation force. Others provide the magnetic field for hysteresis. Since they do not use electrical power, permanent magnet clutches are often used in situations that require greater levels of safety. 
• Electromagnetic: Electromagnetic clutches use a coil that, when energized, creates a magnetic field that pulls the armature toward the rotor. Once in mechanical contact, the rotor turns at the same rate as the armature and affects the drive. When unpowered, the armature retreats back to an air gap from the rotor.
• Hysteresis: Hysteresis uses noncontact magnetic fields to apply resistance or engage load rotation. Torque loading may be applied independently of shaft speed. The magnetic torque is frictionless because the magnetic flux field operates in the air gap between the rotor and the stationary poles. Brakes and clutches that use hysteresis are suitable for precision tensioning and holdback applications where close braking control is important, or when variable clutch engagement of a load is required. 
• Eddy Current: Eddy current products use a magnetic field to induce eddy currents in the load. The load engagement, whether actuation (clutch) or slowing (brake), can be precisely controlled by controlling the magnetic field. Because no surfaces come into physical contact, there is no mechanical wear. Most eddy current devices are used in low-power applications.

Several electric 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.

Features

Additional capacities can be added to electric 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.

Electromagnetic clutches are often chosen for remote operation since no mechanical linkage, hydraulic tubing, or pneumatic piping is required for operation. Automated machinery that transmits control commands as electric signals as part of operation is a natural fit for electromechanical clutches. Small PTO (Power Take Off) applications such as those found in light agricultural and home power equipment are a recognizable favorite application of electric clutches that are found in many home garages. Industrial electric clutches are designed for a wide variety of power transmission applications. They can be found in printing machinery, conveyor drives, copier machines, and factory automation. In vehicle applications an electric clutch replaces clutch pedal by a simple switch. A smaller electric clutch is often used to drive the compressor of an air conditioning system. Though initial cost can be higher for an electric clutch, its compactness, instant actuation, and minimal system connection requirements more than offset this expense providing an optimal system solution.

Related Information

CR4 Community—Help Needed to Estimate Holding Voltage of Electric Clutch

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The Hilliard Corporation