What is a Clutch?

Industrial Clutch products
"A clutch is a frictional device whose primary function is to transmit power intermittently."

A clutch is a device that is used to couple and uncouple two separate bodies of rotation. These two separate bodies may consist of shafts, gears and sprockets, a prime mover or motor, or any combination of these. The driven components are typically pumps, fans, power take-off shafts, compressors, gearboxes and generators. Normally the shaft of the driven or driving component itself is used to transfer power.

Methods of Clutch Actuation

Clutches can be classified by their method of actuation. These include mechanical, electric, hydraulic and pneumatic (air). The last two are frequently combined, as many industrial clutch models suitable for hydraulic actuation can also be used in pneumatic (air) applications.

View an actuation method comparison.

A subtype of the classification is based on whether the actuation method is used to engage or disengage the clutch. These subtypes are spring applied (energy release) clutches and energy applied clutches. They are distinguished by the way the clutch is engaged. A clutch is said to be "engaged" when transmitting torque. It is "disengaged" when no torque is being transmitted through the device.

Spring Applied Clutches vs. Energy Applied Clutches

Type Function Example
Spring Applied
(Energy Release)
Torque transmitted when power absent Artillery elevation drive mechanism
Energy Applied No torque transmitted until actuated Power take-off "PTO" drive

A spring applied clutch is referred to as "normally engaged", meaning that in the absence of actuating power, the clutch will transmit torque. Actuation energy is required to disengage it. This is a useful design when the driven component is only momentarily decoupled during normal operation. An energy applied clutch is said to be "normally disengaged" - no torque is transmitted to the driven device until actuation energy is applied. Most clutched in power transmission applications are of the energy applied subtype.

How They Work

In an energy applied clutch, the end plate is drawn toward a buttress plate when the device is actuated. Between the end plate and the buttress plate are the friction discs. The end plate squeezes the friction discs tightly, engaging the clutch and permitting the transfer of torque.

In a spring applied (energy release) clutch, an armature is located next to the buttress plate and engagement springs are inserted between the armature and the buttress plate. These springs force the armature away from the buttress plate, squeezing the friction discs against the end plate and permitting the transfer of torque through the device. Actuation energy pulls the armature toward the buttress plate, squeezing the springs, releasing the pressure on the friction discs and disengaging the device.

A further breakdown defines the process by which the engagement method transmits the rotating mechanical energy to the driven component, called "power transfer". These include friction and positive engagement.

Friction Clutches vs. Positive Engagement Clutches

Type Function Example
Friction Torque management Vehicle clutch
Positive Engagement Locked positioning Helicopter drive

In Friction power transfers single or multiple-disc friction plates are held together by the force of springs or by the application of energy such as magnetic flux, or by a piston that is pressurized, to transmit torque by friction. Positive Engagement power transfers involve jaw or tooth clutches which move into a known position when engaged.

Industrial Clutch Considerations

Each of these classifications and subtypes are designed to offer equipment designers a complete range of options, and each option has its unique advantages and disadvantages.

When selecting a clutch for a given application, it is important to understand the advantages, disadvantages, and limitations of each type of device. Carlyle Johnson offers a full line of standard industrial clutches at modest cost, and can provide engineering expertise to help solve the most difficult control problems.

Classification Overview
Industrial Clutches

Method of Actuation

Clutch Actuation Subtypes

  • Spring Applied (Energy Release)
  • Energy Applied

Power Transfer Method

  • Friction
  • Positive Engagement

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