Stop and go

Some say clutches and brakes will one day be obsolete, that electronic braking will fulfill the same needs. However, as energy efficiency jumps up the priority list of design objectives, these components play an important role.

Clutches and brakes are typically energy savers, as a motor runs continuously while its clutch is cycled. This benefits ac motors, as they can overheat if started too frequently. Another example of energy savings: When used with a servomotor, a spring-applied, air-released poweroff brake can hold a load without consuming any energy or lowering torque. Servomotors alone, on the other hand, will consume energy and overheat if left in stall mode for long periods of time.

Clutches and brakes’ basic function is to transfer torque between the input and output shafts when starting or stopping. Manufactured in a variety of sizes and configurations, clutches and brakes are actuated electrically, hydraulically, mechanically, or pneumatically. Interfaces or engagement points are friction, electromagnetic, or mechanical. The combinations of features available can make selection seem overwhelming. Defining the brake or clutch’s purpose in a specific application can help narrow selection to a single type.

Friction types

Of all the clutch and brake categories, friction interfaces are the most popular for general purpose clutching and braking. Typically, one mating surface is metallic and the other a composite friction material. Rotating forces transfer via surface contact. These clutches and brakes use a variety of actuation methods, from pneumatic and electromagnetic to hydraulic and mechanical (typically springs). The contacting surfaces are shaped into a variety of other configurations such as single plates, curved shoes, multiple, plate, and caliper pads.

Electromagnetic types

Within the family of electric clutches and brakes there are two types.

Electromagnetic friction clutches and brakes, one option, transmit torque across an air gap via the creation of an electromagnetic field and engage when voltage/current is applied to a coil. The coil produces magnetic flux that is transferred through a small gap. The interfaces are pulled together to create the frictional force.

The other group includes nonfriction types that create an electromagnetic field to cause hysteresis, eddy current, or magnetic particles to engage input and output shafts instead of a friction interface. Non-friction clutches and brakes operate with continuous, controlled slip or can be locked-up if the required torque is less than clutch capacity.

Non-friction hysteresis clutches and brakes can provide any amount of slip, as long as the heat dissipation capacity of the unit is not exceeded. Hysteresis units are used primarily in fractional horsepower applications and do not have wear components, which results in almost infinite life.

In magnetic particle clutches and brakes, the space between the input and output members is filled with dry iron particles. When a coil is energized, magnetic flux lines span this space and the particles line up, forming a rigid bond between the members. Directly proportional to variable current, the amount of particle bonding determines how much torque is transmitted. Because it is independent of speed, torque can remain stable throughout the unit operating speed range. Magnetic particle brakes are commonly used in tension control applications because they can operate efficiently in a constant slip mode and can control torque precisely.

Mechanical types

Jaw or tooth clutches have interlocking square lugs or closely pitched teeth that transmit torque when forced together. Tooth clutches engage at zero or near-zero rpm, do not slip, provide high torque output in a relatively small envelope, and can be used in oily or dry applications. The tooth clutch interface can be arranged to engage in one or more positions for accurate positioning.

Wrap spring and sprag overrunning clutches permit torque transmission in one direction only. Wrap spring clutches link input and output shafts in one direction with a coiled spring. Rotation in one direction tightens the spring around the shafts for torque transmission. Rotation in the opposite direction expands the spring and disengages the shafts. Sprag clutches have an outer race with a cylindrical ID. An inner race with ramps and rollers that are individually spring loaded provides constant roller-race contact. This arrangement assures instant action at all running speeds and immediate driving capability whenever one of the races rotates with respect to the other in the drive direction.

Centrifugal clutches are self-actuating units that work best where motor speed is the control, soft starts are acceptable, and saving energy is important.

Oil-shear clutches and brakes transmit torque through the shearing of an oil film between multiple discs. There is no frictional contact until the disc speeds are equal. Then the film breaks down, allowing full static engagement. The oil film lubricates while transmitting torque, reducing wear. Heat from slippage is carried away by the oil.

Selection criteria

With each type of brake and clutch, certain performance parameters must be considered. Maximum transmitted torque, a function of transmitted horsepower and speed, is the most important consideration in the application of clutches and brakes.

Torque requirements increase as the speed decreases for any given horsepower. Clutches and brakes should be mounted on or near the highest speed shaft, usually the motor shaft. Because the highest speed shaft has the least amount of inertia to start or stop, it has lower torque requirements. Placing a clutch or brake there keeps the unit small and cost-effective.

Torque multiplied by a service factor is the design torque for an application. The service factor depends on the severity of the application. A clutch or brake should not be applied at its maximum torque.

The mode of actuation also determines service factor use. An electromagnetic clutch or brake needs up to five times the static torque rating, while an air or hydraulically actuated unit typically needs a service factor of 1.2 to 2.

Torque is one of many considerations. Another important factor is maximum operating speed. The larger the unit, the lower the operating speed. The operating speed is limited by the maximum safe material peripheral velocity and the unit’s bearing speed rating. The time it takes a clutch or brake to respond to a power at a control with a change in torque is important. Torque, peak thermal input, continuous thermal input, machine inertia, response time, and friction facing life also need evaluation in cyclic start-and-stop applications.

Heat develops in clutches and brakes during dynamic engagement and is one of the most important considerations when selecting a clutch or brake. Thermal capacity is categorized into three distinctly different types.

First is peak input, the highest rate of heat generation at the interface during acceleration or deceleration that will not raise the temperature enough to damage the friction plate or facing. The peak input rate can be estimated as the speed difference between the friction disc and friction facing when the clutch or brake reaches full torque. Second is heat sink values, the total heat that a clutch or brake can dissipate into metal components in a given time period. It is a measure of the heat required to raise the surrounding components from an ambient temperature level without permanent damage. Third is continuous heat dissipation, the average rate at which heat can be generated at the friction interface without damaging the clutch and brake components themselves. Clutches and brakes are typically designed with features to dissipate heat energy. The continuous dissipation rating, expressed as horsepower, is based on empirical data resulting from continuously slipping the units, and monitoring the temperature rise.

Friction material wear life is rated in foot-pounds of “work energy capacity” and is based on the material wear rate and usable volume. In cyclic situations, this is the kinetic energy obtained from the formulation taking the inertia load, speed, and cycle rate into consideration. The maximum number of cycles for a given unit is the work energy capacity divided by the energy per cycle. The result is typically millions of cycles for a fluid power actuated clutch or brake.