Clutches and brakes
Clutches and brakes couple, decouple, accelerate, and decelerate rotating machine components and maintain them at proper speed. The functions of each are so similar that their rolls are often interchangeable. Whether the clutch/brakes are mechanically, electrically, or fluid power actuated, they are considered mechanical devices because they transmit mechanical power.
Key Concepts
Types
Selection
Sections: Selection Oil-shear clutches and brakes
Sidebars: Selection criteria Application notes Troublemakers
Clutches and brakes couple, decouple, accelerate, and decelerate rotating machine components and maintain them at proper speed. The functions of each are so similar that their rolls are often interchangeable. Whether the clutch/brakes are mechanically, electrically, or fluid power actuated, they are considered mechanical devices because they transmit mechanical power.
Clutches and brakes are generally used in rotary motion applications. The clutch or brake must be designed to convert mechanical energy absorbed during relative motion or slippage into heat energy. It must survive both mechanical and thermal steady-state and shock loads imposed by the system during the operational cycle without damage.
Selection
The first step in selecting a clutch or brake is to determine the configuration required, depending on performance characteristics.
After selecting a type to use, the required size must be determined. Selection charts are available for this purpose.
A typical chart is based on the horsepower and speed of the system, which should be the shaft speed at the clutch or brake, not motor speed.
For example, the clutch needed to drive a shaft powered by a 1-hpmotor rotating at 1750 rpm is Model A. If the shaft speed at the clutch is a 2:1 reduction, clutch speed would be 875 rpm, and Model B would be selected.
The relationship between system horsepower and speed is given by the following equation for determining the dynamic torque capability required from the clutch or brake:
D = 5252 x hp x SF
_____________
N
where:
D = dynamic torque, lb-ft
hp = horsepower
SF = service factor (ranges from 1-10)
N = speed of clutch/brake, rpm
As rpm decreases, torque increases. For this reason, the best location for a clutch or brake is on the high-speed shaft of a machine.
Speed should not be too low; below 300 rpm is not recommended for friction-driven units. At low speeds, burnishing or mating wear of the friction faces does not occur, and torque capacity may be reduced dramatically.
When a clutch or brake must be operated at very low speeds, it might be necessary to oversize the unit. In these cases, it is normally sufficient to use a unit with a static torque rating two times the calculated dynamic torque requirement.
Oil-shear clutches and brakes
In a basic oil-shear drive, torque is transmitted through shearing of an oil film between two disks. As the rotating input disk moves toward the stationary output disk, the oil shearing action forces the output disk to begin rotating. There is no friction material-to-metal contact until input and output disk speeds are nearly equal. Then the oil film breaks down, allowing full static engagement. Wear is greatly reduced by the oil film, which lubricates while transmitting most of the dynamic torque of engagement.
Selection criteria
Maximum operating torque
Maximum torque
Type of actuation
Type of engagement
Response time
Cycle rate
Thermal capacity
Space or weight restrictions
Environmental conditions
Acceptable service life
Amount of routine maintenance
Application notes
Pneumatic and hydraulic-powered clutches and brakes have higher torque ratings than electric-powered units.
Electromagnetic tooth clutches have higher torque ratings than electromagnetic disk clutches and don’t slip.
Self-actuating clutches work best where motor speed is an adequate control, soft starts are acceptable, and energy savings are important.
Magnetic particle clutches and brakes are useful in tensioning and positioning when there are continuous speed changes.
Eddy current clutches are useful for providing drag loads needed in tensioning.
Multicaliper clutches, with ventilated disks, are designed for continuous duty.
Square jaw clutches are limited to running engagements under 10 rpm.
Spiral jaw clutches allow engagement speeds up to 150 rpm.
Multitooth clutches can have running engagement speeds up to 300 rpm.
Disk clutches and brakes are preferred over drum-style units for frequent start-stop applications.
Eddy current clutches are used primarily in adjustable speed drives.
Troublemakers
Environmental and service conditions that can cause trouble for clutches and brakes include:
Exposure to gritty dust
Poor ventilation
Operation in wet or damp environments
Exposure to:— oil vapor— salt air— chemical fumes
Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our WTWH Media editorial team and getting the recognition you and your company deserve. Click here to start this process.