Technical Information - Airflex - #3

Depending upon the operating characteristics of the prime mover and the energy demand of the work shaft there are several ways in which the clutch can function to control the energy transfer. These include:

Coupling or disconnect function - This function permits the prime mover; e.g., synchronous motor or diesel engine, to obtain operating speed and/or temperature before being coupled or connected to the work shaft. When energy is not required at the work shaft and it is not desirable to stop the prime mover, the clutch provides the disconnect between the two. This type of application is usually associated with long periods of clutch engagement, as opposed to one in which sev­eral engagements per minute are required.

Starting function - This function permits controlled acceleration of the work shaft with minimal torsional shock when accelerating delicate or breakable materials. For high inertia starts, it permits the prime mover to run continuously at efficient speeds.

Direction or speed change function - Multi-speeds and direction changes in many machines are accomplished with gear boxes or gear trains equipped with clutches. Generally, one clutch is required for each speed and for changing the direction of rotation.

Cyclic function - This function requires that the work shaft be started very frequently while the prime mover runs continuously. A brake is usually required to stop the work shaft in order to obtain the cyclic rate.

Continuous slip function - This function requires that the driven side of the clutch rotate at a speed slower than the driver side. An application example is a rewind stand where material must be wound into a roll under constant tension. By controlling clutch torque, material tension and speed is regulated.

Overload protection function - This function limits the maximum torque which can be transmitted to prevent damage to drive compo­nents. If the maximum torque is exceeded, the clutch will slip. To avoid thermal damage, the clutch should not be allowed to slip for an excessive length of time.

In any given application, the clutch can provide one or a number of these functions. It is important to identify all the functions the clutch will be sub­jected to in the clutch selection process.

Brake functions closely parallel those of clutches, except they absorb and dissipate the energy of the driven shaft. These functions include:

Holding function - In this function the brake prevents an energy transfer to the driven shaft by holding it stationary. Holding may be required ei­ther prior to or after shaft rotation and is usually required to maintain or hold a position or location of a driven component.

Stopping function - This function provides a means to stop the driven shaft or machine in a controlled manner by limiting its coasting time and/or distance. The brake torque determines how quickly the stop occurs.

Emergency stop function - This function rapidly stops all moving com­ponents to protect the operator and/or equipment.

Cyclic function - This function works in conjunction with a clutch and permits frequent starting and stopping of the driven shaft while allow­ing the prime mover to run continuously.

Controlled slip function - This function provides a retarding torque to the driven shaft. There is constant relative movement between the driven shaft and the stationary half of the brake.

In any given application, the brake can provide one or a number of these functions. It is important to identify all the functions the brake will be sub­jected to in the brake selection process.

Friction may be defined as the resistive force occurring between two sur­faces as they slide or tend to slide across each other. The contacting sur­faces can be thought of as consisting of peaks and valleys which mesh together. The resistive force is developed by the effort to slide the peaks on each surface out of the meshing valleys. When the surfaces slide across each other, the peaks and valleys are not as free to mesh as when the sur­faces are stationary. This is the reason frictional resistance decreases when sliding occurs.

It is apparent that the frictional resistive force is proportional to the normal force holding the surfaces in contact with each other. The constant of proportionability is called the dynamic coefficient of friction when sliding occurs, and the static coefficient of friction when sliding does not occur

Copyright Eaton Corporation, 1995, All rights reserved.

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