While most clutches serve to engage and disengage a driving source to or from driven equipment, there are many possible variations – each of which requires varying designs. In order to select the best possible option, it is best to consider the following:
1. Primary Drive
Slip clutches can be used to provide constant tension in winding paper, film, foil, or wire when driven slightly faster than the product line speed. The clutch slips at the desired torque value to match the process line speed. In these applications, care must be taken to minimize the thermal load on the clutch. The heat load is the product of the slip RPM and the torque setting. Slip RPM should be limited to the minimum in order to allow smooth slip. Excessive slip will lead to premature wear, erratic tension, and damaging heat build-up.
- Constant slip speed and torque
When the clutch is used to drive a tension roll or spool of constant diameter the slip RPM is constant and the thermal load is generally easily managed.
- Increasing slip speed and torque
When the clutch is used to drive a roll or spool on which the diameter is increasing with the build-up of material, the thermal load increases dramatically as the roll builds. As the diameter increases, the torque must be increased to maintain the required tension. If the clutch is driven at a constant input RPM, the increasing diameter will require greater and greater slip to match the line speed. The product of the increased slip and higher torque causes an increasing thermal load on the clutch.
Mechanical friction torque limiters and clutches are frequently used to limit the amount of torque between drive and driven components to prevent damage to either. These devices are designed to allow the friction elements to slip when the preset torque of the device is exceeded. The friction surfaces continue to slip until the excess torque load is removed.
- Frequent Overload Occurrence
Torque limiting devices are sometimes used in assembly equipment that requires a brief period of slip at the end of each cycle. For these applications, adjustability and long friction wear life are critical.
For many drive systems, the torque limiter is designed in to prevent damage to expensive drive components in the event of some unforeseen failure – and may never slip.
Means of Engagement
Consideration should be given to which means of activation is most compatible with the location, machinery, and duty.
- Mechanical Lever
- Pneumatic Pressure
- Hydraulic Pressure
- Spring (Power off engagement)
All clutches and brakes have a specified torque capacity which may be fixed or variable. Based on the individual designs, they can have very different values of the following precision characteristics.
- Torsional Stiffness
- Positional Accuracy (Single position, Multi position or Infinite)
- Constantly Engaged (Primary machine drive)
- Cycling (Indexing, packaging)
- Idling (Backup drive)
- Typical Electric Motor Speed (1750 RPM)
- Typical Reducer Output (Less than 1750 RPM)
- High Speed (Greater than 1750 RPM)
- Controlled Temperature/Dry
- Wash Down
- Exposed to the Weather
- Extreme Temperatures (Hot or Cold)
- Exposed to Harsh Chemicals or Lubricants
- Food Safe