- Q1. What is the difference between a clutch and brake?
A: A clutch is a transmission and control device that provides for energy transfer from the driver to the driven shaft.
A brake is a transmission and control device that stops a moving load, regulates movement, or holds a load at rest by transforming kinetic energy into heat.
- Q2. How do I choose a clutch or brake?
A: The two main considerations in the selection process are heat and wear. Other items to consider are:
- Speed
- Horsepower
- Load
- Type of action desired
- Environmental conditions
- Cycle rate
- Torque
- Shaft size
- Service availability
- Service factor
- Q3. How do I calculate torque for my clutch or brake?
A: To calculate torque (T) in foot pounds (lb. ft.) you need to know the following:
The speed (RPM) that the shaft on which the unit is rotating.
The horsepower (HP) of the driving force.
Any desired safety factor (k), optional.
Then calculate static torque based on drive components:
T = HP X 5250 X k/RPM - Q4. What is the difference between static and dynamic torque?
A: A very basic way to explain the difference between static and dynamic torque is that if there is no speed differential between the input and output, then you have a static torque condition. If either the input or output is at speed, and the other side is not, then you have a dynamic condition. If one side is at a higher speed than the other, you again have a dynamic situation.
- Q5. Can brakes & clutches be operated at maximum RPM and rated torque simultaneously?
A: No, because of heat dissipation limits. Brakes and clutches run at rated torque and maximum RPM would overheat quickly. Maximum allowable slip torque and slip RPM can be determined by using the formula for heat dissipation.
- Q6. Is torque available at low or zero RPM?
A: Yes. Full torque is available even at zero RPM. However, slip torque is not perfectly smooth below about 20 RPM (depending on driveline stiffness) due to 'slip-stick', also called 'chatter'. The amount may not be noticeable under many conditions. If the web in not elastic, slow RPM is probably OK.
Unwinding a very elastic web at very low RPM is not recommended. The web will stretch, then the brake will release, then the web will contract.
- Q7. How is torque calculated, to tension a roll of material?
A: First decide desired web tension & maximum roll diameter. Then, use the formula for determining torque.
- Q8. What is the accuracy of the torque?
A: When manufactured, rated torque is adjusted within -0 percent to +25 percent. (Precision torque available: Brakes: -0 to +10 percent. Clutches: -0 to +15%). Torque is precise.
Torque will repeat short term within +/-2 percent from cycle to cycle. But with extended usage, torque can vary +/-5 percent from initial factory settings. After months of continuous usage, torque will decrease due to wear and fretting corrosion of the stainless steel magnetic particles. Higher input current can compensate for the decreased torque for quite a while.
For applications requiring better accuracy, uses a torque transducer as a feedback device, (or a tension transducer on a web), to measure actual torque (or tension). Then vary the voltage to the brake or clutch for high accuracy.