Seven questions and answers about magnetic particle brakes
What’s the difference between a brake and a clutch?
Brakes – When the coil is energized, the brake shaft is coupled to the housing. Typical uses are to provide adjustable tension for unwinding webs (paper, wire, film, etc.), and also to provide an adjustable torque load to a motor under test (typically stepper or gear motors).
Clutches – When the coil is energized, the clutch input shaft is coupled to the clutch output shaft. Typical uses are to provide adjustable tension for rewinding webs, and precision torque limiting (tightening fasteners & bottle caps).
Both brakes and clutches have the same torque and tension characteristics.
Can brakes & clutches be operated at maximum RPM and rated torque simultaneously?
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.
Is torque available at low or zero RPM?
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 is not elastic, slow RPM is probably OK.
Unwinding a very elastic web at a very low RPM is not recommended. The web will stretch, then the brake will release, then the web will contract.
How is torque calculated, to tension a roll of material?
First, decide desired web tension & maximum roll diameter. Then, use the formula for determining torque.
What is the accuracy of the torque?
When manufactured, rated torque is adjusted from -0 percent to +25 percent. (Precision torque available: Brakes: -0 to +10 percent. Clutches: -0 to +15%). The 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. A 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.
Is the torque vs. current graph a single line?
No. Magnetic particle brakes(Magnetic Powder Brakes) & clutches exhibit hysteresis. If the input current is increased from zero to the rated current, and then the input current is decreased from the rated current to zero, 2 different lines are graphed. The 2nd line indicates higher torque.
If the current is increased from zero to about 1/2 of the rated current and then decreased, the decreasing graphed line will be between the upper and lower lines.
When high accuracy is needed, always start at 0 amps, or always start at the rated current. If this is not practical, either use a torque transducer as a feedback device to measure torque, or determine experimentally the current needed at your various settings, and repeat this pattern during usage.
Should magnetic particle brakes and clutches be used as safety devices?
No. Avoid applications where failure could cause personal injury or loss of property.