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 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.

How is torque calculated, to tension a roll of material?

What is the accuracy of the torque?

  • 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.

Is the torque vs. current graph a single line?

  • No. Magnetic particle brakes & clutches exhibit hysteresis. If input current is increased from zero to rated current, and then input current is decreased from rated current to zero, 2 different lines are graphed. The 2nd line indicates higher torque.
  • If current is increased from zero to about 1/2 of 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 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.

What is a hysteresis brake?

  • A hysteresis brake provides adjustable slip torque, which is controlled by electrical current.
  • Torque is frictionless.
  • Torque is generated without contact, so life is long. Torque is very accurate and smooth, throughout the entire speed range.

What is a cogging?

  • Cogging is pulsating output torque.
  • Cogging occurs after input current is greatly reduced, while the shaft is at zero or very low RPM.
  • Placid's hysteresis brake feature the patented 'Cog-Buster', which automatically decogs (stops the brake from cogging) in 1 revolution of the shaft.
  • Hysteresis brakes without the 'Cog-Buster' must be manually decogged. To manually decog: Rotate the shaft while simultaneously reducing the input current from the previous high current to zero. This is quite inconvenient, or impossible if the brake is located remotely.

What are the best applications for hysteresis brakes?

  • Precision torque control applications such as:
    - Winding / Tensioning applications,
    - Load simulation / Torque testing both low and high speed motors.
  • Hysteresis brakes are needed where:
    - Better low speed smoothness is needed than a magnetic particle device can provide
    - RPM is higher than the 'Maximum RPM' rating for a magnetic particle brake
    - The required heat dissipation is too high for a magnetic particle brake. Even moderate RPM and torque generates a lot of heat.
    - The heat dissipation rating is higher for a hysteresis brake with the same torque rating.

What is the worst application for hysteresis brakes?

  • The worst application would require the brake to repeatedly change from high torque to low, while the shaft is moving slowly, or stopped. The brake would cog, or pulse, for 1 revolution each time.

Is torque available at low or zero RPM?

  • Yes. Torque is not affected by RPM.
  • Full torque is available even at zero RPM.

What is the accuracy of the torque?

  • When manufactured, rated torque is adjusted within -0 percent to +10 percent. (Precision torque available: -0, to +5 percent).
  • Torque is precise.
  • Torque will repeat within +/-1 percent from cycle to cycle.
  • Since torque is produced without physical contact, there is nothing to wear out (except the bearings), so there won't be any change in torque vs. current with age or usage.
  • The brake does exhibit hysteresis. If misunderstood, the user can confuse this with inaccuracy or lack of precision.

Is the torque vs. current graph a single line?

  • No. Hysteresis brakes (and magnetic particle brakes & clutches) exhibit hysteresis.
  • If input current is increased from zero to rated current, and then input current is decreased from rated current to zero, 2 different lines are graphed. The 2nd. line indicates higher torque.
  • If current is increased from zero to about 1/2 of 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 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.

What is a constant current' power supply?

  • The user sets the desired output current. The power supply automatically adjusts the output voltage to maintain the desired current.
  • If constant voltage is used instead, output current (and torque) decreases as the brake or clutch temperature increases.
  • As the temperature of copper increases, the resistivity increases, so the (brake or clutch) coil resistance increases (15%-20% from cool to hot).

How is output current controlled?

  • By 2 methods:
    -    +++ MANUAL: by turning a knob of a potentiometer or
    -    +++ '0-10' Voltage: Output current is proportional to a 0-10 volt signal from your computer, PLC, or electronic controller.

How is the method of current control selected?

  • PS power supply: a switch selects the control method.
  • PST power supply: a jumper selects the control method.
  • PSH power supply:
    • Current mode - Set voltage to max (or max desired value), then use the current knob to control current.
    • Voltage mode - Set current to max (or max desired value), then use the voltage knob to control current.
  • '0-10' Input Impedance=2000 ohms, (5 mA current required).
  • All brakes or clutches connected to a single power supply would have the same voltage, but only approximately the same torque. For precisely matched torque, use individual power supplies for each brake or clutch.

Can more than 1 clutch (or brake) connect to a power supply?

  • More than one may be connected, provided maximum power supply output power is not exceeded.
  • Output power:
    • PS (12 volt) supplies: 1.25 Amperes, 15 watts.
    • PS (24 volt) supplies: 0.75 Amperes, 22 watts.
    • PST: 2, 3, and 5 amp versions.
    • PSH: 2, 3, and 5 amp versions.
  • All clutches connected to a single power supply would have the same voltage, but only approximately the same torque.

Should a 12 or 24 volt power supply be used?

  • Connect 12 volt PS supplies to 12 volt brakes and clutches.
  • Connect 24 volt PS supplies to 24 volt brakes and clutches.
  • Always use a 24 volt PS supply with brake B220.
  • For our smallest brakes and clutches: We prefer to sell the 12 volt version (with a 12 volt PS power supply), since 24 volt versions of the smaller units require very small gauge wire for the coil.
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When is a follower needed?

  • The follower is needed for maintaining constant tension while winding rolls, from full to empty roll condition.
  • Without the follower, the same torque would be applied to the axle of the spool from start to finish. Web tension would increase as roll diameter decreases.

How does the follower arm potentiometer maintain tension constant?

  • The follower measures the diameter of the roll to be unwound (or rewound) .
  • The power supply then outputs current proportional to roll diameter to keep tension constant.

Why must the follower arm potentiometer sometimes be used instead of an ultrasonic sensor?

  • The ultrasonic sensor cannot measure narrow webs with large spool flanges because the sound waves would bounce off the flanges.

How is tension adjusted?

  • The potentiometer knob is used to adjust the tension.
  • Tension may be adjusted even while running.

When is an ultrasonic sensor needed?

  • The ultrasonic sensor is needed for maintaining constant tension while winding rolls, from full to empty roll condition.
  • Without the ultrasonic sensor, the same torque would be applied to the axle of the spool from start to finish. Web tension would increase as roll diameter decreases.

How does the ultrasonic sensor maintain tension constant?

  • The ultrasonic sensor measures the diameter of the roll to be unwound (payout) or rewound.
  • The power supply then outputs current proportional to roll diameter to keep tension constant.

Why is the ultrasonic sensor sometimes preferred over the follower arm pot?

  • The ultrasonic sensor measures the roll with ultrasonic sound.
  • Webs that scratch easily, are sticky, or cannot be touched are ideal for this non-contact method.

How is tension adjusted?

  • The potentiometer knob is used to adjust the tension.
  • Tension may be adjusted even while running.

How is the ultrasonic sensor adjusted for different diameter rolls?

  • A push button is used for programming.
  • The button sets the far limit (roll center line) and near limit (maximum roll diameter).