Logo

Placid Industries, Inc.

Frequently Asked Questions

home.gifReturn to Main Page

Magnetic Particle Brake & Clutches

What's the difference between a brake and a clutch?
Can brakes & clutches be operated at maximum RPM and rated torque simultaneously?
Is torque available at low or zero RPM?
How is torque calculated, to tension a roll of material?
What is the accuracy of the torque?
Is the torque vs. current graph a single line?
Should magnetic particle brakes and clutches be used as safety devices?

Hysteresis Brakes

What is a hysteresis brake?
What is cogging?
What are the best applications for hysteresis brakes?
What is the worst application for hysteresis brakes?
Is torque available at low or zero RPM?
What is the accuracy of the torque?
Is the torque vs. current graph a single line?


Power Supplies

What is a 'constant current' power supply?
How is output current controlled?
Should a 12 or 24 volt power supply be used?
Can more than 1 clutch (or brake) connect to a power supply?

Follower Arm Potentiometer

When is a follower be needed?
How does the follower arm potentiometer maintain tension constant?
Why must the follower arm potentiometer sometimes be used instead of an ultrasonic sensor?
How is tension adjusted?

Ultrasonic Sensor

When is an ultrasonic sensor needed?
How does the ultrasonic sensor maintain tension constant?
Why must an ultrasonic sensor sometimes be used instead of a follower arm potentiometer?
How is tension adjusted?
How is the ultrasonic sensor adjusted for different diameter rolls?




- - - -   ANSWERS   - - - -


Magnetic Particle Brakes & Clutches


What's the difference between a brake and a clutch?
Unwind drawing 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).


Rewind Drawing 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.

Back to top

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.
Back to top

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 noticable 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.
Back to top

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.
Back to top

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.
Back to top

Is the torque vs. current graph a single line?
Hysteresis Torque Curve 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.

Back to top

Should magnetic particle brakes and clutches be used as safety devices?
No. Avoid applications where failure could cause personal injury, or loss of property.
Back to top



Hysteresis Brakes


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.
Back to top

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.
Back to top

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
-or-
RPM is higher than the 'Maximum RPM' rating for a magnetic particle brake
-or-
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.
Back to top

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.
Back to top

Is torque available at low or zero RPM?
Yes. Torque is not affected by RPM. Full torque is available even at zero RPM.
Back to top

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.
Back to top

Is the torque vs. current graph a single line?
Hysteresis Torque Curve No. Hysteresis brakes 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.

Back to top


Power Supplies


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 output.
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).
Back to top

How is output current controlled?
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.
'0-10' Input Impedance=2000 ohms, (5 mA current required).
A switch is used to select the 2 methods of control.
Back to top

Should a 12 or 24 volt power supply be used?
Connect 12 volt supplies to 12 volt brakes and clutches.
Connect 24 volt supplies to 24 volt brakes and clutches.
Always use a 24 volt supply with brake B220.
For our smallest brakes and clutches: We prefer to sell the 12 volt version (with a 12 volt power supply), since 24 volt versions of the smaller units require very small gauge wire for the coil.
Back to top

Can more than 1 clutch (or brake) connect to a power supply?
More than 1 may be connected, provided maximum power supply output power is not exceeded.
Output power:
12 volt supplies: 15 watts.
24 volt supplies: 22 watts.
But, all clutches connected to a single power supply would have the same voltage, and approximately the same torque.
Back to top



Follower Arm Potentiometer


When is a follower be needed?
The follower is used for constant tension unwinding & rewinding.
When tensioning an roll of material to be unwound (or rewound), and the brake (or clutch) is applying torque to the axle of the roll, if constant torque is used, tension will vary with roll diameter.
Tension will increase, as the roll diameter decreases.
Back to top

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.
Back to top

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. The sound waves would bounce off the flanges.
Back to top

How is tension adjusted?
On the Placid Industries power supply, a knob connected to a potentiometer is used to adjust tension. Tension may be adjusted even while running.
Back to top



Ultrasonic Sensor


When is an ultrasonic sensor needed?
The ultrasonic sensor is used for constant tension unwinding & rewinding.
When tensioning an roll of material to be unwound (or rewound), and the brake (or clutch) is applying torque to the axle of the roll, if constant torque is used, tension will vary with roll diameter.
Tension will increase, as the roll diameter decreases.
Back to top

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.
Back to top

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.
Back to top

How is tension adjusted?
On the Placid Industries power supply, a knob connected to a potentiometer is used to adjust tension. Tension may be adjusted even while running.
Back to top

How is the ultrasonic sensor adjusted for different diameter rolls?
A push button is used for programming. The button sets the roll center line, maximum roll diameter, and slope (so power supply output current increases as the roll diameter increases).
Back to top


home.gifReturn to Main Page