How to Select a Brake or Clutch

To properly size a brake or clutch, the operating parameters of MAXIMUM TORQUE (T), SLIP SPEED (rpm), and KINETIC POWER (watts) will have to be determined. Once calculated, these parameters can be used to select the proper size brake or clutch from the technical data provided in the Hysteresis Product Data Sheets. These parameters are easily calculated from system operating requirements such as: total web or strand tension (F); the radius of the full reel, roller, pulley, etc. (d/2); and linear velocity/feed rate (v). The following example is given to show the relationship of these system requirements to the calculation of the brake operating parameters. Since this example is not meant to cover all possible applications, the application data sheet is provided in the product catalogs. If additional assistance is needed, simply complete a copy of the data sheet and fax it to Magtrol. Our Application Engineering staff will be pleased to assist you.

Sample Problem - English

Select a brake to tension a 7-inch diameter payoff reel in a system requiring total (web or strand) tension of 2 lb and a process speed of 600 ft/min.
Solution:
Brake Torque (T) =	Force (F) x Radius (d/2)
T =	2 lb × 3.5 in = 7 lb·in
or T =	32 oz × 3.5 in = 112 oz·in
Slip Speed (rpm) =	linear velocity (v) (in/min) / circumference (in)
rpm =	(600 ft/min × 12) / (pi × 7 in)
rpm =	327
Kinetic Power (W) =	Kinetic Power requirement is calculated using basic horsepower formula x 746 watts /hp
W =	(T (lb·in) x rpm/63025) x 746
W =	(7 lb·in x 327 rpm/63025) x 746 = 27 watts

Quick Check:
The curves to the left can be used as a quick check to verify the kinetic power calculation. Simply locate the required torque on the vertical axis, move horizontally until you intersect the appropriate speed line, and then read vertically (up or down) to obtain the resulting watts or horsepower.

Selection:
From the Hysteresis Brake ratings it can be seen that an HB-140 Hysteresis Brake which has a rated torque of 140 oz·in, a maximum speed capability of 12,000 rpm, and a kinetic power capability of 75 watts continuous, would be the proper selection for this application.

Note: In a clutch application, slip speed is the difference in rotational speed between the input and output members of the clutch assembly. If, in the above example, tensioning were being accomplished with a clutch inserted between a take-up reel and a motor driving at 500 rpm, the actual slip speed used to compute the kinetic power requirements would be 500 rpm (clutch input speed) - 327 rpm (clutch output speed = 173 rpm). The difference in speed would obviously impact the result for kinetic power.

Sample Problem - Metric

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