Page 10 of catalogue Rotary actuators of SKF Linearsysteme GmbH

Calculation of motor size To calculate the appropriate size of the motor (drive) it is necessary to know the maximum load (torque) that the actuator must handle and at which rotational speed this takes place. The speed of the drive unit (nin) can be calculated from diagram 1 or with help of this formula.
800

Diagram 1
Actuator input speed rpm

600

15 • U nin = t90° rpm
400 A=CRAB 10, 40 B=CRAB 30 C=CRAB 11 D=CRAB 17, 20 E=CRAB 12, 13 C D E

nin

= The speed of the drive unit in rpm (the actuator input speed in order to meet the requested speed on the output shaft.)

A B 200

t90° = Positioning time for a 90° angular movement on the output shaft of the actuator. U = Gear ratio in actuator.

0 0 10 Example 20 30 40 t=s/90°

The input torque needed (Tin) can be calculated from diagram 2 or with help of this formula.
800

Diagram 2
Actuator input torque Nm F G A

Tin =

Tout U • ha

Nm
B 15

Tout = Maximum output torque (actuator) Nm U ηa = Gear ratio in actuator = Efficiency factor (actuator)
10 C D

Example: You have chosen a CRAB 30 rotary actuator to operate 600 Nm at maximum, with a speed of 7,5°/s. (7,5°/s equal to 90°/12 s) What torque (Nm) and speed (rpm) must the drive unit produce? Alt. 1: See diagram 1 and 2. Alt. 2: Use the formulae

E 5

A=CRAB 40 B=CRAB 30 C=CRAB 11 D=CRAB 20 E=CRAB 12 F=CRAB 13 G=CRAB 17

0 0 500 Example 1 000 1 500 Nm Actuator output torque

Torque (motor) =

Tout U • ha 15 • U =

600 88 • 0,75 15 • 88 = 12 = 110 rpm = 9,09 Nm

N.B. these diagrams are not valid for CRAB 20, 30, or 40 versions with ACME SCREW. In these cases use the calculations formulae. Always contact SKF whenever you consider these ACME SCREW versions.

Speed (motor) =

t90°

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