Page 9 of catalogue Rotary actuators of SKF Linearsysteme GmbH

Calculations
Calculation of service life
Mean torque Nm 1 700 1 600 1 500 1 400 400 1 300 1 200 CRAB 40 1 100 1 000 900 800 700 600 500 400 300 200 100 0 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 106 cycles (180°) L10 life CRAB 12 CRAB 20 CRAB 30 100 90 80 70 60 50 40 30 20 10 0 1 CRAB 40 CRAB 30 200 300 Mean torque Nm Backlash free CRAB 12 (Backlash ±0,05°)

N.B. These diagrams only apply to SKF rotary actuators with BALL SCREW. Always contact SKF whenever you consider ACME SCREW versions. L10 LIFE Calculated L10 life related to mean load.

CRAB 20 CRAB 12 23 456 7 8 9 10 11 12 13 14 15 16 17 106 cycles (180°) L10 life

Ball screw actuators The service life of an SKF rotary actuator is normally determined by the life of the ball screw. L10 is defined as the life that 90 % of a sufficiently large group of apparently identical actuators can be expected to attain or exceed. Sometimes the life of the motor can be shorter than the life of the ball screw, however, the motor can be easily replaced. In order to calculate the service life (L10) of an SKF rotary actuator it is necessary to determine the size of the actual mean torque (Tm). The torque fluctuates often during one working cycle but Tm has the same influence on the actuator as the actual fluctuating torque. The mean torque (Tm) can be calculated using the formula:

One (1) cycle is identical to a 180° rotation on the output shaft of the actuator. (When the actual working cycle is a few degrees or fractions of a degree, the accumulated number of operations must be calculated in order to reach one 180° cycle for L10 calculation). Acme screw actuators The L10 life of a ball screw can be predicted by the rolling bearing theory and experience. The life of an acme screw has different characteristics. While the lifetime of a ball screw ends rather suddenly (due to material stresses in the balls) the wear of an acme screw starts from the beginning and continues until final breakdown. Therefore, when predicting the service life of a rotary actuator incorporating an acme screw, it is necessary to know the ambient temperature, intermittence factor, rotational speed, etc. Always contact SKF whenever you consider acme screw versions.

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Tm=

T13C1+T3C2+T33C3+ --C1+C2+C3+ ---

Nm

T1, T2, T3, etc. is the constant torque (in Nm) during C1, C2, C3, etc. fraction of a cycle.

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