Screw Jack Calculations

Written by jennifer gigantino
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Screw Jack Calculations
Screw jacks are often used as car jacks. (Jupiterimages/ Images)

A screw jack, or jackscrew, is a jack that derives its compressive force from the turning of a leadscrew. A self-locking trapezoidal thread is often used as a leadscrew, though ball screws are sometimes found instead.

Linear Speed

To find the linear velocity (v) of a screw jack in millimetres per minute, multiply the input shaft's rpm by the width in mm of the lifting screw's lead (which can be found by multiplying the screw pitch in mm by the number of starts on the lifting screw). Divide this result by the gear ration.

Input Torque

To find the input torque of a screw jack, multiply the load (in kilonewtons) by the lifting screw lead in mm to get X, then multiply the actuator efficiency by the actuator gear ratio by two by Pi to get Y. Divide X by Y. For the input power, multiply the load by the raise rate (in mm per minute) and divide that by the actuator efficiency times 60,000.

General Motion

The lifting motion in kilowatts can be found by multiplying the mass by the rate of gravity by the lift velocity, and dividing that by the coefficient of efficiency times 1,000. The linear motion in kilowatts can be found by multiplying the friction resistance by the velocity and dividing that by 1,000. The friction resistance can be found by multiplying the mass by the gravitational acceleration by the coefficient of friction.

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