(June 07, 2011 - Rockford, IL)

Under what conditions will a lead screw or ball screw back drive?

Back drive is a critical effect of specifying the correct ball screw or lead screw driven linear actuator, especially in vertical applications. Back-driving is the result of the load (or thrust force) pushing axially on the screw or nut to create rotary motion. All screws, depending on their efficiency, will back drive. The resulting torque is known as “back-driving torque” and is the torque required to hold a load in position.



In a vertical orientation, back drive can occur by the load (or carriage) alone, causing the carriage to drift down. Vibration and other external factors can also cause this movement. Generally, a lead screw with efficiency greater than 50% will have a tendency to back drive.

Use the formula below to calculate the Back Drive Torque being created in a given slide system.



If you wish to have no movement of the carriage, the motor, brake and possibly screw efficiency come into play. The motor will typically have a holding torque value under power or detent torque at stop. This is the value at which the motor will resist movement until this torque is over run. A brake is generally utilized for positive holding, especially a system in vertical orientation.

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