Leadscrew Frictional Torque of the Thrust Collar

Description

A leadscrew (or lead screw), also known as a power screw or translation screw, is a screw used as a linkage in a machine, to translate turning motion into linear motion.
The torque required to lift a load can be calculated by “unwrapping” one revolution of a thread. The force of the load is directed downward, the normal force is perpendicular to the hypotenuse of the triangle, the frictional force is directed in the opposite direction of the direction of motion (perpendicular to the normal force or along the hypotenuse), and an imaginary “effort” force is acting horizontally in the direction opposite the direction of the frictional force. Using this free-body diagram the torque required to *lift * a load can be calculated.

If the leadscrew has a collar in which the load rides on then the frictional forces between the interface must be accounted for in the torque calculations as well. For the following equation the load is assumed to be concentrated at the mean collar diameter.

For collars that use thrust bearings the frictional loss is negligible and the equation shown here can be ignored.

Related formulas

Variables

Tcfrictional torque of the thrust collar (N*m)
Fload on the screw (N)
μccoefficient of friction between the collar on the load (dimensionless)
dcmean collar diameter (m)