Mechanical Power Screws

Power screws (or translation screws) are mechanical devices used to convert rotary motion into linear motion to transmit power in applications such as presses, jacks, and lead screws on lathes.

1. Thread Forms and Mechanics

Common thread forms for power screws are the Square thread, Acme thread, and Buttress thread. The Acme thread is frequently used because it is easier to manufacture and can use a split nut to compensate for wear.

1.1 Torque Required to Raise Load

The torque $T_R$ required to raise a load $F$ using a square-threaded power screw is related to the mean diameter $d_m$, the lead $l$, and the coefficient of friction $f$:

1.2 Torque Required to Lower Load

To lower the load, the required torque $T_L$ is:

$T_L = \frac{F d_m}{2} \left( \frac{\pi f d_m - l}{\pi d_m + f l} \right) + \frac{F f_c d_c}{2}$

2. Self-Locking Condition

A power screw is self-locking if it will not lower the load without an externally applied torque. This occurs when the thread friction is sufficient to prevent the screw from turning under the axial load. The condition for self-locking is:

$f \ge \tan \lambda$

Where $\lambda$ is the lead angle.

3. Efficiency

The efficiency $e$ of a power screw is the ratio of useful work output to work input:

$e = \frac{F l}{2 \pi T_R}$

High efficiency is desirable but generally conflicts with the requirement for self-locking. Self-locking screws typically have an efficiency below 50%.