Power Screw Calculator

Lowering thread torque is (W·dm/2)(π·f·dm − l)/(π·dm + f·l), so its sign flips depending on whether π·f·dm is larger or smaller than the lead l. If the lead is large relative to friction, that term goes negative — meaning the screw would spin backward and lower the load on its own without any applied torque, because friction alone can't hold it. That's the same non-self-locking condition the calculator flags separately, and it means you need a brake or ratchet to safely hold the load.

Self-locking occurs when π·f·dm > l — when the friction moment around the mean thread diameter exceeds the mechanical advantage the lead provides per revolution. Equivalently, it happens when the lead angle (λ = atan(l/(π·dm))) is smaller than the friction angle, so friction can fully resist the load trying to back-drive the screw. Higher thread friction or a finer lead (smaller l) both push a screw toward self-locking.

Collar torque, W·fc·dc/2, acts at the thrust bearing surface where the load pushes against a flat collar, not along the helical thread — so it has its own effective radius (dc/2, not dm/2) and typically a very different friction coefficient (as low as 0.01-0.02 for a ball thrust bearing versus 0.08-0.25 for sliding thread contact). Combining them into one formula would misrepresent both the geometry and the friction behavior, so they're computed and added independently.

Efficiency here is (W·l)/(2π × raising torque) — the useful work per revolution divided by the actual work input. Because power-screw threads slide against the nut rather than roll like a ball screw's recirculating balls, sliding friction consumes a large share of the input torque, which is why well-lubricated square-thread or Acme screws typically land around 30-50% efficient versus 90%+ for ball screws.