Trapezoidal Lead Screw & Self-Locking

A lead screw with a trapezoidal thread that moves by sliding friction, and can lock itself when power is cut.

A trapezoidal lead screw converts rotation into linear motion using a thread with a trapezoidal cross-section; the nut and screw make sliding contact, unlike a ball screw, which has rolling balls in between. It's cheap and mechanically simple, but has high friction and low efficiency. That same friction gives it a useful property called self-locking: when the thread's lead angle is smaller than the friction angle, an axial push on the nut can't force the screw to spin backward, so the load won't drift down on its own when power is cut, and no separate brake is needed. The tradeoff is poor back-drivability — an external force can't easily move the joint — which rules it out for applications needing compliance or force control. It's common in low-cost linear actuators, lift mechanisms, and some dexterous hands that use a small lead screw to push a finger.

ExampleThe Z-axis of a desktop 3D printer often uses a T8 trapezoidal lead screw, so the print bed doesn't sag when power is cut.

Also called

Acme Lead Screw, Sliding Lead Screw, T-Type Lead Screw

Related

Ball Screw · Screw Lead · Micro Lead Screw · Backdrivability · Linear Actuator (Electric Cylinder) · Holding Brake

Sources

Leadscrew - Wikipedia