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Leadscrew

Leadscrew

A leadscrew is a screw specialized for the purpose of translating rotational to linear motion. The mechanical advantage of a leadscrew is determined by the screw pitch or lead.

A leadscrew nut and screw mate with rubbing surfaces, and consequently they have a relatively high friction and stiction compared to mechanical parts which mate with rolling surfaces and bearings. Their efficiency is typically only between 25 and 70%, with higher pitch screws tending to be more efficient. A higher performing, and more expensive, alternative is the ball screw.

The high internal friction means that leadscrew systems are not usually capable of continuous operation at high speed, as they will overheat. Due to inherently high stiction, the typical screw is self-locking (ie. when stopped, a linear force on the nut will not apply a torque to the screw) and are often used in applications where backdriving is unacceptable, like holding vertical loads or in hand cranked machine tools.

Leadscrews are typically used well greased, but, with an appropriate nut, it may be run dry with somewhat higher friction. There is often a choice of nuts, and manufacturers will specify screw and nut combinations as a set.

Leadscrew threads typically have an Acme profile. Backlash can be reduced with the use of a second nut to create a static loading force known as preload; alternately, the nut can be cut along a radius and preloaded by clamping that cut back together.

Leadscrew manufacturing

Leadscrews are manufactured to various tolerances.

Leadscrews are manufactured using various processes. The manufacturing processes can be categorized within two major groups: material removal (machining) and net shape.

Machining

Material removal processes for leadscrew manufacturing include traditional machining by cutting, and machining by grinding.

Net shape

Net shape processes for leadscrew manufacturing include forming, a cold forging technique called 'rolling', and casting.

Back driving characteristics

A lead screw will back drive. A leadscrew's tendency to backdrive depends on its thread helix angle, coefficient of friction of the interface of the components (screw/nut) and the included angle of the thread form. In general, a steel acme thread and bronze nut will back drive when the helix angle of the thread is greater than 20 degrees.

Examples of use

* vice
* lathe

ee also

* Ball screw
* Jackscrew

References


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