Tom Solon
Kerk Motion Products, Inc.
Hollis, N.H.

Leadscrew-driven slides function as product pushers on this shrink bundler.
Photo courtesy Omega Design Corp.

Leadscrews consist of a screw and its riding nut, which is wedged forward as the screw turns. Originally, leadscrews were assembled using low-quality screws from other fastener or screw designs so that a simple, one-piece nut only provided basic power transmission and rotary-to-linear motion conversion. Today, leadscrews feature technology with distinct advantages for motion control.

Modern materials keep friction very low (sometimes with a coefficient under 0.1) without any external lubrication. Self-lubrication is instrumental in combating environmental challenges. For example, cartoning and case packing regularly generate airborne paper dust, coating everything nearby. But because leadscrews can operate without lubricants, grease — which captures dust and creates a harmful, abrasive paste — is eliminated. They're also easier to tune and more predictable than timing belts. These two characteristics mean leadscrews are often maintenance-free. Also, there is lower particulate generation. Particulate generation is a byproduct of wear; by keeping friction to a minimum, wear is minimized. Also, lead screw wear tends to be steady and linear in nature while ball screws fail catastrophically when localized brinnelling or surface failure occurs on the ball or screw. The pitting and resulting debris triggers additional failure and heavy debris generation. Finally, ball screws rely on external lubrication. Lubrication breakdown, contamination and loss all are frequent causes for early failure.

Leadscrews also operate more quietly. Balls generate noise, especially traveling through the return tubes as they recirculate.

Other leadscrew characteristics include:

• High helices for fast leads — greater than 100 mm per rev. No balls means lead screws are not constrained to available ball geometries and dynamics. (Getting balls to fit and roll in a high helix screw is a gating design parameter.) Second, thread rolling allows the manufacture of leads that cannot be cut or ground because the cutter will not clear itself. The cutter wipes out adjacent threads if the angle is to high.
  
  High helix, fast leads are possible in smaller diameters. Screws with 6-mm diameters and 25 mm-per-rev leads are very common in all types of equipment, including scanning, data storage, medical analysis, and semiconductor handling.
• Very fine threads — as small as 0.5 mm per rev. Thread rolling, a cold forming process can produce very uniform, precise threads in high volume, at a fraction of the cost of machining, and without the problems of cutter wear.
• Multifunction nuts. Nuts can include other functional components such as guide bushings, mounting features for other components, sensor flags, and gears. This results in part consolidation and reduced assembly time.
• Easily customized nut designs. It is much simpler to incorporate a threaded hole in a custom molded or machined nut than it is to provide a full ball bearing recirculation circuit.