Leadscrews roll

Dec. 8, 2005
Leadscrews provide finer pitch and faster motion than comparable ball screws.

Tom Solon
Kerk Motion Products
Hollis, N.H.

These leadscrews are all less than 80-mm long with both right and left-hand threads and leads as fine as 0.65 mm.

Leadscrews can have fast leads up to 100 mm/rev that are both efficient and accurate. This type of thread is successful in high-speed automation, including semiconductor handling, laser scanning and engraving, and transportation door and valve actuation.

Screws with leads of more than 75 mm/rev and diameters from 20 to 25 mm come in lengths over 4 m, virtually impossible for a ball screw. Thread grinding cannot produce these high-helix leads and the cost of a 4-m-long ground screw is prohibitive. Yet the best leadscrews are produced in many leads with standard accuracy of 0.6 m/mm and special accuracy to 0.1 m/mm. Rolled multistart threads also avoid thread drunkenness caused by pitch-to-pitch error of ground or cut multistart threads.

These 4 and 6-mm-diameter leadscrews find use in all types of equipment, including printing and scanning, data-storage, medical-analysis, paper-handling, semiconductor-handling, and light-industrial applications. The complex precision-molded nuts reduce part count and tolerance stack-up compared to more expensive ball screws.

A single composite molded nut incorporates the carriage and linear guide block. Although this is a highvalue leadscrew, the composite molded nut reduced component costs and simplified assembly and alignment.

Once thought of as a cheap substitute for ball screws, leadscrews found use in less-demanding applications. Early leadscrew construction often used low-quality ball screw or fastener threads. A simple, one-piece nut provided basic power transmission and rotary-to-linear motion conversion. That has all changed today.

Primarily, leadscrews use sliding friction between the nut and screw rather than the rolling friction of ball bearings used in ball screws. Modern materials keep leadscrew friction between the nut and screw low — typically less than a factor of 0.10 — without external lubrication.

Leadscrews built with today's technology now offer many distinct advantages for motion control. A quick list includes: practically zero maintenance, no lubrication requirements, lower particulate generation, longer life with noncatastrophic failure, quieter operation without recirculating ball noise, high helix for fast leads greater than 100 mm/rev, and also fine threads with some as small as 0.5 mm/rev. Additional features include nonbackdriving or selflocking leads, multifunction nuts, customized nut designs, almost zero backlash with very light preloads and low drag, and a lower cost/selling price — sometimes 25 to 90% less than an equivalent ball screw.

Leadscrews assembled with precision rolled threads offer lead accuracy to 0.1 m/mm at lengths up to 4 m. Poly-mer composite nuts provide high strength for dynamic loads up to 250 kg and long life with travels over 750 million cm. Composites molded to custom shapes adapt leadscrews to specific applications while antibacklash nut designs automatically compensate for wear.

Ball screws have higher theoretical efficiencies over leadscrews thanks to the mechanics of their rolling elements. In practice, the differences are often small because of the effects of lubricant viscosity and manufacturing tolerances. It is these same efficiencies that prevent ball screws from offering selflocking, nonbackdriving leads.

The best ground ball screws offer greater maximum speed, lead accuracy, and load capacity. Rolled ball screws are less expensive than ground ball screws; but also have compromises that reduce their advantages. Users get high load ratings but are left with higher maintenance components, shorter screw life, and less design flexibility. And rolled ball screws may still cost several times more than precision rolled leadscrews.

Top quality leadscrews outperform more expensive ball screws. They are an enabling technology without which some products would never make it to market. For example, leadscrews work well in wash-down environments. Materials used for lubricant-free operation allow total immersion in water or many other fluids. Miniature leadscrews having 2 to 4-mm diameter with and without antibacklash compensation helped bring the latest data-storage drives and telecommunications equipment to market.


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