Leadscrews roll over ball screws

Dec. 11, 2003
Leadscrew technology has made vast strides. Now, leadscrews offer higher performance at lower cost.
These leadscrews are all less than 80-mm long, with both right-hand and left-hand threads, and leads as fine as 0.65 mm/rev.
Fast leadscrews used in data-storage systems have diameters of 4 and 6 mm and complex precision molded nuts. The nuts reduce part count and tolerance stack-up.
Complex assemblies can be reduced to single components. Complete leadscrew assemblies are custom engineered and delivered at a lower cost, and usually in less time than standard offerings.
A leadscrew nut incorporates the carriage and linear guide block. The design reduces component cost and simplifies assembly and alignment.

Tom Solon
Applications/Sales Engineer
Kerk Motion Products
Hollis, N.H.

Leadscrews were once considered a cheap substitute for ball screws to be used mostly in less demanding applications. They were often assembled using low-quality ball-screw or fastener designs, where a simple, one-piece nut provided basic power transmission and rotary-to-linear motion conversion.

Today, leadscrews offer many distinct advantages for motion control. For instance, precision thread rolling yields lead accuracy of 0.0001 mm/mm at approximately one-tenth the cost of ground screws in lengths to 4 m. Polymer composite nut materials provide high strength, capable of supporting dynamic loads to 250 kg, long life (over 750 million cm of travel), and can be molded to custom shapes. In addition, antibacklash nuts automatically compensate for wear.

Among some of the other benefits are high helix, fast leads (greater than 100 mm/rev) with threads as small as 0.5 mm/rev. Plus, nonbackdriving, self-locking leads and multifunction nuts are easily customized, and can offer zero backlash with light preload and low drag.

Leadscrews differ from ball screws in that they use sliding rather than rolling friction between the nut and screw. This leads to quieter operation because of no recirculating ball noise. And modern materials keep friction low, with dynamic coefficients ranging from 0.07 to 0.09, eliminating the need for external lubrication and cutting maintenance costs.

Another major concern is the amount of particulates generated and their impact on operation. Metal-on-metal contact with rolling elements has no sliding friction. But imperfections in feature sizes can lead to localized stress concentrations that, over time, create surface imperfections. These imperfections have the cumulative effect of increasing friction stress. This leads to sliding and later brinnelling, which generates significant debris and eventually catastrophic failure.

Ground ball screws and bearings start out better. But lower-cost rolled ball screws are more likely to start out with sliding friction and can generate significant amounts of debris quite quickly. Metal-on-metal leadscrews also have these limitations.

Polymer nuts with self-lubricating components on metal screws offer low friction without external lubrication. The particulate generation rate is generally flat over the life of the product, compared to the exponential rate of metal-on-metal ball nuts. The rated life for a polymer-nut leadscrew assembly is usually higher than competitive ball screws, and catastrophic failure is uncommon.

Absence of external lubricants also minimizes particulate generation. The lubrication in ball screws and bearings can be a significant source of particulates. As lubricant collects and accumulates, it can dry out and flake or crumble. Another benefit is that particulates from a polymer nut are relatively inert in contrast to conductive metal debris, which can cause failure of nearby electronics.

Ball versus leadscrews

The best ground ball screws offer greater maximum speed, lead accuracy, and load capability. However, drawbacks include higher cost and more noise. Due to the mechanics of rolling elements, ball screws have higher theoretical efficiencies than leadscrews with their sliding elements. In practice, the differences are often smaller because of the effects of lubricant viscosity and manufacturing tolerances. These same efficiencies prevent ball screws from offering self-locking, nonbackdriving leads.

Rolled ball screws are less expensive than ground ball screws but have compromises that reduce performance advantages. A higher load rating comes with a higher maintenance component, shorter life, and less design flexibility. Plus, it may still cost several times more than a precision rolled leadscrew assembly.

Top quality leadscrews outperform both inexpensive leadscrews and more-expensive ball screws. They also cost less. Specifically, leadscrews work well in washdown environments and can be totally immersed in water and other fluids.

Miniature leadscrews, with and without antibacklash compensation, provide precision motion in a compact package. For example, the ability to produce a high-accuracy screw and nut, 2 to 4 mm in diameter with custom features, has helped bring the latest data storage drives and telecommunications equipment to market.

At the other end of the spectrum, leadscrews can have fast leads (up to 100 mm/rev) that are efficient and accurate. This type of thread is used in high-speed automation including semiconductor handling, laser scanning and engraving, transportation door actuation, and valve actuation.

Some screws can have leads of more than 75 mm/rev and diameters of 20 to 25 mm readily supplied in lengths over 4 m, which would be impossible for a ball screw. Thread grinding cannot produce these high-helix leads, and the cost of a 4-m ground screw would be huge. Yet the best leadscrews are produced in many leads with standard accuracy of 0.0006 mm/mm and special accuracy to 0.0001 mm/mm. Rolled multistart threads also avoid thread drunkenness caused by pitch-to-pitch error of ground or cut multistart threads.

High-helix, fast leads are possible in smaller diameters as well. Screws as small as 3-mm diameter with leads of 10 mm/rev are possible. Screws with 6-mm diameter and 25-mm/rev leads are common in all types of equipment including printing and scanning, data storage, medical analysis, paper handling, semiconductor handling, and light industrial applications.

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