Lead screws transform rotary motion into linear motion to quickly and efficiently position loads. They use sliding surfaces between the nut and screw. Ball screws, a competing design, recirculate ball bearings between the nut and screw.

Unfortunately, lead screws got a bad rap from some people who thought the devices were just inexpensive substitutes for ball screws and were only fit for less-demanding applications. It didn't help that a few older lead-screw designs were made with simple nuts and low-quality screws that used fastener threads.

Today's lead screws, however, come with the latest in motion control and sport many distinct advantages.

A few key plusses of lead screws include:

• Zero maintenance

• No lubrication required

• Generates few particulates

• Longer life with noncatastrophic failure

• Quieter operation without recirculating-ball noise

• High helix, fast leads. Over 100 mm/rev

• Threads as small as 0.3 mm/rev

• Compact packages. Miniature lead screws, with and without antibacklash compensation, provide precise motion in small volumes.

• Backdriving or nonbackdriving (self locking leads)

• Multifunction nuts

• Easily customized nut designs

• Zero-backlash with light preload and low drag

• Costs are 10 to 75% of similarly rated ball screws

Some lead screws use advanced technology such as:

• Precision thread rolling for lead accuracies of 0.0001 mm/mm at about one tenth the cost of ground screws, and lengths to 4 m.

• Polymer-composite nuts have high strength (dynamic loads of 250 kg), long life (over 750 million cm of travel), and can be molded to custom shapes for additional capabilities. In addition, modern materials keep friction quite low, less than 0.10 Cf, and without external lubrication.

• Antibacklash nut self adjusts for wear

Top-quality lead screws can outperform more expensive ball screws. They also cost less. And without lead screws, some products might not get to market. For instance, lead screws are essential for wash-down environments. Their materials and lubricant-free operation withstands total immersion in water and other fluids. And high-accuracy screws and nuts, 2 to 4-mm diameter, were important in bringing the latest data-storage drive and telecommunication equipment to market.

Lead screws also can have fast or high leads, up to 100 mm/rev so far. These too are efficient and accurate, and have been successful in high-speed automation, including semiconductor handling, laser scanning and engraving, and valve actuation.

Screws with leads of more than 75 mm/rev and diameters of 20 to 25 mm can be readily supplied in lengths over 4 m. This length would be impractical for a ball screw. Thread grinding cannot produce high-helix leads, and the cost of a 4-m ground screw would be astronomical. Yet the best lead screws are produced in many different pitches with accuracies of 0.0006 mm/mm and special accuracy to 0.0001 mm/mm. Rolled multi-start threads also avoid what's called thread drunkenness — a pitch to pitch error of ground or cut multi-start threads.

Screws with 3-mm diameters and leads of 10 mm/rev provide examples of high helix, fast leads in smaller diameters. Screws with 6-mm diameters and 25 mm/rev leads are widely used in all types of equipment, including medical analysis, printing and scanning, data storage, paper handling, semiconductor handling, and light-industrial applications.

Additional lead-screw benefits come from the nuts. A variety of nut materials let lead screws offer higher performance at lower costs. A few illustrations show examples of multifunction nuts. Imagine how many parts would be needed to perform the function if only a standard nut configuration was available.

Get rid of the ball screws, get rid of the noise

Replacing ball screws with lead-screw linear-positioning devices let engineers at PerkinElmer Life Sciences quiet several machines and reduce their lubrication requirements. Most ball-screw noise comes from ball-return tubes. But there is no recirculation system or ball-return tubes in nonball-screw devices. The only movement screws have comes from the self-lubricated nut on the TFE coated screw, so most noise is gone.

The company had been using 0.625-in. diameter screws, a common size. While the performance was adequate, noise was a nuisance.

Kerk replaced the ball screws in several PerkinElmer machines, in particular the MiniTrak, a compact liquid-handling model, and PlateTrak, a liquid-handling system for high-throughput screening and other drug discovery and biomolecular protocols. Besides lead screws, the machines also use a ScrewRail for Z-axis positioning.

Prior to using ScrewRails, the machines used a vertical ball screw with a bearing on top but a free end of the bottom. This created vibration in a carriage assembly, creating additional noise when in action.

The ScrewRail, a self-contained actuator, consists of a precision lead screw and concentric guide rail, both with KerkoteTFE coatings. The unit captures a bearing on the lower end to tighten up radial “slop” and provides an additional guiding surface. Consequently, vibration is smoothed out and noise almost eliminated. What's more, the module actually has a more pleasing physical appearance.

Self-lubricating lead screws reduced lubrication and routine maintenance problems. Better yet, machine assembly was simplified, as have installation and adjustment procedures.