Lead screw technology rolls over ball screws

Lead screws offer the latest technology for motion control, with many distinct advantages, says Tom Solon of Kerk Motion Products.

Lead screws were once believed to be a cheap substitute for ball screws, to be used in less demanding applications.

They were often assembled using low-quality screws from ball screws or fastener designs, where a simple, one-piece nut provided basic power transmission and rotary-to-linear motion conversion.

Today, lead screws offer the latest technology for motion control, with many distinct advantages.

Lead screws differ from ball screws through the use of sliding, rather than rolling, friction between the nut and screw.

Modern materials keep friction very low (less than 0.10), without any external lubrication.

Some of the key advantages include: zero maintenance; no lubrication required; lower particulate generation; longer life with noncatastrophic failure; quieter operation (no recirculating ball noise); high helix/fast leads (greater than 100mm/rev); very fine threads (as small as 0.5mm/rev); non-backdriving (self-locking) leads; multifunction nuts; easily customised nut designs; zero-backlash with very light preload/low drag; and much lower cost/selling price - 25 to 90% less.

Companies like Kerk Motion Products are now taking this technology a step further.

By building on these lead screw advantages, using both patented and proprietary technology, additional benefits are created.

Precision thread rolling offers lead accuracy of 0.0001mm/mm, at approximately one tenth the costs of ground screws, at lengths up to 4m.

Polymer composite nut materials provide high strength (dynamic loads of 250kg) and long life (over 750 million centimetres of travel), and can be moulded to custom shapes to provide additional functionality.

Antibacklash nut designs can automatically compensate for wear.

So, when should ball screws be used? If the application can tolerate the high cost, the design limitations, the noise and the reduced life, the best ground ball screws offer greater maximum speed, lead accuracy and load capability.

Due to the mechanics of rolling elements, ball screws have higher theoretical efficiencies than lead screws with their sliding elements.

In practice, the differences are often smaller because of the effects of lubricant viscosity and manufacturing tolerances.

It is these same efficiencies that prevent ball screws from offering self-locking, non-backdriving leads.

Rolled ball screws are less expensive than ground ball screws but have compromises that reduce these advantages.

The user gets a high load rating, but is left with a higher maintenance component, shorter life and less design flexibility, that may still cost several times more than a precision rolled lead screw assembly.

When should lead screws be used? Top quality lead screws outperform both inexpensive lead screws and more expensive ball screws.

They also cost less to use.

Lead screws are an enabling technology.

Without them, some products would never make it to market.

Specifically, lead screws work well in wash down environments.

The materials used and the lubricant-free operation allows total immersion in water and many other fluids.

Miniature lead screws, with and without antibacklash compensation, provide precision motion in a package size unmatched by other technologies.

The ability to produce a high accuracy screw and nut, 2 to 4mm in diameter with custom features, has succeeded in bringing the latest data storage drives and telecommunications equipment to market.

At the other end of the spectrum, lead screws can have very fast leads (up to 100mm/rev so far) that are efficient and accurate.

This type of thread has been very successful in high-speed automation, including semiconductor handling, laser scanning and engraving, transportation door actuation and valve actuation.

Some non-ball lead screws have leads of more than 75mm/rev and diameters of 20 to 25mm and can be readily supplied in lengths over 4m.

This would be impossible for a ball screw.

Thread grinding cannot produce these high-helix leads, and the cost of a 4m ground screw would be huge.

Yet the best lead screws are produced in many leads with standard accuracy of 0.0006 mm/mm and special accuracy to 0.0001mm/mm.

Rolled multi-start threads also avoid thread drunkenness caused by pitch-to-pitch error of ground or cut multi-start threads.

High helix, fast leads are possible in smaller diameters as well.

Screws as small as 3mm diameter with leads of 10mm/rev can be produced.

Screws with 6mm diameter and 25mm/rev leads are very popular in all types of equipment, including printing and scanning, data storage, medical analysis, paper handling, semiconductor handling and light industrial applications.

Some fast lead screws are used in data storage systems with 4 and 6mm diameters, with complex precision moulded nuts.

These nuts have added functions that reduce part count, reduce tolerance stack-up, outlive, outperform and reduce overall product cost compared to more expensive ball screws and less expensive, low quality lead screws.

The benefits are clear.

The benefits of multifunction, custom-moulded lead nuts are great.

Using a variety of materials, lead screws offer higher performance at lower costs.

Imagine how many parts would be needed to perform the functions if only a standard nut configuration was available.

Then, try to imagine getting this type of customisation in a ball nut.

Complex assemblies can be reduced to single components.

Complete solutions are custom engineered and delivered at a lower cost, and usually in less time, than standard offerings from competitive technologies.

In some screws, the nut incorporates the carriage and linear guide block.

Although these are high value lead screws, the customer sees reduced component costs and a dramatic simplification of assembly and alignment.

Lead screw technology has made vast strides, keeping pace with the latest advances in motion control.

Now that lead screws can provide precision performance at a low cost, and offer flexible design solutions, they have become the premier choice of engineers.

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