Ground vs Rolled Ball Screw

Ground vs Rolled Ball Screw
Ball Screws
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Ball screws have one fundamental use: to convert rotary motion into linear motion. Many automation applications will require ball screws to offer precise, repeatable linear motion. Here we look at the differences between ground vs rolled ball screw manufacturing techniques. Furthermore, we also factor in precision and how each type might be implemented into a machine design.

Ground vs Rolled Ball Screw

Estimated reading time: 5 minutes

Ground vs rolled ball screw manufacturing methods

A rolled ball screw is produced by forcing round blank bar material through rotating tool dies. As the blank material is moved through these rolling dies, the threads are formed as the material is pushed out of the way. The rolling dies come in a variety of shapes and sizes, to form the available pitches associated with ball screws.

Rolled Ball Screw Shaft Material | Ground vs Rolled Ball Screw

This rolling method is super-efficient as no material needs to be removed – as would be the case with the grinding process associated with ground ball screws – and is perfect for ball screw manufacturers as they can achieve high production rates along with maintaining a precision linear product.

Ground ball screw is manufactured by way of grinding away the material of the blank bar to form the ball screw threads. The blank bar material is machined by two machining centres which grind a few threads at a time.

Ground Ball Screw Shaft Material | Ground vs Rolled Ball Screw

This type of machining process means the material blanks need to be cut to length before grinding, meaning a ground ball screw is very much a bespoke product that is ‘made to order’. A similar process is carried out to form the associated ball nut.

Ground vs rolled ball screw advantages and disadvantages

Rolled ball screws

Rolled Ball screw Advantages

When looking at ground vs rolled ball screw affordability, a rolled ball screw is undoubtedly a cheaper option than a ground ball screw, sometimes costing more than 50% less.

As we have seen, this is due to the highly efficient manufacturing process of pushing blank material through rolling dies. Generally, a rolled ball screw will also be available for delivery much faster than a ground ball screw.

Although it seems counterintuitive to say, an advantage of a rolled ball screw is the precision and accuracy they offer.

There is no doubt a ground ball screw is a more accurate, repeatable product, however, a C5 rolled ball screw can still achieve 23-micron (0.023mm) accuracy, more than suitable for a multitude of applications.

Rolled Ball Screw and Nuts

Rolled Ball screw Disadvantages

Rolled ball screws are heat treated and the bearing ends are machined after the rolling process. This can mean that concentricity between screw thread and bearing ends can be hard to maintain.

Also, as the rolled thread material is heat-treated after the threads have been formed, any unpredictable expansion cannot be rectified.

This in turn can mean higher noise levels and higher lead errors over the length of travel.

Ground ball screws

Ground Ball screw Advantages

When looking at ground vs rolled ball screw performance, a ground ball screw is the superior linear motion product when it comes to accuracy and repeatability. A C0 ground ball screw can achieve just 3 micron (0.003mm) mean travel deviation over a 100mm length.

A ground ball screw is manufactured using the same centres to grind all surfaces. This grinding process maintains concentricity and as a ground ball screw is cut to length and the bearing journals machined before the grinding process, this ensures complete concentricity with the ground ball threads.

Ground ball screws will be much quieter in operation than their rolled counterparts. The mating ball nut is ground and paired with the ground screw ensuring ball-to-screw contact is optimised.

Ground Ball Screw and Nuts

Ground Ball screw Disadvantages

The price for a ground ball screw will nearly always be considerably higher than an equivalent-sized rolled ball screw. However, if the application requirements outstrip the capability of a rolled ball screw, then the higher price tag becomes insignificant.

Due to the nature of the grinding process, a ground ball screw can be on a longer delivery lead time than a rolled ball screw. This is something to consider when starting the process of designing a ball screw system into your machine design. 

Ground vs rolled ball screw, which to choose?

When selecting a ball screw, a rolled ball screw will generally cost much less than a ground ball screw and the performance capabilities of a rolled ball screw will suit a wide and varied range of applications.

If the machine design requires extremely high accuracy and repeatability, then a ground ball screw may be the best option. Both types are available in a wide variety of diameters and pitches to suit nearly any application.

Kiwi Motion are a supplier of both rolled and ground ball screws to the whole of the UK & Ireland. As linear motion experts we can advise and guide you on what ball screw is best for your bespoke machine design.

Kiwi Motion supplies rolled ball screws and ball nuts from stock with custom machining available for all rolled ball screws. Contact us with your requirements for ground ball screws.

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