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Time to get the ball rolling

Author : Phil Nicholas is Business Development Manager for Linear and Actuation Technology at SKF

07 November 2016

Roller screws are often an order of magnitude more expensive than common ball screws – but are the technology of choice for many demanding applications, says Phil Nicholas, Business Development Manager, Linear and Actuation Technology at SKF.

SKF planetary roller screw

Design engineers are programmed to be cost-conscious, so convincing them to spend more on components can be difficult. However, if the switch comes with huge engineering advantages – and their paymasters are amenable – then the job becomes a lot easier. Such a situation exists for replacing ball screws with roller screws.

Both perform a similar function, of converting rotational motion into linear motion with minimal friction. Ball screws use a series of ball bearings to transfer force from the nut to the screw. The balls themselves are recirculated, to allow motion to continue. Roller screws, on the other hand, use multiple barrel-shaped rollers – arranged around the shaft – to transfer the force. Because the rollers are guided and positively driven they never come into contact with one another, consequently they can be driven faster with less heat generation than a ball screw. 

In a typical planetary roller screw, the screw (or shaft) has a triangular thread form with an included angle of 90°. The nut is internally threaded and has the same type and number of threads as the screw shaft. The rollers are threaded with a single start triangular thread form, with an included angle of 90°. The roller screw has a much greater number of contact points – and a far greater contact area – than a ball screw. This improves load-carrying capacity and can boost service life by up to 10 times, which helps to reduce the total cost of ownership.

With positively driven satellite rollers, rotational speeds can be up to 50 percent higher than for ball screw systems – while acceleration is up to three times faster. At the same time, the absence of a recirculation system – a major failure mode in ball screw systems – makes the system more reliable.

Overall, roller screws offer a number of advantages, including: greater load-carrying capacity; superior power density; faster speed and acceleration; and, higher positional accuracy.

What’s the difference?

However, in order to achieve this higher performance they must be made to far more exacting standards. For this reason, they are often substantially more expensive than ball screws.

For a start, components must be manufactured to more precise tolerances. This means using precision engineering techniques that allow grinding to sub-micron accuracies. Roller screws also tend to include greater numbers of precision components – barrelled profiles on both roller threads, and a timing gear at the end of each roller, for instance. These all help to push the cost up.

Ball screws

Although the materials used to make roller screws are usually no different to those used for ball screws – typically bearing steels – alternatives can easily be selected to enhance performance: higher tempering temperatures will raise the working temperature and boost wear resistance and stainless steel for corrosion resistance.  

However, the added cost of these processes is usually justified in terms of the improved performance and longevity that roller screws allow.

Why switch to rollers?

The combination of advantages offered by roller screws means that they are now specified in a wide variety of industrial applications – from plastics machinery to microchip production, and offshore applications to clean rooms. For this reason and to provide greater value and shorter lead times, principal suppliers such as SKF offer a ‘preferred range’, which includes the most frequently used sizes of planetary roller screws and recirculating roller screws – along with standard size nuts. Shafts are manufactured according to customer requirements.

In injection moulding machinery, for instance, SKF’s Ultra Power roller screws have been used in the injection and clamping mechanism of models that exert a clamping pressure of up to 200,000kg. Roller screws are also used in thermoforming and blow moulding machines, and help to boost machine productivity, save energy and improve cleanliness – by replacing hydraulic mechanisms with electric ones.

Roller screws have also been used in factory automation systems – for operations such as gluing, riveting and welding – thanks to their accuracy and repeatability, faster resetting, reduced downtime and compactness. In microchip encapsulation, the reduced footprint, increased duty cycle and longer lifetime – coupled with reduced maintenance – are the main reasons for choosing roller screws.

And, in an example of a precision application, roller screws were used in the electromechanical drive of a radiotherapy couch – which must be positioned accurately before gamma rays are directed into the patient.

In terms of applications, roller screws are usually chosen when ball screws can no longer cope: so, if a ball screw-based design is right on the edge of its speed or load capabilities, or is likely to undergo premature failure, it’s time to consider stepping up to roller screws.

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