SKF recirculating roller screws
The roller screw: an essential element in modern machine design
2017 August 15, 09:00 GMT
Rollers screws may be an expensive alternative to traditional ball screws, but they are fast becoming the de facto choice for the modern machine builder seeking compactness, precision and superior dynamic performance, as Phil Nicholas, Business Development Manager, Linear and Actuation Technology at SKF explains.
Automated industrial systems such as assembly cells and production lines are getting smaller; manufacturers are seeking to make these systems more compact so they take up less space, while at the same time demanding greater reliability, lower power consumption and higher levels of precision. The electronic devices that monitor and control these systems have an important role to play but so, too, do the mechanical components: linear motion devices and actuators, for example; and like the electronics, there is a desire for miniaturisation here too.
Traditionally, conventional ball screw driven assemblies have provided relatively inexpensive and accurate linear motion for production machines but they tend to be bulky and designers are presented with problems when attempting to integrate them into today's more compact machine configurations. Moreover, there is a requirement for modern production machines to deliver greater positional accuracy at high speeds and this poses a challenge for the ball screw actuator, which has limitations in terms of its accuracy and dynamics.
An alternative to the ball screw is the roller screw, which is not only more compact, but also capable of carrying greater loads in relation to its size while achieving considerably better positional accuracy compared with a ball screw. At the same time, a roller screw will operate at much higher speeds and accelerations, and deliver superior power density, better reliability and longer service life.
However, all this comes at a price; achieving this superior performance requires some rather specialised methods of machining – for example, 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, complicating assembly and testing, which all helps to push up the cost.
This cost premium is difficult to ignore; after all, design engineers are programmed to be cost-conscious, so convincing them to spend more on components is not an easy task – particularly so in the case of a roller screw which can be as much as an order of magnitude more expensive than a ball screw. However, if the engineering advantages of roller screws over ball screws can be demonstrated – and where precision and reliability cannot be sacrificed – the roller screw becomes the technology of choice. Indeed, in many lighter duty applications of roller screws, the extra investment will ultimately reap returns in the form of a manifold increase in service life thanks to the higher load-carrying capacity.
A recent example where a food industry OEM switched from SKF SX ball screws to SKF Type SRC planetary roller screws in one of their ranges of pressurised depositing manifolds is expected to see a fourfold gain in basic dynamic load carrying capacity and a calculated service life extension from little over a month with the original ball screws to three years with the replacement roller screws – that’s about thirty times longer life! This retrofit was carried out with minimal modification using the existing drives and controls to achieve faster cycle times and higher speeds of operation in addition to the extended service life.
The combination of advantages offered by roller screws is compelling and, despite their cost, they are now specified for a wide variety of industrial applications – from plastics moulding machinery to microchip production, and offshore applications to clean rooms. For this reason and to provide greater value and shorter lead times, principle suppliers in this market, 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, with shafts being manufactured to customer specifications.
The extraordinary load capacity of roller screws is used to advantage in injection moulding machinery; for instance, SKF’s Ultra Power planetary roller screws (which create forces approaching 500kN) can exert a clamping pressure of up to 200 tonnes in these machines. The reliability and load capacity of roller screws is also exploited in aerospace applications where they provide the necessary actuation for flight control surfaces.
And thanks to their superior positional accuracy and repeatability, roller screws are now also widely used in automated assembly cells for operations that require precise and repeatable linear motion such as gluing, riveting and welding. A further example of a precision application is their use in the electromechanical drives of radiotherapy couches which must be positioned with pinpoint accuracy to ensure that patients receive correctly targeted radiation doses.
Combined with a suitable electric servomotor in a compact actuator format, roller screw based electromechanical cylinders are now increasingly being used as drop-in replacements for hydraulic rams in applications that require high actuation forces without the attendant problems of hydraulics such as failed seals and oil contamination. Indeed, roller screw-based electromechanical actuators are now finding their way into many applications that might previously have involved the use of hydraulics or pneumatics.
Precision electromechanical cylinders do cost more per equivalent unit than their fluid power alternatives; however, electromechanical technology provides additional value over the lifetime of a machine that more than offsets the higher initial outlay. The environmental and performance benefits of electromechanical cylinders have already been touched upon; other advantages include a much simpler machine design that dispenses with pumps, accumulators, valve manifolds, pipework, seals and so on, as well as a substantially reduced maintenance burden.
In terms of applications, roller screws are likely to be 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 envelope, lacks the required accuracy, provides short service life or is likely to suffer premature failure, then it’s time to do the cost benefit analysis and consider stepping up to roller screws.