Traditionally, conventional ball screw driven assemblies have provided relatively inexpensive and accurate linear motion for production machines. However, they tend to be bulky and designers often have problems when attempting to integrate them into 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 ball screws, which have limitations in terms of accuracy and dynamics.
An alternative to the ball screw is the roller screw, which is not only more compact, but is 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 performance requires some specialised methods of machining such as grinding to sub-micron accuracies. It means roller screws tend to include greater numbers of precision components such as barrelled profiles on both roller threads and a timing-gear at the end of each roller, 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, and roller screws 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 service life thanks to the higher load-carrying capacity.
Making the change
A recent example includes a food OEM, which switched from SKF SX ball screws to SKF Type SRC planetary roller screws for a range of pressurised depositing manifolds. It is expecting to see a fourfold gain in basic dynamic load carrying capacity and a calculated service life extension from little over a month to three years with the replacement roller screws.
The 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 plastic moulding machinery to microchip production. For this reason, and to provide greater value and shorter lead times, principle suppliers offer a ‘preferred range’, which includes the most frequently used sizes of planetary and recirculating roller screws, along with standard size nuts, with shafts manufactured to customer specifications.
The extraordinary load capacity of roller screws is used to advantage in injection moulding machinery, where, for instance, SKF’s Ultra Power planetary roller screws create forces approaching 500kN. They can exert a clamping pressure of up to 200T 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 also widely used in automated assembly cells for operations that require precise and repeatable linear motion such as riveting and welding. A further example of a precision application is their use in the electromechanical drives of radiotherapy couches that 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 increasingly 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 finding their way into applications that might previously have involved the use of hydraulics or pneumatics.
Precision electromechanical cylinders do, again, 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 with other advantages including a much simpler machine design that dispenses with pumps, accumulators, pipework, seals and so on, as well as substantially reduced maintenance.
In terms of applications, roller screws are likely to be chosen when ball screws can’t 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 roller screws.
Phil Nicholas is business development manager for linear and actuation technology at SKF