Rolling with it: boosting linear performance
06 November 2017
Despite their higher cost, roller screws are well worth considering as a replacement for ball screws thanks to their premium performance in linear actuation applications. Phil Nicholas, Business Development Manager Linear & Actuation Technology at SKF explains why.
In the world of engineering, cost pressures are becoming ever more critical. However, as in all walks of life, it is always worth paying a premium for something if it offers clear benefits. Roller screws are a case in point. Though more expensive than ball screws, they offer more in terms of load capacity, positional accuracy, speed and reliability – while also fitting into a more compact space.
They cost more because they are manufactured to more precise tolerances, and use precision components. This superior performance makes them suitable for a variety of demanding linear electro-mechanical actuation applications, including injection moulding machinery and industrial spray guns – and many others in the medical, marine and aerospace industries.
Like a ball screw, a roller screw uses a rotating externally threaded shaft to control the linear position of an internally threaded ‘nut’. The difference is in the design of the elements used to transmit loads while minimising friction and backlash.
In a planetary roller screw – the most common design in general industrial use – threaded rollers are held in position around the shaft by guide rings at both ends of the nut. Gear teeth are cut into the ends of the rollers, and mesh with internally toothed rings inside the nut – ensuring the correct rolling motion at all times.
Other advantages of roller screws include: the need for minimal lubrication; high power density; a mechanical efficiency that usually exceeds 80 percent; limited wear – especially for screws that have been ‘run in’ at the factory; and quiet operation.
The overwhelming advantage compared to a ball screw is the load carrying capacity. In ball screws, the load is transmitted from the nut to the shaft through balls engaged in the groove. Here, the ball diameter is limited to around 70 percent of the screw lead.
So, the number of contacts is small even if there are multiple starts. In roller screws, the load is transmitted through the surfaces of all the engaged threaded rollers. This increases both the diameter of the contact surface and the number of contact points. The result is that – for a given size – a roller screw will carry a greater load, for longer, without wearing out. This can lead to a tenfold increase in service life, which far outweighs the extra initial investment.
Other configurations of roller screw offer different benefits. The recirculating roller screw, for instance, has grooved rollers that turn within a separate cage mounted inside the nut. This design allows the construction of screws with an exceptionally fine lead, which are often used in precision applications such as scientific instruments, medical and semiconductor manufacturing.
A third configuration, the inverted roller screw, combines the benefits of planetary and recirculating designs. Here, a set of planetary rollers translates along the inside of the nut during operation, moving a separate push tube that carries the load. Inverted roller screws allow either the nut or shaft to be driven, and are sometimes built into a motor to create a powerful, accurate short-stroke actuator – with a minimal footprint.
As well as replacing ball screws, roller screws have also ousted hydraulic actuators in some applications, thanks to their compactness, and their high power density and robustness.
In general, a roller screw will allow actuation to be performed in a smaller space, and in more demanding environments. In one extreme example, they have been used in the control of valves for deep sea oil & gas applications, where they are exposed to extreme temperatures and pressures. There are many other examples of them being specified in challenging applications:
In injection moulding machines they have been used in both the injection and clamping mechanisms, in models that exert clamping pressures of up to 200 tonnes – while boosting energy efficiency and banishing the need for hydraulic oil; as part of the electromechanical drive of a radiotherapy couch, which must be positioned accurately before gamma rays are directed into the patient; in racing yachts, hydraulic cylinders have been replaced with lighter roller screws in the mechanism that tilts the keel in order to maximise righting moment; and, on the European Southern Observatory’s Very Large Telescope in Chile, four 8.2m diameter mirrors each rest on 150 recirculating roller screws. The screws move automatically every 25 seconds, to reposition the mirror constantly.
Cost control has become increasingly important when specifying components for a design project, and this is unlikely to change. However, looking beyond cost by choosing the most appropriate component – such as a roller screw, when its advantages are clear – will make it more likely that the final product is robust enough to work efficiently under challenging conditions.
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