Five starts to maximum efficiency

Tom Shelley reports on some recent advances in linear systems and bearings that give striking enhancements to performance.

A new linear screw achieves greater performance per unit size and volume by using a fine screw thread with five starts per revolution instead of one, while ranges of new bearings reduce friction by 25% and new bearing based products for the automotive sector contribute significantly to fuel efficiency. The enhancements come from both improved design and analysis and from lower tolerance improved manufacturing to ensure optimum fits. One of the results of this advanced design is SKF's planetary roller screws, which use a series of threaded rollers to interact with a fine-threaded shaft with five or six starts, to achieve greater forces per unit volume and lower friction than either recirculating ballscrews or nuts driven on shafts with a single screw thread. Having a fine thread means that a screwed shaft is able to offer a higher proportion of its outer diameter to load bearing, as well as reducing opportunities for crack starting. Multiple starts permit faster linear movement for the same angular rotation. Friction is reduced relative to ballscrews because there is no mismatch between the linear speed of the elements that transfer load from nut to shaft. In a ballscrew, the balls have to go at a different speed, in order that they can recirculate. Asked why the thread in the roller screws had five or six starts rather than one or seven, Marek Pajek, business leader, SKF actuators and drives, replied that five starts was optimal, while six starts allowed the nut to be made smaller, but lowered load capacity. He added that the shaft thread could in some cases, have two or three starts, depending on the application, but in order to design a roller screw to work best in a particular application, he said: "It is necessary to work with the customer from the beginning". One of the applications is power presses, developed by H&T Produktions Technologie based in Crimmitschau in Germany. These machines use two, three or four roller screws to apply forces of up to 65 to 250 tonnes, with 400 and 600 tonne machines in development. Using roller screws instead of hydraulics reduces noise, vibration and energy consumption, and allows very precise movements, particularly rapid accelerations and quick stops. It is also possible to coordinate movements of the actuators very precisely, even when the loading on the actuators is very uneven. Although flow dividers will ensure the delivery of similar amounts of oil to actuators bearing different loads, there is always some tendency for the least loaded actuator to move furthest. Shaft diameters range from 8mm to 210mm. In a 2 tonne system, the actuators are able to achieve a precision of 23µm in 210mm of stroke. As well as power presses, the actuators are used in plastic injection moulding machines, broaching machines and on spot welding robots, where they are said to reduce consumption of welding tips by 15% because of their more precisely positioned movements. Typical service lives are more than 20 million cycles. Manufacturing technologies are key in a new range of 'Super precision' ball bearing units for small turbo chargers, which are expected to become the norm for all diesel engines by 2012, plus 25% of petrol engines. The design brief for the turbochargers is running speeds of 240,000 rpm and working temperatures of more than 250ºC. The SKF solution uses a double row angular contact ball bearing unit with ceramic balls which is said to save 2% fuel and 3% CO2 compared with the current plain bearing solution. One of the other new automotive bearing products developed by SKF are rocker arms with bearings in steel wheels in cam followers, instead of the direct frictional contact that has been traditional in the past. SKF's first target market for these is motor scooters. The scooter industry has moved from two-stoke to four-stroke engines which require camshafts and rockers. Roller camshaft followers offer reduced wear, friction and noise.