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Planetary screw drive offers high load carrying capacity and high power density

Planetary screw drive offers high load carrying capacity and high power density

Precision bearing manufacturer Schaeffler (UK) has extended its range of spindles by introducing a screw drive with a new design and functional principle, which provides a high load carrying capacity and high power density.

The technical characteristics of the new PWG range of planetary screw drives make it the ideal addition to the Schaeffler range, which also includes the ballscrew drive (KGT) and roller screw drive (RGT).

The high power density of the PWG means that it can also be used as an alternative to hydraulic drives. In terms of speed, precision and load carrying capacity, Schaeffler can provide a complete range of screw drives for every application. The PWG also paves the way towards "smart actuators" - electromechanical linear actuators that combine the highest possible efficiencies with the smallest possible design envelope.

The ballscrew drive and roller screw drive are both well established in the screw drives market. The ballscrew drive's outstanding strengths include high dynamics (due to the high spindle pitch), low-friction running and high positioning/repeatability. However, for geometrical reasons, low spindle pitches cannot be achieved. In comparison, the roller screw drive offers a higher load carrying capacity at spindle pitches from approx. 2-10mm, as well as high positioning accuracy.

The missing third element is a design that can support even higher loads at small spindle pitches of less than 5mm. For these applications, the PWG planetary screw drive is ideal. The design incorporates planetary gears with V-shaped parallel grooves, which roll up and down the spindle. The rotation of these planetary gears and the planetary screw drive is achieved by using a two-piece screw drive nut, which also has grooves at the ends that engage with the ends of the planetary gears.

A high number of rolling contacts means that the PWG achieves a higher load carrying capacity and rigidity than the KGT and RGT ranges. Friction levels remain low due to good internal load distribution and the optimised osculation between the spindle thread flanks and the crowned flanks of the planetary gear grooves. When planetary gears with the correct groove diameter are selected, overall pitches of just 0.75mm to 5mm can be achieved.

Other important technical characteristics include how the PWG range is manufactured. The spindles and planetary gears are produced using forming methods, which ensures good material compression, optimum grain flow, and high strength, resulting in a 15% increase in load ratings compared to conventional technologies. This manufacturing method also reduces the costs to a level comparable with that of ballscrew drives manufactured using forming methods. Clearance-free, preloaded units can easily be produced by adding a spacer washer between the two halves of the spindle nut.

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