Planetary screw drive has high load-carrying capacity

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 high efficiencies with small design envelopes.

Schaeffler's 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. The PWG planetary screw drive is well suited for these applications. 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 per cent increase in load ratings compared with 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.

Reduced design envelope and higher power density

The PWG plugs a gap in the Schaeffler range between the RGT and KGT for spindle diameters between 5mm and approximately 30mm, where very high load-carrying capacities and a small overall pitch are required.

For example, the PWG generates 200N of axial force from just 40Ncm with an overall pitch of 0.75mm. As a result, very high axial forces can be achieved using small electric motors. The PWG therefore enables not only actuators driven by electric motors with a high power density, long rating life and low maintenance requirements to be developed, but also for low-cost motors to be used. The electric drive can be easily integrated using a feather key connection on the outside diameter of the spindle nut.

Initial applications can be found in mirror tracking systems for solar power systems, azimuth adjustment systems in wind energy, feed units for sheet metal forming, sheet metal bending machines, locking cylinders for plastic injection moulding machines, riveting and cutting devices, and in adhesive dispensing and metering systems. The PWG is already being volume produced for use in clutch actuators for the automotive industry.

For more information on the PWG range of planetary screw drives, please go to www.schaeffler.com.