Schaeffler is expanding its range of spindles through the addition of a screw drive that features a new design and functional principle and delivers both a high load carrying capacity and a high power density. The technical characteristics of the so-called "planetary screw drive" or PWG make it the perfect addition to Schaeffler's range, which includes the ball screw drive (KGT) and roller screw drive (RGT). The high power density of the PWG means that it can also be used as a substitute for hydraulic drives. Schaeffler thus has a complete range of screw drives and can deliver the perfect solution for every application in terms of speed, precision, and load carrying capacity. The planetary screw drive (PWG) paves the way towards "smart actuators": Electromechanical linear actuators that combine the highest possible efficiency with the smallest possible design envelope.
The ball screw drive and roller screw drive have both become well established on the screw drive market. The ball screw drive's outstanding strengths include high dynamics due to the high spindle pitch, low-friction running, and a high level of positioning and repeat accuracy. For geometrical reasons, however, low spindle pitches cannot be achieved. By comparison, the roller screw drive offers a higher load carrying capacity at spindle pitches from approx. 10 mm and high positioning accuracy.
The missing third element is a design that can support even higher loads at small spindle pitches of less than 5 mm. For this type of application, Schaeffler has developed the so-called planetary screw drive (PWG): This design features planetary gears with v-shaped parallel grooves that roll up and down the spindle. The rotation of these planetary gears and the planetary screw drive is ensured by the two-piece screw drive nut, which also has grooves at the ends that engage with the ends of the planetary gears. The very high number of rolling contacts means that the PWG achieves a higher load carrying capacity and rigidity than the other two designs. Friction levels remain low thanks to the good internal load distribution and the optimized 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 0.75 to 5 mm can be achieved.
The special features of this design include the manufacturing process itself: The spindles and planetary gears are manufactured using forming methods, which makes good material compression possible together with optimum grain flow, the highest possible strength, and therefore a further 15% increase in load rating compared to conventional technologies. This manufacturing method also reduces the costs to a level comparable with that of ball screw drives manufactured using forming methods. Clearance free, preloaded units can easily be created by adding a spacer washer between the two halves of the spindle nut.