Drives and Controls Magazine
Novel linear actuators challenge hydraulic drives
Published:  16 March, 2015

Schaeffler has come up with a new form of linear spindle actuator that can support heavy loads at spindle pitches of less than 5mm. Called the planetary screw drive – or PWG – the new device has large enough power densities and load-carrying capacities to replace hydraulic drives in some applications.

Schaeffler says that the new drive will pave the way towards “smart” actuators – electromechanical linear actuators that combine the highest possible efficiencies with the smallest possible design envelopes.

Two types of electromechanical actuator – ballscrew and rollerscrew drives – are already well-established in the screw drive market. The ballscrew drive’s key strengths include its high dynamics due to high spindle pitches, low-friction running, and high level of positioning and repeat accuracy. For geometrical reasons, however, they cannot achieve low spindle pitches. Rollerscrew drives, by comparison, offer higher load-carrying capacities at spindle pitches larger than about 10mm, as well as high positioning accuracies.

According to Schaeffler, the planetary screw drive is the missing third element that can support high loads at spindle pitches of less than 5mm. The new planetary screw drive incorporates planetary gears with V-shaped parallel grooves that roll up and down the spindle. A two-piece screw drive nut ensures the rotation of these planetary gears and the planetary screw drive. The nut also has grooves at the ends that engage with the ends of the planetary gears.

The high number of rolling contacts means that the PWG achieves a higher load-carrying capacity and rigidity than the two older designs. Friction levels remain low thanks to the 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 used, overall pitches of just 0.75–5mm are possible.

The spindles and planetary gears are manufactured using a forming technique which results in good material compression together with an optimum grain flow, high strength, and a 15% increase in load rating compared to conventional technologies. This manufacturing method also cuts the costs to a level comparable to those of ballscrew drives produced using forming methods. Clearance-free, preloaded versions can be created by adding a spacer washer between the two halves of the spindle nut.

Schaeffler's planetary screw drive will support high loads at spindle pitches of less than 5mm

Schaeffler says that the planetary screw drive – which makes its public debut at the Hannover Fair in April – closes the gap between ballscrew and rollerscrew drives for spindle diameters from 5–30mm in terms of high load-carrying capacities and small pitches. For example, the PWG can generate 200N of axial force from only 40Ncm with an overall pitch of 0.75mm. This allows high axial forces to be achieved even with small motors.

The new actuators can be driven by specially developed motors with high power densities, long lives, and low maintenance costs – or by low-cost motors. The motors can be integrated easily using a feather key connection on the outside of the spindle nut.

Schaeffler says that with the new arrival, it now has a complete range of screw drives and can offer the ideal drive for every application in terms of speed, precision, and load-carrying capacity.

Initial applications for the planetary screw drive are likely to be in areas such feed systems for sheet metal forming, mirror tracking systems for solar power, azimuth adjustment systems for wind turbines, sheet metal bending machines, locking cylinders for injection moulding machines, as well as in riveting and cutting devices. The technology is already being used in automotive clutch actuators.