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Hydrostatic bearings mount a real challenge to roller guides

01 October, 2006

Hydrostatic bearings mount a challenge to roller guides

Hydrostatic bearings — which move on a cushion of high-pressure fluid, thus avoiding metal-to-metal contact — offer many potential advantages over conventional rolling element bearings in linear guides. These benefits include improved damping, high dynamic rigidity, and the elimination of vibrations.

But, to date, hydrostatic bearings have not been adopted widely because they have been expensive, time-consuming to mount, and needed more space that traditional profiled rail guides.

This could change, however, with the development of a new generation of precision hydrostatic linear guides that are said to be easy to mount, and will fit in the same space as an equivalent rolling element guidance system. The new guides have been developed by engineers` from Schaeffler`s linear division, working with researchers from the University of Aachen.

The hydrostatic guides (above) are aimed at applications such as machine tools, where their high dynamic stiffness will help to boost cutting capacities, resist chatter, and provide improved damping. The result, says Schaeffler, will be longer tool lives and better surface finishes for the machined parts.

Another claimed advantage of the hydrostatic approach is that the fluid film damps — or even eliminates — vibrations at resonant frequencies which can affect the accuracy and surface finish of workpieces. These vibrations, caused by blade contact shocks or tool wear, normally result in surface imperfections.

A further attraction of the hydrostatic bearings is that wear is negligible, resulting in extremely long service lives for the bearings and the machines they are built into. And, because the oil pressure in the pocket increases as the cutting load is applied, the fluid gap is prevented from closing under high impact loads, thus protecting against machine crashes.

At this stage, Schaeffler is producing the hydrostatic linear guides as prototypes and is working with customers to develop the designs for volume production. The prototypes have a pocket pressure of 100 bar, a load-carrying capacity of up to 18kN, a throughflow rate of less than 3 litres/min, and a gap height of 0.015mm. Their load-carrying capacity is almost as good as a monorail guidance system of the same size, and their static rigidity in the compressive direction similar to that of a conventional guide.

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