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Hydrostatics ready for prime-time machining.

Stiffness and pressure problems are overcome.

Hydrostatic design has taken a major leap forward as an alternative to traditional slide and roller systems used in guideways, as well as for leadscrews, spindles, and bearings. The Hyprostatik design developed by Hyprostatik Schonfeld GmbH, Goppingen, Germany, for hydrostatic systems overcomes the principal problems of traditional hydrostatics in machine tool applications--lack of stiffness and pressure control.

The Hyprostatik system works essentially like any other hydrostatic system. Hydrostatic-guided components are suspended on an oil film that is distributed to pockets in the slide or hydrostatic bearings. These pockets hold slides and spindles in suspension and provide damping against a wide range of mechanical forces. In a fraction of a second after the machine is turned on, the pressure in the pocket increases so that the slide or the spindle lifts. Since there is no contact between the surfaces, the slide can be moved without any wear. Friction is confined to the oil and not the metal surfaces.

Traditional systems have lacked sufficient stiffness and require holding strips to add stiffness, especially in horizontal applications. In addition oil pressure has been difficult to optimize through the capillary tube delivery system. The Hyprostatik system uses an innovative PM-flow controller complete with the necessary computer algorithms to regulate and optimize the flow of oil and pressure to every pocket according to the varying loads required. The flow of oil is regulated to the guideway or spindle by an ultrasensitive pressure membrane in the controller. The net effect is to reduce the demand for oil and the demand on pump pressure required by the system.

A number of major European machine tool builders have adopted the Hyprostatik system for various types of precision machines, including but not limited to cylindrical, gear, and crankshaft grinders, large milling machines, and vertical CNC turning centers.

According to Dipl Ing Jochen Schonfeld, president, Hyprostatik Schonfeld, stiffness in guideways is four to five times higher than the stiffness of hydrostatic systems equipped with capillary control systems. The pocket pressure can reach 90% of the pump pressure without contact so the loadability is also improved. The Hyprostatik design minimizes pump energy needed and the heat created so that the hydrostatic guideways normally do not need a cooling device. Cleanliness of the continuous flow of filtered oil through the frame and the pockets makes the hydrostatic systems less sensitive to pollution than rolling systems.

Other benefits of the Hyprostatik system include elimination of backlash while reversing, no stick-slip, no friction fluctuation or vibration due to recirculating balls, and no wear and tear on critical parts. The overall result is higher machining precision, improved productivity, lower maintenance costs, and longer machine life, he says.

Mr Schonfeld says that designing a hydrostatic guideway into a new machine, most of the time, is comparable to using alternative systems.

When hydrostatic systems are used instead of large roller or ball bearings, e.g. in round tables, the characteristic features are better and the costs are less. If there are no lift or tilt forces on the slide, the hydrostatic guideway can be designed open, e.g. as a V-flat guideway, which is less expensive to produce. The stiffness of open guideways can be improved by prestress with vacuum pockets. Because the cost for hydrostatic guideways is almost independent of the size, large hydrostatic bearings and guideways are generally less expensive.

Hydrostatic guideways can be driven by linear motors, ballscrews, or hydrostatic leadscrews. They are very effective with linear motors, which tend to generate heat in ball or roller guide systems. Because the friction of a hydrostatic guide system is very low and the high normal force has no influence to the friction force, there is no problem with friction, heating, high speed, and acceleration.

At EMO 97, Hyprostatik Schonfeld introduced a hydrostatic leadscrew with high stiffness for heavy loads. Unlike ballscrews, the nut is not stored between balls, but floats between hydrostatic pockets. Because of the internal flow controller, only one pressure connection is necessary. For spindles, the Hyprostatik system makes it possible to do rough cutting at high load and precision finishing with the same spindle. As a rule, says Mr Schonfeld, Hyprostatik spindles can be designed with larger diameters than conventional ball bearing spindles because the bore of the spindle case has the same size as the spindle's outer diameter.

Hyprostatik's systems are used in precision machine tools like grinders, lathes, machining centers, milling machines, measuring machines, and testing devices in single or series production.
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Title Annotation:Hyprostatik Schonfeld GmbH system
Comment:Hydrostatics ready for prime-time machining.(Hyprostatik Schonfeld GmbH system)
Publication:Tooling & Production
Article Type:Brief Article
Geographic Code:4EUGE
Date:Jan 1, 1999
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