Hydrostatic Linear Slides

When developing new processing machines, more and more manufacturers are turning to external procurement of complete assemblies that have already been optimised in terms of efficiency, precision and costs. These include slide units that are conventionally guided by rolling elements. However, these are subject to wear, cause a significant surge in the driving force when the direction of movement is reversed and have, at best, only minimal damping. These inadequacies are circumvented by hydrostatic guides, which is why many machine manufacturers prefer hydrostatically guided slide units.

Hydrostatically guided slides: contactless and wear-free
The principle of hydrostatic components is based on non-contact movement, which is why hydrostatically guided slides are wear-free and thus have an unlimited service life. The hydrostatically guided linear slide units from Hyprostatik Schönfeld, the technology leader in the field of hydrostatic systems, are characterised by high reliability, long-term availability and low service costs. Since these properties exist independently of the service life, the machine end-customer can produce more effectively and economically in the long term.

Functionality of the hydrostatic slide
Due to the low friction, no force surge occurs when reversing the direction of movement of a hydrostatic slide. As a result, position deviations of less than 0.1 µm are achieved if linear scales are used for displacement measurement. Hyprostatik Schönfeld manufactures the guideway and slide on a high-precision surface grinding machine with hydrostatic guideways in all three axes and hydrostatically mounted grinding spindle. Since a high degree of straightness of the slide movement is achieved, this enables the use of the linear guides even on ultra-precision machines.

Slide variants
These slide units are only offered with hydrostatic guidance and alternatively with feed drive via linear motor or via screw drive with a hydrostatic nut and hydrostatic bearing for the threaded spindle.

No friction, absolutely minimal position deviations
Since no sliding seals are used, the entire friction of the slide movement in both drive versions arises from hydrostatic components. Thus friction is minimal and proportional to the speed. This means that at low speed friction is close to zero. At zero speed it is zero, which is why no force surge occurs when the direction of movement is reversed.

Depending on the possibilities of feed control and the quality of the path measurement, position deviations of less than 0.1 µm can be achieved, whereby path deviations of less than 0.1 µm are achieved with compound slides. It is thus possible, for example, to produce different diameters of a jig grinder by a circular movement of the compound slide. This means that a stable and powerful internal grinding spindle can be used on the vertical slide of these machines. A rotating sleeve with an unstable lateral slide for the internal grinding spindle is no longer required.

Quality of the slide guide
In addition to position deviation, the quality of a slide guide is also determined by the straightness of the slide movement in both directions transverse to the direction of movement. Prerequisites for best results are high rigidity of the hydrostatic guides and low transverse forces from the drive. Furthermore, an excellent degree of straightness of the guide surfaces on the guideway, i.e. a correspondingly precise production machine, is decisive. Both requirements are perfectly met by the hydrostatically guided slides from Hyprostatik Schönfeld.

A blessing from hydrostatics

The compensation of short-wave surface defects of the guideway is a boon from hydrostatics. Surface defects with a significantly smaller area than the area of the hydrostatic pocket are largely compensated due to the integrating effect of hydrostatics.

Which feed drive for which task

The following feed drives are available for different tasks:

• For ultra-precision machines with the highest demands on planarity at low feed forces, non-ferrous linear motors with as little lateral constraint as possible are suitable.
• Significantly higher, approx. four times the feed forces are possible with normal linear motors, which are therefore intended for lighter machining forces such as light alloy machining. In relation to the required feed force, linear motors for higher feed forces are very wide and require wider guides. Linear motors can also be thermally overloaded if oscillating loads in the direction of movement continuously generate reaction forces which are above the nominal force of the linear motor.
• Hydrostatic screw drives are recommended for high feed forces, e.g. for steel and cast iron machining, but also for high alternating forces or oscillating loads, even at high frequency, and for vertical axes. Hydrostatics are immune to these alternating forces. This is demonstrated by the deployment of a threaded drive with a hydrostatic nut with a load capacity of 340 kN in a broaching machine which, after nine years of three-shift operation, only became defective through failure of the motor control system.

In addition to customer-specific designs, Hyprostatik Schönfeld offers standardised solutions in slide widths of 300 mm, 450 mm and 600 mm. The slide length and slide stroke are designed according to customer requirements.

The difference: Hyprostatik Schönfeld competence
"For more than 25 years we have been calculating hydrostatic linear guides and also rotary tables for machine tools, so that users can manufacture these components without themselves having competencies in this technology.

We have designed several hundred guides and gained extensive knowledge and experience in the process. This experience and expertise in the field of hydrostatics enables us to design and manufacture optimal hydrostatically guided slide units.", explains company founder Robert Schönfeld.

This competence is reinforced by comprehensive calculation programs for the best possible design of the hydrostatics to counter static loads and for optimum vibration damping. These calculation programs are used exclusively by Hyprostatik Schönfeld.

A further Hyprostatik Schönfeld asset is the progressive volume control. This controls the oil flow into the hydrostatic pocket. Compared to solutions with capillaries, significantly higher loads are possible and four to five times higher stiffness is achieved.

Furthermore, the manufacturing conditions are also decisive. Hyprostatik Schönfeld manufactures the extraordinarily precise guides on a surface grinding machine with a stable granite bed. The machine is equipped with hydrostatic guides in all axes, a hydrostatic screw drive in the workpiece axis and a hydrostatic grinding spindle.