19.10.2010 19:02 Tempo: 4 yrs

Accuracy to the micrometre. Hydrostatics and aerostatics for precision rotary tables.

Categoria: Machine building, Bearings
Por: Raik Flämig

ZOLLERN expands its range of hydro and aerostatic rotary tables for precision mechanical engineering. The new product series are the first to use direct drives developed and produced by Zollern.

With its precision solutions in the field of hydrostatic rotary tables, ZOLLERN has long been an established partner of the engineering industry, supplying hydrostatic bearing precision rotary tables with diameters from 300 to 4,000 mm. Apart from complete tables, ZOLLERN also supplies ready-to-install hydrostatic bearing components as radial-axial bearings for rotary tables up to a diameter of 10,000 mm.

In response to continuing strong demand for precision solutions from the European and particularly the German machine tool industry, ZOLLERN has extended its product range to include aerostatic rotary tables with table diameters of 200 to 1,200 mm. While hydrostatic models achieve a flatness and rotation accuracy of 1 µm, aerostatic solutions improve this by a factor of five, or down to less than 0.2 µm. What's more, they can be connected to existing pneumatic systems, and because they don't require hydraulic oil, they are cleaner. This makes aerostatic rotary tables ideal for measuring and high-precision machine tool manufacturing, while hydrostatic solutions are predominantly used for gear cutting machines, (vertical) grinding and milling machines, vertical lathes and machining centres.

Application determines technical solution

However, not only the precision requirements determine where the different types of rotary table are used. Equally important are the bearing capacity and machining speed. While hydrostatic rotary tables work with a feed pressure of the medium of approximately 30 to 40 bar, aerostatic systems use only around 6 bar. This difference in feed pressure and compressibility of the support medium means hydrostatic tables have five to ten times the bearing capacity of aerostatic tables of the same size.

At the same time, the higher bearing capacity of hydrostatic rotary tables and the associated use of hydraulic oil limits the possible processing speed compared to aerostatic tables. The viscosity of hydraulic oil comes with a shearing friction that leads to power loss and heating of the assembly. In contrast, the circumferential speed of aerostatic bearing tables is theoretically unlimited because shearing friction can not occur with air. In practice, circumferential speeds of 40 to 50 m/s are achieved. The figure for hydrostatic solutions is approximately10 m/s.

Ultimately it's the application that determines whether customers choose aerostatic or hydrostatic rotary tables from ZOLLERN. The advantages of both types compared to conventional, antifriction bearing tables are firstly the freedom from wear which guarantees sustained precision. Secondly, both systems come with better damping characteristics. The damping characteristics of the two media, hydraulic oil or air, reduce the upswing of the whole system. That results in much higher machined surface qualities. Tool wear is reduced, and higher machining speeds are possible.

Precision bearings require precision drives

Because the precision of rotary tables doesn’t only depend on the type of bearing, but also on the quality of the direct drives, ZOLLERN has developed and now produces its own torque motors. They are an integral part of the hydrostatic and aerostatic rotary tables. Although they are tailored to each customer's application, what they all have in common is a much lower torque ripple than the usual solutions on the market. The power loss is up to 25 percent and the nominal current up to 15 percent lower. The heat acting on the surrounding components is correspondingly low.

Talking about the product launch, Michael Knaus, Head of Hydrostatics/ Aerostatics at ZOLLERN, says: "This combination of high-precision bearings and precision drives is just what customers want. They benefit from the fact that all the components in our rotary tables come from the ZOLLERN group. The result is top technical standards plus attractive prices".