New hydrostatic linear guides

A new range of hydrostatic linear guides has been developed that combines excellent damping characteristics with the compact dimensions of a profiled rail linear guide.

Following extensive research and development and customer trials, a new range of hydrostatic linear guides is now available from the Schaeffler Group.

The new HLE range of hydrostatic linear guides is ideal for linear motion applications in which high damping characteristics are required and where the damping itself must take place at the point where the load is applied, for example, in high speed machining and high precision grinding applications.

With a demand for increased dynamics and power densities in the machine tool industry, linear guidance systems are now required that offer both high dynamic rigidity and reduced vibration. If machine vibration can be dampened, machine efficiencies can be improved, resulting in higher cutting capacities, higher speeds, improved surface quality and improved machining accuracy.

Unlike conventional ball linear guides or roller linear guides, the hydrostatic guides feature no metal-to-metal contact. The guide carriages move on a thin cushion of high pressure fluid, the end result being zero wear of the guideways. An integral hydraulic controller means the hydrostatic guides are ready-to-fit and the mounting dimensions are interchangeable with monorail guidance systems.

Bad vibes
Vibration in machine tools can be a problem. Vibrations are induced as a result of blade contact shocks, tool wear, regenerative effects and other disruptive forces from outside influences. Due to the large number of ‘springs’ and ‘masses’, numerous resonant frequencies occur with, in some cases, strong effects on the accuracy and surface quality of the workpieces. In this case, the lubricant film separating the components in hydrodynamic and hydrostatic guidance systems provides vibration damping by means of the so-called ‘squeeze film’ effect.

This damps or even eliminates the occurrence of amplitude elevations in the resonant frequencies. The dynamic rigidity of the machine is improved and higher precision is achieved. However, hydrostatic guidance systems have until now been associated with design and economic disadvantages: a larger design envelope is required, increased fitting and adjustment work is needed and costs are therefore higher.

Schaeffler’s size 45 (45mm rail width) HLE hydrostatic linear guide, for example, has a damping value of higher than 470,000kg per second. The system operates at 100 bar nominal pressure with an oil viscosity class of ISO VG 46. The flow rate of the carriage is around 1.2 litres per minute. The tensile rigidity almost equates to a corresponding rolling bearing support.

The carriage comprises a steel saddle plate with pressure pockets and two end pieces. At the oil supply side, a corresponding pressure regulation is integrated in the end piece. The second end piece is only for oil recirculation purposes. The design and geometrical dimensions of the contact surfaces of the saddle plate and rails are critical to the performance of the hydrostatic guide.

Bearing life of hydrostatic guides can be considered infinite, as there is no bearing wear, resulting in longer machine life and extended rigidity and precision. Crash protection also improves, because the oil pressure in the pocket increases as the cutting load is applied, creating a restoring force that prevents the fluid gap from closing under very high impact load.

Customer Applications
Extensive tests have been conducted on the hydrostatic guides in numerous customer applications. One of these tests involved evaluating the hydrostatic guide on a three-axis machining centre.

The transverse axis (Y-axis), which guides the main spindle assembly and the Z-axis, was fitted with a size 45 hydrostatic guide. Measuring the deflection showed that the HLE had almost identical values to the roller monorail guidance system, indicating that there were no disadvantages in terms of axis rigidity.

However, on examination of the dynamic resilience in the machine tool centre point (TCP), this revealed a clear reduction in vibration peaks in all axes when using the HLE. This enabled a significant increase in the maximum machining depth. Tests by the customer showed that the cut becomes unstable at a certain machining depth when using monorail guidance systems, with regenerative chatter effects occurring. However, under the same conditions but using HLE guides, the cut is stabilised.

Similar positive results were revealed on workpiece table vibration on a grinding machine. Significant first-order plate vibration was found when excitation occurred in the centre of the workpiece table. When the table was guided using four hydrostatic guide carriages rather than four conventional roller monorail carriages, the resilience frequency measurements showed a significant reduction in vibration amplitudes. The HLE achieved a clear improvement in decay behaviour at the centre of the table. On assessing the results of the grinding by surface measurement, it was found that the HLE optimised both the ‘long waviness’ and ‘short waviness’ of the table guide.

To date, most hydrostatic guides have been relatively expensive, time-consuming to mount and require a larger design envelope compared to profiled rail guides. However, the new INA HLE guides are manufactured to fit within the standard design space of a profiled rail linear guide. For machine builders, this ease of mounting means that design and assembly costs can be reduced. And, because the guides comply with ISO/DIN design envelopes and mounting dimensions for monorail guidance systems, a design engineer could, for example, use both a roller guidance system and a hydrostatic guiding element in the same machine design, but for a variety of machine tool performance classes.

For more information on Schaeffler Group’s range of hydrostatic miniature linear guides, please visit www.schaeffler.co.uk or telephone the marketing department on 0121 313 5870 or email info.uk@schaeffler.com

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