Schaeffler Developed An Innovative Range of Linear Guides

Schaeffler (UK) has developed an innovative range of high precision, hydrostatic linear guides, that are not only cost effective and easy to mount, but also fit the same design space as a rolling element guidance system. When it comes to selecting bearing supports for linear motion applications, most machine builders tend to opt for profiled rail linear guides, which provide optimum performance (rigidity, load capacity and dynamics) and mounting. Hydrostatic linear guides are now a cost effective option, as they offer both high precision and better damping characteristics for high dynamic rigidity. However, to date, most hydrostatic guides have been relatively expensive, time-consuming to mount and require a larger design envelope compared to profiled rail guides. Engineers from the Schaeffler Group's Linear Division, working closely with the Machine Tool Laboratory at the University of Aachen in Germany, have developed a range of hydrostatic linear prototype guides that are ideal for high precision machining and machine tool applications, offering design engineers and machine builders much greater design freedom. Standard Design Space The hydrostatic guides are manufactured to fit within the standard design space (DIN 645) of a profiled rail linear guide and the load carrying capacity has been found to be equal to the operating load of a profiled rail linear guide of similar size. Compressive rigidity is also comparable to a roller monorail system. For machine builders and end users, the ease of mounting means design and assembly costs are reduced. And, because the guides comply with DIN design envelopes and DIN 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. Longer Life, Fewer Breakdowns 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. Impact vibration tests and torsional load (continuous vibration) testing on machine tools, for example, have shown that hydrostatic guideways provide improved dynamic stiffness for heavy cutting capability and chatter resistance, as well as better damping characteristics compared to conventional linear ball guides. The result is longer tool life and improved part surface finish and roundness. 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. 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. Peter Lutz, head of the Roller Guidance Systems Product Group at Schaeffler Group summarised the benefits of the hydrostatic guides: 'The novel feature of this new concept is that the hydrostatic guidance system conforms to the standard design envelope in accordance with DIN 645 for a monorail guidance system'. 'The prototypes have a load carrying capacity approaching the operating load range of a monorail guidance system of the same size'. 'The pocket geometry and hydraulic control system have been adapted in such a way that the static rigidity in the compressive direction is equivalent to that of a monorail guidance system'. For the hydrostatic linear guide prototypes, pocket pressure is 100 bar, load carrying capacity is up to 18kN and through flow rate is less than 3 litres/min. Gap height is 0.015mm. Schaeffler is currently working with its customers to develop designs for volume production.