Throughout the machine tool industry, hydrostatic linear guides are considered the best technical solution for vibration damping at the point of system loading. The guides also carry heavy loads and can withstand contaminated environments. However, there was always a catch: Most traditional hydrostatic guides are relatively expensive, time consuming to mount, and require a larger design envelope than conventional linear guides. For these reasons, they're infrequently specified.

Now, newer compact hydrostatic linear guides address old issues. Built to the dimensions of standard profiled-rail linear guides, they fit within the same design space as typical recirculating bearing and guideway assemblies — to give engineers and machine builders the benefit of unmatched damping without requiring installation compromises.

Basic operation

Hydrostatic guides consist of carriages with pressure pockets that ride on precision-honed guideways energized by a steady layer of pressurized oil. Each carriage is comprised of a steel saddle plate featuring two endpieces. At one endpiece, an integrated pressure regulator supplies oil to the carriage's pressure pockets; at the other suction-side endpiece, depressurized oil is extracted via an oil circuit. It's upon the oil-filled pressure pockets that a hydrostatic guide is loaded.

Unlike conventional ball or roller-based linear guides, hydrostatic guidance systems eliminate all metal-to-metal contact. The guide carriages move on a thin cushion of high-pressure fluid, which prevents wear to the guideways. Their ability to damp vibration is superior to that of rolling guide systems.

More specifically, hydraulic oil is fed to the pressure side and chambers under constant pressure: Integrated chokes are set so that the carriage is optimally positioned on the guideway and lifted off evenly to a gap height of 0.015 mm.

This integral hydraulic control system is what allows the unit to fit into the same standard design envelope as a conventional roller profile rail system.

Glasses of water show how a roller-based guide transmits significantly more vibration than a hydrostatic guide.

How much oil and pressure is required? A hydrostatic guide with a 45-mm wide rail has a flow rate of approximately 1.2 liters per minute at an operating pressure of 100 bar with an oil viscosity class of ISO VG 46.

To prevent damage in the event of a crash due to a partial or complete loss of pressure, the working pressure pockets are lined with a sliding material to create a failsafe for the system. Note that to protect this lining during installation, the carriage must be oiled before being pushed on to the guideway.

Rigidity, accuracy, and performance

One might assume that moving loads on a film of oil would not provide the rigidity necessary for maximum machine performance. To the contrary, when standard design specifications are met, the tensile rigidity of a hydrostatic linear guide is roughly equal to that of a similarly sized roller bearing and guideway assembly. Note that this is based on a two-rail, four-carriage system with two carriages per rail. With this arrangement, rigidity in the compressive direction is 1,300 N/µm; in tensile and lateral directions, it is 1,200 N/µm and 950 N/µm respectively. This includes deformation of the hydrostatic guide and screw connections to the adjacent construction. The rigidity of the carriage prevents any deformation of the pressure pockets, which is critical in maintaining equal pressure distribution within the carriage.

Rigidity can also be enhanced by increasing the oil pressure via a valve setting. In mechanical systems, higher preloads on the rolling elements are required to boost rigidity. Unfortunately, what also results is increased rolling friction and an exponential decrease in bearing life.

Hydrostatic carriages, on the other hand, run on pressurized oil, so there is no increase in friction and no effect on the system's operating life. Friction is independent of load until the load limit is reached.