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A hydrostatic bearing builds up its own load-bearing lubricating film, provided the shaft rotates fast enough. However, during startup, shutdown and when the shaft is at a standstill, this effect is absent, and the bearing surfaces and the shaft come into contact, resulting in direct material contact. Hydrostatic bearings are available for applications that cannot tolerate this.
In this article, you will learn:
A hydrostatic bearing is a bearing with an active, externally generated lubricant film pressure. Instead of waiting for relative motion to build up a load-bearing pressure, a pump or other external pressure source continuously forces the lubricant between the bearing and the shaft at a defined pressure. Oil is usually used as the lubricant.
The difference from the hydrodynamic principle lies in the way pressure is generated. In a hydrodynamic bearing, the load-bearing lubricating film is created by the motion itself: The shaft draws lubricant into a narrowing gap, and the resulting wedge effect builds up the load-bearing pressure. When the bearing is at a standstill, this motion-induced pressure generation is completely absent, and during startup, the lubricating film must first build up.
In a hydrostatic bearing, the lubricating film can form even before the shaft and bearing begin to move relative to each other. When properly designed, the working surfaces are always completely separated from one another. Hydrostatic and hydrodynamic bearings therefore cannot be generally classified as “better” or “worse.” Which principle is suitable depends on operating conditions, application requirements and available infrastructure.
A hydrostatic bearing is therefore always part of a system, not just a single component. The bearing assembly must include an external pressure supply. This is also the key difference from other types of bearings, which can operate, for example, with low-pressure pumps or sump lubrication.
Our experts will be happy to advise you on which type of bearing is best suited for your application.
A hydrostatic bearing always consists of two functional components: the bearing itself and the supply system that feeds it. Typically, these include:
The bearing seat itself contains several lubricant pockets, which are incorporated into the bearing surface. The lubricant is fed into these pockets through inlet channels and distributed from there across the circumferential bearing gap. Each inlet channel contains a restrictor that limits the flow to the respective pocket. In bearings designed to carry one-sided loads, the lubricant is intentionally supplied at several points distributed around the circumference so that a separate, independently adjustable pressure can be established on each side.
The pump provides the supply pressure. Through throttle or control elements, the lubricant is fed in a controlled manner into the bearing pockets and flows from there through the narrow bearing clearance. The resulting pressure field absorbs the external load and maintains the clearance between the bearing and the shaft, regardless of speed and load.
Throttling is critical to the system’s operation. Two throttling points act in series within the system: the fixed throttle in the inlet duct and the bearing clearance itself, which behaves like a second, variable throttle. If the bearing clearance changes due to an external load, the flow resistance, flow rate and pressure in the corresponding bearing pocket also change. When the design is properly tuned, this results in a self-resetting effect. Step by step:
On the opposite side, the same mechanism operates in reverse: the gap widens, the resistance decreases and the pocket pressure drops. Taken together, this creates a self-regulating restoring force, without any active control technology, solely through the fluid dynamics of the throttle assembly.
| Hydrostatic Bearing | Hydrodynamic Bearing | |
| Formation of the lubricating film | Supplied externally via a pump or pressure source | Due to relative motion and the wedge effect in the lubrication gap |
| Behavior during start and stop | A stable film can exist even before movement begins | The full film only takes shape as the relative velocity increases |
| Speed dependence | Load-carrying capacity depends only slightly on rotational speed | Bearing capacity is primarily determined by rotational speed, viscosity, load and geometry |
| Load capacity when stationary | Possible with active power supply | Without an additional lift or pressure supply, no hydrodynamic pressure build-up occurs |
| Breakaway torque | Low | May be higher due to boundary or mixed friction during startup |
| Auxiliary systems/energy requirements | Pump, filter, piping, monitoring; redundancy may be required | Can be operated with a low-pressure pump or sump lubrication |
| Complexity/maintenance | Higher | Often simpler, depending on the application |
| Typical applications | Precision machines, low-speed heavy-duty applications, test benches | Applications that run continuously at a sufficient operating speed |
Ultimately, the direct comparison of the two principles is what matters when making a design decision:
The comparison shows that a hydrodynamic bearing has a simpler design and does not require an external pressure supply to maintain a load-bearing lubrication gap. Provided that the application ensures a sufficiently high operating speed and can tolerate brief solid-to-solid contact, it is often the more economical solution. This is particularly true when wear and service life requirements can be reliably met through appropriate measures such as optimized material selection or the use of coatings. A hydrostatic bearing pays off precisely where this is not the case: at very low or fluctuating speeds, when there are high requirements for concentricity, or when static loads must be supported continuously.
In practice, hybrid forms also exist: hydrodynamic-hydrostatic hybrid bearings use an external pressure supply only during start-up and shutdown and switch to the purely hydrodynamic principle during normal operation. They thus combine the reduction of solid-solid contact during start-up with lower system requirements during continuous operation.
Since the load-bearing lubricating film forms independently of movement, there is no contact between the bearing and the shaft. This can help reduce the breakaway torque and enable controlled movements from a standstill.
As a bearing solution for start-stop operation, the hydrostatic principle is particularly advantageous when high loads are applied before or immediately after movement. The external lubrication system enables more consistent lubrication conditions, provided it is activated before startup and maintained during the relevant operating phases.
During start-up and shutdown, hydrodynamic bearings typically operate within speed ranges where a complete lubricating film is not present. A hydrostatic lubrication system can largely prevent this critical phase.
Pressure buildup does not depend on high relative speeds. A hydrostatic bearing can therefore support loads even at very low speeds or when stationary. This is relevant, for example, for heavy machine tables, large shafts, or slowly moving components
If the working surfaces are reliably separated by a lubricating film, solid-to-solid friction decreases. Hydrostatic bearings thus offer the potential for a low-wear solution, provided the system is optimally tuned.
The carefully coordinated combination of bearing geometry, bearing seats and pressure control enables high bearing stiffness. This supports precise guidance and limits displacement under varying forces.
The use of hydrostatic bearings involves additional technical and economic requirements. Pumps, filters, piping and pressure regulation require installation space and increase the number of system components. In addition, there are capital costs and energy requirements for the external pressure supply, since the pump continues to run even at low speeds or when the system is at a standstill. Maintenance and monitoring of the supply, filters, leak detection and pressure control must be taken into account from the outset.
Particular attention must be paid to potential pressure loss: If the external supply fails, the bearing may lose its load-bearing capacity. Depending on the risk assessment, pressure, flow or temperature monitoring, storage solutions, redundant components, or defined shutdown sequences may be required.
Miba is also pursuing a new approach to harness and store the oil pressures that naturally occur in the lubrication gap during operation and to inject the oil in a controlled and targeted manner via a valve system.
Which of these applications actually benefits from a purely hydrostatic, a hybrid or an otherwise optimized bearing system can only be determined based on the complete operating profile.
A thorough evaluation of such issues requires comprehensive knowledge of applications, design and materials. As a development partner, Miba supports customers throughout the entire process. Using modern simulation methods, extensive calculation tools, and numerous test benches, different bearing concepts can be evaluated under realistic conditions. This lays the foundation for a technically and economically optimal solution, tailored to the specific requirements of the respective application.
Our experts are happy to assist you with their extensive knowledge of applications, design and materials.
Hydrostatic bearings offer significant advantages, particularly at low speeds, under high loads and during frequent start-stop cycles. Since the load-bearing lubricating film is generated externally, it can exist even when the system is at a standstill. Under suitable conditions, this makes it possible to avoid mixed friction, reduce the breakaway torque and meet high requirements for stiffness and positioning accuracy.
At the same time, a hydrostatic bearing requires an external pressure supply and additional system components. Energy requirements, installation space, maintenance, monitoring and behavior in the event of a pressure loss must therefore be taken into account in the design from the very beginning.
A thorough analysis of your operating conditions lays the foundation for making the right technology decision. Our experts will guide you through the process, from evaluating your requirements to selecting the appropriate bearing solution.
