Hydrostatic bearings and hydrodynamic bearings are fluid film bearings that rely on a film of oil or air to create a clearance between the moving and stationary elements. Although they may be similar in appearance, hydrostatic and hydrodynamic bearings are different in operation from plain bearings. Plain bearings are simple, inexpensive bearings inwhich the shaft comes in contact with the bearing surface during operation, and uses a lubricant to reduce the friction between the sliding surfaces. In contrast to plain bearings, hydrostatic and hydrodynamic bearings are more expensive and complex in design. Typical applications for hydrostatic and hydrodynamic bearings are machine tool spindles and slides; applications that require precise movement and high stiffness.

Hydrostatic bearings and hydrodynamic bearings are both classified as types of fluid bearings. Since there are no direct contact betweenthe bearing and the moving surface, the generation of heat during machine operation is due to oil shear. Although hydrostatic and hydrodynamic (or fluid dynamic) bearings both support the load on a thing film of fluid (oil or air), they differ in how the fluid layer is generated.

Hydrostatic Bearing

A hydrostatic bearing employs a positive pressure supply that maintains clearance between the rotating and stationary elements. With a hydrostatically-lubricated bearing, the lubrication is introduced under pressure between the moving surfaces.

Hydrostatic bearing spindles feature high stiffness and long bearing life, and are often used for fine machining and finishing. Because hydrostatic lubrication does not depend on relative motion to maintain the lubrication film, it can accommodate heavy loads at low speeds. They can also provide large (accurate and controllable) direct stiffness as well as damping (energy dissipation) coefficients.  

Hydrostatic bearings can be designed for use in linear or rotary motion with radial loads, depending on the application. This makes them ideal to support elements in process fluid pumps. The design of the bearing can be very complex and requires precision pressure and gap control to operate properly.

Chart credit: Rotolab

Hydrodynamic Bearing

In hydrodynamic bearings, the gap is generated dynamically by the bearing motion. Hydrodynamic bearings are used in rotary applications, and may require external pressure on one of the bearing pads or a secondary bearing to avoid excess friction when starting rotation. Hydrodynamic bearings can be designed for radial or thrust loads.

A hydrodynamic bearing is typically a low-clearance assembly that relies on a film of oil (and occasionally air) that develops space while the spindle is rotating. The bearings transmit (float) the load on self-renewing film of lubricant.

The most basic hydrodynamic bearing is the journal bearing. It has a cylindrical bore, typically with two axial grooves for lubrication. This bearing has a high load capacity and the simple design is compact, bi-rotational, and easy to manufacture. As the design speeds of machines increased, it was found that this bearing had limitations requiring modifications.

The pivoted shoe concept was applied to journal bearings and extensive tests have proved this is the most effective design. The pivot allows the shoe to rotate and form a wedge. The pivot surface is spherical to allow 360° rolling freedom. Hydrodynamic pivoted shoe bearings provide considerable benefits. They are smaller, less expensive, require less maintenance, last longer, and are more efficient than previous bearing design. 

The three basic types of hydrodynamic bearings are circumferential groove bearings, pressure bearings, and multiple groove bearings. Hydrodynamic bearing spindles feature high stiffness and long bearing life, and are often used for fine machining and finishing.

Machines that use hydrodynamically lubricated bearings should not endure high load during start up. Hydrodynamic bearings can be found in industrial applications including steam turbines, electric motors, cooling pumps, and rock crushers. They are also commonly found on ships in use in the clutch, blowers, pumps, and auxiliary machinery.

Specifications

Lubrication

A suitable lubricant must always be present to ensure safe operation of hydrostatic and hydrodynamic bearings. The lubricant needs to be cooled to remove the heat generated by the oil shear and it must be warm enough to flow freely. The lubricant must be filtered so that the average particle size is less than the minimum film thickness.

Allowable outlet (minimum) film thickness for a given surface finish

Table Credit: RPI

Speed Limits

Hydrostatic and hydrodynamic bearings have only viscous friction associated with a fluid film layer being sheared during the motion of the bearing. They can experience hydrodynamic effects in a high speed condition if the lands are too wide and considerable heat can be generated as a result. The approximate maximum speed is 1,000,000 DN. The DN number describes the bearing diameter in mm (D) and top speed in rpm (N).

Accuracy

The accuracy of these bearings types is determined by the accuracy of the components. Hydraulic linear motion bearings have been built with submicron/meter accuracy.

Stiffness and Damping

Hydrostatic and hydrodynamic bearings do not have loss of contact problems that sliding or rolling contact bearings that are preloaded against each other have. They can easily be in the Newton per nanometer range for stiffness.

Due to the thin oil film in the bearing gap, these bearings have excellent damping capabilities in both normal and tangential bearing directions. When air is used, the low-viscosity air film in the bearing gap gives bearings moderate to low damping capabilities in the normal and tangential bearing directions, respectively.

Thermal Performance

In general, hydrostatic bearings are not used where speeds greater than 2 m/s are encountered because viscous shear of the fluid in the bearing gap generates too much heat. Hydrostatic and hydrodynamic bearings get energy in the form of a flow at a pressure. The oil comes out of the bearing and into a drip pan. In the pan its flow rate and pressure are essentially zero, so all the power that is represented by the initial flow and pressure is expended in the viscous shear the fluid undergoes as it oozes out of the bearings. This power is dissipated as heat. The temperature rise of the oil depends on how much heat is conducted by the machine.

Size and Weight

These bearings take up very little space; however the plumbing requirements may be significant. They have very high performance -to-weight ratios if the size and weight of the pump, oil collection/distribution system, and oil temperature control systems are excluded.

Maintenance and Required Life

The oil level and cleanliness must be monitored and the filter on the pump must be changed regularly. Oil quality should be monitored to make sure that its pH level remains within the desired limits and that it is not contaminated.

Since these bearings are noncontact devices, they have an essentially infinite life.

Standards

Hydrostatic and hydrodynamic bearings must adhere to specific standards to ensure proper design and functionality. For example ESDU 92026 is used to determine calculation methods for hydrostatic journal bearings and ISO 12167-1 discusses the calculation of oil-lubricated hydrostatic bearings with drainage grooves. Hydrodynamic bearings standardd include BS ISO 6281 for testing under conditions of hydrodynamic and mixed lubrication and BS ISO 12130-2 for calculation of tilting pad thrust bearings.  

References

Comparison of Bearings (pdf)

Hydrostatic Journal Bearings (pdf)

Hydrodynamic and Hydrostatic Bearings (pdf)

Image Credits

ebearings | Pioneer |  Machinery Lubrication | NewAge Industries