Hydrodynamic bearings are a type of fluid film bearing which relies on an oil film or an air film to create a clearance between the surfaces of the stationary part and the moving part. Though the hydrodynamic bearings look similar to plain bearings, they differ in the working principle. For the typical plain bearings, their construction is simple: a shaft comes in contact with the bearing surface during operation and a lubricant is used during the process to reduce the friction between the surfaces. As for the hydrodynamic bearings, they are more complex in design so to speak. They are used primarily in machine tool spindles and slides where high levels of precision movement and stiffness are required.
One significant feature of hydrodynamic bearings is that the surface of them does not have a direct contact with the surface of the moving part. The generation of heat is involved in a machine operation but it is generated not because of the friction, but the oil shear. Oil shear is described as the oil’s permanent loss of viscosity which takes place when the long chain polymers that maintain the oil’s viscosity are broken at machine operating temperature. The hydrodynamic bearings support the load with a thin layer of fluid, either oil or air, instead of direct contact.
In hydrodynamic bearings, the clearance between the bearing surface and that of a moving component is created dynamically by the bearing motion and the gap is filled with a fluid film, hence this type of bearing is called hydrodynamic bearing. The lubrication film is basically maintained based on the relative motion of the shaft within the bearing. They are used in high performance rotary applications such as machining spindles. Since the clearance is generated with a film of fluid while the spindle is rotating, the load is transmitted by the bearing in a floating fashion.
Normally, a hydrodynamic bearing has a cylindrical bore with two axial grooves for lubrication. It has a high load capacity with a compact design. The design of a hydrodynamic bearing is based on the pivoted shoe concept. The shoe refers to the tilting pads on the bearing. The pivot allows the shoe to rotate and form a wedge. The pivot surface is spherical to provide a 360-degree rolling capability. The hydrodynamic bearings, just like other types of bearing, can be made to take axial loads or radial loads.
The advantages of this pivoted shoe design includes smaller size of the bearing, less expensive, less maintenance, longer lifespan, and more efficient in transmitting the load. The machining spindle that applies the hydrodynamic bearings usually has higher stiffness and longer life. They are able to perform fine machining and finishing operations. The industrial application of hydrodynamic bearings includes steam turbines, electric motors, pumps, etc.
Hydrostatic bearings are another type of fluid film bearing and they are very similar to hydrodynamic bearings. The hydrostatic bearings employ a positive pressure supply to create the gap between the surfaces of the rotating part and the stationary part. The lubrication required is introduced under the pressure between the two surfaces. This type of bearing is ideal for taking heavy loads at lower speeds. In addition, they are able to offer accurate and controllable stiffness and damping coefficient during machine operation.
Though hydrostatic bearings are able to support larger loads without depending on the film thickness and lubricant viscosity, they need ancillary equipment and more costs in installation and maintenance. Also, the fluid filtration equipment is required because the performance of a hydrostatic bearing can be compromised once the fluid is contaminated.
Both hydrodynamic bearings and hydrostatic bearings require the presence of a suitable lubricant to assure the applicable operation of machinery. The lubricant is responsible for removing the heat that is generated by the oil shear while it has to stay at a certain temperature at the same time so that it can flow freely inside the machinery. The viscous friction has to do with the fluid film that is being sheared during operation. If the operating speed is too high, excess heat will be generated and the performance of the bearings may lose. As a result, the just adequate amount of lubrication is important.
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