When it comes to journal bearings in the Turbomachinery industry, the purpose of a bearing is to support a load and reduce friction between stationary and rotating parts. A common type of bearing used in this industry is a fluid film bearing. These bearings use a lubrication system to supply oil in order to create an oil film to prevent metal to metal contact. As opposed to ball bearings, which do have metal-to-metal contact. Fluid film journal bearings are in the sliding contact category of bearings, while ball bearings are in the rolling contact category.
Bearings which support rotating shafts can be classified into four categories:
-Rolling contact: the load is supported by balls or rollers.
-Sliding - Hydrostatic: Load is supported by a high pressure fluid.
-Sliding - Hydrodynamic: Load is supported by a lubricant film.
-Magnetic - Load is supported by magnetic fields.
Fluid-film bearings are used in high speed, high load or precision applications. In these applications, Ball bearings will have limited life and cause excessive noise and vibration leading to Rotordynamic issues.
Hydrostatic vs Hydrodynamic:
There are two types of fluid film bearings: hydrostatic and hydrodynamic. They both support a load using a thin fluid film, but they are different in how they generate the fluid film pressure. Hydrostatic bearings are pressured externally using a fluid such as: oil, water, or air. A pump is used to pressurize the fluid. A Hydrodynamic bearing uses the high speed of the journal (i.e. area where the bearing rests on a shaft) to self-generate the fluid pressure in a wedge-shaped film that forms from the relative motion between the surfaces.
Hydrostatic bearings do not depend on the relative motion between surfaces to maintain the fluid film, which means it can handle heavy loads at low speeds. These types of bearings utilize a recessed portion in between equally spaced stationary pads. Pressure is maintained in this recessed portion by pumping fluid through a flow restrictor. This pressure helps lift the shaft until flow out of the recessed portions and over the pads equals flow in. This forms a film thickness that stays constant for a certain recess pressure and bearing load. A flow restrictor is needed to create a pressure drop between the inlet feed annulus and the pad recesses to ensure that the required pressure for any individual pad never exceeds supply pressure. This is important because as the load direction changes, pressure on the loaded pad will change and the unloaded pads will decrease in pressure. The typical load restrictor used in hydrostatic bearings are orifices, which requires careful design to determine the optimum sizing.
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| Hydrodynamic Type Bearing: Tilt Pad Journal Bearing |
For high speed shafts in the Turbomachinery industry, the most common type of Hydrodynamic bearing used today is the Tilt Pad Journal Bearing. In Integrally Geared Centrifugal Compressors, tilt pad journal bearings are used for the high speed pinion rotor assemblies.
There are two major categories of Hydrodynamic bearings:
-Fixed geometry bearings: 360° plain, partial arc, pressure dam and lobed bearings.
-Tilting pad bearings: bearing pads that are free to move about a pivot, line or a point. They have a lot of variety in their parameters: number of pads, preload, pivot offset, and load direction.
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