Bearings – Rotary bearing – Fluid bearing
Patent
1996-10-25
1998-06-30
Hannon, Thomas R.
Bearings
Rotary bearing
Fluid bearing
384263, 384267, F16C 3206, F16C 2502
Patent
active
057723346
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to fluid film bearings and, more specifically, to hydrodynamic bearings.
BACKGROUND OF THE INVENTION
In the most basic form of hydrodynamic bearing, a journal is rotatably mounted relative to a housing with a small radial clearance therebetween. The axes of the journal and housing are offset by a small distance, such that in operation, when a lubricant at relatively low pressure is introduced via a suitable opening in the housing, the journal exerts a frictional drag on the lubricant drawing it into the wedge-shaped part of the space between the surfaces, and a convergent film is formed. The action of the relatively moving surfaces creates in the convergent film a relatively high pressure zone which holds the surfaces apart.
Establishing a lubricant film of satisfactory thickness is dependent upon a number of different conditions. During operation of such a bearing, factors such as the operating speed and the operating load affect the lubrication conditions and hence the bearing performance. Over the life of the bearing, wear can clearly also affect its performance.
A more recent approach to hydrodynamic bearings has involved the provision of segmental bearings, wherein the bearing surface is formed of a plurality of circumferentially spaced pads. Each pad is supported in the housing for limited pivotal movement and during operation the pad tilts such that the moments acting on it due to the fluid film pressures are in equilibrium.
Examples of this type of bearing construction, known as a `tilting pad` bearing, are disclosed in U.S. Pat. No 4,714,357, U.S. Pat. No 4,490,054 and U.S. Pat. No 4,636,095.
Although tilting pad bearings have application in a wide variety of different fields, they may exhibit disadvantages particularly at high rotational speeds and when subjected to transverse loads. At high speed, and especially under no-load or low-load conditions, oscillations of the rotating shaft can be significant. Moreover, under transverse loads, there can be considerable changes in the eccentricity of the shaft (the displacement of the shaft axis from that of the housing). Both these effects are the result of the fact that the segmental bearing accommodates oscillations and transverse movement by the pivoting of the tilting pads, and there is no possibility of intervention to control the operation of the bearing apart from the provision of shims to alter the unloaded setting (sometimes known as the `preload`) of the bearing pads, which of course involves decommissioning the bearing to strip it down. Moreover, the load capacity at low speeds, such as at starting-up and slowing-down speeds, is very low, since the hydrodynamic lubricant pressure is low in the fluid film.
A further fluid film bearing arrangement is disclosed in GB-1 010 547, in which a circumferentially-spaced series of bearing elements surrounding a shaft are mounted for limited radial displacement against a linked series of pressure chambers. It is intended that any relative displacements as the shaft vibrates are effectively damped by the fact that each element can find its own equilibrium position, as determined by the balance between the force produced by its associated pressure chamber and the opposing force resulting from the fluid film pressure. This arrangement provides no means of actually controlling the conditions in the fluid film. It can be said that the instantaneous fluid film pressure acts to determine the instantaneous position of the bearing elements.
Another bearing, and the method of its manufacture, is disclosed in GB-1 251 160, in which a plurality of bearing surfaces support a rotating shaft. The bearing surfaces can be radially moved while assembling the bearing, and then fixed in a support ring to set their bearing position. No further adjustment of any type is allowed or envisaged with this bearing.
An alternative form of hydrodynamic sliding bearing and one appropriate specifically to lubrication by a gas, such as air, is known as the foil bearing. In a foil bearing
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Martin James Keith
Parkins David Walter
British Technology Group Limited
Hannon Thomas R.
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