Product Code Database
Fluid bearings are bearings in which the load is supported by a thin layer of rapidly moving pressurized liquid or gas between the bearing surfaces. Since there is no contact between the moving parts, there is no sliding friction, allowing fluid bearings to have lower friction, wear and vibration than many other types of bearings. Thus, it is possible for some fluid bearings to have near-zero wear if operated correctly.
They can be broadly classified into two types: fluid dynamic bearings (also known as hydrodynamic bearings) and hydrostatic bearings. Hydrostatic bearings are externally pressurized fluid bearings, where the fluid is usually oil, water or air, and is pressurized by a pump. Hydrodynamic bearings rely on the high speed of the journal (the part of the shaft resting on the fluid) to pressurize the fluid in a wedge between the faces. Fluid bearings are frequently used in high load, high speed or high precision applications where ordinary would have shortened life or caused high noise and vibration. They are also used increasingly to reduce cost. For example, hard disk drive motor fluid bearings are both quieter and cheaper than the ball bearings they replace. Applications are very versatile and may even be used in complex geometries such as Leadscrew.
The fluid bearing may have been invented by French civil engineer L. D. Girard, who in 1852 proposed a system of railway propulsion incorporating water-fed hydraulic bearings.
Hydrostatic bearings rely on an external pump. The power required by that pump contributes to system energy loss, just as bearing friction otherwise would. Better seals can reduce leak rates and pumping power, but may increase friction.
Hydrodynamic bearings rely on bearing motion to suck fluid into the bearing, and may have high friction and short life at speeds lower than design, or during starts and stops. An external pump or secondary bearing may be used for startup and shutdown to prevent damage to the hydrodynamic bearing. A secondary bearing may have high friction and short operating life, but good overall service life if bearing starts and stops are infrequent.
Hydrodynamic (full film) lubrication is obtained when two mating surfaces are completely separated by a cohesive film of lubricant.
The thickness of the film thus exceeds the combined roughness of the surfaces. The coefficient of friction is lower than with boundary-layer lubrication. Hydrodynamic lubrication prevents wear in moving parts, and metal to metal contact is prevented.
Hydrodynamic lubrication requires thin, converging fluid films. These fluids can be liquid or gas, so long as they exhibit viscosity. In computer fan and spinning device, like a hard disk drive, heads are supported by hydrodynamic lubrication in which the fluid film is the atmosphere.
The scale of these films is on the order of micrometers. Their convergence creates pressures normal to the surfaces they contact, forcing them apart. Three types of bearings include:
Conceptually the bearings can be thought of as two major geometric classes: bearing-journal (anti-friction), and plane-slider (friction).
The Reynolds equations can be used to derive the governing principles for the fluids. Note that when gases are used, their derivation is much more involved.
The thin films can be thought to have pressure and viscous forces acting on them. Because there is a difference in velocity there will be a difference in the surface traction vectors. Because of mass conservation we can also assume an increase in pressure, making the body forces different.
Bearing characteristic number: Since viscosity, velocity, and load determine the characteristics of a hydrodynamic condition, a bearing characteristic number was developed based on the effects of these on film thickness.
Therefore,
C is known as the bearing characteristic number.
The value of C, to some extent, gives an indication of whether there will be hydrodynamic lubrication or not
Most fluid bearings require little or no maintenance, and have almost unlimited life. Conventional rolling-element bearings usually have shorter life and require regular maintenance. Pumped hydrostatic and aerostatic (gas) bearing designs retain low friction down to zero speed and need not suffer start/stop wear, provided the pump does not fail.
Fluid bearings generally have very low friction—far better than mechanical bearings. One source of friction in a fluid bearing is the viscosity of the fluid leading to dynamic friction that increases with speed, but static friction is typically negligible. Hydrostatic gas bearings are among the lowest friction bearings even at very high speeds. However, lower fluid viscosity also typically means fluid leaks faster from the bearing surfaces, thus requiring increased power for pumps or friction from seals.
When a roller or ball is heavily loaded, fluid bearings have clearances that change less under load (are "stiffer") than mechanical bearings. It might seem that bearing stiffness, as with maximum design load, would be a simple function of average fluid pressure and the bearing surface area. In practice, when bearing surfaces are pressed together, the fluid outflow is constricted. This significantly increases the pressure of the fluid between the bearing faces. As fluid bearing faces can be comparatively larger than rolling surfaces, even small fluid pressure differences cause large restoring forces, maintaining the gap.
However, in lightly loaded bearings, such as disk drives, the typical ball bearing stiffnesses are ~10^7 MN/m. Comparable fluid bearings have stiffness of ~10^6 MN/m. Because of this, some fluid bearings, particularly hydrostatic bearings, are deliberately designed to pre-load the bearing to increase the stiffness.
Fluid bearings often inherently add significant damping. This helps attenuate resonances at the gyroscopic frequencies of journal bearings (sometimes called conical or rocking modes).
It is very difficult to make a mechanical bearing which is atomically smooth and round; and mechanical bearings deform in high-speed operation due to centripetal force. In contrast, fluid bearings self-correct for minor imperfections and slight deformations.
Fluid bearings are typically quieter and smoother (more consistent friction) than rolling-element bearings. For example, hard disk drives manufactured with fluid bearings have noise ratings for bearings/motors on the order of 20–24 decibel, which is a little more than the background noise of a quiet room. Drives based on rolling-element bearings are typically at least 4 dB noisier.
Fluid bearings can be made with a lower NRRO (non repeatable run out) than a ball or rolling element bearing. This can be critical in modern hard disk drive and ultra precision spindles.
Tilting pad bearings are used as radial bearings for supporting and locating shafts in compressors.
The fluid film of the bearing is air that flows through the bearing itself to the bearing surface. The design of the air bearing is such that, although the air constantly escapes from the bearing gap, the pressure between the faces of the bearing is enough to support the working loads. This pressure may be generated externally (aerostatic) or internally (aerodynamic).
Aerodynamic bearings can only be operated in high-speed applications, aerostatic bearings are required for load bearing at low speed. Both types require highly finished surfaces and precise manufacturing.
The bearing has circle shoes, or pads on pivots. When the bearing is in operation, the rotating part of the bearing carries fresh oil in to the pad area through viscous drag. Fluid pressure causes the pad to tilt slightly, creating a narrow constriction between the shoe and the other bearing surface. A wedge of pressurised fluid builds behind this constriction, separating the moving parts. The tilt of the pad adaptively changes with bearing load and speed. Various design details ensure continued replenishment of the oil to avoid overheating and pad damage.
Michell/Kingsbury fluid bearings are used in a wider variety of heavy-duty rotating equipment, including in hydroelectric plants to support turbines and generators weighing hundreds of tons. They are also used in very heavy machinery, such as marine .
It is likely the first tilting pad bearing in service was built in 1907 by George Weymoth (Pty) Ltd (under A.G.M. Michell's guidance) for a centrifugal pump at Cohuna on the Murray River, Victoria, Australia, just two years after Michell had published and patented his three-dimensional solution to Reynold's equation. By 1913, the great merits of the tilting-pad bearing had been recognised for marine applications. The first British ship to be fitted out with the bearing was the cross-channel steamboat the Paris, but many naval vessels were similarly equipped during the First World War. The practical results were spectacular – the troublesome thrust block became dramatically smaller and lighter, significantly more efficient, and remarkably free from maintenance troubles. It was estimated that the Royal Navy saved coal to a value of £500,000 in 1918 alone as a result of fitting Michell's tilting-pad bearings.
According to the ASME (see reference link), the first Michell/Kingsbury fluid bearing in the US was installed in the Holtwood Dam (on the Susquehanna River, near Lancaster, Pennsylvania, US) in 1912. The 2.25-tonne bearing supports a water turbine and electric generator with a rotating mass of about 165 tonnes and water turbine pressure adding another 40 tonnes. The bearing has been in nearly continuous service since 1912, with no parts replaced. The ASME reported it was still in service as of 2000. As of 2002, the manufacturer estimated the bearings at Holtwood should have a maintenance-free life of about 1,300 years.
Until now tilting pad bearings play an essential role for rotating equipment like expanders, pumps, gas or steam turbines or compressors. Next to the traditional babbitt bearings which were used since the early 20th century modern manufacturers like Miba use other materials for example Bronze or Copper-Chromium as well to improve the bearings' performance.
|
|
