19-01-2013, 09:54 AM
BEARING
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INTRODUCTION
WHAT IS BEARING?
The term "bearing" comes ultimately from the verb "to bear" and a bearing is thus a machine element that allows one part to bear another.
A bearing is a mechanical element that permits relative motion between two parts,with minimum friction. This is typically to allow and promote free rotation around a fixed axis or free linear movement; it may also be to prevent any motion, such as by controlling the vectors of normal forces. [9]
FUNCTION OF BEARING
• A bearing permits relative motion between two machine members while minimizing Frictional resistance.
• A bearing consists of an inner and outer member separate either by a thin film of Lubricant or a rolling element
• A bearing bears the load.
• It locates the moving parts in correct position.
• It provides free motion to the moving part by reducing friction.
For example-in shaft and the housing the function of bearing are as follow-
1-The bearing ensures free rotation of shaft with minimum friction.
2-The bearing supports the shaft and hold it in correct position.
3- The bearing takes up the force that act on the shaft and transmits them to the frame or foundation.[9]
FRICTION
Reducing friction in bearings is often important for efficiency, to reduce wear and to facilitate extended use at high speeds and to avoid overheating and of the bearing. Essentially, a bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces or by separating the surfaces with an electromagnetic field.
By shape, gains advantage usually by using spheres or rollers, or by forming flexure bearings.
By material, exploits the nature of the bearing material used. (An example would be using plastics that have low surface friction.)
By fluid, exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurized medium to keep the two solid parts from touching, or by reducing the normal force between them.
By fields, exploits electromagnetic fields, such as magnetic fields, to keep solid parts from touching.
Combinations of these can even be employed within the same bearing. An example of this is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.[9]
WHAT IS AN AIR BEARING
A air bearing is defined as a device having two accurately machined surfaces which are separated by a thin film of gas and arranged in such a way that any tendency to change the clearance between the surfaces is resisted by a change in pressure in the air film. Air bearings, which are also known as gas bearings, allow designers to push the envelope on precision and high-speed applications. The fluid film of the bearing is achieved by supplying a flow of air through the bearing itself to the bearing surface. The design of the air bearing is such that although the air constantly dissipates from the bearing site, the continual flow of pressurized air through the bearing is sufficient to support working loads.Being non-contact, air bearings avoid the traditional bearing-related problems of friction, wear, and lubricant handling, and offer distinct advantages in precision positioning and high speed applications. Most people, when they think of a bearing, would normally think of a ball bearing. In this type of bearing a fixed surface and a moving surface are separated by a series of lubricated balls that run in special tracks or races. Perhaps the most common application is when a shaft rotates inside a fixed hub, for example a drive shaft on a car or the front wheel of a bicycle.In an air bearing, the balls are replaced by a cushion of air.[1][15]
It is possible to use this same principle to act as a bearing for a rotating shaft. High pressure air is fed into the gap between the rotating shaft and a stationary bearing. The gap is extremely small allowing the air pressure to be maintained within the gap. This small gap also significantly reduces the 'springiness' of the air cushion resulting in a shaft that is very accurately located i.e. has low dynamic run out. The shaft can then rotate freely.[15]
HISTROY OF AIR BEARING
The earliest experimental work on compressible fluid bearings was conducted by Hirn and published in 1854. The work highlighted the use of a thin-film of high-pressure air to reduce friction in machinery. In 1897, Kingsbury experimented with a six inch dia gas journal bearing, and later in 1904, Westinghouse developed an air thrust bearing to support a vertical steam turbine. In 1920, the externally pressurized air journal bearing was patented by Abbott .The limitation in manufacturing capability impaired the further development of gas bearings.
In the years following World War II, significant improvements were achieved in gas bearings as they were developed for applications in nuclear power and defense industries. Advances in computing technology allow the design of gas bearings to be better approximated. The use of finite difference and finite element analysis methods is widely applied to gas bearing analysis. The majority of the new work is focused on expanding the applications of gas bearings.[1]
TYPE OF BEARING
Mainly there are two type of air bearing:
AEROSTATIC BEARINGS
In contrast to aerodynamic bearings, aerostatic bearings can bear loads at a zero speed. Air bearings offer a solution for many high-tech applications where a high performance and high accuracy are required. Aerostatic bearings require an external pressurized air source due to which aerostatic bearings are also sometimes known as passive air bearings. Pressurized air is introduced between the bearing surfaces through precision holes, grooves, steps or by using porous compensation techniques and discharges through the edges of the bearings (Figure 7.2). If the correct design is used, a very high stiffness can be obtained. The aerostatic bearing is able to support a higher load than the aerodynamic bearing, but it requires a continuous source of power for supplying pressurized air. Overall, aerostatic bearings perform well in most aspects such as having a long life, noise-free operations and are free from contamination.[7]
Principle of Aerostatic Bearings
Figure 2.3 shows how gas at a supply pressure, P o is admitted into the clearance through a restricting device, which reduces the supply pressure. The pressure drop is due to the acceleration of the gas as it expands. The air will flow through the bearing and back to the atmosphere where the pressure further reduces to the atmospheric pressure, P a. A smaller clearance will reduce the pressure drop that gives a higher load capacity. It is desirable to achieve an optimum condition at which a maximum stiffness occurs where the rate of change of load when divided by the rate of change of clearance is a maximum.[1][6]
AERODYNAMIC BEARINGS
Aerodynamic bearings, which are sometimes known as active gas bearings, function depending on the relative motion between the bearing surfaces and usually some type of spiral grooves to draw the air between the bearing lands. This bearing action is very similar to hydroplaning on a puddle of water in the case of automobiles moving at high speeds. At a lower speed, the tyre cuts through the water on the road. In a similar way, aerodynamic bearings require a relative motion between surfaces, when there is no motion or when the motion is not fast enough to generate an air film, the bearing surfaces will come into contact.
Aerodynamic bearings are often referred to as foil bearings or self-acting bearings, and they generate pressure within the gas film by viscous shearing. This type of bearing is relatively simple because it is independent of an external pressure source and mechanism. However, its application is limited due to the fact that the surfaces require a very high standard of accuracy and a low load capacity. It is also not suitable for applications where frequent starts and stops or change of direction is required. The aerodynamic bearing system is however simpler and cheaper to operate compared to the aerostatic system. Examples of this type of bearing include the read–write head flying over a spinning disk,crankshaft journals, camshaft journals, and thrust bearings for electrical generator turbines.[3]
Orifice and Porous Media Technology
Air bearings are typically classed as ‘orifice’ or ‘porous media’ bearings.Porous media air bearings are quite different in that the air is supplied through the entire surface of the bearing. The porous material controls the airflow in the same way an orifice bearing would if
it had millions of miniature holes across its surface. With the exception of water and oil, it is usually not affected by dirt and rust. Porous air bearings have a substantially greater stiffness and a load capacity, but in practice, it is difficult to obtain a sufficiently small bearing clearance and to maintain the openings of the pores. The porous surfaces can also take the form of inserts. For an external flow control, a sensor is used to monitor the bearing clearance, and a feedback control system is devised using pneumatic amplifiers to vary the supply pressure accordinglyBroadly speaking there are two techniques for achieving the compensating effect in air bearings. Orifice compensation is traditionally the most widely used method, but porous surface compensation is rapidly emerging as the method of choice due to its many advantages and increasing availability.[10][8]