Main type of Bearing - Classification and Selection of Bearing

 

 

 

Moving parts in machinery involve relative sliding or rolling motion. Examples of relative motion are linear sliding motion, such as in machine tools, and rotation motion, such as in motor vehicle wheels. Most bearings are used to support rotating shafts in machines. Rubbing of two bodies that are loaded by a normal force (in the direction normal to the contact area) generates energy losses by friction and wear. Appropriate bearing design can minimize friction and wear as well as early failure of machinery. The most important objectives of bearing design are to extend bearing life in machines, reduce friction energy losses and wear, and minimize maintenance expenses and downtime of machinery due to frequent bearing failure. In manufacturing plants, unexpected bearing failure often causes expensive loss of production. Moreover, in certain cases, such as in aircraft, there are very important safety considerations, and unexpected bearing failures must be prevented at any cost. During the past century, there has been an ever-increasing interest in the friction and wear characteristics of various bearing designs, lubricants, and materials for bearings.

 

Classification and Selection of Bearings

 

Radial and Thrust Bearings

 

Bearings can also be classified according to their geometry related to the relative motion of elements in machinery. Examples are journal, plane-slider, and spherical bearings. A journal bearing, also referred to as a sleeve bearing, is widely used in machinery for rotating shafts. It consists of a bushing (sleeve) supported by a housing, which can be part of the frame of a machine. The shaft (journal) rotates inside the bore of the sleeve. 

There is a small clearance between the inner diameter of the sleeve and the journal, to allow for free rotation. In contrast, a plane-slider bearing is used mostly for linear motion, such as the slides in machine tools. A bearing can also be classified as a radial bearing or a thrust bearing, depending on whether the bearing load is in the radial or axial direction, respectively, of the shaft. The shafts in machines are loaded by such forces as reactions between gears and tension in belts, gravity, and centrifugal forces. All the forces on the shaft must be supported by the bearings, and the force on the bearing is referred to as a bearing load. The load on the shaft can be divided into radial and axial components. The axial component (also referred to as thrust load) is in the direction of the shaft axis (see Fig. 1), while the radial load component is in the direction normal to the shaft axis.

 

 

Bearing Classification

 

Machines could not operate at high speed in their familiar way without some means of reducing friction and the wear of moving parts. Several important engineering inventions made it possible to successfully operate heavily loaded shafts at high speed, including the rolling-element bearing and hydrodynamic, hydrostatic, and magnetic bearings.

 

• Rolling-element bearings are characterized by rolling motion, such as in ball bearings or cylindrical rolling-element bearings. The advantage of rolling motion is that it involves much less friction and wear, in comparison to the sliding motion of regular sleeve bearings.

 

• The term hydrodynamic bearing refers to a sleeve bearing or an inclined plane-slider where the sliding plane floats on a thin film of lubrication. The fluid film is maintained at a high pressure that supports the bearing load and completely separates the sliding surfaces. The lubricant can be fed into the bearing at atmospheric or higher pressure. The pressure wave in the lubrication film is generated by hydrodynamic action due to the rapid rotation of the journal. The fluid film acts like a viscous wedge and generates high pressure and load-carrying capacity. The sliding surface floats on the fluid film, and wear is prevented.

 

• In contrast to hydrodynamic bearing, hydrostatic bearing refers to a configuration where the pressure in the fluid film is generated by an external high-pressure pump. The lubricant at high pressure is fed into the bearing recesses from an external pump through high-pressure tubing. The fluid, under high pressure in the bearing recesses, carries the load and separates the sliding surfaces, thus preventing high friction and wear.

 

• A recent introduction is the electromagnetic bearing. It is still in development but has already been used in some unique applications. The concept of operation is that a magnetic force is used to support the bearing load. Several electromagnets are mounted on the bearing side (stator poles). The bearing load capacity is generated by the magnetic field between rotating laminators, mounted on the journal, and stator poles, on the stationary bearing side. Active feedback control keeps the journal floating without any contact with the bearing surface. The advantage is that there is no contact between the sliding surfaces, so wear is completely prevented as long as there is magnetic levitation.

 

DRY AND BOUNDARY LUBRICATION BEARINGS

 

Whenever the load on the bearing is light and the shaft speed is low, wear is not a critical problem and a sleeve bearing or plane-slider lubricated by a very thin layer of oil (boundary lubrication) can be adequate. Sintered bronzes with additives of other elements are widely used as bearing materials. Liquid or solid lubricants are often inserted into the porosity of the material and make it self-lubricated.

 

However, in heavy-duty machinery namely, bearings operating for long periods of time under heavy load relative to the contact area and at high speeds better bearing types should be selected to prevent excessive wear rates and to achieve acceptable bearing life. Bearings from the aforementioned list can be selected, namely, rolling-element bearings or fluid film bearings. In most applications, the sliding surfaces of the bearing are lubricated. However, bearings with dry surfaces are used in unique situations where lubrication is not desirable.

 

 

HYDRODYNAMIC BEARING

 

An inclined plane-slider is shown in Figure. It carries a load F and has horizontal velocity, U , relative to a stationary horizontal plane surface. The plane slider is inclined at an angle a relative to the horizontal plane. If the surfaces were dry, there would be direct contact between the two surfaces, resulting in significant friction and wear. It is well known that friction and wear can be reduced by lubrication. If a sufficient quantity of lubricant is provided and the liding velocity is high, the surfaces would be completely separated by a very thin lubrication film having the shape of a fluid wedge. In the case of complete separation, full hydrodynamic lubrication is obtained. The plane-slider is inclined, to form a converging viscous wedge of lubricant.

 

A commonly used bearing in machinery is the hydrodynamic journal bearing, as 3. Similar to the inclined plane-slider, it can support a radial load without any direct contact between the rotating shaft (journal) and the bearing sleeve. The viscous fluid film is shaped like a wedge due to the eccentricity, e, of the centers of the journal relative to that of bearing bore. As with the plane-slider, a pressure wave is generated in the lubricant, and a thin fluid.

 

 

film completely separates the journal and bearing surfaces. Due to the hydrodynamic effect, there is low friction and there is no significant wear as long as a complete separation is maintained between the sliding surfaces. The pressure wave inside the hydrodynamic film carries the journal weight together with the external load on the journal.

 

The principle of operation is the uneven clearance around the bearing formed by a small eccentricity, e, between the journal and bearing centers, as shown in Figure. The clearance is full of lubricant and forms a thin fluid film of variable thickness. A pressure wave is generated in the converging part of the clearance. The resultant force of the fluid film pressure wave is the load-carrying capacity, W , of the bearing. For bearings operating at steady conditions (constant journal speed and bearing load), the load- carrying capacity is equal to the external load, F, on the bearing. But the two forces of action and reaction act in opposite directions.

 

In a hydrodynamic journal bearing, the load capacity (equal in magnitude to the bearing force) increases with the eccentricity, e, of the journal. Under steady conditions, the center of the journal always finds its equilibrium point, where the load capacity is equal to the external load on the journal. Figure  indicates that the eccentricity displacement, e, of the journal center, away from the bearing center, is not in the vertical direction but at a certain attitude angle, f, from the vertical direction. In this configuration, the resultant load capacity, due to the pressure wave, is in the vertical direction, opposing the vertical external force. The fluid film pressure is generated mostly in the converging part of the clearance, and the attitude angle is required to allow the converging region to be below the journal to provide the required lift force in the vertical direction and, in this way, to support the external load.

 

In real machinery, there are always vibrations and disturbances that can cause occasional contact between the surface asperities (surface roughness), resulting in severe wear. In order to minimize this risk, the task of the engineer is to design the hydrodynamic journal bearing so that it will operate with a minimum lubrication-film thickness, h n, much thicker than the size of the surface asperities. Bearing designers must keep in mind that if the size of the surface asperities is of the order of magnitude of 1 micron, the minimum film thickness, h n, should be 10–100 microns, depending on the bearing size and the level of vibrations expected in the machine.

 

Disadvantages of Hydrodynamic Bearings

One major disadvantage of hydrodynamic bearings is that a certain minimum speed is required to generate a full fluid film that completely separates the sliding surfaces. Below that speed, there is mixed or boundary lubrication, with direct contact between the asperities of the rubbing surfaces. For this reason, even if the bearing is well designed and successfully operating at the high rated speed of the machine, it can be subjected to excessive friction and wear at low speed, such as during starting and stopping of journal rotation. In particular, hydrodynamic bearings undergo severe wear during start-up, when they accelerate from zero speed, because static friction is higher than dynamic friction.

A second important disadvantage is that hydrodynamic bearings are completely dependent on a continuous supply of lubricant. If the oil supply is interrupted, even for a short time for some unexpected reason, it can cause overheating and sudden bearing failure. It is well known that motor vehicle engines do not last a long time if run without oil. In that case, the hydrodynamic bearings fail first due to the melting of the white metal lining on the bearing. This risk of failure is the reason why hydrodynamic bearings are never used in critical applications where there are safety concerns, such as in aircraft engines. Failure of a motor vehicle engine, although it is highly undesirable, does not involve risk of loss of life; therefore, hydrodynamic bearings are commonly used in motor vehicle engines for their superior performance and particularly for their relatively long operation life.

A third important disadvantage is that the hydrodynamic journal bearing has a low stiffness to radial displacement of the journal (low resistance to radial run-out), particularly when the eccentricity is low. This characteristic rule out the application of hydrodynamic bearings in precision machines, e.g., machine tools. Under dynamic loads, the low stiffness of the bearings can result in dynamic instability, particularly with lightly loaded high-speed journals. The low stiffness causes an additional serious problem of bearing whirl at high journal speeds. The bearing whirl phenomenon results from instability in the oil film, which often results in bearing failure.

 

HYDROSTATIC BEARING

 

The introduction of externally pressurized hydrostatic bearings can solve the problem of wear at low speed that exists in hydrodynamic bearings. In hydrostatic bearings, a fluid film completely separates the sliding surfaces at all speeds, including zero speed. However, hydrostatic bearings involve higher cost in comparison to hydrodynamic bearings. Unlike hydrodynamic bearings, where the pressure wave in the oil film is generated inside the bearing by the rotation of the journal, an external oil pump pressurizes the hydrostatic bearing. In this way, the hydrostatic bearing is not subjected to excessive friction and wear rate at low speed.

 

The hydrostatic operation has the advantage that it can maintain complete separation of the sliding surfaces by means of high fluid pressure during the starting and stopping of journal rotation. Hydrostatic bearings are more expensive than hydrodynamic bearings, since they require a hydraulic system to pump and circulate the lubricant and there are higher energy losses involved in the circulation of the fluid. The complexity and higher cost are reasons that hydrostatic bearings are used only in special circumstances where these extra expenses can be financially justified.

 

 

MAGNETIC BEARING

 

A magnetic bearing is shown in Figure. The concept of operation is that a magnetic field is applied to support the bearing load. Several electromagnets are mounted on the bearing side (stator poles).

Electrical current in the stator poles generates a magnetic field. The load-carrying capacity of the bearing is due to the magnetic field between the rotating laminators mounted on the journal and the coils of the stator poles on the stationary bearing side. Active feedback control is required to keep the journal floating without its making any contact with the bearing. The control entails on-line measurement of the shaft displacement from the bearing center, namely, the magnitude of the eccentricity and its direction. The measurement is fed into the controller for active feedback control of the bearing support forces in each pole in order to keep the journal close to the bearing center. This is achieved by varying the magnetic field of each pole around the bearing. In this way, it is possible to control the magnitude and direction of the resultant magnetic force on the shaft. This closed-loop control results in stable bearing operation. During the last decade, a lot of research work on magnetic bearings has been conducted in order to optimize the performance of the magnetic bearing. The research work included optimization of the direction of magnetic flux, comparison between electromagnetic and permanent magnets, and optimization of the number of magnetic poles. This research work has resulted in improved load capacity and lower energy losses. In addition, research has been conducted to improve the design of the control system, which resulted in a better control of rotor vibrations, particularly at the critical speeds of the shaft.

 

Disadvantages of Magnetic Bearings

Although significant improvement has been achieved, there are still several disadvantages in comparison with other, conventional bearings. The most important limitations follow.

 

• Electromagnetic bearings are relatively much more expensive than other noncontact bearings, such as the hydrostatic bearing. In most cases, this fact makes the electromagnetic bearing an uneconomical alternative.
• Electromagnetic bearings have less damping of journal vibrations in comparison to hydrostatic oil bearings.
• In machine tools and other manufacturing environments, the magnetic force attracts steel or iron chips.
• Magnetic bearings must be quite large in comparison to conventional noncontact bearings in order to generate equivalent load capacity. An acceptable-size magnetic bearing has a limited static and dynamic load capacity. The magnetic force that supports static loads is limited by the saturation properties of the electromagnet core material. The maximum magnetic field is reduced with temperature. In addition, the dynamic. load capacity of the bearing is limited by the available electrical power supply from the amplifier.

 

ROLLING-ELEMENT BEARINGS

Rolling-element bearings, such as ball, cylindrical, or conical rolling bearings, are the bearings most widely used in machinery. Rolling bearings are often referred to as antifriction bearings. The most important advantage of rolling-element bearings is the low friction and wear of rolling relative to that of sliding. Rolling bearings are used in a wide range of applications. When selected and applied properly, they can operate successfully over a long period of time. Rolling friction is lower than sliding friction; therefore, rolling bearings have lower friction energy losses as well as reduced wear in comparison to sliding bearings. Nevertheless, the designer must keep in mind that the life of a rolling- element bearing can be limited due to fatigue. Ball bearings involve a point contact between the balls and the races, resulting in high stresses at the contact, often named hertz stresses, after Hertz (1881), who analyzed for the first time the stress distribution in a point contact.

 

When a rolling-element bearing is in operation, the rolling contacts are subjected to alternating stresses at high frequency that result in metal fatigue. At high speed, the centrifugal forces of the rolling elements, high temperature (due to friction-energy losses) and alternating stresses all combine to reduce the fatigue life of the bearing. For bearings operating at low and medium speeds, relatively long fatigue life can be achieved in most cases. But at very high speeds, the fatigue life of rolling element bearings can be too short, so other bearing types should be selected. Bearing speed is an important consideration in the selection of a proper type of bearing. High-quality rolling-element bearings, which involve much higher cost, are available for critical high-speed applications, such as in aircraft turbines. Over the last few decades, a continuous improvement in materials and the methods of manufacturing of rolling-element bearings have resulted in a significant improvement in fatigue life, specifically for aircraft applications. But the trend in modern machinery is to increase the speed of shafts more and more in order to reduce the size of machinery. Therefore, the limitations of rolling element bearings at very high speeds are expected to be more significant in the future.

 

The fatigue life of a rolling bearing is a function of the magnitude of the oscillating stresses at the contact. If the stresses are low, the fatigue life can be practically unlimited. The stresses in dry contact can be calculated by the theory of elasticity. However, the surfaces are usually lubricated, and there is a very thin lubrication film at very high pressure separating the rolling surfaces. This thin film prevents direct contact and plays an important role in wear reduction. The analysis of this film is based on the electrohydrodynamic (EHD) theory, which considers the fluid dynamics of the film in a way similar to that of hydrodynamic bearings.

 

NONCONTACT BEARINGS FOR PRECISION APPLICATIONS

Three types of noncontact bearings are of special interest for precision machining, because they can run without any contact between the sliding surfaces in the bearing. These noncontact bearings are hydrostatic, hydrodynamic, and electro-magnetic bearings. The bearings are noncontact in the sense that there is a thin clearance of lubricant or air between the journal (spindle in machine tools) and the sleeve. In addition to the obvious advantages of low friction and the absence of wear, other characteristics of noncontact bearings are important for ultra-high precision applications. One important characteristic is the isolation of the spindle from vibrations. Noncontact bearings isolate the spindle from sources of vibrations in the machine or even outside the machine. Moreover, direct contact friction can induce noise and vibrations, such as in stick-slip friction; therefore, noncontact bearings offer the significant advantage of smooth operation for high precision applications. The following discussion makes the case that hydrostatic bearings are the most suitable noncontact bearing for high-precision applications such as ultra-high-precision machine tools.