Function Of A Bearing & Types And Applications Of Bearing - SKengineers

 

WHAT IS A BEARING? HOW DOES IT WORKS AND TYPES OF BEARINGS -

Bearing (mechanical) -

What is a Bearing?

The bearing in its current form was developed towards the end of the 19th century. It was initially made by hand.

Nowadays, bearings are one of the most commonly used machine parts because their rolling motion make almost all movements easier and they help reduce friction.

Bearings have two key functions -

They transfer motion, i.e. they support and guide components which turn relative to one another

They transmit forces.

Rolling bearings and sleeve bearings -

In a sleeve or plain bearing, the axle and the bearing move in opposite directions on a sliding surface. By contrast, the two components of a rolling bearing that move towards one another – the inner and outer rings – are separated by rolling elements. This design generates significantly less friction than a sleeve bearing.

Radial bearings and axial bearings -

Bearings can transmit loads in a radial direction or an axial direction (thrust) and in many cases there is a combination of both radial and axial loads to transmit.

Both designs are available as ball bearings or roller bearings. The choice of bearing design depends upon the application in question.

Components -

Bearings usually consist of the following components:

Two rings or discs with raceways

Rolling elements in the form of rollers or balls

A cage which keeps the rolling elements apart and guides them.

Inner Ring / Outer Ring -

The inner and outer ring are usually made from a special high-purity, chrome alloy steel. This material has the necessary hardness and purity – both important factors for a high load rating and a long service life.

The raceways are hardened, ground and honed.

Special materials such as ceramic and plastics are also used. Although plastics cannot withstand extremely high temperatures, they are considerably lighter than steel. This makes them invaluable in sectors such as the automotive industry, where every gram matters.

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Inner Ring

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Outer Ring

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Rolling Elements -

Rolling elements are either balls, rollers, cones, spheres or needles. They are usually made from a special high-purity, chrome alloy steel. Special materials such as ceramic and plastics are also used.

The rolling elements roll on the specially formed raceways of the rings or discs and are kept apart and guided by the cage.

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Cage -

The cage is responsible for keeping the rolling elements apart and guiding them. The materials used include steel, brass and plastic. Solid metal cages can be produced using machining techniques, while pressed cages are made from sheet metal. Similarly, plastic cages can be machined from solid plastic or injection moulded.

Ball bearing -

A bearing is a machine element that constrains relative motion to only the desired motion, and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or to the directions of the loads (forces) applied to the parts.

Rotary bearings hold rotating components such as shafts or axles within mechanical systems, and transfer axial and radial loads from the source of the load to the structure supporting it. The simplest form of bearing, the plain bearing, consists of a shaft rotating in a hole. Lubrication is used to reduce friction. In the ball bearing and roller bearing, to reduce sliding friction, rolling elements such as rollers or balls with a circular cross-section are located between the races or journals of the bearing assembly. A wide variety of bearing designs exists to allow the demands of the application to be correctly met for maximum efficiency, reliability, durability and performance.

The term "bearing" is derived from the verb "to bear" a bearing being a machine element that allows one part to bear (i.e., to support) another. The simplest bearings are bearing surfaces, cut or formed into a part, with varying degrees of control over the form, size, roughness and location of the surface. Other bearings are separate devices installed into a machine or machine part. The most sophisticated bearings for the most demanding applications are very precise devices; their manufacture requires some of the highest standards of current technology.

Types of Bearings and Their Uses -

We take a deeper look into the different types of bearings and their uses.

Ball Bearings -

Ball Bearings are mechanical assemblies that consist of rolling spherical elements that are captured between circular inner and outer races. They provide a means of supporting rotating shafts and minimizing friction between shafts and stationary machine members. Ball bearings are used primarily in machinery that has shafts requiring support for low friction rotation. There are several configurations, most notably shielded or sealed. Ball bearings are standardized to permit interchangeability. Ball bearings are also known as rolling element bearings or anti-friction bearings. Considerations include -

Roller Bearings -

Roller Bearings are mechanical assemblies that consist of cylindrical or tapered rolling elements usually captured between inner and outer races. They provide a means of supporting rotating shafts and minimizing friction between shafts and stationary machine members. Roller bearings are used primarily in machinery with rotating shafts that require the support of heavier loads than ball bearings provide. Tapered roller bearings are often used to accommodate higher thrust loads in addition to the radial loads. Types range from cylindrical to spherical rollers. Roller bearings are standardized like ball bearings, albeit to a lesser degree. Considerations include -

Mounted Bearings -

Mounted Bearings are mechanical assemblies that consist of bearings housed within bolt-on or threaded mounting components and include pillow blocks, flanged units, etc. They provide means of supporting rotating shafts and minimizing friction between shafts and stationary machine members. Mounted bearings are used primarily in machinery with exposed rotating shafting. They are used as take-up devices on the ends of conveyors and as flanged units along intermediate points. The bearings can be rolling element or journal bearing configurations. Mounted bearings are designed for bolt-on mounting and ease of replacement. Other varieties of mounted bearings include rod end bearings and cam followers. Considerations include -

Linear Bearings -

Linear Bearings are mechanical assemblies that consist of ball or roller elements captured in housings and used to provide linear movement along shafts. Linear bearings are used primarily in machinery that requires linear movement and positioning along shafts. They also may have

secondary rotational features depending on the design. Considerations include

Lower friction and higher accuracies compared with bushings

Costlier and more complex than bushings.

Slide Bearings -

Slide bearings are mechanical assemblies designed to provide free motion in one dimension between structural elements. Slide bearings are used primarily in the structural support of bridges as well as commercial and industrial buildings. These parts accommodate thermal movement, allow for end-beam rotation, and isolate components of the structure against vibration, noise, and shock. Other types of slide bearings include those used on truss base plates, heat exchangers, and process equipment.

Jewel Bearings -

Jewel bearings are mechanical devices used in light rotating applications such as watches, meter movements, gyroscopes, etc. where loads are small and the supported rotating shafts are tiny. Jewel bearings are constructed from a range of synthetics, with ruby and sapphire being particularly common. 

Frictionless Bearings -

Frictionless bearings are mechanical or electro-mechanical alternatives to conventional bearings that provide controllable shaft support through air, magnetic fields, etc. for critical, high precision applications.

Motions -

Common motions permitted by bearings are:

Radial rotation e.g. shaft rotation;

linear motion e.g. drawer;

spherical rotation e.g. ball and socket joint;

hinge motion e.g. door, elbow, knee.

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 premature failure 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.

Air pressure exploits air pressure 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.

Loads -

Bearing design varies depending on the size and directions of the forces that they are required to support. Forces can be predominately radial, axial (thrust bearings), or bending moments perpendicular to the main axis.

Speeds -

Different bearing types have different operating speed limits. Speed is typically specified as maximum relative surface speeds, often specified ft/s or m/s. Rotational bearings typically describe performance in terms of the product DN where D is the mean diameter (often in mm) of the bearing and N is the rotation rate in revolutions per minute.

Generally, there is considerable speed range overlap between bearing types. Plain bearings typically handle only lower speeds, rolling element bearings are faster, followed by fluid bearings and finally magnetic bearings which are limited ultimately by centripetal force overcoming material strength.

Stiffness -

A second source of motion is elasticity in the bearing itself. For example, the balls in a ball bearing are like stiff rubber, and under load deform from round to a slightly flattened shape. The race is also elastic and develops a slight dent where the ball presses on it.

 The stiffness of a bearing is how the distance between the parts which are separated by the bearing varies with applied load. With rolling element bearings this is due to the strain of the ball and race. With fluid bearings it is due to how the pressure of the fluid varies with the gap (when correctly loaded, fluid bearings are typically stiffer than rolling element bearings).

Service life -

Fluid and magnetic bearings -

Fluid and magnetic bearings can have practically indefinite service lives. In practice, there are fluid bearings supporting high loads in hydroelectric plants that have been in nearly continuous service since about 1900 and which show no signs of wear.

Rolling element bearings -

Rolling element bearing life is determined by load, temperature, maintenance, lubrication, material defects, contamination, handling, installation and other factors. These factors can all have a significant effect on bearing life. For example, the service life of bearings in one application was extended dramatically by changing how the bearings were stored before installation and use, as vibrations during storage caused lubricant failure even when the only load on the bearing was its own weight; the resulting damage is often false brinelling. Bearing life is statistical: several samples of a given bearing will often exhibit a bell curve of service life, with a few samples showing significantly better or worse life. Bearing life varies because microscopic structure and contamination vary greatly even where macroscopically they seem identical.

L10 life -

Bearings are often specified to give an "L10" life (outside the USA, it may be referred to as "B10" life.) This is the life at which ten percent of the bearings in that application can be expected to have failed due to classical fatigue failure (and not any other mode of failure like lubrication starvation, wrong mounting etc.), or, alternatively, the life at which ninety percent will still be operating. The L10 life of the bearing is theoretical life and may not represent service life of the bearing. Bearings are also rated using C0 (static loading) value. This is the basic load rating as a reference, and not an actual load value.

Plain bearings -

For plain bearings, some materials give much longer life than others. Some of the John Harrison clocks still operate after hundreds of years because of the lignum vitae wood employed in their construction, whereas his metal clocks are seldom run due to potential wear.

Flexure bearings -

Flexure bearings rely on elastic properties of a material. Flexure bearings bend a piece of material repeatedly. Some materials fail after repeated bending, even at low loads, but careful material selection and bearing design can make flexure bearing life indefinite.

Short-life bearings -

Although long bearing life is often desirable, it is sometimes not necessary. Harris 2001 describes a bearing for a rocket motor oxygen pump that gave several hours life, far in excess of the several tens of minutes life needed.

Composite bearings -

Depending on the customized specifications (backing material and PTFE compounds), composite bearings can operate up to 30 years without maintenance.

Oscillating bearings -

For bearings which are used in oscillating applications, customized approaches to calculate L10 are used.

External factors -

The service life of the bearing is affected by many parameters that are not controlled by the bearing manufacturers. For example, bearing mounting, temperature, exposure to external environment, lubricant cleanliness and electrical currents through bearings etc. High frequency PWM inverters can induce currents in a bearing, which can be suppressed by the use of ferrite chokes.

The temperature and terrain of the micro-surface will determine the amount of friction by the touching of solid parts.

Certain elements and fields reduce friction while increasing speeds.

Strength and mobility help determine the amount of load the bearing type can carry.

Alignment factors can play a damaging role in wear and tear, yet overcome by computer aid signalling and non-rubbing bearing types, such as magnetic levitation or air field pressure.

Maintenance and lubrication -

Many bearings require periodic maintenance to prevent premature failure, but many others require little maintenance. The latter include various kinds of polymer, fluid and magnetic bearings, as well as rolling-element bearings that are described with terms including sealed bearing and sealed for life. These contain seals to keep the dirt out and the grease in. They work successfully in many applications, providing maintenance-free operation. Some applications cannot use them effectively.

Non-sealed bearings often have a grease fitting, for periodic lubrication with a grease gun, or an oil cup for periodic filling with oil. Before the 1970s, sealed bearings were not encountered on most machinery, and oiling and greasing were a more common activity than they are today. For example, automotive chassis used to require "lube jobs" nearly as often as engine oil changes, but today's car chassis are mostly sealed for life. From the late 1700s through the mid-1900s, industry relied on many workers called oilers to lubricate machinery frequently with oil cans.

 Factory machines today usually have lube systems, in which a central pump serves periodic charges of oil or grease from a reservoir through lube lines to the various lube points in the machine's bearing surfaces, bearing journals, pillow blocks, and so on. The timing and number of such lube cycles is controlled by the machine's computerized control, such as PLC or CNC, as well as by manual override functions when occasionally needed. This automated process is how all modern CNC machine tools and many other modern factory machines are lubricated. Similar lube systems are also used on nonautomated machines, in which case there is a hand pump that a machine operator is supposed to pump once daily (for machines in constant use) or once weekly. These are called one-shot systems from their chief selling point: one pull on one handle to lube the whole machine, instead of a dozen pumps of an alemite gun or oil can in a dozen different positions around the machine.

The oiling system inside a modern automotive or truck engine is similar in concept to the lube systems mentioned above, except that oil is pumped continuously. Much of this oil flows through passages drilled or cast into the engine block and cylinder heads, escaping through ports directly onto bearings, and squirting elsewhere to provide an oil bath. The oil pump simply pumps constantly, and any excess pumped oil continuously escapes through a relief valve back into the sump.

Many bearings in high-cycle industrial operations need periodic lubrication and cleaning, and many require occasional adjustment, such as pre-load adjustment, to minimize the effects of wear.

Bearing life is often much better when the bearing is kept clean and well lubricated. However, many applications make good maintenance difficult. One example is bearings in the conveyor of a rock crusher are exposed continually to hard abrasive particles. Cleaning is of little use because cleaning is expensive yet the bearing is contaminated again as soon as the conveyor resumes operation. Thus, a good maintenance program might lubricate the bearings frequently but not include any disassembly for cleaning. The frequent lubrication, by its nature, provides a limited kind of cleaning action, by displacing older (grit-filled) oil or grease with a fresh charge, which itself collects grit before being displaced by the next cycle. Another example are bearings in wind turbines, which makes maintenance difficult since the nacelle is placed high up in the air in strong wind areas. In addition, the turbine does not always run and is subjected to different operating behaviour in different weather conditions, which makes proper lubrication a challenge.

Rolling-element bearing outer race fault detection -

Rolling-element bearings are widely used in the industries today, and hence maintenance of these bearings becomes an important task for the maintenance professionals. The rolling-element bearings wear out easily due to metal-to-metal contact, which creates faults in the outer race, inner race and ball. It is also the most vulnerable component of a machine because it is often under high load and high running speed conditions. Regular diagnostics of rolling-element bearing faults is critical for industrial safety and operations of the machines along with reducing the maintenance costs or avoiding shutdown time. Among the outer race, inner race and ball, the outer race tends to be more vulnerable to faults and defects.

 There is still room for discussion as to whether the rolling element excites the natural frequencies of bearing component when it passes the fault on the outer race. Hence we need to identify the bearing outer race natural frequency and its harmonics. The bearing faults create impulses and results in strong harmonics of the fault frequencies in the spectrum of vibration signals. These fault frequencies are sometimes masked by adjacent frequencies in the spectra due to their little energy. Hence, a very high spectral resolution is often needed to identify these frequencies during a FFT analysis. The natural frequencies of a rolling element bearing with the free boundary conditions are 3 kHz. Therefore, in order to use the bearing component resonance bandwidth method to detect the bearing fault at an initial stage a high frequency range accelerometer should be adopted, and data obtained from a long duration needs to be acquired. A fault characteristic frequency can only be identified when the fault extent is severe, such as that of the presence of a hole in the outer race. The harmonics of fault frequency is a more sensitive indicator of a bearing outer race fault. For a more serious detection of defected bearing faults waveform, spectrum and envelope techniques will help reveal these faults. However, if a high frequency demodulation is used in the envelope analysis in order to detect bearing fault characteristic frequencies, the maintenance professionals have to be more careful in the analysis because of resonance, as it may or may not contain fault frequency components.

Using spectral analysis as a tool to identify the faults in the bearings faces challenges due to issues like low energy, signal smearing, cyclo-stationarity etc. High resolution is often desired to differentiate the fault frequency components from the other high-amplitude adjacent frequencies. Hence, when the signal is sampled for FFT analysis, the sample length should be large enough to give adequate frequency resolution in the spectrum. Also, keeping the computation time and memory within limits and avoiding unwanted aliasing may be demanding. However, a minimal frequency resolution required can be obtained by estimating the bearing fault frequencies and other vibration frequency components and its harmonics due to shaft speed, misalignment, line frequency, gearbox etc.

Packing -

Some bearings use a thick grease for lubrication, which is pushed into the gaps between the bearing surfaces, also known as packing. The grease is held in place by a plastic, leather, or rubber gasket (also called a gland) that covers the inside and outside edges of the bearing race to keep the grease from escaping.

Bearings may also be packed with other materials. Historically, the wheels on railroad cars used sleeve bearings packed with waste or loose scraps of cotton or wool fibre soaked in oil, then later used solid pads of cotton.

Ring oiler -

Bearings can be lubricated by a metal ring that rides loosely on the central rotating shaft of the bearing. The ring hangs down into a chamber containing lubricating oil. As the bearing rotates, viscous adhesion draws oil up the ring and onto the shaft, where the oil migrates into the bearing to lubricate it. Excess oil is flung off and collects in the pool again.

Splash lubrication -

A rudimentary form of lubrication is splash lubrication. Some machines contain a pool of lubricant in the bottom, with gears partially immersed in the liquid, or crank rods that can swing down into the pool as the device operates. The spinning wheels fling oil into the air around them, while the crank rods slap at the surface of the oil, splashing it randomly on the interior surfaces of the engine. Some small internal combustion engines specifically contain special plastic flinger wheels which randomly scatter oil around the interior of the mechanism.

Pressure lubrication -

For high speed and high power machines, a loss of lubricant can result in rapid bearing heating and damage due to friction. Also in dirty environments, the oil can become contaminated with dust or debris that increases friction. In these applications, a fresh supply of lubricant can be continuously supplied to the bearing and all other contact surfaces, and the excess can be collected for filtration, cooling, and possibly reuse. Pressure oiling is commonly used in large and complex internal combustion engines in parts of the engine where directly splashed oil cannot reach, such as up into overhead valve assemblies. High speed turbochargers also typically require a pressurized oil system to cool the bearings and keep them from burning up due to the heat from the turbine.

Composite bearings -

Composite bearings are designed with a self-lubricating polytetrafluroethylene (PTFE) liner with a laminated metal backing. The PTFE liner offers consistent, controlled friction as well as durability whilst the metal backing ensures the composite bearing is robust and capable of withstanding high loads and stresses throughout its long life. Its design also makes it lightweight-one tenth the weight of a traditional rolling element bearing.

Applications and Industries -

Bearing applications span across virtually every industry which employs moving components and equipment. For example -

1.      Ball and roller bearings are used in machinery of all kinds, from boiler feed pumps to automotive transmissions.

2.      Mounted bearings are especially common on conveyors, in shaft linkages, and particularly where long lengths of shafting must be supported by housed units where the bearing is not protected by another housing such as a transmission case.

3.      Linear bearings are used exclusively in linear applications such as slide tables.

4.      Slide bearings are used primarily for load-bearing application in large civil engineering projects such as bridges where they accommodate a limited range of movement, unlike the other bearings here, where motion—either radial or linear—is the main concern.

5.      Jewel bearings are restricted to very small devices and movements and do not rely on any rolling elements.

6.      Frictionless bearings are any of the other special-purpose designs that include air bearings, magnetic bearings, etc.

7.      While bearings are used nearly everywhere, there are some industries that use so many or have specific requirements for durability, cleanliness, etc. that they warrant mentioning here. Some of these industries are:

Aerospace.

Agricultural.

Automotive.

Machine Tools.

Medical.

Mining.