STRESS

STRESS

Stress is the internal resistance of a material to the distorting effects of an external force or load.

Stress, s = f/a

When a metal is subjected to a load (force), it is distorted or deformed, no matter how strong the metal or light the load. If the load is small, the distortion will probably disappear when the load is removed. The intensity, or degree, of distortion is known as strain. If the distortion disappears and the metal returns to its original dimensions upon removal of the load, the strain is called elastic strain. If the distortion disappears and the metal remains distorted, the strain type is called plastic strain.

Stress is the internal resistance, or counterforce, of a material to the distorting effects of an external force or load. These counterforces tend to return the atoms to their normal positions. The total resistance developed is equal to the external load. This resistance is known as stress.

Although it is impossible to measure the intensity of this stress, the external load and the area to which it is applied can be measured. Stress (s) can be equated to the load per unit area or the force (f) applied per cross-sectional area (a) perpendicular to the force as shown

Stress, s = f/a

Where:

s = stress (psi or lbs of force per in.2)

F = applied force (lbs of force per in.2)

A = cross-sectional area (in.2)

Types of stress

Stresses occur in any material that is subject to a load or any applied force. There are many types of stresses, but they can all be generally classified in one of six categories

1) Residual stress

Residual stresses are due to the manufacturing processes that leave stresses in a material. Welding leaves residual stresses in the metals welded

2) Structural stress

Structural stresses are stresses produced in structural members because of the weights they support. The weights provide the loadings. These stresses are found in building foundations and frameworks, as well as in machinery parts.

3) Pressure stress

Pressure stresses are stresses induced in vessels containing pressurized materials. The loading is provided by the same force producing the pressure. In a reactor facility, the reactor vessel is a prime example of a pressure vessel.

4) Flow stress

Flow stresses occur when a mass of flowing fluid induces a dynamic pressure on a

conduit wall. The force of the fluid striking the wall acts as the load. This type of

Stress may be applied in an unsteady fashion when flow rates fluctuate. Water hammer is an example of a transient flow stress.

5) Thermal stress

Thermal stresses exist whenever temperature gradients are present in a material.

Different temperatures produce different expansions and subject materials to internal stress. This type of stress is particularly noticeable in mechanisms operating at high temperatures that are cooled by a cold fluid

6) Fatigue stress

Fatigue stresses are due to cyclic application of a stress. The stresses could be due to vibration or thermal cycling.

Types of applied stresses

These are known as tensile, compressive, and shear

As illustrated in figure, the plane of a tensile or compressive stress lies perpendicular to the axis of operation of the force from which it originates. The plane of a shear stress lies in the plane of the force system from which it originates.

a) Tensile stress

Tensile stress is that type of stress in which the two sections of material on either side of a stress plane tend to pull apart or elongate

b) Compressive stress

Compressive stress is the reverse of tensile stress. Adjacent parts of the material tend to press against each other through a typical stress plane

c) Shear stress

Shear stress exists when two parts of a material tend to slide across each other in any typical plane of shear upon application of force parallel to that plane

Assessment of mechanical properties is made by addressing the three basic stress types. Because tensile and compressive loads produce stresses that act across a plane, in a direction perpendicular (normal) to the plane, tensile and compressive stresses are called normal stresses.

Two types of stress can be present simultaneously in one plane, provided that one of the stresses is shear stress. Under certain conditions, different basic stress type combinations may be simultaneously present in the material. An example would be a reactor vessel during operation. The wall has tensile stress at various locations due to the temperature and pressure of the fluid acting on the wall. Compressive stress is applied from the outside at other locations on the wall due to outside pressure, temperature, and constriction of the supports associated with the vessel. In this situation, the tensile and compressive stresses are considered principal stresses. If present, shear stress will act at a 90° angle to the principal stress.

COMPUTER BASICS FOR EVERYONE

COMPUTER BASICS FOR EVERYONE

What is a Computer?

An electronic device for processing information and performing calculations; follows a program to perform sequences of mathematical and logical operations

Computer Parts

A computer has various parts, and each part performs a specific function.

Part Description

Input Devices

You use input devices to provide information to a computer, such as typing a letter or giving instructions to a computer to perform a task. Some examples of input devices are described as follows

Mouse:

A device that you use to interact with items displayed on the computer screen. A standard mouse has a left and a right button. You use the left button to select items and provide instructions by clicking an active area on the screen. You use the right button to display commonly used menu items on the screen.

Keyboard:

A set of keys that resembles a typewriter keyboard. You use the keyboard to type text, such as letters or numbers into the computer.

Microphone:

A device that you can use to talk to people in different parts of the world. You can record sound into the computer by using a microphone. You can also use a microphone to record your speech and let the computer convert it into text.

Scanner:

A device that is similar to a photocopy machine. You can use this device to transfer an exact copy of a photograph or document into a computer. A scanner reads the page and translates it into a digital format, which a computer can read. For example, you can scan photographs of your family using a scanner.

Webcam:

A device that is similar to a video camera. It allows you to capture and send the live pictures to the other user. For example, a webcam allows your friends and family to see you when communicating with them.

Output Devices

You use output devices to get feedback from a computer after it performs a task. Some examples of output devices are described as follows.

Monitor:

A device that is similar to a television. It is used to display information, such as text and graphics, on the computer.

Printer:

A device that you use to transfer text and images from a computer to a paper or to another medium, such as a transparency film. You can use a printer to create a paper copy of whatever you see on your monitor.

Speaker/Headphone

Devices that allow you to hear sounds. Speakers may either be external or built into the computer.

Central Processing Unit and Memory

The central processing unit (CPU) is a device that interprets and runs the commands that you give to the computer. It is the control unit of a computer. The CPU is also referred to as the processor.

Memory is where information is stored and retrieved by the CPU. There are two main types of memory.

Random Access Memory (RAM):

It is the main memory and allows you to temporarily store commands and data. The CPU reads data and commands from RAM to perform specific tasks. RAM is volatile, which means it is available only while the computer is turned on. The contents of RAM must be copied to a storage device if you want to save the data in the RAM.

Read Only Memory (ROM):

It is the memory that retains its contents even after the computer is turned off. ROM is nonvolatile, or permanent, memory that is commonly used to store commands, such as the commands that check whether everything is working properly or not.

Motherboard

The motherboard is the main circuit board inside the computer. It has tiny electronic circuits and other components on it. A motherboard connects input, output, and processing devices together and tells the CPU how to run. Other components on the motherboard include the video card, the sound card, and the circuits that allow the computer to communicate with devices like the printer. The motherboard is sometimes called a system board.

Expansion Cards

An expansion card is a circuit board that can be attached to the motherboard to add features such as video display and audio capability to your computer. An expansion card either improves the performance of your computer or enhances its features. Expansion cards are also called expansion boards. Some types of expansion cards are described below.

Video Card:

It is connected to the computer monitor and is used to display information on the

monitor.

Network Interface Card (NIC):

It allows the computer to be connected to other computers so that information can be exchanged between them.

Sound Card:

It converts audio signals from a microphone, audio tape, or some other source to digital signals, which can be stored as a computer audio file. Sound cards also convert computer audio files to electrical signals, which you can play through a speaker or a headphone. The microphone and the speakers or the headphones connect to the sound card.

Storage Devices

You use storage devices to store computer information. Storage devices come in many forms. Some examples are hard drive or disk, CDROM, floppy disk, and DVD‐ROM. Storage devices can be divided into two types, internal storage devices and external storage devices. Some common storage devices are described below

Hard Disk:

A magnetic disk that is usually the main storage device on most computers. It can be an external or an internal device.

Floppy Disk:

A portable storage device that allows you to store a small amount of data. A disadvantage of this disk is that it can be easily damaged by heat, dust, or magnetic fields.

CD‐ROM:

A portable storage medium that allows you to store 400 times more data than on a floppy disk. It is less prone to damage than a floppy disk.

DVD‐ROM:

A portable storage medium that is similar to a CD‐ROM; however, it can store larger amounts of data than a floppy disk or a CD‐ROM. A DVD‐ROM is commonly used to store movies and videos.

Ports and Connections

A port is a channel through which data is transferred between input/output devices and the processor. There are several types of ports that you can use to connect the computer to external devices and networks. Some types of ports below

Universal Serial Bus (USB) Port:

You use this to connect peripheral devices such as a mouse, a modem, a keyboard, or a printer to a computer.

FireWire:

You use this to connect devices such as a digital camera. It is faster than the USB.

Network Port:

You use this to connect a computer to other computers to exchange information between the computers.

Parallel Port and Serial Port:

You use these ports to connect printers and other devices to a personal computer. However, the USB is now the preferred method for connecting peripheral devices because it is faster and easier to use.

Display Adapter:

You connect a monitor to the display adapter on your computer. The display adapter generates the video signal received from the computer, and sends it to a monitor through a cable. The display adapter may be on the motherboard, or on an expansion card.

Power:

The motherboard and other components inside a computer use direct current (DC). A power supply takes the alternating current (AC) from the wall outlet and converts it into DC power.

TRANSFORMER

TRANSFORMERS

TRANSFORMER

Transformer is used to convert electrical energy of higher voltage (usually 11-22-33kV) to a lower voltage (250 or 433V) with frequency identical before and after the transformation. Its main application is mainly within suburban areas, public supply authorities and industrial customers. With given secondary voltage, distribution transformer is usually the last in the chain of electrical energy supply to households and industrial enterprises.
INTRODUCTION A transformer has no internal moving parts, and it transfers energy from one circuit to another by electromagnetic induction. External cooling may include heat exchangers, radiators, fans, and oil pumps. Radiators and fans are evident in figure. The large horizontal tank at the top is a conservator. Transformers are typically used because a change in voltage is needed. Power transformers are defined as transformers rated 500 kVA and larger. Larger transformers are oil-filled for insulation and cooling. Transformers smaller than 500 kVA are generally called distribution transformers.
Radiator : Any of numerous devices, units, or surfaces that emit heat, mainly by radiation, to objects in the space in which they are installed.
All electrical devices using coils (in this case, transformers) are constant wattage devices. This means voltage multiplied by current must remain constant; therefore, when voltage is “stepped-up,” the current is “stepped-down” (and vice versa). Transformers transfer electrical energy between circuits completely insulated from each other. This makes it possible to use very high (stepped-up) voltages for transmission lines, resulting in a lower (stepped-down) current. Higher voltage and lower current reduce the required size and cost of transmission lines and reduce transmission losses as well. Transformers have made possible economic delivery of electric power over long distances. The transformer cannot change the frequency of the supply. If the supply is 60 hertz, the output will also be 60 hertz.

Basic principle

A simple transformer consists of two electrical conductors called the primary winding and secondary winding, and a steel core that magnetically links them together These two windings can be considered as a pair of mutually coupled coils. At the instant a transformer primary is energized with AC, a flow of electrons (current) begins. During the instant of switch closing, buildup of current and magnetic field occurs. As current begins the positive portion of the sine wave, lines of magnetic force (flux) develop outward from the coil and continue to expand until the current is at its positive peak. The magnetic field is also at its positive peak. The current sine wave then begins to decrease, crosses zero, and goes negative until it reaches its negative peak. The magnetic flux switches direction and also reaches its peak in the opposite direction.

NOTE: Direct current (DC) is not transformed, as DC does not vary its magnetic fields. A transformer usually consists of two insulated windings on a common iron (steel) core:With an AC power circuit, the current changes (alternates) continually 60 times per second, which is standard in the United States. Other countries may use other frequencies. In Europe, 50 cycles per second is common. Strength of a magnetic field depends on the amount of current and number of turns in the winding. When current is reduced, the magnetic field shrinks. When the current is switched off, the magnetic field collapses

Transformer Voltage and Current If the secondary coil has twice as many turns as the primary, it will be cut twice as many times by the flux, and twice the applied primary voltage will be induced in the secondary. The total induced voltage in each winding is proportional to the number of turns in that winding. If E1 is the primary voltage and I1 the primary current, E2 the secondary voltage and I2 the secondary current, N1 the primary turns and N2 the secondary turns, then:
E1/E2 = N1/N2 = I2/I1

Note that the current is inversely proportional to both voltage and number of turns. This means that if voltage is stepped up, the current must be stepped down and vice versa. The number of turns remains constant unless there is a tap changer. The power output or input of a transformer equals volts times amperes (E x I). If the small amount of transformer loss is disregarded, input equals output or:
E1 x I1 = E2 x I2Transformers are adapted to numerous engineering applications and may be classified in many ways:

§ By power level (from fraction of a watt to many megawatts),§ By application (power supply, impedance matching, circuit isolation),§ By frequency range (power, audio, RF)§ By voltage class (a few volts to about 750 kilovolts)§ By cooling type (air cooled, oil filled, fan cooled, water cooled, etc.)§ By purpose (rectifier, arc furnace, amplifier output, etc.).§ By ratio of the number of turns in the coils

Step Down TransformersIf there are fewer turns in the secondary winding than inthe primary winding, the secondary voltage will be lower than the primary.

Step Up TransformersIf there are fewer turns in the primary winding than in the secondary winding, the secondary voltage will be higher than the secondary circuit. IsolatingWhen the primary winding and the secondary winding havethe same amount of turns there is no change voltage, the ratio is 1/1 unity. VariableThe primary and secondary have an adjustable number of turns which can be selected without reconnecting the transformer.Note: The primary winding is the winding which receives the energy; it is not always the high-voltage winding.
CONSTRUCTIONThere are 3 main parts in the distribution transformer:
1) Coils/windingwhere incoming alternating current (through primary winding) generates magnetic flux, which in turn induces a voltage in the secondary coil.
2) Magnetic corematerial allowing transfer of magnetic field generated by primary winding to secondary winding by the principle of electromagnetic induction. A transformers core and windings are called its Active Parts. This is because these two are responsible for transformer s operation.
3) Tankserving as a mechanical package to protect active parts, as a holding vessel for transformer oil used for cooling and insulation.
Transformer AccessoriesBucholz relayBreatherPressure relief device etc
(Breather pipe A pipe that opens into a container for ventilation, as in a crankcase or oil tank. Also known as crankcase breather.)

A Buchholz relay, also called a gas relay or a sudden pressure relay, is a safety device mounted on some oil-filled power transformers, equipped with an external overhead oil reservoir called a conservator. The Buchholz Relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment
LossesAn ideal transformer would have no losses, and would therefore be 100% efficient. In practice energy is dissipated due both to the resistance of the windings (known as copper loss), and to magnetic effects primarily attributable to the core (known as iron loss). Transformers are in general highly efficient, and large power transformers (around 100 MVA and larger) may attain an efficiency as high as 99.75%. Small transformers such as a plug-in "power brick" used to power small consumer electronics may be less than 85% efficient.

The losses arise from:1) Winding resistanceCurrent flowing through the windings causes resistive heating of the conductors.2) Eddy currentsInduced currents circulate in the core and cause its resistive heating.3) Stray lossesNot all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive objects such as the transformer's support structure, and be converted to heat. The familiar hum or buzzing noise heard near transformers is a result of stray fields causing components of the tank to vibrate, and is also from magnetostriction vibration of the core.4) Hysteresis lossesEach time the magnetic field is reversed, a small amount of energy is lost to hysteresis in the magnetic core. The level of hysteresis is affected by the core material.5) Mechanical lossesThe alternating magnetic field causes fluctuating electromagnetic forces between the coils of wire, the core and any nearby metalwork, causing vibrations and noise which consume power.6) MagnetostrictionThe flux in the core causes it to physically expand and contract slightly with the alternating magnetic field, an effect known as magnetostriction. This in turn causes losses due to frictional heating in susceptible ferromagnetic cores.

The magnetic circuit and windings are the principal sources of losses and resulting temperature rise in various parts of a transformer. Core loss, copper loss in windings (I2R loss), stray loss in windings and stray loss due to leakage/high current field are mainly responsible for heat generation within the transformer.

§ Cooling systemLarge power transformers may be equipped with cooling fans, oil pumps or water-cooled heat exchangers designed to remove the heat caused by copper and iron losses. The power used to operate the cooling system is typically considered part of the losses of the transformer.

Modes of Heat Transfer
The heat transfer mechanism in a transformer takes place by three modes, viz. conduction, convection and radiation. In the oil cooled transformers, convection plays the most important role and conduction the least important.

Conduction
Almost all the types of transformers are either oil or gas filled, and heat flows from the core and windings into the cooling medium. From the core, heat can flow directly, but from the winding it flows through the insulation provided on the winding conductor. In large transformers, at least one side of insulated conductors is exposed to the cooling medium, and the heat flows through a small thickness of the conductor insulation. But in small transformers the heat may have to flow through several layers of copper and insulation before reaching the cooling medium.

Radiation
Any body, at a raised temperature compared to its surroundings, radiates heat energy in the form of waves. The heat dissipation from a transformer tank occurs by means of both radiation and natural convection. The cooling of radiators also occurs by radiation, but it is far less as compared to that by convection.

Convection
The oil, being a liquid, has one important mechanical property that its volume changes with temperature and pressure .The change of volume with temperature provides the essential convective or thermosiphon cooling. The change of volume with pressure affects the amount of transferred vibrations from the core to tank.

The heat dissipation from the core and windings occurs mainly due to convection. When a heated surface is immersed in a fluid, heat flows from the surface to the cooling medium. Due to increase in the fluid temperature, its density (or specific gravity) reduces. The fluid (oil) in oil-cooled transformers, rises upwards and transfers its heat to outside ambient through tank and radiators. The rising oil is replaced by the colder oil from the bottom, and thus the continuous oil circulation occurs.

Common Cooling Arrangements ONAN/OA cooling (Oil Natural and Air Natural)
In small rating transformers, the tank surface area may be able to dissipate heat directly to the atmosphere; while the bigger rating transformers usually require much larger dissipating surface in the form of radiators/tubes mounted directly on the tank or mounted on a separate structure. If the number of radiators is small, they are preferably mounted directly on the tank so that it results in smaller overall dimensions.

Oil is kept in circulation by the gravitational buoyancy in the closed-loop cooling system as shown in figure. The heat developed in active parts is passed on to the surrounding oil through the surface transfer (convection) mechanism. The oil temperature increases and its specific gravity drops, due to which it flows upwards and then into the coolers. The oil heat gets dissipated along the colder surfaces of the coolers which increases its specific gravity, and it flows downwards and enters the transformer tank from the inlet at the bottom level.
Since the heat dissipation from the oil to atmospheric air is by natural means (the circulation mechanism for oil is the natural thermosiphon flow in the cooling
equipment and windings), the cooling is termed as ONAN (Oil Natural and Air Natural) or OA type of cooling. Since the heat dissipation from the oil to atmospheric air is by natural means (the circulation mechanism for oil is the natural thermosiphon flow in the cooling equipment and windings), the cooling is termed as ONAN (Oil Natural and Air Natural) or OA type of cooling.

ONAF/FA cooling (Oil Natural and Air Forced)
If fans are used to blow air on to the cooling surfaces of the radiators, the heat transfer coefficient is significantly increased. For a given set of ambient air temperature and oil temperature, a compact arrangement is possible since less number of radiators is required to cool the oil. This type of cooling is termed as ONAF (Oil Natural and Air Forced) or FA type of cooling.

OTHER TRANSFORMER TYPESCurrent Transformer(CT) Current transformers are used in electric metering for large load situations to reduce the current level presented to the metering circuit in order to make it more manageable and safe. A current transformer is a type of "instrument transformer" that is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary.

Potential Transformer

A Potential Transformer is a special type of transformer that allows meters to take readings from electrical service connections with higher voltage (potential) than the meter is normally capable of handling without at potential transformer. Potential transformers are used with voltmeters, wattmeters, watt-hour meters, power-factor meters, frequency meters, synchroscopes and synchronizing apparatus, protective and regulating relays, and undervoltage and overvoltage trip coils of circuit breakers. One potential transformer can be used for a number of instruments if the total current required by the instruments connected to the secondary winding does not exceed the transformer rating.

Autotransformers It is possible to obtain transformer action by means of a single coil, provided that there is a “tap connection” somewhere along the winding. Transformers having only one winding are called autotransformers, shown schematically in figure . An autotransformer has the usual magnetic core but only one winding, which is common to both the primary and secondary circuits. The primary is always the portion of the winding connected to the AC power source. This transformer may be used to step voltage up or down. If the primary is the total winding and is connected to a supply, and the secondary circuit is connected across only a portion of the winding (as shown), the secondary voltage is “stepped-down.”
Step-up and step-down transformersA transformer designed to increase voltage from primary to secondary is called a stepuptransformer. A transformer designed to reduce voltage from primary to secondary iscalled a step-down transformer.
Power TransformerPower transformers are selected based on the application, with the emphasis toward custom design being more apparent the larger the unit. Power transformers are available for step-up operation, primarily used at the generator and referred to as generator step-up (GSU) transformers, and for step-down operation, mainly used to feed distribution circuits. Power transformers are available as single-phase or three-phase apparatus.The construction of a transformer depends upon the application. Transformers intended for indoor use are primarily of the dry type but can also be liquid immersed. For outdoor use, transformers are usually liquid immersed.

AccessoriesThere are many different accessories used to monitor and protect power transformers, some of which are considered standard features, and others of which are used based on miscellaneous requirements. A few of the basic accessories are briefly discussed here.

Liquid-Level IndicatorA liquid-level indicator is a standard feature on liquid-filled transformer tanks, since the liquid medium is critical for cooling and insulation. This indicator is typically a round-faced gauge on the side of the tank, with a float and float arm that moves a dial pointer as the liquid level changes.

Pressure-Relief DevicesPressure-relief devices are mounted on transformer tanks to relieve excess internal pressuresthat might build up during operating conditions. These devices are intended to avoid damage to the tank. On larger transformers, several pressure-relief devices may be required due to the large quantities of oil.

Liquid-Temperature IndicatorLiquid-temperature indicators measure the temperature of the internal liquid at a point near the top of the liquid using a probe inserted in a well and mounted through the side of the transformer tank.

Sudden-Pressure RelayA sudden- (or rapid-) pressure relay is intended to indicate a quick increase in internal pressure that can occur when there is an internal fault. These relays can be mounted on the top or side of the transformer, or they can operate in liquid or gas space.

Desiccant (Dehydrating) BreathersDesiccant breathers use a material such as silica gel to allow air to enter and exit the tank, removing moisture as the air passes through. Most tanks are somewhat free breathing, and such a device, if properly maintained, allows a degree of control over the quality of air entering the transformer

‘‘Buchholz’’ RelayOn power transformers using a conservator liquid-preservation system, a ‘‘Buchholz’’ relay can be installed in the piping between the main transformer tank and the conservator. The purpose of the Buchholz relay is to detect faults that may occur in the transformer. One mode of operation is based on the generation of gases in the transformer during certain minor internal faults. Gases accumulate in the relay, displacing the liquid in the relay, until a specified volume is collected, at which time a float actuates a contact or switch. Another mode of operation involves sudden increases in pressure in the main transformer tank, a sign of a major fault in the transformer. Such an increase in pressure forces the liquid to surge through the piping between the main tank and the conservator, through the ‘‘Buchholz’’ relay, which actuates another contact or switch.

BEARINGS

BEARINGS

Bearings permit smooth, low-friction movement between two or more surfaces. Conventional bearings provide support to rotating machinery by allowing relative movement. They allow rotation and provide support in either radial or axial planes of rotation. The most common types of bearings are rolling element bearings, and oil film or journal bearings. Bearings reduce friction by providing smooth metal balls or rollers. These balls or rollers "bear" the load, allowing the device to spin smoothly

Bearings typically have to deal with two kinds of loading, radial and thrust. Depending on where the bearing is being used, it may see all radial loading, all thrust loading or a combination of both e.g. electric motor bearing supports a radial load , a revolving stool bearing supports axial or thrust load

What is load?
Load is the force applied to the bearing, which the bearing has to withstand. Generally there are two types of load
1) Radial load is the load which is applied perpendicular to the shaft axis.
2) Axial load is the load applied parallel to the shaft axis
3) Combined load is radial plus axial load

Principle of operationRolling element bearings consist of a stationary outer race and a rotating inner race; in between them are the rolling elements, most common are spherical balls, but cylinders or tapered pins are also used. During rotation, these 3 items are in contact with each other and the weight being supported is transferred through the rolling elements between the inner race and outer race.

Oil film bearings have no rolling elements, but make use of pressurized oil to provide a film of support, and prevent galling between the shaft and the bearing journal. The oil is circulated so that fresh, cool oil is constantly entering the space between the stationary and rotating pieces. The shaft rotation shears the oil in this gap, causing it to heat up. The oil then exits the journal for cooling, filtering, and recirculation.

Types of bearings

1) Ball bearingsBall bearings are very commonly used. They are found in everything from skate boards, to washing machines to PC hard drives. These bearings are capable of taking both radial and thrust loads, and are usually found in applications where the load is light to medium and is constant in nature (i.e. not shock loading). In a ball bearing, the load is transmitted from the outer race to the ball and from the ball to the inner race

2) Roller bearings
Roller bearings are used in heavy duty applications such as conveyor belt rollers, where they must hold heavy radial loads. In these bearings the roller is a cylinder, so the contact between the inner and outer race is not a point (like the ball bearing above) but a line. This spreads the load out over a larger area, allowing the roller bearing to handle much greater loads than a ball bearing. However, this type of bearing cannot handle thrust loads to any significant degree. A variation of this bearing design is called the needle bearing. The needle roller bearing uses cylindrical rollers but with a very small diameter. This allows the bearing to fit into tight places such as gear boxes that rotate at higher speeds.

3) Thrust ball bearings
Ball thrust bearings are mostly used for low-speed non precision applications. They cannot take much radial load.

4) Roller thrust bearing
Roller thrust bearings can support very large thrust loads. They are often found in gearsets like car transmissions between gears

5) Taper roller bearing
Tapered roller bearings are designed to support large radial and large thrust loads. These loads can take the form of constant loads or shock loads. Tapered roller bearings are used in many car hubs, where they are usually mounted in pairs facing opposite directions. This gives them the ability to take thrust loads in both directions.

6) Magnetic Bearing
Magnetic bearing systems represent a completely different approach to the support of rotating equipment. Magnetic bearings are a non-contact technology, which means negligible friction loss and no wear, and higher reliability. It also enables previously unachievable surface speeds to be attained. Lubrication is eliminated, meaning that these bearings can be incorporated into processes that are sensitive to contamination, such as the vacuum chambers in which many semiconductor manufacturing processes take place.

General Application Guidelines:
Ball bearings are the less expensive choice in the smaller sizes and under lighter loads, while roller bearings are less expensive for larger sizes and heavier loads. Roller bearings are more satisfactory under shock or impact loading than ball bearings. Ball-thrust bearings are for pure thrust loading only. At high speeds, a deep-groove or angular-contact ball bearing usually will be a better choice, even for pure thrust loads. Self-aligning ball bearings and cylindrical roller bearings have very low friction coefficients. Deep-groove ball bearings are available with seals built into the bearing so that the bearing can be pre-lubricated to operate for long periods reducing maintenance requirements.
Rolling bearing types
Numerous rolling bearing types with standardized main dimensions are available for the various requirements. Rolling bearings are differentiated according to: – (1) the direction of main load: radial bearings and thrust bearings. Radial bearings have a nominal contact angle a0 of 0° to 45°. Thrust bearings have a nominal contact angle a0 of over 45° to 90°. – (2) The type of rolling elements: ball bearings and roller bearings.

Difference between Ball & Roller bearing
The essential differences between ball bearings and roller bearings are,
– Ball bearings: lower load carrying capacity, higher speeds
– Roller bearings: higher load carrying capacity, lower speeds

Rolling bearing components
Rolling bearings generally consist of bearing rings (inner ring and outer ring), rolling elements which roll on the raceways of the rings, and a cage which surrounds the rolling elements. The lubricant also has to be regarded as a rolling bearing component as a bearing can hardly operate without a lubricant. Seals are also increasingly being integrated into the bearings.

Types of Rolling elements
Rolling elements are classified, according to their shape, into balls, cylindrical rollers, needle rollers, tapered rollers and barrel rollers. The rolling elements function is to transmit the force acting on the bearing from one ring to the other. For a high load carrying capacity it is important that as many rolling elements as possible, which are as large as possible, are accommodated between the bearing rings. Their number and size depend on the cross section of the bearing. Also it is just as important for loadability that the rolling elements within the bearing are of identical size.