Bearing | Mechanical Engineering SSC JE (Technical) PDF Download

BEARING

• A bearing is a machine element which support another moving machine element known as Journal. It permits a relative motion between the contact surfaces of the members, while carrying the load. The lubricant used to separate the journal and bearing is usually a mineral oil refined from petroleum, but vegetable oils, silicon oils, grease etc. may be used.
   
• Classification of Bearings 
  • Depending upon the direction of load to be supported.
    • In radial bearings the load acts perpendicular to the direction of motion of the moving element.
      
    • In thrust bearings, the load acts along the axis of rotation. 
• These bearings may move in either of the direction.

• Depending upon the nature of contact
  • In sliding contact bearing, the sliding takes place along the surfaces of the contact between the moving element and the fixed element. These are also known as plain bearing.
  • In rolling contact bearings, the steel balls or rollers , are interposed between the moving and fixed elements. The balls offer rolling friction at two points for each ball or roller.

• Sliding Contact Bearings 
  • The sliding bearings in which the sliding action is along the circumference of a circle or an arc of a circle and carrying radial loads are known as journal or sleeve bearings, when the angle of contact of the bearing with the journal is 360° then the bearing is called full journal bearing. This type of bearing is commonly used in industrial machinery to accommodate bearing loads in any radial direction.

• When the angle of contact of the bearing with the journal is 120°, then the bearing is said to be partial jouranl bearing. This type of bearing has less friction than full journal bearing, but it can be used only where the load is always in one direction.
  The most common application of the partial journal bearings is found in rail road car axle. The full and partial journal bearings may be called as clearance bearings because diameter of the journal is less than that of bearing.

• When a partial journal bearings has zero clearance i.e. the diameters of the journal and bearing are equal, then bearing is called a fitted bearings.
   
• Hydrodynamic Lubricated Bearing 
  • The thick film bearings are those in which the working surfaces are completelyseparated from each other by the lubricant. Such type of bearings are also called as hydrodynamic lubricated bearings. 
  • When the bearing is supplied with sufficient lubricant, a pressure is build up in the clearance space when the journal is rotating about an axis that is eccentric with the bearing axis. The load supporting pressure in hydrodynamic bearings arises from either :                      
    • The flow of a viscous fluid in a converging channel (wedge film lubrication).
    • The resistance of a viscous fluid to being squeezed out from between approaching surface (known as squeeze film lubrication). In certain cases, the bearing oscillate or rotate so slowly, that the wedge film cannot provide a satisfactory film thickness. If the load is uniform or varying in magnitude while acting in a constant direction, this becomes a thin film or possibly zero film . But if the load reverses its direction, the squeeze film develop sufficient capacity to carry the dynamic loads without contact between the journal and the bearing. Such bearings are known as squeeze film journal bearings.
       
• Properties of Sliding Contact Bearing Material 
  • When the journal and the bearings are having proper lubrication i.e. there is a film of clean, non corrosive lubricant in between separating the two surfaces in contact, the only requirement of the bearing material is that they should have sufficient strength and rigidity.
  • Compressive strength. The maximum bearing pressure is considerably greater than the average pressure obtained by dividing the load to the projected area.
    Therefore the bearing material should have high compressive strength to withstand this maximum pressure so as to prevent extrusion or other permanent deformation of the bearing.
  • Fatigue strength. By sufficient fatigue strength, it can withstand repeated loads without developing surface fatigue cracks. It is of major importance in aircraft and automotive engines. 
  • Comformability. It is the ability of bearing material to accomodate shaft deflections and bearing inaccuracies by plastic deformation without excessive wear and heating.
  • Embedability. It is the ability of bearing material to accomodate small particle of dust, grit etc., without scoring the material of journal. 
  • Bondability. Many high capacity bearings are made by bonding one or more thin layers of a bearing material to a high strength steel shell. Thus the strength of the bond i.e. bondability is an important consideration in selecting bearing material. 
  • Corrosion Resistance. This property is of particular importance in internalcombustion engines where the same oil is used to lubricate the cylinder walls and bearings. In the cylinder, the lubricating oil comes into contact with hot cylinder walls and may oxidise and collect carbon deposits from the walls. 
  • Thermal conductivity. The bearing material should have high thermal conductivity so as to permit the rapid removal of the heat generated by friction.
     
• Materials used for sliding contact Bearings 
  • Babbit Metal. The tin base and lead base babbits are widely used as a bearing material, because it satisfy most requirements of general applications.
  • Tin base babbits : Tin 90%, copper 4.5%, Antinomy 5%, Lead 0.5%. Lead base babbits : Lead 84%, Tin 6%, Antimony 9.5%, copper 0.5%. 
  • Bronzes. The bronzes are generally used in the form of machined bushes pressed into the shell. (alloys of copper, tin and zinc).
    • The Gun metal (cooper 88%, Tin 10%, Zinc 2%) for high grade bearings subjected to high pressure and high speeds.
    • The phosphor bronze (copper 80%, Tin 10%, Lead 9%, Phosphorus 1%) is used for bearings subjected to very high pressure and speed. 
  • Cast Iron. The cast iron bearings are generally used with steel journals. 
  • Silver. The silver and silver lead bearing are mostly used in aircraft engines where the fatigue strength is the most important consideration. 
  • Non metallic bearings. The various non-metallic bearings are made of carbon graphite rubber, wood and plastics. The carbon-graphite bearings are self lubricating, chemically inert and can operate at high temperature than other bearings. The commonly used plastic materials for bearings is Nylon and Teflon and can be used dry. 
  • Lubricants. The liquid lubricants usually used in bearings are mineral oils and synthetic oils. A grease is a semi-liquid lubricant having higher viscosity than oils. The grease are employed where slow speed and heavy pressure exist. The solid lubricants are useful in reducing friction where oil films cannot be maintained because of pressures and temperatures. A graphite is the most common of the solid lubricants either alone or mixed with oil or grease. 
• Viscosity. The viscosity of the lubricant is measured by Saybolt universal viscometer.

where, Z = absolute viscosity at temperature t, in kg/m-s
S = Saybolt universal viscosity, in seconds.

• Terms used in Hydrodynamic Journal bearing

Let, D = diameter of the bearing
d = diameter of the journal
l = length of the bearing 

• Diametral clearances. It is the difference between the diameters of the bearing and the journal C = D – d 
• Radial clearance. It is difference between radii of bearing and the journal.

• Diametral clearance Ratio. It is the ratio of the diametral clearance to the diametral of the journal.

• Eccentricity, It is the radial distance between the centre (O) of the bearing and the displaced centre (O1) of the bearing under load. It is denoted by e. 
• Minimum oil film thickness. It is the minimum distance between the bearing and journal, under complete lubrication condition. It is denoted by ho and occurs at the line of centres. Its value may be assumed as C/4. 
• Attitude or eccentricity ratio. It is the ratio of the eccentricity to the radial clearance.

• The coefficient of friction of a bearing is a function of three variables

where, N = speed of journal
l = length of bearing

• The factor ZN/p is termed as bearing characteristic number and is dimension less number.

The minimum amount of friction occurs at A and at this point the value of ZN/p is known as bearing modulus, which is denoted by K.

(when Z is in Kg/m-s and p is in N/mm2) k = factor to correct end leakage.  = 0.002 for l/d = 0.75 to 0.8

• Sommerfeld Number. It is dimensionless parameter, Sommerfled number

= 14.3 × 106 (for design purpose) Z ® kg/m-s, P ® N/mm2 

• Heat Generated in Journal Bearing. The heat generated due to the fluid friction and friction of the parts having relative motion.
  H_g = µ.W.V (N-m/S or J/S or watts) where,
  µ = Coefficient of friction
  W = load on the bearing in
  N, = p (l × d) V = Rubbing velocity in m/s
  
  N = Speed of the Journal in r.p.m. 
   
• Heat dissipated by the bearing H_d = C.A (t_b – t_a) J/S or W
  where, C = Heat dissipation coefficient in W/m2/°C
  A = Projected area of the bearing
  t_b = temperature of the bearing surface
  t_a = temperature of the surrounding air
   
• The temperature of bearing (t_b) is approximately midway between the temperature of the oil film (t_o) and the temperature eof outside air (t_a), So

• For well designed bearing the temperature of the oil film should not be more than 60°C otherwise the viscosity of the oil decreases rapidly and the operation of the bearing is found to suffer. 
• In case the temperature of the oil film is higher, then the bearing may be cooled by circulating water through coils built in the bearing. 
• The mass of oil to remove the heat generated at the bearing may be obtained by equating the heat generated to the heat taken away by the oil.
  Heat = m.S.t watts where,
  m = mass of oil in kg/s
  s = specific heat of oil
  t = Difference between outlet and inlet temperature of the oil.

• Bushed Bearing 
  • A bushed bearing is an improvement of solid bearing in which a bush of brass of gun metal is provided. The outside of the bush is a driving fit in the hole of the casting whereas the inside is a running fit for the shaft. When the bush get worn out, it can be easily replaced.
     
• Thrust Bearings 
  • A thrust bearing is used to guide or support the shaft which is subjected to a load along the axis of the shaft. Such type of bearings are mainly used in turbines and propeller shafts. These are two types In foot step or pivot bearing, the loaded shaft is vertical and the end of the shaft rests within the bearing. Incase of collar bearing, the shaft continues through the bearing. The shaft may be vertical or horizontal with single collar or many collars.

• Collar Bearings 
  • The shaft may be vertical or horizontal with single collar or many collars. In designing collar bearings, it is assumed that the pressure is uniformly distributed over the bearing surface. The bearing pressure for a single collar and water cooled multicollared bearings may be taken same as for foot step bearings.

• Rolling Contact Bearing 
  • In this type of bearings, the contact between the bearing surfaces is rolling instead of sliding as in sliding contact bearing. The ordinary sliding bearing start from rest with practically metal to metal contact and has a high coefficient of friction. It is an advantage of a rolling contact bearing over a sliding bearing that it has a low starting friction. Due to this local friction offered by rolling contact bearings, these are called antifriction bearings. 
  • The ball and roller bearings consists of an inner race which is mounted on the shaft or journal and an outer race which is carried by the housing or casing. A number of balls or rollers are used and these are held at proper distances by retainers so that they do not touch each other. The retainers are thin strips and is usually in two parts which are assembled after the balls have been properly spaced. The ball bearings are used for light loads and the roller bearings are used for heavier loads.
     
• Types of Radial Ball Bearings 
  • Single row deep grooves bearing. During assembly of these bearings, the races are offset and the maximum number of balls are placed between the races. The deep groove ball bearings are used due to their high load carrying capacity and suitability for high running speed. The load carrying capacity of a ball bearing is related to the size and number of balls.

• Filling notch bearings. These bearings have notches in the inner and outer races which permit more balls to be inserted than in a deep groove balls bearings. This type of bearing contains larger number of balls than a corresponding unnotched one therefore it has a larger bearing load capacity.

• Angular contact bearings. These bearings have one side of the outer race cut away to permit the insertion of more balls than in a deep groove bearing but without having a notch cut into both races. This permits the bearing to carryng a relatively large axial load in one direction while also carrying a relatively large radial load. The angular contact bearings are used in pairs so that thrust load may be carried in either direction.

• Double row bearing. These bearings may be made with radial or angular contact between the balls and races. The double row bearings is appreciably narrower than two single row bearings. The load carrying capacity of such bearings is slightly less than twice that of a single row bearing.

• Self-aligning bearing. These bearings permit shaft deflections within 2 – 3 degrees.
  It may be noted that normal clearaces in a ball bearing are to small to accommodate any appreciable misalignment of the shaft relative to the housing. The internally selfalighing ball bearing is inter changeable with other ball bearings.

• Thrust Ball Bearings. The thrust ball bearings are used for carrying thrust loads exclusively and at speeds below 2000 r.p.m. At high speeds, centrifugal force causes the balls to be forced out of the races. Therefore at high speeds, it is recommended that angular contact ball bearings should be used in place of thrust ball bearings.

• Types of Roller Bearings 
  • Cylindrical roller bearings. These bearings have short rollers guided in a cage.
    These bearings are relatively rigid against radial motion and have the lowest coefficient of friction of any form of heavy duty rolling contact bearings. Such type of bearings are used in high speed service.

• Spherical roller bearings. These bearings are self-aligning bearings. These bearings can normally tolerate angular misalignment in the order of
  
  and when used with a double row of rollers, these can carry thrust loads in either direction.

• Needle roler bearings. These bearings are relatively slender and completely fill the space so that neither a cage nor a ratainer is needed. These bearings are used when heavy loads are to be carried with an oscillatory motion.

• Tapered roller bearing. The rollers and race ways of these bearings are truncated cones whose elements interest at a common point. Such type of bearing can carry both radial and thrust loads.

• Load Life Relationship 
  • To relationship between the dynamic load carrying capacity, the equivalent dynamic load and bearing life is

where, L — bearing life (million revolutions)
C = Dynamic load capacity in Newton
P = equivalent dynamic load
p = 3 for ball bearing p = 10/3 for roller bearing
This relationship between life in million revolutions and life in working hours is

where, L_h = bearing life (in hours) n = speed of rotation (rpm) 

• The radius of friction circle is g sin f where,
  r = radius of journal
  f = friction angle