Design of Sliding Contact Bearings Dr. Chandan Sharma Department - PowerPoint PPT Presentation

Design of Sliding Contact Bearings Dr. Chandan Sharma Department of Mechanical Engineering Engineering College Ajmer

Contents • Basic modes of lubrication – Thick and Thin film lubrication • Common bearing materials • Desirable properties of a good bearing material • Bearing design – Selection of parameters • Properties of lubricating oils • Bearing failures – Causes and Remedies • Comparison of Sliding and Rolling Contact Bearings • Design procedure of hydrodynamic bearing

Basic modes of lubrication The objectives of lubrication are as follows:  To reduce friction  To reduce or prevent wear  To carry away heat generated due to friction  To protect the journal and bearing from corrosion The lubricants can be :  Liquid lubricant like mineral or vegetable oils  Semi-solid lubricants like grease  Solid lubricants like graphite or Molybdenum disulphide

Basic modes of lubrication There are two basic modes of lubrication:  Thick film lubrication  Thin film lubrication Further thick film lubrication can be of two types:  Hydrodynamic lubrication  Hydrostatic lubrication

Hydrodynamic lubrication  Also called as self-acting bearings  Used in bearings mounted on engines and centrifugal pumps

Hydrodynamic journal bearings These bearings can be of two types:  Full journal bearing  Partial bearing

Full journal bearing versus partial bearings The advantages of partial bearings compared to full journal bearing are as follows:  Partial bearings are simple in construction  It is easy to supply lubricating oil to Partial bearing  The frictional losses in partial bearing are less hence temperature rise is low But Partial bearings can take load in only one radial direction  Clearance bearings (diameter of bearing > diameter of journal)  Fitted bearings (diameter of bearing = diameter of journal)

Hydrostatic lubrication  Also called as externally pressurized bearings  Used in vertical turbo-generators, centrifuges and ball mills

Hydrodynamic bearings versus Hydrostatic bearings Hydrodynamic bearings:  Simple in construction  Easy to maintain  Lower in initial as well as maintenance cost Hydrostatic bearings:  High load carrying capacity even at low speeds  No starting friction  No rubbing action at any operating speed or load

Thin film lubrication • Also called as boundary lubrication • There is partial metal to metal contact • Found in door hinges and machine tool slides

Newton’s law of viscosity  The constant of proportionality μ is called as absolute viscosity (N-sec/m 2 or MPa-sec). U U U   1 2 h h h  Popular unit is Poise (Dyne-sec/cm 2 ) 1 2  Viscosity in centipoise (cp) denoted by z.       P U U        or P A   z  A   h   h    9 10

Petroff’s equation It is used to determine coefficient of friction in journal bearings. It is based on following assumptions:  Shaft is concentric with the bearing  The bearing is subjected to light load

Petroff’s equation      r n     2 s f (2 )      c  p    Petroff’s equation indicates that there are two important dimensionless parameters namely (r/c) and ( μ n s /p).  They govern the coefficient of friction and other frictional properties like frictional torque, frictional power loss and temperature rise in the bearing.

Mckee’s investigation • In region BC, there is partial metal to metal contact (thin or boundary lubrication) • In region CD, there is relatively thick film of lubricant and hydrodynamic lubrication takes place • Coefficient of friction is minimum at C. Bearing characteristic no. corresponding to minimum coefficient of friction is called as Bearing modulus (K)

Mckee’s investigation • In thin film or unstable region, variations are compounding. • In thick film or stable region, variations are self-correcting • In order to avoid seizure, operating value of bearing characteristic number should be at least 5 to 6 times bearing modulus. • If the bearing is subjected to fluctuating of impact loads, it should be 15 times bearing modulus.

Reynold’s equation The theory of HD lubrication is based on differential equation derived by Reynolds. This equation is based on following assumptions: • Lubricant follows Newton’s law of viscosity • The lubricant is incompressible • The viscosity of lubricant is constant • It is assumed that the film is so thin that the pressure is constant across the film thickness • The shaft and the bearing are rigid • There is continuous supply of lubricant

Reynold’s equation • There is no exact analytical solution for this equation for bearings with finite length. • Theoretically exact solution can be obtained if the bearing is assumed to be either infinitely long or very short • Approximate solutions using numerical methods are available for bearings with finite length

Raymondi and Boyd method • Reynolds equation was solved on computers using iterative technique • This method predicts that performance of the bearing can be expressed in terms of dimensionless parameters  c R - r e   Eccentrici ty ratio c    Since R e r h 0   R - r e h 0      c e h or c c h 0 0 h h      1 0 or 1 - 0 c c

Dimensionless parameters Length to diameter ratio L/D Radial clearance ratio r/c Coefficient of friction variable (CFV) (r/c)f Flow variable (FV) Q/rcnL Sommerfeld number (S) (r/c) 2 ( μ n s /p) Minimum film thickness variable h 0 /c Eccentricity ratio ( ε ) e/c Pressure ratio p max /p

Dimensionless performance parameters L/D ε h 0 /c S (r/c)f Q/rcnL Q s /Q p/p max φ 0.1 0.9 1.33 79.5 26.4 3.37 0.150 0.540 1.0 0.2 0.8 0.631 74.02 12.8 3.59 0.280 0.529

Desirable properties of a good bearing material • Should not stick or weld to the journal surface in case of metal to metal contact • Should have high compressive strength • Should have high fatigue strength • Should have high ‘Conformability’ (ability to adapt shape of journal) • Should have high ‘Embeddability’ (accommodating dirt particles in oil) • Should have high ‘Bondability’ (ability to bond with high strength steel shell) • Should have sufficient corrosion resistance • Should have high thermal conductivity and low thermal expansion • Should have low coefficient of friction • Should be of reasonable cost and easily available

Common bearing materials • Babbits or white alloys (Sn 89.3% + Sb 8.9% + Cu 1.8%)  Tend to loose their strength quite rapidly at higher temperature  Have relatively low fatigue strength  Have excellent ‘Conformability’ and ‘Embeddability’ • Copper lead alloys (more hardness and fatigue strength, used in heavy duty applications) • Bronzes (2 nd most preferred after Babbits, excellent casting and machining properties) • Aluminium alloys • Silver (most costly) • Cast Iron • Teflon (low coefficient of friction and requires no external lubricant) • Rubber (used in marine applications)

Bearing design – Selection of parameters • Length to diameter ratio (L/D) • Unit Bearing Pressure • Start up load • Radial clearance • Minimum oil film thickness • Maximum oil film temperature

Length to diameter ratio (L/D) • Length to diameter ratio affects the performance of the bearing • A long bearing has more load carrying capacity but are more susceptible to metal to metal contact • A short bearing has greater side flow • L/D > 1 (long bearing) • L/D < 1 (short bearing) • L/D = 1 (square bearing)

Unit Bearing Pressure • It is load per unit of projected area of the bearing in running condition • It depends on bearing materials, operating temperature, nature and frequency of load and service conditions. • The values of unit bearing pressure based on past experience is provided in the design data book

Start up load • It is static load when the shaft is stationary • Mainly consist of dead weight of shaft and its accessories • The unit bearing pressure for the starting conditions should not exceed 2 MPa

Radial clearance (c) • Should be small to provide the necessary velocity gradient • This requires: • Costly finishing operations • Rigid mounting of the bearing assembly • Clean lubricating oil without any foreign particle • Practical value of c is 0.001 r Material Radial clearance Babbits 0.001 r to 0.00167 r Copper lead 0.001 r to 0.01 r Aluminium alloys 0.002 r to 0.0025 r

Minimum oil film thickness (h 0 ) • The surface finish of the journal and the bearing is governed by the value of minimum oil film thickness selected by the designer and vice versa. • There is a lower limit for the minimum oil film thickness below which metal to metal contact occurs and hydrodynamic film breaks • The lower limit is given by h 0 = 0.0002 r