Bearing Technologies: An Overview

Transcription

1 Bearing Technologies: An Overview Dr. H. Hirani Assistant Professor, Mechanical Engineering INDIAN INSTITUTE OF TECHNOLOGY BOMBAY I.I.T. Bombay 1

2 I.I.T. Bombay Computer Hard disk with read/write head Tribo-Pair Real area vs. Apparent area. Low contact area stressstate > elastic/fracture point of materials. In such a condition, RELATIVE MOTION between surfaces causes excessive wear. 2

3 Speed > 25,000 rpm 700 k rpm Pressure < 0.05 N/mm 2 Temperature < 100 degree C Magnetic bearing. Magnetic levitation. Problem? Magnetic materials Negative Stiffness i F 2 h It is a tribological solution but does not belong to fluid film lubrication 2 3

4 No restoring force. Instability causes further instability Requires feed back loop control system. High speed and very low load Speed > 25,000 rpm 700 k rpm Pressure < 0.05 N/mm 2 Temperature range [-200 to 2000 degree C]. Aerodynamic lubrication Any gas Problems? Increase in friction with speed Sophisticated manufacturing Requires other means to separate surface during start/stop operations. Negligible damping Instability 4

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6 Moderate load, moderate speed Pressure 3.0 N/mm 2 Speed < 20,000 rpm Temperature < 200 C Problems? Start/stop h U min W High friction at relatively high velocity Chances of instability at relatively high speed F = ηu h A 6

7 Friction or Coefficient of friction F ηulrπ 2 + ε CεW = + 2 C 1 ε 1+ ε 2R φ For journal bearing Eccentricity ratio 0.5, 0.6, 0.7, Friction force is: 36.5 N, 39N, 43 N, 52N, 74 N Load capacity 4.5kN, 8k N, 12.2kN, 23.5kN, 57kN Coeff. of friction.008,.0048,.0035,.0022, Variable load, moderate speed 7

8 Squeeze Films If one surface approaches another Very thin films difficulty squeeze out Under fluctuating load, it is probable that squeezing out takes more time, the load will reverse before the separating film breaks down and surfaces actually touch. 8

9 Under fluctuating load, it is probable that squeezing out takes more time, the load will reverse before the separating film breaks down and surfaces actually touch. Large load, Moderate speed High fluid pressure (maximum ranging between 0.5 to 3 GPa) Elasto-Hydrodynamic Lubrication (EHL) As the name suggests this lubrication mechanism utilizes: (1)elastic deformation, and (2) hydrodynamics. Soft surfaces?? 9

10 Soft EHL Elastic deformation + Hydrodynamic Hard EHL Elastic deformation + Hydrodynamic + increase in viscosity hmin W hmin U Simplest way to analyze EHL: Assume film thickness, Pressure solution using hydrodynamic Evaluate elastic deformation caused by pressure Modify film thickness and iterate. Iteration continues until modified film thickness distribution matches with new film thickness distribution. Large load, low speed Hydrostatic lubrication Mixed lubrication Boundary lubrication Hydrostatic lubrication separates two surfaces by an external pressure source. Suitable for highly controlled precision works. Ex. Large telescopes, radar tracking units, machine tools and gyroscopes 10

11 Hydrostatic As pressure is generated and supplied by external sources, it is one of the expensive approaches to separate two surfaces. If a load greater than design load is applied the tribo-surface will not be able separated. To reduce load-film thickness sensitivity feed back control system is used, which further increases the cost of overall system. Hybrid? Hydrodynamic + Hydrostatic Fig: Mixed lubrication Fig. Boundary lubrication 11

12 Qualitative or Quantitative info Dimensionless film parameter Λ ( Specific film thickness) Λ = h min 2 2 R rms, a + Rrms, b Classification of four important lubricant regimes Boundary lubrication, Λ<1 Mixed lubrication, 1<Λ<3 Hydrodynamic lubrication, Λ>5 Elastohydrodynamic, 3<Λ<5 12

13 Physical or chemical properties of lubricant In HL and EHL, viscosity determines: friction loss, load carrying capacity, film thickness, lubricant flow rate Viscosity Physical property-resistance to flow. Due to internal friction and molecular phenomena Kinematic viscosity -- centistokes (1cSt = 1 mm 2 /s) Dynamic viscosity centipoises (1cP = Pa.s).!"#$% & ' "#$ % " $ & '# & ("#$) Note: 1 Centistoke = 10-6 m 2 /s! 13

14 Viscosity-Temperature relation Increase or decrease with temp. " Variation with Temperature More viscous oil is more susceptible to change in viscosity with temp. Walther s equation: Form the basis of ASTM viscosity temperature chart log log( η / ρ + 0.6) = constant Vogel s equation: Most accurate; very useful in engineering calculations η = ke b /( t+ θ ) k gives inherent viscosity. - clogt b has units of temp. b increases with increase in viscosity. 14

15 Viscosity Index Measure of lubricant s consistency of viscosity with temperature change. VI= Pennsylvanian oil~vi=100 gulf coast oil ~ VI=0 L U *100 L H High VI indicates little change in viscosity with temp. # * 15

16 As viscosity is a very influencing parameter in designing tribo-pair based on the fluid film lubrication, it is important to keep this parameter within certain limits. Use of multi-grade oil is a way to reduce sensitivity of viscositytemperature. Therefore most oils on shelf today are MULTIGRADE oils, such as 10W30 or 20W50. The two numbers in oil grade indicate two separate grades: one grade at 0 F and another a higher grade at 210 F. 10W30 indicates 2100 cp at 0 F & viscosity of SAE30 at 210 F. Lower the first number better is the performance in extremely cold conditions. Higher the second number the oil will protect better at higher temperatures. These oils are made by adding polymers in mineral oils to enhance viscosity indices (about 150). At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up, the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. Zip/Unzip 16

17 Non-Newtonian Behavior Multigrade oil exhibits a non-newtonian behavior Fig. Shear thinning effect of multi-grade oils µ = µ 1 K K + µ 2γ + µ γ 1 Importance?? + (,-. /---0/1,2. /340/)5 6(-7 ) 8 k for oil A=1500 Pa, and k for oil B=20000 Pa. 9 ' '': ;5 '2---%/ 17

18 Table: Effect of Shear stability + ( * ( ) Bearing technology??? Design A company X, decided to design aircirculators for paint shops. Length of two meters and diameter of sixty centimeters was designed for rotor of air-circulator. Company X wanted to design suitable bearings that consume minimum power, and sustain rotor load. On studying the air-circulator we found rotor length could be reduced from 200 cm to 140 cm by relocating the drive-motor. Reduction in length of rotor itself solved the problem. 18

19 A company Y increased speed of diesel engine without considering its effect of lubrication system. As a result crankshaft and its six bearings failed in no time. A structural design of hydrodynamic bearing subjected to dynamic load, such as engine load, results in larger bearing dimensions compared to dimensions of bearing subjected to static load of same magnitude. In reality it happens in reverse. Bearing dimension under engine load is smaller than dimension of bearing designed under static load. Taylor [1993] pointed out the following examples of camfollower design, where lack of bearing knowledge caused early failure: Cam-nose radius of 8 mm vs mm: As per common structural design, cam with larger nose radius, that reduces Hertzian stress, was designed. However, better results were shown by smaller cam-nose radius. In actuality, reduction in cam-nose radius increases the relative velocity and provides thicker film that operates satisfactorily whereas large nose radius reduces relative velocity, provides thinner film and fails prematurely. Direct acting overhead cam. Usage of direct acting cam mechanism is favored as it reduces the number of linkages. However due to unavailability of suitable lubricant, there were a number of cam failures. Now with help of tribological knowledge failures are decreased. 19