MEL 417 Lubrication Minor I Date: Tuesday, 08/02/2011 Time: 4 – 5 pm. Venue: Bl. V, 419

2 Extreme Pressure Boundary Lubrication Active chemicals, such as chlorine, sulfur, and phosphorus form inorganic film of low shear strength (chlorides, sulfides, phosphides). – EP additives react with sliding surfaces under severe conditions in contact zone to give compound with low shear strength, thus forming a lubricating film at precisely a location where it is needed.

3 Boundary vs. EP lubrication BL is restricted to those systems where there is thermodynamic reversibility. A small change in temperature or concentration, up or down, brought about a related change in film coverage. If lubricant reacts chemically with metal, then lubrication should properly be considered a type of extreme pressure lubrication, which is considered in next slide. – EP lubricants are inorganic molecules that provide good lubrication at elevated temperature & pressure – Reaction of E.P. additive does not occur rapidly at low temp.

4 Extreme Pressure Lubricants Major difficulty - carcinogenic nature, environmental pollutant. – Removal of sulfur compound present in small amounts in petroleum based lubricants requires elaborate and expensive refining techniques.  Lubricant containing chlorine forming CuCl or CuCl 2 on surface, providing protection against adhesive wear  C. Iron Chloride- 649°C  Sulfur and phosphorus are common additives for iron and steel. Films have relatively high melting point Iron sulphide- 1,170°C

Hydrodynamic and sqeeze film lubrication Load PRESSURE DISTRIBUTION Relative movement of bearing surfaces HYDRODYNAMIC LUBRICATION SQUEEZE FILM LUBRICATION  m

Elastohydrodynamic lubrication Pressure distribution Rolling Squeeze film Load Rolling Squeeze film Thin fluid film, high pressures RIGID BEARINGS Larger area, low pressures Pressure distribution DEFORMABLE BEARINGS (Elastohydrodynamic)

7 Weeping lubrication in bone joints When the cartilage tissue on both sides come in contact, fluid is squeezed out into the joint space cartilage Space filled with a layer of fluid

8 Dimensionless film parameter  (“Specific film thickness”) Boundary lubrication,  <1 Hydrodynamic lubrication,  >5 Mixed lubrication, 1<  <3 Elastohydrodynamic, 3<  <5 Lubrication Mechanisms

Streibeck curve abc Boundary friction/lubrication Fluid film friction/lubrication Mixed film friction/lubrication Coefficient of friction  Left of a- boundary lubrication Between a and c- mixed film lubrication Right of c- fluid film lubrication b- Minimum coefficient of friction

10 Lubrication system TG- Temperature gauge PG- Pressure gauge ENGINEShaft Cooler Storage tank Pump Filter Bearings PG TGPG TG

Viscosity

Ideal fluid (Pascalian) Frictionless Incompressible Experience only normal force and no drag force Does not adhere to a solid surface Relative tangential velocity with surface is known as slip

Real fluid Experience both tangential and normal force during motion Attaches to a solid boundary in contact (no slip) There is resistance to flow between molecules of the fluid and is known as “internal friction” Viscosity is a measure of the internal friction Also known as viscous fluids

Viscosity The property characterizing internal resistance to fluid flow Defines the frictional force between layers of the fluid Direction of flowResistance to flow Fluid layers Solid surface

Laminar fluid flow between parallel plates Direction of motion of top plate Top layer of fluid moves with same velocity as the plate t = 0Time (t)t = dt Velocity of top plate = u Velocity of bottom plate = 0 A is area of the plate Newton’s law: shear stress  shear strain Shear stress = y Shear force F Velocity profile

Fluid element subject to shear stress Velocity = u + du Velocity = u t = 0t = dt dy dd du.dt For slow viscous flow, tan(d  ) ~ d  or strain rate Fluid element

Newton’s hypothesis of viscosity Shear force is proportional to area Shear force is proportional to velocity Shear force is inversely proportional to film thickness A is the surface area of fluid layer du is the velocity dy is the film thickness Therefore shear force F 

Coefficient of dynamic viscosity Therefore Shear stress  =  is the coefficient of dynamic viscosity or absolute viscosity or

Coeff. of dynamic viscosity (units) In F.P.S system (Named after Reynolds) (Named after Poiseuille) In C.G.S system In S.I. units

Coefficient of kinematic viscosity The ratio of absolute viscosity to density Coefficient of kinematic viscosity Used when flow of fluid is influenced by gravity (as density influences weight) The units in C.G.S is the stoke or centistoke (1/100 stokes)

Effect of temperature on viscosity Viscosity decreases with rise in temperature Amount of decrease depends upon source of oil, refining and blending procedures and the presence of additives Temperature-viscosity relationship is important as viscosity determines the load bearing capacity Walther’s formula: log ( +C) = A/R m A and m are constants, R is the temperature in degree Rankine, C ranges from 0.6 to 0.8

ASTM (American Society for Testing Materials) viscosity equation Log.[log( + C)] = log(A) – m.log(T) A and m are constants is the kinematic viscosity T is the temperature in Kelvin C is a function of

ASTM chart for viscosity Log[log(  + C)] Log T T1T1 T2T2 P P1P1 P1P1 P 1 and P 2 obtained from experiment, P obtained from chart

Viscosity index (number indicating change in viscosity with temperature) Viscosity 40 o C L- viscosity of low-grade oil at 40 o C H- viscosity of high-grade oil at 40 o C U- viscosity of test oil at 40 o C V- viscosity of test, low grade, and high grade oils at 100 o C (same and known) L U H V 100 o C Temperature T High grade: Less viscosity change with temperature change Low grade: More viscosity change with temperature change VI is basically calculated using a scale of 0 to 100. The value can come to above 100 (Dean and Davis method)

Viscosity index calculation Viscosity index (VI) = L is the viscosity of low grade oil H is the viscosity of high grade oil U is the viscosity of test oil VI of high grade oil is taken as 100 VI of low grade oil is taken as 0 Therefore above formula gives VI of the test oil on a scale of 0 to 100

Viscosity index calculation (contd.) If the viscosity is lower than H at 40 o C, the VI > 100 In such cases, the ASTM equation for viscosity calculation is

Pressure effect on viscosity Viscosity increases with pressure as  o is the dynamic viscosity at atmospheric pressure (Ns/m 2 )  is the dynamic viscosity at pressure p (Ns/m 2 )  is the pressure viscosity coefficient (m 2 /N)

Newtonian fluid Fluids obeying Newton’s law of viscous flow Viscosity is independent of shear rate and shear stress Shear strain under action of shear stress linearly increases with time Strain is not recovered when the shear stress is withdrawn Linear stress-shear relationship is for a particular temperature and pressure under laminar flow

Newtonian fluid: shear stress-shear strain relationship  Linear dependence Slope is 1/  Shear stress  Shear rate

Non-Newtonian Fluids Do not behave like Newtonian fluids E.g. Slurries, greases, jams, toothpaste Deformation and flow behavior varies

Bingham plastics Do not flow unless shear stress exceeds a minimum value Behave like Newtonian fluids beyond this value E.g. lubricating greases, pastes, waxy crude oo Shear stress  Shear rate

Bingham plastics (contd.) Below a shear stress value (shown  o ) it behaves like an elastic solid (deformation is finite and disappears when shear stress is withdrawn) Below this value of shear stress, it behaves as Newtonian If shear stress is withdrawn, the deformation is reduced showing partly plastic behavior If  >  0 If  <  0

Pseudoplastic fluids E.g. dripless paint, paper pulp, cutting oils, polymeric solutions Viscosity decreases with increase in shear rate Due to shear thinning (orientation and formation of polymeric molecules) Due to rupture of ingredients

Pseudoplastic: Shear stress-shear rate curve Governing equation: (A, B, are constants, B < 1) Shear stress  Shear rate

Dilatant fluids E.g. pastes, inks, butter Increase in viscosity with rate of shear (shear thickening) Governing equation same as for pseudoplastics (A, B, are constants, B > 1) Shear stress  Shear rate

Boltzmann (thixotropic) fluids Time dependent behavior E.g. heavy oils, asphalts, greases, ice cream, chocolates Decrease in viscosity over time under the action of a constant shear stress When shearing is stopped, the thixotropic substance may recover its viscosity with time

Boltzman fluids Time (t) Viscosity (  ) constant removed recovery