Transcript MEL 417 Lubrication Minor I

MEL 417 Lubrication
Minor I
Date: Tuesday, 08/02/2011
Time: 4 – 5 pm.
Venue: Bl. V, 419
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.
2
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.
3
Extreme Pressure Lubricants
 Lubricant containing chlorine forming CuCl or CuCl2 on surface,
providing protection against adhesive wear. .. 800C. Iron Chloride649°C
 Sulfur and phosphorus are common additives for iron and steel. Films
have relatively high melting point
Iron sulphide- 1,170°C
• Major difficulty - carcinogenic nature,
environmental pollutant.
– Removal of sulfur compound present in small
amounts in petroleum based lubricants
requires elaborate and expensive refining
techniques.
4
Hydrodynamic and sqeeze film lubrication
SQUEEZE FILM
LUBRICATION
HYDRODYNAMIC
LUBRICATION
Load
Load
PRESSURE DISTRIBUTION
10-50 mm
Relative movement of
bearing surfaces
Relative movement
of bearing surfaces
Elastohydrodynamic lubrication
RIGID BEARINGS
Pressure
distribution
Load
Pressure
distribution
Load
Squeeze
film
Rolling
Thin fluid film, high pressures
DEFORMABLE BEARINGS (Elastohydrodynamic)
Pressure
distribution
Load
Pressure
distribution
Load
Squeeze
film
Rolling
Larger area, low pressures
Weeping lubrication in bone joints
When the cartilage tissue on both sides
come in contact, fluid is squeezed out into
the joint space
cartilage
cartilage
Space filled with a
layer of fluid
7
Lubrication Mechanisms
hmin
• Boundary lubrication,

2
2
<1
Rrms

R
,a
rms,b
• Hydrodynamic
• Dimensionless film
lubrication, >5
parameter  (“Specific film
• Mixed lubrication,
thickness”)
1<<3
• Elastohydrodynamic,
3<<5
8
Streibeck curve
Left of a- boundary lubrication
Coefficient of friction m
Between a and c- mixed film lubrication
Mixed film
friction/lubrication
Right of c- fluid film lubrication
b- Minimum coefficient of friction
Fluid film
friction/lubrication
a
b
Boundary
friction/lubrication
c
Viscosity  velocity
Load
Lubrication system
•
•
TG- Temperature gauge
PG- Pressure gauge
PG
TG
ENGINE
Shaft
Bearings
PG
Cooler
TG
PG
Storage tank
Pump
Filter
10
10
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 flow
Fluid layers
Resistance to flow
Solid surface
Laminar fluid flow between parallel plates
Velocity of top plate = u
Shear force F
y
Time (t)
t=0
Velocity
profile
Direction of motion
of top plate
Top layer of fluid moves with
same velocity as the plate
t = dt
Velocity of bottom plate = 0
A is area of the plate
Newton’s law: shear stress  shear strain
F du
Shear stress =   
A dy
Fluid element subject to shear stress
du.dt
Velocity = u + du
dy
Fluid
element
Velocity = u
t=0
d
Fluid
element
t = dt
For slow viscous flow, tan(d) ~ d
du.dt
tan( d) 
dy
or strain rate
d du

dt dy
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
Therefore shear force F 
• A is the surface area of fluid layer
• du is the velocity
• dy is the film thickness
A.du
dy
Coefficient of dynamic viscosity
• Therefore
Shear stress  =
or
F du

A dy
du
h
dy
h is the coefficient of dynamic viscosity or absolute
viscosity
Coeff. of dynamic viscosity (units)
F dy
h .
A du
In F.P.S system
In C.G.S system
In S.I. units
lb . sec
h
 reyn
2
in .
(Named after
Reynolds)
dyne . sec
h
 poise
2
cm.
(Named after
Poiseuille)
h  pressure . sec .  pascal. sec  pa.s
Coefficient of kinematic viscosity
• The ratio of absolute viscosity to density
Coefficient of kinematic viscosity
h


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/Rm
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)]
P1
P1 and P2 obtained from
experiment, P obtained from chart
P
P1
T1
T2
Log T
Viscosity index (number indicating change in
viscosity with temperature)
L- viscosity of low-grade oil at 40 oC
(Dean and
L Davis method)
V- viscosity of test, low grade, and high
grade oils at 100 oC (same and known)
Viscosity 
VI is basically calculated using a
scale of 0 to 100. The value can
come to above 100
H- viscosity of high-grade oil at 40 oC
U- viscosity of test oil at 40 oC
U
H
V
High grade: Less viscosity change with
temperature change
Low grade: More viscosity change with
temperature change
40 oC
Temperature T
100 oC
Viscosity index calculation
L U
L-U
V
V
.100 
.100
Viscosity index (VI) =
L H
L-H
V V
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 oC, the VI > 100
• In such cases, the ASTM equation for viscosity
calculation is
10 - 1
VI 
 100
0.00715
N
log H - log U
N
log U
Pressure effect on viscosity
• Viscosity increases with pressure as
h  h0 e
p
ho is the dynamic viscosity at atmospheric pressure
(Ns/m2)
h is the dynamic viscosity at pressure p (Ns/m2)
 is the pressure viscosity coefficient (m2/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
Shear rate
Newtonian fluid: shear stress-shear strain
relationship
du
h
dy
du
dy
•Linear dependence
•Slope is 1/h

Shear stress 
Non-Newtonian Fluids
• Do not behave like Newtonian fluids
• E.g. Slurries, greases, jams, toothpaste
• Deformation and flow behavior varies
Bingham plastics
Shear rate
• Do not flow unless shear stress exceeds a minimum value
• Behave like Newtonian fluids beyond this value
• E.g. lubricating greases, pastes, waxy crude
du
dy
o
Shear stress 
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
u
  h  0
y
u
0
y
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
Shear rate
Pseudoplastic: Shear stress-shear rate curve
du
dy
Shear stress 
Governing equation:
(A, B, are constants, B < 1)
 u 
  A  
 y 
B-1
 u 
. 
 y 
Dilatant fluids
Governing equation
same as for
pseudoplastics
(A, B, are constants, B >
1)
Shear rate
• E.g. pastes, inks, butter
• Increase in viscosity with rate of shear (shear thickening)
du
dy
 u 
  A  
 y 
Shear stress 
B-1
 u 
. 
 y 
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
Viscosity
(h)
recovery
du
dy
constant
du
dy
Time (t)
removed