lubrication & journal bearings [compatibility mode]
TRANSCRIPT
I/C: KALLURI VINAYAK
Introduction
• Objective of lubrication is to reduce friction,
wear and heating of machine parts which move
relative to each other.
• Lubricant is exactly that substance which does
the above when inserted between moving
surfaces.surfaces.
• Lubrication is needed everywhere, for example,
sleeve bearings, antifriction bearing, cam and
follower, gear teeth, piston in cylinder, crank
shaft and connecting rod bearings.
Introduction
• In a sleeve bearing, a shaft or journal, rotates
within a sleeve or bushing, and the relative
motion is sliding.
• Frequently used in high load, high speed or high
precision applications where ordinary ball
bearings have short life or high noise and vibration.bearings have short life or high noise and vibration.
• In applications requiring low load bearing
capacity, nylon bearings requiring no lubrication,
a powder metallurgy bearing with lubricant built-
in, a bronze bearing with ring oiling, solid
lubricant film or grease lubrication may be
satisfactory
CLASSIFICATION
• Bearings are classified in two ways.
1. Based on type of load carried a. Radial bearings
b. Thrust bearings or axial bearings
c. Radial – thrust bearings
2. Based on lubrication mechanism 2. Based on lubrication mechanism a. Hydrodynamic lubricated bearings
b. Hydrostatic lubricated bearings
c. Elastohydrodynamic lubricated bearings
d. Boundary lubricated bearings
e. Solid film lubricated bearings
Radial bearings
Thrust bearings / axial bearings / Collar bearings
Single collar thrust bearing Multiple collar thrust bearing
Radial – thrust bearings
Types of Lubrication
1. Hydrodynamic
2. Hydrostatic
3. Elastohydrodynamic
4. Boundary
5. Solid film5. Solid film
Hydrodynamic (full-film) Lubrication• Metal-to-metal contact is prevented by a thick
film of lubricant present in between the bearingsurfaces.
• The film pressure is created by the moving surfaceitself by pulling the lubricant into a wedge-shapedzone at a velocity sufficiently high to create thepressure necessary to separate the surfaces againstpressure necessary to separate the surfaces againstthe load on the bearing
• Stability can be explained by the laws of fluidmechanics.
• Lubricant is introduced into the load-bearing area
at a pressure high enough to separate the surfaces
with a relatively thick film of lubricant.
• Lubrication does not require motion of one surface
relative to another.
• Considered in designing where the velocities are
Hydrostatic Lubrication
• Considered in designing where the velocities are
small or the frictional resistance is to be an
absolute minimum.
Elastohydrodynamic Lubrication
• Lubricant is introduced between surfaces that are in
rolling contact, such as mating gears, rolling
bearings and cams etc.
• The mathematical explanation requires the
Hertzian theory of contact stress and fluid
mechanics.mechanics.
Boundary lubrication.
• Insufficient surface area, drop in velocity, lessening of
lubricant quantity, increase in bearing load, or increase
in lubricant temperature lead to a decrease in viscosity—
any one of these—may prevent the buildup of a film
thick enough for full-film/ hydrodynamic lubrication.
• Bearings operating in above situations are called boundary
lubricated bearings.
• Mixed hydrodynamic- and boundary-type lubrication
occurs first, and as the surfaces move closer together, the
boundary-type lubrication becomes predominant.
Solid-film Lubrication
• Necessary when operation is to be at extremely
high temperatures because ordinary minerals oils
degrade;
• Graphite and Molybdenum disulphide are often
used
• Composite bearing materials are being researched• Composite bearing materials are being researched
because liquid lubricants also proved to be
environmentally non-sustainable
Design Considerations
• Values either given or are under the control of
the designer are
1. The viscosity µ
2. The load per unit of projected bearing area, P
3. The speed N
4. The bearing dimensions r, c, β, and l
• The dependent variables (designer cannot control • The dependent variables (designer cannot control
these except indirectly by changing one or more
of the above group) are
1. The coefficient of friction f
2. The temperature rise T
3. The volume flow rate of oil Q
4. The minimum film thickness h0
PETROFF’S EQUATION:
�Imagine the film as composed of a
series of horizontal layers and the
force F causing these layers to
deform or slide on one another just
like a deck of cards
�Intermediate layers have velocities
that depend upon their distances y
from the stationary surface
Contd0.
Stable Lubrication
�McKee brothers explained
the difference between
boundary (unstable) and
hydrodynamic (stable)
lubrication in an actual test
of friction by reference to
Fig.
�Region to the right of line B A defines stable lubrication�Region to the right of line B A defines stable lubrication
because variations are self-correcting.
�Region to the left of line B A represents unstable
lubrication.
� Point C represents what is probably the beginning of
metal-to-metal contact as µN/P becomes smaller.
Design Constraint: 6107.1 −
×≥P
Nµ
Thick Film Lubrication
An eccentricity ratio,
c
e=ε
ε−=⇒−= 100
c
hech
Significant Angular Speed
It has been discovered that the angular speed N that is
significant to hydrodynamic film bearing performance is
fbj NNNN 2−+=
The Relations of the Variables• Albert A. Raimondi and John Boyd, of
Westinghouse Research Laboratories, used aniteration technique to solve Reynolds’hydrodynamic equation
• charts are used to define the variables for length-diameter (l/d) ratios of 1:4, 1:2, and 1 and for beta angles of 60 to 360◦.angles of 60 to 360◦.
• The charts appearing in text book are for full journal bearings (β = 360◦) only.
• For other categories, referA. A. Raimondi and John Boyd, “A Solution for the Finite Journal
Bearing and Its Application to Analysis and Design, Parts I, II, and III,” Trans. ASLE, vol. 1, no. 1, in Lubrication Science and Technology, Pergamon, New York, 1958, pp. 159–209.
Fig. 12.12Viscosity Charts: I
viscosity used in the
analysis must
correspond to T .correspond to Tav.
Viscosity Charts: II
viscosity used in the
analysis must
correspond to T .
Fig. 12.13
correspond to Tav.
The remaining charts from Raimondi and Boyd relate several bearing
design variables to the Sommerfeld number. These variables are
– Minimum film thickness
– Coefficient of friction
– Lubricant flow
– Film pressure
Raimondi and Boyd Charts:
Film–pressure distribution notation
W = bearing load (N)
N = speed (rps)
h0 = minimum film-thickness (mm)
e = eccentricity (mm)
P = film pressure (MPa)
Pmax= max fill pressure (MPa)
Φ= position of the minimum film thickness
θpo = terminating position of the lubricant film
θpmax = the position of maximum film pressure. Fig. 12.15
Film–pressure distribution notation
Chart for minimum film-thickness variable and eccentricity ratio.
Fig. 12.16
c
e=εh0 = minimum film-thickness (mm)
e = eccentricity (mm), c= radial clearance (mm)Eccentricity ratio,
Chart for the position of the minimum film thickness h0.
Fig. 12.17
Chart for coefficient-of-friction variable;
Fig. 12.18
Chart for flow variable
Fig. 12.19
Chart for determining the ratio of side flow to total flow.
Fig. 12.20
Chart for determining the maximum film pressure.
Fig. 12.21
Chart for the terminating position of the lubricant film
and the position of maximum film pressure.
Fig. 12.22
Problem:
A full journal bearing has a journal diameter of 40 mm,
with a unilateral tolerance of −0.025 mm. The bushing
bore has a diameter of 40.08 mm and a unilateral
tolerance of 0.075 mm. The bearing is 40 mm long. The
journal load is 2.2 kN and it runs at a speed of 1800
rev/min. Using an average viscosity of 25 mPa.s, find
the minimum film thickness, eccentricity, position ofthe minimum film thickness, eccentricity, position of
minimum film thickness, coefficient of friction, the
torque to overcome the friction, the power loss to
friction, total volumetric flow rate of lubricant, side flow
rate of lubricant, the maximum film pressure, and the
location of maximum and terminating pressures, for the
minimum clearance assembly.
Temperature Rise Dimensionless Variable
Fig. 12.24
Trumpler’s Design Criteria
• A throat of at least 200 µ is necessary to pass the debris
particles from the ground surface. To achieve this,
• To avoid the degrading of lubricant properties at high
temperatures, therefore
• In starting under load there is metal to metal contact,
mmdh 00004.000508.00 +≥
CT 0
max 121≤
• In starting under load there is metal to metal contact,
abrasion, and the generation of wear particles between
journal and bushing, which, over time, can change the
geometry of the bushing,
• Design load factor is used for different load applications
except in starting load calculation.
MPald
W st 068.2≤
2, ≥dnfactordesign
PROBLEM
• A full journal bearing has a shaft diameter of 80.00
mm with a unilateral tolerance of −0.01 mm. The
l/d ratio is unity. The bushing has a bore diameter
of 80.08 mm with a unilateral tolerance of 0.03
mm. The SAE 30 oil supply is in an axial-groove
sump with a steady-state temperature of 60◦C. The
radial load is 3000 N. Estimate the average film radial load is 3000 N. Estimate the average film
temperature, the minimum film thickness, the heat
loss rate, and the lubricant side-flow rate for the
minimum clearance assembly, if the journal speed
is 8 rev/s.
PROBLEM (12.11):
• A full journal bearing has a shaft diameter of 80.00
mm with a unilateral tolerance of −0.01 mm. The
l/d ratio is unity. The bushing has a bore diameter
of 80.08 mm with a unilateral tolerance of 0.03
mm. The SAE 30 oil supply is in an axial-groove
sump with a steady-state temperature of 60◦C. The
radial load is 3000 N. The rise in film temperature radial load is 3000 N. The rise in film temperature
is 100C and estimate the minimum film thickness,
the heat loss rate, and the lubricant side-flow rate
for the minimum clearance assembly, if the journal
speed is 8 rev/s.
(Refer viscosity chart II)