Unit-5 Sliding Contact Bearings
A bearing supports a loaded rotating
member (e.g. a shaft) and transmits the load
from the rotating member to a stationery
member such as frame or housing.
Bearings permit relative motion of members
in one or two directions. The relative motion
is one of Sliding in Sliding-Contact
Bearings. Rolling contact bearings have
point or line contact whereas slidingcontact bearing have surface contact
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Sliding Contact Bearings
Classification
There are in general three ways of classification:
- Based on direction of load carried
- Based on type of lubrication
- Based on lubrication mechanism
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Sliding Contact Bearings
Classification based on the direction of
load:
– Radial bearings support loads which are
perpendicular to the axis of the shaft.
– Thrust bearings support axial loads.
– Combined Radial and thrust Bearings support
both radial and axial loads.
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Sliding Contact Bearings
Radial Bearing (Fig. 5.1)
Radial Load is transferred through lubricant film between
journal and bearing
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Single Collar and Multiple Collar Thrust
Bearings (Fig. 5.2a and 5.2b )
Thrust Load is transferred through lubricant film between
thrust collar on rotor and thrust collar on housing
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Combined radial and thrust bearing (Fig. 5.3)
The bearing carries both radial and thrust load.
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Bearing Classification based on Type of
Lubrication (Fig. 5.4a, 5.4b, 5.4c)
The type of lubrications means the extent to which the
contacting surfaces are separated in a shaft-bearing
combination
The classification includes
- Thick film lubrication (a) : also called Hydrodynamic lubrication
- Thin film lubrication (b) : some metal to metal contact
- Boundary lubrication (c) : almost continuous surface contact
(a)
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(b)
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(c)
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Bearing Classification based on type of
lubrication (Fig. 5.5)
Type of lubrication Coefficient of friction VS Bearing Modulus
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Bearing Classification based on Lubrication
Mechanism
a.
b.
c.
d.
e.
Hydrodynamic lubrication
Hydrostatic lubrication
Elastohydrodynamic lubrication
Boundary lubrication
Solid film lubrication
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Lubrication Mechanism: Hydrodynamic
Lubrication
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Lubrication Mechanism: Hydrostatic Lubrication
(Fig. 5.7)
Hydrostatic lubricated bearings are supplied externally
pressurized lubricant (gas or liquid). They can work from
very low speeds to very high speeds
Hydrostatic lubrication
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Lubrication Mechanism : Elastohydrodynamic
Lubrication (Fig. 5.8)
The oil film thickness is very small because the contact
pressures are very high. To reduce wear and prevent metal
to metal contact, the surface hardness and finishes are
high. Examples of this kind of lubrication are gear teeth,
rolling element bearings and cams.
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Lubrication Mechanism
Boundary lubrication
When the speed of the bearing is inadequate, less
quantity of lubricant is delivered to the bearing. An
increase in the bearing load, or an increase in the
lubricant temperature results in drop in viscosity –
any one of these may prevent the formation of thick
film lubrication and establish continuous metal-tometal contact extensively. Often bearings operating
in such situations are called boundary lubricated
bearings.
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Lubrication Mechanism
Solid film lubrication
For extreme temperature operations, ordinary
mineral oils are not satisfactory. Solid film
lubricants such as graphite, molybdenum disulfide
or their combinations which withstand high
operating temperature are used. These types of
bearings are common in furnace applications, or
trunnion bearings of liquid metal handling systems,
hot drawing mills etc.
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Hydro -dynamically Lubricated Bearings
Among all the sliding bearings, hydro-dynamically
lubricated radial bearings (called sleeve or journal
bearings) and thrust bearings find wide
applications. Hence these bearings will be discussed
in more detail
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Important Milestones in the Development of Theory
and Design of Hydro-dynamically Lubricated bearings
1871. Newton Law of Viscosity
1883. Petroff’s Equation for Bearing Friction
Torque
1883, 1885. Tower’s Reports on Friction Experiments
1886. Reynolds Analytical Investigations
1932. McKee and McKee, Journal Bearing Friction
in the Regime of Thin Film Lubrication
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Newton’s Law of Viscous Friction
Viscosity is a very important physical property of a
lubricant in the design and operation of sliding bearings.
Viscosity is the internal friction that resists the motion in
fluids.
Consider an unloaded plate of area A m2 moving parallel
to a stationary horizontal surface with a velocity of U
m/s (Fig. 5.9). Further, the space between the plates is
filled with a fluid
Area A
Fig. 5.9
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Newton’s Law of Viscous Friction (Contd.)
Area A
Fig. 5.9
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Units of Viscosity
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Units of Viscosity (Contd.)
1cP = 1 mPa-s = 10-9 MPa-s = 0.145 μ reyn
(5.3)
The absolute viscosity of water at 200 C is 1.0020 cP that
is about 1 cP
The absolute viscosities of most lubricating oils fall in
the range of 5-400 cP
The absolute viscosity of liquids decreases with
increasing temperature whereas for gases, it increases
with increasing temperature
A plot of variation of absolute viscosity with temperature
for a number of typical lubricating oils is given in Fig.
5.10
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Fig. 5.10 Variation of Absolute Viscosity with
temperature of SAE graded oils
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Assumptions made in the derivation of viscous law:
- The adhesion between the oil and the plates is perfect
- There is no transverse force; and
- The dimensions of the plates are large enough to
neglect the edge effects.
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Kinematic Viscosity
The concept of viscosity or absolute viscosity is based on
viscous friction, the measurement of which is rather
difficult. For liquids, a simple alternative has been
devised in the form of kinematic viscosity which is
defined as (dynamic viscosity/mass density). Symbol ν (a
Greek alphabet pronounced as ‘nu’) is used for kinematic
viscosity.
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Kinematic Viscosity (Contd.)
The SI unit for kinematic viscosity is [m2/s]. The cgs unit
for kinematic viscosity is [cm2/s] and it is called stokes,
St, named after Gabriel Stokes. However, it is generally
expressed in centistokes (cSt):
1 St = 1 cm2 s-1 = 10-4 m2 s-1 and 1 cSt = 1 mm2 s-1 = 10-6
m2 s-1
(5.4)
Water at 200 C has a kinematic viscosity of about 1 cSt;;
and as stated earlier the absolute viscosity of water at 200
C is also about 1 cP.
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Measurement of Kinematic Viscosity
The kinematic viscosity is measured as time required for
a specified volume of liquid to pass through either a
capillary tube or a short-tube orifice of standard
dimensions. Thus there are two types of viscometers:
- Capillary viscometers, e.g. , Ostwald viscometer
- Efflux viscometers based on short tube orifice, for
example:
• Saybolt universal viscometer
• Redwood viscometer
• Engler viscometer
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Saybolt Universal Viscometer
Saybolt viscometer is an efflux type of viscometer and it
is widely used in USA for determining the kinematic
viscosity of oils. The viscometer is immersed in a
constant temperature bath. When the temperature
stabilizes, the time taken by a 60-ml sample to pass
through a 12.25 mm long, 1.76-mm diameter nozzle is
noted.
Fig. 5.11
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Experimental Determine of Visosity
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Experimental Determine of Visosity (Contd.)
For petroleum based lubricating oils, the density at 15.60
C is 0.89 and as such
ρ = 0.89 – 0.00063(T-15.6) (5.12)
Thus μ(in cP) at T0C = (0.22s – 180/s)(0.89 – 0.00063
(T-15.6))
(5.7)
where Saybolt seconds, s, has also been measured a T0C.
Dynamic Viscosities may be determined at two reference
temperature as above or under high shear conditions, for
example, the cone on plate or cylinder viscometer in
which the viscous shear stress is measured as friction
torque between two cylinders ( see next section)
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Variation Viscosity with Temperature
With the viscosity determined at two reference
temperatures, the viscosity for intermediate temperatures
is calculated using standard interpolation functions.
Variation of viscosity with temperature for several SAE
Viscosity Single Grade Oils is given in Fig. 5.10. The
variation for viscosity with temperature for SAE Multigrade Oils is given in Fig. 5.12.
Variation of viscosity with temperature for several ISO
VG Grade Oils is given in Fig. 5.13.
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Fig. 5.12 Viscosity-temperature curves for multiviscosity lubricants
derived from known viscosities at 40 and 100 deg C
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Fig. 5.13 Viscosity-temperature curves for ISO VG
graded oils
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Multi-grade Oils and Viscosity Index
SAE 10W30 is a multi-grade oil and SAE 30 is a monograde oil. The first umber (10W) refers to the viscosity
grade at low temperature (W from Winter), whereas the
second number (30) refers to viscosity grade at high
temperature.
In many cases the variation of viscosity with temperature
is expressed in terms of Viscosity Index (VI). It was
observed that crude oils from Pennsylvania had least
change in viscosity with temperature whereas Gulf Coast
Oils had the greatest change in viscosity with
temperature.
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Viscosity Index (Contd.)
Pennsylvania oil was
assigned a Viscosity
Index of 100 and the
Gulf oil’s VI = 0.
Let U be the VI of
the same oil is then
determined as
follows:
Fig. 5.141Graphical explanation of
viscosity index
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Viscosity Index (Contd.)
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Kinematic Viscosity of ISO VG Oils
The classification in 18 grades on the basis of kinematic
viscosity at 400 C and VI are given in ISO 3448.
Variation of kinematic viscosity with temperature of 14
out of 18 grades are given in Fig. 5.13.
The kinematic viscosity has an accuracy of +10%. For
example an ISO VG32 oil has the kinematic viscosity of
32 cSt + 10% at 400C.
The viscosity index of petroleum oils can be increased
by mixing appropriate additives.
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Pressure Effects on Viscosity
The viscosity of lubricating oils increases with pressure.
This effect is important in elastohydrodynamic
lubrication.
However,
the
lubricant
pressures
encountered in sliding bearings are not so high to
influence the viscosity significantly.
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Desirable Properties of a Lubricant
Properties of a good lubricant are:
1. It should give rise to low friction.
2. It should adhere to the surface and reduce the wear.
3. It should protect the system from corrosion.
4. It should have good cleaning effect on the surface.
5. It should carry away as much heat from the surface as
possible.
6. It should have thermal and oxidative stability.
7. It should have antifoaming ability.
8. It should be compatible with seal materials.
9. It should be inexpensive and available in plenty.
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