MD 2: LEC 3 - SLIDING CONTACT BEARINGS TERMS

is a machine element which supports, guides or controls
a moving machine element (known as journal).

A bearing

Classification of bearings depending upon the direction of load to be supported

(a) Radial bearings
(b) Thrust bearings

• the load acts perpendicular to the direction of
  motion of the moving element

(a) Radial bearings

• the load acts along the axis of rotation.

(b) Thrust bearings

Classification of bearings depending upon the nature of contact

(a) Sliding contact bearings (plain bearings)
(b) Rolling contact bearings

Types of sliding contact bearings

Slipper or guide bearings
Journal or sleeve bearings

sliding contact bearings in which the
sliding action is guided in a straight line and carrying radial loads

Slipper or guide bearings

sliding contact bearings in which the
sliding action is along the circumference of a circle or an arc of a
circle and carrying radial loads

Journal or sleeve bearings

When the angle of contact of the bearing with the journal is 360°
then the bearing is called a (blank)

full journal bearing

When the angle of contact of the bearing with the journal is 120°, the
bearing is said to be (blank)

partial journal bearing

The full and partial journal bearings may be called as (blank) because the diameter of the journal is less than that of bearing

clearance bearings

When a partial journal bearing has no clearance i.e. the diameters of
the journal and bearing are equal, then it is called a (blank)

fitted bearing

In (blank), there is a thick film of lubricant
between the journal and the bearing

hydrodynamic lubricated bearings

The bearing material should have high
(blank) to withstand this maximum pressure so as to
prevent extrusion or other permanent deformation of the bearing.

Compressive strength.

The bearing material should have sufficient
(blank) so that it can withstand repeated loads without
developing surface fatigue cracks.

Fatigue strength.

is the ability of the bearing material to
accommodate shaft deflections and bearing inaccuracies by plastic
deformation (or creep) without excessive wear and heating.

Comformability

is the ability of bearing material to accommodate
(or embed) small particles of dust, grit etc., without scoring the material
of the journal.

Embeddability

Many high capacity bearings are made by bonding one
or more thin layers of a bearing material to a high strength steel shell.

Bondability

The bearing material should not corrode
away under the action of lubricating oil.

Corrosion resistance

The bearing material should be of high
(blank) so as to permit the rapid removal of the heat
generated by friction.

Thermal conductivity

The bearing material should be of low
coefficient of (blank), so that when the bearing operates
over a wide range of temperature, there is no undue change in the
clearance.

Thermal expansion

The tin base and lead base babbit are widely used as a
bearing material. The composition of babbit metals can be tin or lead based

Babbit metal

Tin 90% ; Copper 4.5% ; Antimony 5% ; Lead 0.5%

Tin base babbits

Lead 84% ; Tin 6% ; Antimony 9.5% ; Copper 0.5%

Lead base babbits

The (blank) (alloys of copper, tin and zinc) are generally used in
the form of machined bushes pressed into the shell. The most common used are gun metal and phosphorus bronze

Bronze

(Copper 88% ; Tin 10% ; Zinc 2%)

Gun metal

(Copper 80% ; Tin 10% ; Lead 9% ; Phosphorus 1%)

Phosphor bronze

The (blank) bearings are usually used with steel journals.

Cast iron

The (blank) bearings are mostly used in aircraft
engines where the fatigue strength is the most important consideration.

Silver and Silver Lead

The various (blank) are made of
carbon-graphite, rubber, wood and plastics.

Non-metallic bearings

are used in bearings to reduce friction between the
rubbing surfaces and to carry away the heat generated by friction. It also protects the bearing against corrosion.

The lubricants

used in bearings are mineral oils and synthetic oils.

1. Liquid: The liquid lubricants

A grease is a (blank) having higher viscosity than oil.

2. Semi-liquid

The (blank) lubricants are useful in reducing friction where oil films cannot be maintained because of pressures or temperatures. They should be softer than materials being lubricated.

3. Solid

is the most common of the solid lubricants either alone or mixed with oil or grease

A graphite

It is the measure of degree of fluidity of a liquid. It is a
physical property by virtue of which an oil is able to form, retain and
offer resistance to shearing a buffer film-under heat and pressure.

1. Viscosity.

It is a joint property of the lubricant and the bearing
surfaces in contact. It is a measure of the lubricating qualities under
boundary conditions where base metal to metal is prevented only by
absorbed film.

2. Oiliness.

This property has no relation to lubricating value but is
useful in changing the kinematic viscosity to absolute viscosity.

3. Density.

= absolute viscosity in kgm/s

μ

= kinematic viscosity in m2/s

v

= density of lube oil in kg/m3

p

The term (blank) is used to denote the
degree of variation of viscosity with temperature.

4. Viscosity index.

It is the lowest temperature at which an oil gives off
sufficient vapour to support a momentary flash without actually setting
fire to the oil when a flame is brought within 6 mm at the surface of the
oil.

5. Flash point.

It is the temperature at which an oil gives off sufficient
vapour to burn it continuously when ignited.

6. Fire point.

It is the temperature at which an oil
will cease to flow when cooled.

7. Pour point or freezing point.

= center of the journal

O

= center of the bearing

O'

= diameter of the bearing

D

= diameter of the journal

d

= length of the bearing

l

It is the difference between the diameters
of the bearing and the journal.

1. Diametral clearance.

It is the difference between the radii of the
bearing and the journal.

2. Radial clearance.

It is the ratio of the diametral clearance
to the diameter of the journal.

3. Diametral clearance ratio.

It is the radial distance between the centre of the
bearing and the displaced centre of the journal.

4. Eccentricity (e).

• It is the minimum distance
  between bearing and journal, under complete lubrication condition.

5. Minimum oil film thickness (h0)

It is the ratio of the eccentricity to the
radial clearance.

6. Attitude or eccentricity ratio.

If ratio of length to diameter of the journal
(i.e. l/d) is less than 1, it is short bearing. If l/d is greater than 1, it is
long bearing. If l/d is 1, it is square bearing.

7. Short and long bearing.

= bearing pressure on the projected bearing area in MPA

p

= load on the bearing

W

= factor to correct for end leakage

k

= coefficient of friction for journal bearings

f

= heat generated in a journal bearing in W

Qg

= rubbing velocity in m/s

v

= speed of the journal in rpm

N

= heat dissipated by the bearing in W

Qd

= heat dissipation coefficient in W/m2-C

C

= projected area of the bearing in mm2 or m2

Ab

= temperature of the bearing surface in C

tb

= temperature of surrounding air in C

ta

= temperature of oil film or operating temperature in C

to

are employed where slow speed and heavy pressure exist and where oil drip from the bearing is undesirable.

grease