2. Design of Bearings
The main function of a rotating shaft is to transmit power from one end of
the line to the other.
It needs a good supports to ensure stability and frictionless rotation.
The support for the shaft is known as bearing.
• A bearing is a machine element which support another moving machine
element (known as journal). It permits a relative motion between the
contact surfaces of the members, while carrying the load.
• Due to the relative motion between the contact surfaces, a certain amount
of power is wasted in overcoming frictional resistance and if the rubbing
surfaces are in direct contact, there will be rapid wear.
• To reduce frictional resistance and wear and in some cases to carry away
the heat generated, a layer of fluid (known as lubricant) may be provided.
• The lubricant used to separate the journal and bearing is usually a mineral
oil refined from petroleum, but vegetable oils, silicon oils, greases etc., may
4. CLASSIFICATION OF THE SLIDING CONTACT BEARING:
Collar thrust bearing
Journal bearing – in this the bearing pressure is exerted at right angles to the axis of the axis of
the shaft. The portion of the shaft lying within the bearing in known as journal. shaft are
generally made of mild steel.
Foot step or pivot bearing – in this bearing the bearing pressure is excreted parallel to the
shaft whose axis is vertical. Note that in this case the end of the shaft rests within the bearing.
Thrust bearing – in this bearing supporting pressure is parallel to the axis of the shaft having
end thrust. Thrust bearing are used in bevel mountings, propeller drives, turbines, etc. note
here the shaft ,unlike foot-strep bearing passes through and beyond the bearing.
Thrust bearings also known as “collar bearing”.
8. 4. The sliding contact bearings, according to the thickness of
layer of the lubricant
1. Thick film bearings. The thick film bearings are those in which
the working surfaces are
completely separated from each other by the lubricant. Such type
of bearings are also called as hydrodynamic lubricated bearings.
2. Thin film bearings. The thin film bearings are those in which,
although lubricant is present, the working surfaces partially
contact each other atleast part of the time. Such type of bearings
are also called boundary lubricated bearings.
3. Zero film bearings. The zero film bearings are those which
operate without any lubricant present.
4. Hydrostatic or externally pressurized lubricated bearings. The
hydrostatic bearings are those which can support steady loads
without any relative motion between the journal and the bearing.
This is achieved by forcing externally pressurized lubricant
between the members.
11. 6. Properties of Sliding Contact Bearing Materials
1. Compressive strength: The maximum bearing pressure is
considerably greater than the average pressure obtained by
dividing the load to the projected area. Therefore the bearing
material should have high compressive strength to withstand
this maximum pressure so as to prevent extrusion or other
permanent deformation of the bearing.
2. Fatigue strength: The bearing material should have sufficient
fatigue strength so that it can withstand repeated loads
without developing surface fatigue cracks. It is of major
importance in aircraft and automotive engines.
3. Comformability: It is the ability of the bearing material to
accommodate shaft deflections and bearing inaccuracies by
plastic deformation (or creep) without excessive wear and
4. Embeddability. It is the ability of bearing material to accommodate
(or embed) small particles of dust, grit etc., without scoring the
material of the journal.
5. Bondability. Many high capacity bearings are made by bonding one
or more thin layers of a bearing material to a high strength steel
shell. Thus, the strength of the bond i.e. bondability is an important
consideration in selecting bearing material.
6. Corrosion resistance. The bearing material should not corrode away
under the action of lubricating oil. This property is of particular importance
in internal combustion engines where the same oil is used to lubricate the
cylinder walls and bearings. In the cylinder, the lubricating oil comes into
contact with hot cylinder walls and may oxidise and collect carbon deposits
from the walls.
7. Thermal conductivity. The bearing material should be of high thermal
conductivity so as to permit the rapid removal of the heat generated by
8. Thermal expansion. The bearing material should be of low coefficient of
thermal expansion, so that when the bearing operates over a wide range of
temperature, there is no undue change in the clearance.
15. 7. Lubricants
• The lubricants 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.
• All lubricants are classified into the following three groups :
1. Liquid: Minerals oil & Synthetic oil
2. Semi-liquid : Grease (heavy viscosity), use where high pressure and
low velocity exist.
3. Solid: Graphite's are useful in reducing friction where oil films
cannot be maintained because of pressures or temperatures.
17. Journal bearing
Simple journal or solid bearing
It is simply a block of cast iron with a hole for the shaft providing running fit. An oil hole
is drilled at the top for lubrication.
In this the bush of soft material like brass or gun metal is provided and the
body or main block is made of cast iron. Bush is hollow cylindrical piece which is
fitted in a housing to accommodate the mating part. When the bush gets worn out it
can be easily replaced.
20. The outside of the bush is a driving fit (interference fit) in the hole of the casting where
as the inside is a running fit for the shaft.
The bearing material used may be white metal (Babbit – Tin/Cu/Lead/antimony) ,
copper alloy (brass, gunmetal) or aluminium alloy.
Solid bushes are entirely made of bearing material and find the general application. In
lined bush as the bearing material is applied as a lining to a backing material .
Applications: turbines, large diesel engines etc.
21. Direct lined housings
In this type of the housing is lined directly by means of metallurgical
Low-melting point white metal is used as a lining on the cast iron housing
Bush - solid or lined
23. Plummer block or Pedestal bearing
• It is a split type of bearing. This type of bearing is used for higher speeds, heavy loads and
• The component of the bearing:
• Cast iron pedestal or block with a sole
• Brass or gun-metal or phosphorus-bronze “Brasses”, bushes or steps made in two halves.
• Cast iron cap.
• Two mild steel bolts and nuts.
Care is taken that the brasses do not move axially nor are allowed to rotate. For
preventing this rotation , usually a snug at the bottom fitting inside a recess at the bottom of the
pedestal is provided.
This bearing facilitates the placements and removal of the of the shaft from the bearing.
Unlike the solid bearing which are to be inserted end-wise and hence are kept near the ends of the
shaft, these can be placed anywhere. This bearing ensures a perfect adjustment for wear in the
brasses by screwing the cap.
26. Prevention of rotation of brasses
The steps are made octagonal on the outside and they are made to fit inside a
A snug is cast on the lower brass top which fits a corresponding hole in the casting.
The oil hole is drilled through the sung.
Snug are provided at the side, and the corresponding recesses left in the casting
The steps on the lower brass are made rectangular on the outside and they are made
to fit inside a corresponding hole.