Bearings

Finals

bearings

a machine element which support another moving machine element (known as journal).

Bearings

It permits a relative motion between the contact surfaces of the members, while carrying the load.

lubricant

In order to reduce frictional resistance and wear and in some cases to carry away the heat generated, a layer of fluid (known as [ ] ) may be provided

lubricant

used to separate the journal and bearing is usually a mineral oil refined from petroleum, but vegetable oils, silicon oils, greases etc., may be used.

1. direction of load to be supported

a. radial bearings

b. Thrust bearings

2. nature of contact

a. Sliding contact bearings

b. Rolling contact bearings

Classifications of Bearings

1. Depending upon the

2. Depending upon the

radial bearing

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

thrust bearings

the load acts along the axis of rotation.

sliding contact bearings

the sliding takes place along the surfaces of contact between the moving element and the fixed element.

• are also known as plain bearings.
  

rolling contact bearings

steel balls or rollers, are interposed between the moving and fixed elements. The balls offer rolling friction at two points for each ball or roller.

full journal bearing
partial journal bearing.
fitted journal bearing

Types of Sliding Contact Bearings

slipper or guide bearings

The sliding contact bearings in which the sliding action is guided in a straight line and carrying radial loads
usually found in cross-head of steam engines.

journal or sleeve bearings

The 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

full journal bearing

When the angle of contact of the bearing with the journal is 360°

full journal bearing

This type of bearing is commonly used in industrial machinery to accommodate bearing loads in any radial direction.

partial journal bearing

When the angle of contact of the bearing with the journal is 120°

partial journal bearing

This type of bearing has less friction than full journal bearing, but it can be used only where the load is always in one direction.

partial journal bearing

The most common application of the partial journal bearings is found in rail road car axles

clearance bearings

The full and partial journal bearings may be called as [ ] because the diameter of the journal is less than that of bearing.

fitted bearing

When a partial journal bearing has no clearance i.e. the diameters of the journal and bearing are equal,

Compressive strength

The maximum bearing pressure is considerably greater than the average pressure obtained by dividing the load to the projected area.

high compressive strength

Therefore the bearing material should have [ ] to withstand this maximum pressure so as to prevent extrusion or other permanent deformation of the bearing.

Fatigue strength

The bearing material should have sufficient [ ] so that it can withstand repeated loads without developing surface fatigue cracks. It is of major importance in aircraft and automotive engines.

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 heating.

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.

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. [ ]

is an important consideration in selecting bearing material.

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.

Thermal conductivity.

The bearing material should be of [ ] so as to permit the rapid removal of the heat generated by

friction.

Thermal expansion

. The bearing material should be of low coefficient of

[ ], so that when the bearing operates over a wide

range of temperature, there is no undue change in the clearance.

Lubricants

used in bearings to reduce friction between the rubbing surfaces and to carry away the heat generated by friction.

Lubricants

It also protects the bearing against corrosion.

liquid lubricants

• used in bearings are mineral oils and synthetic oils.
  

• mineral oils are most commonly used because of their cheapness and stability.
  

• usually preferred where they may be retained.
  

semi-liquid lubricant

• grease - higher viscosity than oils, employed where slow speed and heavy pressure exist and where oil drip from the bearing is undesirable
  
  

solid lubricants

useful in reducing friction where oil films cannot bemaintained because of pressures or temperatures.

softer than materials being lubricated.

graphite

is the most common of the solid

lubricants either alone or mixed with oil or grease.

14.3 ×10 6

Sommerfeld Number S

critical pressure or the minimum operating pressure

The pressure at which the oil film breaks down so that metal to metal contact begins, is known as

Sommerfeld number

is also a dimensionless parameter used extensively in the design of journal bearings.

Series No.

Numbering system “Y”

Number of 5 mms in the bearing bore

Numbering System: “XX” times how many?

1

1 reyn = (lb-sec)/in2

1

1 poise = (dyne-sec)/cm2

Petroff’s equation for frictional torque

In bearings

Condition of a hydrodynamic lubrication

In bearings

heat

generated in a bearing is due to the fluid friction and friction of the parts having relative motion.

thermal equilibrium

After the [ ] has been reached, heat will be dissipated at the outer surface of the bearing at the same rate at which it is generated in the oil film.

500

K1 of unhardened steel

700, 1500

K1 of hardened carbon steel

1000

K1 of alloy steel

2000

K1 of hardened alloy steel on grooved races

7000

K2 for hardened carbon steel

10000

K2 for hardened alloy steel

Hydrostatic bearing

is one which the lube oil is suppliec under pressure

Oil film pressure

In hydrodynamic bearings, [ ] is generated only by the roatation of the journal

Full bearing

bearing surface that completely surrounds the journal is called

full bearing

a type of bearing that totally encloses the shaft.

babbitt

a metal that assits lubrication or a lubricant itself

babbitt

a composition of antimony, lead, or tin alloy

low carbon steel

unsuitable as a bearing

offset bearing

also called eccentrically loaded bearing

0.0010

as a rule of thumb in journal bearing design, the clearance ratio should be:

kinematic viscosity

the absolute viscosity of the fluid divided by its density expressed in the same terms of units

graphite

the most known lubricants being utilized in watever category of load and speed are oil, air, grease and dry lubricants like.

to lighten the load

ZN/p

Ball or roller bearing

needle bearing

heavy load

Tapered roller bearing