1 1. Definition.
Bearing – a machine element which support another moving machine element. It permits a relative motion between the contact surfaces of the members, while carrying the load.
2. Classification of Bearings
2.1 Depending upon the direction of load to be supported
2.1.1 Radial bearings – where the load acts perpendicular to the direction of motion of the moving element.
2.1.2 Thrust bearings - where the load acts along the axis of rotation.
2.2 Depending upon the nature of contact.
2.2.1 Sliding contact bearings – where the sliding takes place along the surfaces of contact between the moving element and the fixed element.
2.2.2 Rolling contact bearings – where the 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.
3. Common Types of Sliding Contact Bearings 3.1 Journal, or sleeve, bearing.
Journal bearings – are used to furnish lateral support to rotating shafts. The journal is the part of the shaft that runs in the bushing or sleeve.
3.2 Full journal bearing.
Full journal bearing –where the angle of contact of the bushing with the journal is 360o. 3.3 Partial journal bearing
Partial journal bearing – where the angle of contact of bushing the journal is 180o or less, 120o being a common value.
3.4 Clearance bearing
Clearance – refers to the thickness of the space allowed for the lubricant that separates the parts having relative motion.
Clearance bearing – is one which the radius of the journal is less than the radius of the bushing.
3.5 Fitted bearing
Fitted bearing – is one in which the radius of the journal and bushing are equal.
3.6 Thrust bearing
Thrust bearing – a bearing designed to support an axial load.
3.6.1 Step bearing – in which the end of the shaft is in contact with a bearing surface.
3.6.2 Collar bearing – in which a collar is attached to or formed integral with the shaft.
3.6.3 Pivoted-segment bearing – was developed in order to provide a converging film in a thrust bearing.
4. Properties of Lubricants
4.1 Viscosity - 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.
The viscosity of the lubricant is measured by Saybolt universal viscometer. It determines the time required for a standard volume of oil at a certain temperature to flow under a certain head through a tube of standard diameter and length. The time so determined in seconds is the Saybolt universal viscosity. In order to convert Saybolt universal viscosity in seconds to absolute viscosity (centipoise), the following formula may be used:
R. BEARINGS - LECTURE
Z = is the absolute viscosity at temperature t, centipoises S = Saybolt Universal viscosity, sec
SG = specific gravity at temperature t Viscosity conversion:
6.9 x 106 centipoises = 1 reyn = 1 lb-sec/in2.
4.2 Specific gravity (SG) – this property has no relation to lubricating value but is useful in changing the kinematic viscosity to absolute viscosity. Mathematically
Absolute viscosity in centipoises = SG x kinematic viscosity in centistokes
(
60)
SGt = specific gravity at temperature t SG60 = specific gravity at temperature 60 F t = temperature of oil
4.3 Viscosity index – is used to denote the degree of variation of viscosity with temperature.
4.4 Flash point – is the lowest temperature at which an oil gives off sufficient vapor 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.
4.5 Fire point – is the temperature at which an oil gives off sufficient vapor to burn it continuously when ignited.
4.6 Pour point or freezing point – is the temperature at which oil will cease to flow when cooled.
5. Terms used in Hydrodynamic Journal Bearing Figure 11.6 (Faires)
5.1 Diametral clearance, cd - is the difference between the diameters of the bearing and the journal.
5.2 Radial clearance, cr – is the difference between the radii of the bearing and the journal.
2
d r
c =c
5.3 Diametral clearance ratio – is the ratio of the diametral clearance to the diameter of the journal.
r
5.4 Eccentricity, e – is the radial distance between the centre of the bearing and the displaced centre (O′) of the bearing under load.
o
5.5 Minimum oil film thickness - is the minimum distance between the bearing and the journal, under complete lubrication condition. It is denoted by ho and occurs at the line of centres as shown in Fig. 26.7. Its value may be assumed as cd / 4.
R. BEARINGS - LECTURE
3
5.6 Attitude or eccentricity ratio, ε. It is the ratio of the eccentricity to the radial clearance.
If the ratio of the length to the diameter of the journal (i.e. L / d) is less than 1, then the bearing is said to be short bearing. On the other hand, if L / d is greater than 1, then the bearing is known as long bearing.
6. Dimensional Analysis
( )
SCoefficient of friction
( )
Scd = diametral clearance, in.
µ = absolute viscosity, reyns or lb-sec/in2. ns = angular speed of the journal, rps r = journal radius, in.
D = journal diameter, in.
p = unit load or bearing pressure, psi = W / (LD) = W (2rL).
L = bearing length, in.
W = bearing load, lb.
S = Sommerfield number or bearing characteristic number, dimensionless.
2
R. BEARINGS - LECTURE
4 8. Types of Rolling Contact Bearings
1. Ball Bearing 2. Roller Bearing
The ball and roller bearings consist of an inner race which is mounted on the shaft or journal and an outer race which is carried by the housing or casing.
9. Types of Radial Ball Bearings
Following are the various types of radial ball bearings:
1. Deep-groove ball bearing – is one in which the balls are assembled by the eccentric displacement of the inner ring.
2. Filling-slot type ball bearing – has slots or notches that permit the assembly of more balls, giving a bearing of larger radial load capacity.
3. Self-aligning ball bearing – compensate for angular misalignment that arise from shaft or foundation deflection or errors in mounting.
4. Angular-contact bearings – where the line through the areas carrying the load makes an angle with the plane of the face of the bearing.
5. Double-row ball bearings – are similar to single-row ball bearings, except that each rings has two grooves.
6. Cylindrical roller bearings – where the contact is a line instead of a point as in ball bearings, which results in a greater area carrying the load and hence, for a particular size, in a larger radial capacity.
7. Self-aligning roller bearings – with spherical rollers running in a double-grooved inner ring, have curved outer rings that look like the outer ring of a self-aligning ball bearing.
8. Needle bearings – cylindrical bearings made with relatively long bearings.
9. Tapered roller bearings – where the rolling elements are frustums of a cone.
10. Static Load Capacity
2 b b s
s CN D
F = Where,
Fs = basic static load rating, lb.
Nb = number of balls or rollers.
Db = ball diameter. in
Cs = a proportionality constant dependent on the type of bearing and the materials.
11. Dynamic Load Capacity.
(
N)
D lbCN
Fd= b23 rcosα 0.7 1.8 Where,
Fd = the dynamic capacity of the bearing.
Nr = number of rows of balles
C = a constant that varies with the type of ball bearing α = locates the plane of the resultant force.
R. BEARINGS - LECTURE
5 12. Equivalent Dynamic Load.
[
F C F Q]
F C F
x r z
x r e
≤
=
Or
[
F C F Q]
F C F C F
x r z
z t x r e
>
+
= 560.
Where,
Fx – is the radial load Fz – is the thrust load
Cr – is a rotation factor (Cr = 1 for inner race rotating, Cr = 1.2 for outer race rotating).
Ct – is a thrust factor.
Q - is from Table 12.2 of Faires.
13. Basic Dynamic Load Rating.
( )
er B F
F = 10 13
Where B10 mr is the desired number of revolution before 10% failures have occurred.