Rolling Contact
Bearings
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.
Rolling Contact Bearings
► that class of bearing in which the main load is transferred through elements in
rolling contact rather than in sliding contact
► Also Called anti-friction bearing
Nomenclature of a ball bearing
Rolling Contact Bearings
Radial bearings
► Force that is applied towards the direction of radius of bearing.
► Perpendicular to shaft
Radial bearings
► Radial bearings accommodate loads that are predominantly perpendicular to the
shaft.
► Some radial bearings can support only pure radial loads, while most can
additionally accommodate some axial loads in one direction and, in some cases,
both directions
Thrust load
► Load is perpendicular of the radius of bearing
► Are in direction of shaft
Thrust load
► Thrust bearings accommodate loads that act predominantly along the axis of the
shaft.
► Depending on their design, thrust bearings may support pure axial loads in one or
both directions and some can additionally accommodate radial loads (combined
loads)
Types of Rolling bearings
Types of Rolling bearings
Ball Bearing shield and Seals
► Shields and seals are used to keep the dirt out and to keep lubrication in
► Shields are steel stampings that cover the rolling element but avoid contact with
them.
Bearing Part Number
► The first digit or letter or combination of letters identifies the bearing type and
eventually a basic variant.
► The following two digits identify the ISO dimension series. The first digit
indicates the width or height series (dimensions B, T or H). The second digit
identifies the diameter series (dimension D). •
► The last two digits of the basic designation identify the size code of the bearing
bore. The size code multiplied by 5 gives the bore diameter (d) in mm.
► Bearings with a bore diameter of 10, 12, 15 or 17 mm have the following size
code identifications:
► 00 = 10 mm
► 01 = 12 mm
► 02 = 15 mm
► 03 = 17 mm
► 04=20 mm
► 05=25 mm
Bearing Life
► When the ball or roller of rolling-contact bearings rolls, contact stresses occur on
the inner ring, the rolling element, and on the outer ring
Common life measures are
► Number of revolutions of the inner ring (outer ring stationary) until the first
tangible evidence of fatigue.
► Number of hours of use at a standard angular speed until the first tangible
evidence of fatigue
► The life measure of an individual bearing is defined as the total number of
revolutions (or hours at a constant speed) of bearing operation until the failure
criterion is developed
Rating Life L10
► The number of revolutions (or hours at a constant speed) that 90 percent of a
group of bearings will achieve or exceed before the failure criterion develops
Median Life
Median life is the 50th percentile life of a group of bearings. The term average life
has been used as a synonym for median life
Bearing Load Life at Rated Reliability
► When nominally identical groups are tested to the life-failure criterion at different
loads, the data are plotted on a graph as depicted in Fig. 11–4 using a log-log
transformation. To establish a single point, load F1 and the rating life of group
one(L10)1 are the coordinates that are logarithmically transformed
Bearing Load Life at Rated Reliability
► The rating life is a term sanctioned by the ABMA and used by most manufacturers.
The rating life of a group of nominally identical ball or roller bearings is defined
as the number of revolutions (or hours at a constant speed) that 90 percent of a
group of bearings will achieve or exceed before the failure criterion develops.
► A catalog load rating is defined as the radial load that causes 10 percent of a
group of bearings to fail at the bearing manufacturer’s rating life C10.
Bearing Load Life at Rated Reliability
► In selecting a bearing for a given application, it is necessary to relate the desired load and
life requirements to the published catalog load rating corresponding to the catalog rating
life. From Eq. (11–1) we can write
where the subscripts 1 and 2 can refer to any set of load and life conditions.
Letting F1 and L1 correlate with the catalog load rating and rating life, and F2 and
L2 correlate with desired load and life for the application, we can express Eq.
(11–2) as
Bearing Load Life at Rated Reliability
► It is sometimes convenient to express the life in hours at a given speed. Accordingly, any
life L in revolutions can be expressed as
► Where l is in hours, n is in rev/min, and 60 min/h is the appropriate conversion
factor. Incorporating Eq. (b) into Eq. (a),
Bearing Load Life at Rated Reliability
► Solving Eq. (c) for FR, and noting that it is simply an alternate notation for the
catalog load rating C10, we obtain an expression for a catalog load rating as a
function of the desired load, desired life, and catalog rating life.
Bearing Survival: Reliability versus Life
► Mean dimensionless life
The median dimensionless life
standard deviation
► The coefficient of variation of the dimensionless life
The rating life is L10 = LR =L RnR60 = 106 revolutions. From Eq. (11–3),
Relating Load, Life, and Reliability
For the bearing application specifications given in the table for the assigned problem, determine
the Basic Load Rating for a ball bearing with which to enter a bearing catalog.
Radial load 650lbf. Design life=5 yrs, 40 h/week, 400 rev/min for reliability of 95%.
