Technical Specifications of Bearings

Tolerance of a Bearing
TOLERANCES You will find interesting to note that so as to get bearing 'Perfect'. The scientists have now succeeded in space in their sky-lab 1st mission (ie. in 0 gravity) by the help of Astronauts.

The term bearing tolerances refers to the very close dimensional limits of bore, outside diameter and width of a bearing.

Tolerances of boundary dimension are internationally standardized and are maintained in order to ensure accurate mounting of the bearing on the shaft and in the housing; these tolerances also ensure precise operations of the equipment in which the bearings are used.

One main factor must be noted that average dimension of any standard bearings are always on Minus side. This means that if you take the measurement of bore, outside diameter or width of any kind of bearings, then it should be measured from eight different points and the average of all the eight reading will always be on Minus side.

Radial Clearance of a Bearing
In order to freely rotate, a ball bearing must have a certain amount of internal freedom of movement (Internal Clearance, or the space between the raceway and ball). Without this internal clearance, the bearing can be difficult to rotate or may even freeze-up and be impossible to rotate. On the other hand, too much internal clearance will result in an unstable bearing that may generate excessive noise or allow the shaft to wobble. The internal clearance is measured in terms of the direction of the load (Radial Internal Clearance and Axial Internal Clearance).

In our country radial clearance of the bearings known as 'dug' or 'play' and certain class of people do not prefer to use the bearings having radial clearance. But for most of the application radial clearance is a must. This statement is supported by the following arguments. Infact, redial clearance in the bearing is an important factor.

Measuring Radial Internal Clearance
The example shows a radial ball bearing, so the radial internal clearance is measured. The bearing is grasped at one point on the inner ring and at another point on the outer ring, directly opposite (see large arrows). The bearing is held together to assure radial contact between the inner raceway, balls, and outer raceway. This allows measurement of the bearing’s internal clearance at a point on the opposite side of the bearing--180°-- from where the points of contact are being made. The small gap between the top ball and the raceway represents the bearing’s radial internal clearance.

There is a designation system for indicating the radial clearance of bearings. Following designation indicates the magnitudes and clearance.

Redial Internal Clearance of Deep groove ball bearing

Life of Bearing

Definitions

Life:
For an individual rolling bearing, the number of revolutions which one of the bearing rings (or washers) makes in relation to the other rings (or washers) under the prevailing working conditions before the first evidence of fatigue develops in the material of one of the rings (or washers) or rolling elements.

Reliability:
For a group of apparently identical rolling bearings, operating under the same conditions, the percentage of the group that is expected to attain or exceed a specified life.

Basis for Calculation
Bearing life is defined as the length of time, or the number of revolutions, until a fatigue spell of a specific size develops. This spell size, regardless of the size of the bearing, is defined by an area of 0.01 inch² (6 mm)². This life depends on many different factors such as loading, speed, lubrication, fitting, setting, operating temperature, contamination, maintenance, plus many other environmental factors. Due to all these factors, the life of an individual bearing is impossible to predict precisely. Also, bearings that may appear to be identical can exhibit considerable life scatter when tested under identical conditions. Remember also that statistically the life of multiple rows will always be less then the life of any given row in the system.

L₁₀ Life
L₁₀ life is the life that 90 percent of a group of apparently identical bearings will complete or exceed before the area of spalling reaches the defined 0.01 inch² (6 mm² size criterion. If handled, mounted, maintained, lubricated and used in the right way, the life of your tapered roller bearing will normally reach and even exceed the calculated L₁₀ life.

If a sample of apparently identical bearings is run under specific laboratory conditions, 90 percent of these bearings can be expected to exhibit lives greater than the rated life. Then, only 10 percent of the bearings tested would have lives less than this rated life.

Bearing Life Equation
As you will see it in the following, there is more than just one bearing life calculation method, but in all cases the bearing life equation is :

L₁₀ = (C / P)^10/3 × (B / n) × a

L₁₀ in hours

C = radial rating of the bearing in lbf or N

P = radial load or dynamic equivalent radial load applied on the bearing in lbf or N. The calculation of P depends on the method (ISO) with combined axial and radial loading

B = factor dependent on the method ; B = 1.5 × 10⁶ for the Timken method (3000 hours at 500 rev/min) and 10⁶ /60 for the ISO method

a = life adjustment factor ; a = 1, when environmental conditions are not considered ;

n = rotational speed in rev/min.

This can be illustrated as follows :

• Doubling load reduces life to one tenth. Reducing load by one half increases life by ten,

• Doubling speed reduces life by one half. Reducing speed by one half doubles life.

In fact, the different life calculation methods applied (ISO 281) differ by the selection of the parameters used.

Bearing Ratings
Depending on the life calculation method used, the bearing ratings have to be selected accordingly. The C_r rating, based on one million revolutions, is used for the ISO method.

However, a direct comparison between ratings of various manufacturers can be misleading due to differences in rating philosophy, material, manufacturing and design. In order to make a true geometrical comparison between the ratings of different bearing suppliers, only the rating defined following the ISO 281 equation should be used. However, by doing this, you do not take into account the different steel qualities from one supplier to another.

ISO 281 Dynamic Radial Load Rating C_r
This bearing rating equation is published by the International Organization for Standardization (ISO) and AFBMA. These ratings are not published by any bearing manufacturers. However, they can be obtained by contacting our company.

The basic dynamic load rating is function of :

C_r = b _m × f_c × (i × L_we × cos a)^7/9 × Z^3/4 × D_we^29/27

C_r = radial rating

b_m = material constant (ISO 281 latest issue specifies a factor of 1.1)

f_c = geometry dependent factor

i = number of bearing rows within the assembly

L_we = effective roller contact length

a = bearing half-included outer race angle

Z = number of rollers per bearing row

D_we = mean roller diameter

Manufacturing of Bearing

The Basic Bearing Numbering System
Usually, with all the bearings discussed so far, the last two digits of the base bearing number indicate the diameter (size) of the bearing’s bore in millimeters. The first four (4) must be memorized.

When the last two digits of the base bearing number are 04 or larger, simply multiply the double digit number by five (5) and you have the bore size in millimeters, i.e., 04 = 20mm, 10 = 50mm, and 24 = 120mm. ball bearings.

Angular Contact Ball Bearings
Self Aligning Ball Bearings
Ball & Roller Thrust Bearings

Ball Bearings

600	6400
6000	6700
6200	6800
6300	6900

Angular Contact Bearing Series

7000

7200

7300

Double Row Angular Contact Bearing Series

3200	5200
3300	5300

Double Row Self Aligning Bearing Series

1200	2200
1300	2300

Thrust Ball Bearing Series

51100	51300
51200	51400