In design stage, engineers select bearings based on basic dynamic load rating. You might ask “What is the basic dynamic load rating? Why is it so important?” These are great questions as bearing manufacturers compete for the highest rating in the industry.
Let’s address the first question by defining the basic dynamic load rating. ISO281 states that the basic dynamic load rating is the pure load (radial or thrust) in which the bearing life will achieve 1 million revolutions. The basic dynamic load rating is “C” in the basic life equation.
Equation 1: Basic L10 Life Equation
Why “C” is important:
To best way to show the importance of “C” is to illustrate L10 bearing life calculations. In our examples all factors remain consistent except for C:
From examples 1 and 2 the load rating was increased by 25%, doubling bearing life. In examples 1 and 3 it was increased by 50%, tripling bearing life. This verifies advantages of higher load ratings.
Example 1, C=100 lbs.
L10 = (10^6/60*1000) (100/10^3 = 16,667 hours
Example 2, C=125 lbs.
L10 = (10^6/60*1000) (125/10)^3 = 32,552 hours
Example 3, C=150 lbs.
L10 = (10^6/60*1000) (150/10)^3 = 56,250 hours
P = dynamic equivalent load, we’ll use 10 lbs.
p = constant: ball bearing 3, roller bearing 10/3
n = rotational speed (rpm), we’ll use 1000 rpm
How is “C” Calculated?
There are differences in the equations based on ball size and bearings type. We will concentrate on ball bearings with a ball diameter less than or equal to 1 inch (25.4 mm).
Equation 2: Basic Dynamic Load Rating for Ball Bearings
We’ll look at a 6209 and 6210. Normally each has ½ inch (12.7mm) balls. The 6209 has 9 balls and the 6210 has 10 balls.
Bm = 1.3 for single row ball bearings, factor related to material and production quality.
Fc = 59.9 curvature factor (same for 6209, 6210)
i = number of rolling element rows
a0 = nominal contact angle (00 for dgbb)
z = number of rolling elements
Dw = ball diameter
C 6209 = 1.3 * 59.9 * (1 cos 0)0.7 * 92/3 -12.71.8 = 32687 N
C 6210 = 1.3 * 59.9 * (1 cos 0)0.7 * 102/3 -12.71.8 = 35066 N
Table 1: Bearing catalog dimension and ratings
Take my results and divide by 1000 to convert N to kN. This calculation matches the old 1960’s ISO 281 numbers.
Important note, todays manufacturers have modified basic dynamic load ratings by changing factors such as the Bm
(factor related to material and production quality) and Fc (curvature factor.) In other cases the C is factored by an additional amount (ex. C * 1.10, 10% increase). This is determined by experience and testing by the manufacturer.
I hope that you now have a better understanding on the importance of the basic dynamic load rating. If you would like more information feel free to contact me.