Practical Lubrication
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Practical Lubrication Strategies For Bearing Life Extension
by Joseph B. Conyers
Abstract
This paper presents the SKF bearing life prediction equation and explains its use. It begins with the earliest form of the equation and shows its development thru the years up to its refined version as the “New Life Method” which takes into account the degree of contamination present within the bearing. The SKF bearing life equation takes into account factors such as bearing size, actual bearing load versus rated bearing load, machine speed, percent reliability, and actual lubricant viscosity versus required lubricant viscosity to estimate the bearing life. The paper provides not only a description on how to use the equation, but presents two examples of bearings whose life is altered by varying one of variable at a time within the equation. These examples provide good insight into how these factors affect bearing life.
PREVIEW
“Two general conclusions we can draw from the Basic Life Equation (Eq. (1)) are: Bearing Life varies inversely with speed (double the speed, life is reduced by half.) Bearing Life varies exponentially with applied load (P) (double the Load, life is reduced to 0.125 of its original value for Ball Bearings, even more for Roller Bearings)
The effects of these conclusions can be very dramatic operationally. Consider a conveyor belt system whose speed we wish to increase 25%. Perhaps we can accept a bearing life reduction of 25% due to the speed increase. Are there any other considerations? More material will be conveyed per unit time, increasing the weight on the system. Loads on the bearings will increase. If sheaves are changed out to achieve the speed increase, the V-belts may begin to slip on the sheaves, which will be tightened to compensate. If the combined effect of these changes increased the overall load only 10%, life for the ball bearings in this application will be reduced an additional 25%. The overall loss of life may be unacceptable. Significant design changes may be required to offset the reduced bearing life from something that appears at the outset as only a straightforward speed increase.
Adjusting the Basic Life Equation: The advent of cleaner bearing steels in the late 1950’s and had a dramatic improvement on bearing life.
Bearing manufacturing processes also continued to improve, and the basic life equation was extended to reflect these improvements.
Adjusted Life Equation: Lna = a1a23(C/P)p and Lnah = 1,000,000/60n a1a23(C/P)p Eq. (2)
Lna = Adjusted Rating Life, revolutions Lnah = Adjusted Rating Life, hours
a1 = Reliability Adjustment Factor a23 = Material and Lubrication Adjustment Factor
The a1 factor allows us to adjust the basic Life Rating equation for reliability greater than 90%. (Fig. 1) This factor can be used to down-rate applications where safety is critical. Another use of the a1 factor is in high reliability applications. Bearings in aircraft turbine engines, for example, require extreme reliability. The weight penalty from enlarging the bearings and support components to meet reliability needs would be unacceptable. A more cost-effective decision in this case is to use smaller bearings and change them out at an operational life well below the predicted fatigue failure life to achieve an adequate safety margin.”
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