Page 261 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Gear and Coupling Best Practices Be st Practice 4.3

Fig 4.3.1 Anti-friction bearings

‘B-’ or ‘L-’ 10 life is defined as the life in hours that 9 out of 10
The D-N number is a measure of the rotational speed of the randomly selected bearings would exceed in a specific
anti-friction bearing elements application.
×
D-N number = bearing bore (millimeters) speed (RPM) 3
‘B-’ or ‘L-’ 10 life = 16700 C
Approximate lubrication ranges
N F
Lubrication type D-N Range
S e e l a d b o l e w 1 0 0 , 0 0 0 Where: N = RPM
R e e r g a s e a e l b 1 0 0 , 0 0 0–300,000 C = Load in LBS that will result in a bearing element life of
Oil lube (unpressurized) 300,000–500,000 1,000,000 revolutions
Oil lube (pressurized) above 500,000 F = Actual load in LBS

Fig 4.3.2 D-N number Fig 4.3.3 ‘B-’ or ‘L-’ 10 life

The exception to this rule is aircraft gas turbines, which bearing loads. As a result, a small change in the journal
can have D-N numbers in excess of 2,000,000. In these appli- bearing loads can significantly reduce the bearing life.
cations, hydrodynamic bearings are not used, since the size and When anti-friction bearings suddenly start failing where they
weight of the required lubrication system would be prohibitive. did not in the past, investigate all possible sources of in-
Anti-friction bearings have a finite life which is usually creased bearing loads (piping forces, foundation forces, mis-
specified as ‘B-10’ or ‘L-10’ life; as defined in Figure 4.3.3. alignment, unbalance, etc.). Anti-friction bearings are usually
An important fact to note is that the life of any anti- designed for a minimum life of 25,000 hours’ continuous
friction bearing is inversely proportional to cube of the operation.

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