Machinery's Handbook, 31st Edition
2414 Journal Bearings 20) Total flow of lubricant Q: This value is obtained by adding the hydrodynamic flow and the pressure flow. Q Q Q 1 2 = + 21) Bearing temperature rise Δt: This temperature rise in degrees F is obtained from the formula: t Q X P f ∆ = ^ h 22) Comparison of actual and assumed temperature rises: At this point if Δ t a and Δ t differ by more than 5°F, Steps 7 through 22 are repeated using a Δ t new halfway between the former Δ t a and Δ t . 23) Minimum film thickness h o : When Step 22 has been satisfied, the minimum film thickness in inches is calculated from the formula: h o = 1 ∕ 2 C d (1 − ε ). A new diametral clearance c d is now assumed and Steps 5 through 23 are repeated. When this repetition has been done for a sufficient number of values for c d , the full lubri- cation study is plotted as shown in Fig. 11. From this chart a working range of diametral clearance can be determined that optimizes film thickness, differential temperature, fric - tion horsepower and oil flow.
0 20 40 60 Oil Temp. Rise, t , F.
Min. Acceptable h o 50 100 150 Min. Film Thick h o , in. x 10 –6
Oil Flow, Q , gpm
Friction Power Loss, P f , hp
0
Max. Acceptable T
Mfg. Limit
Mfg. Limit
Max. Acceptable h o
Fig. 11. Example of Lubrication Analysis Curves for Journal Bearing. Use of Lubrication Analysis.— Once the lubrication analysis has been completed and plotted as shown in Fig. 11, the following steps lead to the optimum bearing design, taking into consideration both basic operating requirements and requirements peculiar to the application. 1) Examine the curve (Fig. 11 ) for minimum film thickness and determine the acceptable range of diametral clearance, c d , based on a) a minimum of 200 × 10 − 6 inches for small bearings under 1 inch diameter b) a minimum of 500 × 10 − 6 inches for bearings from 1 to 4 inches diameter
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