Question 16.2: Design a boundary-lubricated plain surface bearing to carry ...

We will use the design procedure previously outlined.
Step 1. Trial diameter: Try D = 75 mm.
Steps 2–4. Try L/D = 1.0. Then L = D = 75 mm.
Step 5. Bearing pressure:

p = F/LD = (2500 N)/(75 mm)(75 mm) = 0.444 N/mm{}^2 = 0.444 MPa
Step 6. Journal speed:
V = πDn/(60 000) = π(75)(1150)/(60 000) = 4.52 m/s
Step 7. pV factor:
pV = (0.444 MPa)(4.52 m/s) = 2.008 MPa . m/s
Step 8. Design value for pV = 2(2.008) = 4.016 MPa . m/s.

Step 9. From Table 16–1, we can specify aluminum bronze (C95400) having a pV rating of 4.375 MPa . m/s.
Steps 10–11. From Figure 16–4, we can recommend a minimum C_d = 100 mm (0.100 mm or 0.004 in) based on D = 75 mm and 1150 rpm.

Alternate design: The pV factor for the initial design, while satisfactory, is somewhat high and may require careful lubrication. Consider the following alternate design having a larger bearing diameter.
Step 1. Try D = 150 mm.
Step 2. Let L/D = 1.25.
Step 3. Then
L = D(L/D) = (150 mm)(1.25) = 187.5 mm

Step 4. Let’s use the more convenient value of 175 mm for L.
Step 5. Bearing pressure:
p = F/LD = (2500 N)/(175 mm)(150 mm) = 0.095 N/mm{}^2 = 0.095 MPa
Step 6. Journal speed:
V = πDn/(60 000) = π(150)(1150)/(60 000) = 9.03 m/s
Step 7. pV factor:
pV = (0.095 MPa)(9.03 m/s) = 0.860 MPa . m/s

Step 8. Design value of pV = 2(0.860) = 1.720 MPa . m/s.
Step 9. From Table 16–1, we can specify an oil-impregnated porous bronze bearing having a pV rating of 1.750 MPa . m/s or a BU dry-lubricated bearing having a pV rating of 1.785 MPa . m/s.
Steps 10–11. From Figure 16–4, we can recommend a minimum C_d = 150 μm (0.150 mm or 0.006 in) based on D = 150 mm and 1150 rpm. Other design details are dependent on the system into which the bearing will be placed.