Page 378 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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Solution
Steps Computation Results Units
1. Select diameter d = 100 mm d = 100 mm
2. Select the ratio Select B/d = 1.2 B = 120 mm
of B/d Therefore
B
B = × d = 1.2 × 100 = 120 mm
d
3. Calculate unit From Eq. (12.3) p = 0.833 MPa
bearing load p = F = 10000 = 0.833 MPa
dB 100 × 120
4. Calculate From Eq. (12.4) v = 3.925 m s −1
journal velocity dn × 100 × 750
v = = = 3.925 m∕s
60 × 1000 60 × 1000
5. Calculate pv From Eq. (12.5) pv = 3.27 MPa m s −1
factor F dn
pv = ⋅ = 0.833 × 3.925
Bd 60 × 1000
= 3.27MPa ⋅ m∕s
6. Select materials Select aluminium bronze, from Table 12.5, the Aluminium
for the bearings allowable values are [p] = 15–20 MPa, [v] = 5m s −1 bronze
−1
and [pv] = 15 MPa m s .
Therefore, the design values are less than the limits.

Example Problem 12.2
Design a sliding journal bearing on a 1000 rpm steam turbine rotor supports a constant
radial load of 17 kN. The shaft stress analysis determines that the minimum acceptable
diameter at the journal is 150 mm. The shaft is part of a machine requiring good preci-
sion. The lubricant is supplied to the bearing at atmospheric pressure.

Solution

Steps Computation Results Units
Assumptions Viscosity is constant and corresponds to the average
temperature of the oil flowing to and from the bearing.
The influence on flow rate of any oil holes or grooves
is negligible.
The entire heat generated in the bearing is carried
away by oil
1. Select the ratio of B/d = 0.5 B/d = 0.5
bearing width to
diameter
2. Compute the width B =(B∕d)× d =0.5×0.15 = 0.075 m B = 0.075 m
of bearing
dn × 150 × 1000
3. Compute the v = = = 7.85 m∕s v = 7.85 m s −1
journal speed 60 × 1000 60 × 1000
(continued)