Page 300 - Marks Calculation for Machine Design

P1: Shashi
January 4, 2005
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Brown˙C07
Brown.cls
STRENGTH OF MACHINES
282
Load Type Factor. The load type factor (k c ) for axial loading is given in Eq. (7.13) as
0.923 S ut ≤ 220 kpsi

U.S. Customary: k c =
1 S ut > 220 kpsi
(7.13)
0.923 S ut ≤ 1,540 MPa

SI/metric: k c =
1 S ut > 1,540 MPa
For bending, torsion, or shear, the load type factor (k c ) is given in Eq. (7.14) as
1 bending

k c = (7.14)
0.577 torsion and shear
where the value for torsion and shear is related to the distortion-energy theory for deter-
mining whether a design is safe under static loading conditions.
Temperature Factor. For temperatures very much lower than room temperature materials
like ductile steel become brittle. Materials like aluminum seem to be unaffected by similar
low temperatures.
The temperature factor (k d ) is given in Eq. (7.15) as
S T
k d = (7.15)
S RT
where (S T ) is the ultimate tensile strength at some specific temperature (T ) and (S RT ) is
the ultimate tensile strength at room temperature (RT). Values of the ratio (S T /S RT ), which
is actually the temperature factor (k d ), are given in Table 7.2.
TABLE 7.2 Temperature Factors
◦ ◦
F k d C k d
70 1.000 20 1.000
100 1.008 50 1.010
200 1.020 100 1.020
300 1.024 150 1.025
400 1.018 200 1.020
500 0.995 250 1.000
600 0.963 300 0.975
700 0.927 350 0.927
800 0.872 400 0.922
900 0.797 450 0.840
1000 0.698 500 0.766
1100 0.567 550 0.670

Notice that the temperature factor (k d ) initially increases as the temperature increases,
then begins to decrease as the temperature continues to increase. The temperature of most
materials can reach values that induce creep and yielding becomes more important than
fatigue.
Miscellaneous Effects Factor. All the following effects are important in the dynamic
loading of machine elements, however, only one can be quantified. These effects are residual