380
T. A. Musalli et al.
Fig. 2 Chrome-silicon AISI
9254 stress-life curve
0
500
1000
1500
2000
2500
3000
3500
1.0E+00
1.0E+03
1.0E+08
1.0E+11
ALTERNATING STRESS [MPa]
CYCLES
For the stress-life fatigue analysis approach, the relation between alternative stress
and the number of cycles [7] considered for the present analysis is shown in Fig. 2.
3 Theoretical Calculation
3.1 Stress in Helical Spring
In order to calculate stress in the spring, consider an axial load F which is applied at
the central axis of helical spring. The spring maintains equilibrium by generating an
internal shear force F and torque T, which is shown Fig. 3.
The maximum shear stress is determined by,
τ max = ±
T r
J
+
F
A
(1)
where J is polar moment of inertia, A is cross-section area, r is wire radius and d is
diameter.
In general, the following relationship holds for calculating the shear stress [6]
Fig. 3 Force and torque
acting on the helical spring