MFGE 316 – MACHINE ELEMENTS
Homework 4- Fatigue Failure Criteria
Question 1) The flat steel spring illustrated is loaded in bending by the force F. The
spring supports a static weight of exactly 9.36 kN. During operation, the total load on
the spring is estimated to fluctuate up to 10.67 kN maximum. The spring is forged of a
95-point carbon steel and after heat treatment has the following minimum properties:
Sut = 1400 Mpa; Syt = 950 Mpa; HB = 399, and 32 percent reduction in area. Estimate
the factor of safety if the spring is 18 mm thick, by using Goodman Equation. Question 2) The below figure is for a rotating shaft loaded in completely reversed
bending by the the 500 N and 2000 N forces. Rotating speed of the shaft is 980 rpm
and the shaft is made of 30CrNiMo8 (EN 10083) in quenched and tempered condition
with a ground finish in critical places. The shaft has 2 mm shoulder fillets.
a) Draw shear force and bending moment diagrams and calculate the stress at
the most critical point.
b) Based on an infinite life, find the factor of safety by using Goodman Equation.
c) If shaft has not an infinite life, calculate life for the shaft.
d) If material for the shaft is C35 (EN 10083) in normalized condition, what will
be factor of safety in terms of infinite life (Goodman Equation) and life for
the shaft? Mechanical Engineering