MFGE 316 – MACHINE ELEMENTS

Homework 4- Fatigue Failure Criteria

Due:

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