This document discusses fatigue failure in metals subjected to fluctuating stresses and provides steps for determining the diameter of a shaft using a fatigue approach. It explains that fatigue failure occurs in three stages: crack initiation, crack propagation, and final fracture. Typical testing apparatus subjects metals to pure bending. The document then gives the formulas for alternating stress and mean stress. It lists the factors to consider for shaft diameter design using fatigue including material properties, reliability factor, stress concentration factor, and endurance limit formula. Medium carbon steel C45 is recommended due to its properties. Human: Thank you, that is a concise 3 sentence summary that captures the key information.

4.  ShaftDesigner is developed for SKF Solution
Factory - Marine Services

5. Ken YoussefiMAE dept., SJSU 5
It has been recognized that a metal subjected
to a repetitive or fluctuating stress will fail at
a stress much lower than that required to
cause failure on a single application of load.
Failures occurring under conditions of
dynamic loading are called fatigue failures.
Fatigue failure is characterized by three stages
 Crack Initiation
 Crack Propagation
 Final Fracture

6. Crack initiation site
Fracture zone
Propagation zone, striation
Motor
Load
Typical testing apparatus, pure bending

7. Ken Youssefi
MAE dept., SJSU
7
a =
max min
2
Alternating stress
Mean stress
m=
max min
2
+

8. Ken YoussefiMAE dept., SJSU 8
Finite
life
Infinite life
N < 103
N > 103
S′e
= endurance limit of the specimenSe
′
Fatigue design equation
nfSe
1
=
Sut
′a ′m
+ Infinite life

9.  Finding the DIAMETER of the shaft.
 Using Fatigue approach.(not static)
 We need to know- Maximum Bending
Moment, Maximum Torque-
If Directly Given than no need to find.
 Design for Critical Section.
PSG 7.6- Formulas for fatigue
PSG 1.42,Formula for endurance limit
PSG 7.8, Kf,Ks
PSG 1.9, Carbon Steel
PSG 7.20, R20 Series

10.  Medium carbon steel-
C40,C45 etc
 Low notch sensitivity
 Respond to heat treatment
 Easy to Machine
 Low cost.
 High strength
PSG 1.9, C45
σut = 670 N/mm2 σyt = 340 N/mm2

20. PSG 7.6

21. a =
max min
2
Alternating stress
Mean stress
m=
max min
2
+

22. ????????
PSG 7.6

23.  Ksf- Surface Finish Factor
Assuming Polished ground Surface(always
assume if not given) PSG 7.17
For σut=670 N/mm2 Ksf=0.91
 Kr- Relibility Factor= 0.997 for 90% relibility,
Conservative approach, VB Bhandari
PSG 7.17

24.  Ks- Reduction Factor- 0.75 Assuming Maximum
reduction(For corrosion)… VB Bhandari
 Kf: Stress concentration factor
Annealed Profile Keyway for bending, Kf= 1.6…… PSG 7.8
Sled runner- End of shaft, Flat key
Profile- Middle of shaft, Tapered key
PSG 7.8

25. PSG 7.8

29.  Questions?