ShaftDesigner is developed for SKF Solution

Factory - Marine Services

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

Crack initiation site

Fracture zone

Propagation zone, striation

Motor

Load

Typical testing apparatus, pure bending

Ken YoussefiMAE dept., SJSU

7

a =max min

2

Alternating stress

Mean stress

m=max min

2

+

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

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

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

PSG 7.6

a =max min

2

Alternating stress

Mean stress

m=max min

2

+

????????

PSG 7.6

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

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

PSG 7.8

Questions?