Machine Design – II ( ME – 308 )

Project # 1 : Design of Shaft and Selection of Bearings

Summary of Class # 2 : Static and Fatigue Analysis

 

Step # 1: Select a material for the shaft.

 

Step # 2: Static Analysis

Static analysis will be done at a location on the shaft where we have either torque or moment. By using the Maximum shear stress theory and using the equation (18-10) in the text book, we do static analysis on diameters d3, d2, d4 respectively. We assume the diameter d1 = 0.8 (d2).

Step # 3: Fatigue analysis

For fatigue analysis, first identify the critical locations.

A critical location is defined as a point where the shaft is more likely to fail, such as a Stress concentrated area with a high bending moment. Stress concentration can occur if there is a sudden change in the cross sectional area (Case 1) or if there is any non-uniformity (Case 2).

 

  1. Diameter d1 at shoulder – ignore fatigue analysis. Case 1, but low moment.
  2. Diameter d2 at shoulder – perform fatigue analysis ( eqn. 18-28). Case 1 and high moment.
  3. Diameter d2 at keyway – perform fatigue analysis ( eqn. 18-28). Case 2 and high moment.
  4. Diameter d3 at shoulder – perform fatigue analysis (eqn 18-28). Case 1 and high moment.
  5. Diameter d4 at keyway – ignore fatigue analysis (eqn 18-28). Case 2 but low moment.

Note: In eqn 18-28 ignore factor Kf in the numerator.

Se = kakbkckdke Se

All the factors can be found out from Chapter 7 in the text book.

You can assume kt for the keyways to be 2.5 and for the shoulders you have to calculate it from Fig (A-15-9) in the text book.

Notch radius

0 r 0.08 (For sharp corners)

0.08 r 0.16 ( For rounded corners)

Wherever there is a bearing fitted on to the shaft, the fillet is taken as well rounded and if there is no bearing then the fillet is taken as sharp..

Calculate the notch sensitivity (q) from fig. 5-16.

 

After the analysis, select the maximum diameters to be the final diameters of the shaft.

 

Back