Subject: Machine Design -I

Topic: Machine Design consideration

Difficulty: Medium

Stress concentration:

It is defined as the localization of high stresses due to the irregularities present in the component and abrupt changes of the cross section.

Methods to reduce stress concentration:

Although it is not possible to completely eliminate the effect of stress concentration, there are methods to reduce stress concentrations. This is achieved by providing a specific geometric shape to the component.

(a) Additional Notches and Holes in Tension Member:

• A flat plate with a V-notch subjected to tensile force is shown in Fig. (a). It is observed that a single notch results in a high degree of stress concentration.
• The stress concentration can be reduced by three methods:

(I) Use of multiple notches as shown in fig (b)

(II) Drilling additional holes as shown in fig (c)

(III) Removal of undesired material as shown in fig (c)

(b) Fillet Radius, Undercutting and Notch for Member in Bending:

• A bar of circular cross-section with a shoulder and subjected to bending moment is shown in Fig. (a).
• Ball bearings, gears or pulleys are mounted against this shoulder. The shoulder creates a change in cross-section of the shaft, which results in stress concentration.
• There are three methods to reduce stress concentration at the base of this shoulder.

(I) By providing fillet radius at shoulder as shown Fig (b).

(II) By undercutting the shoulder as illustrated in Fig. (c).

(III) By additional notch at the top of shoulder as shown in Fig. (d).

(c)Drilling Additional Holes for Shaft:

• A transmission shaft with a keyway is shown in Fig. (a). The keyway is a discontinuity and results in stress concentration at the corners of the keyway , therefore holes are drilled to minimise the stress concentration at the corners of keyway as shown in fig (b).

(d) Reduction of Stress Concentration in Threaded Members:

• A threaded component is shown in Fig. (a).
• There are three methods to reduce stress concentration

(I) Undercutting as shown in Fig. (b)

(II) Reduction in Shank Diameter shown in Fig. (c)