Subject: Machine Design -I

Topic: Design against static Loads, Bolted and welded joints under eccentric loading. Power Screw

Difficulty: Medium

• A bolt is a form of threaded fastener with an external male thread as shown in fig below.

• When a bolt is subjected to shock loading, as in case of a cylinder head bolt of an internal combustion engine, the resilience of the bolt should be considered in order to prevent breakage at the thread.
• In an ordinary bolt shown in Fig. (a), the effect of the impulsive loads applied axially is concentrated on the weakest part of the bolt i.e. the cross-sectional area at the root/ core of the threads. In other words, the stress in the threaded part of the bolt will be higher than that in the shank.
• Hence a great portion of the energy will be absorbed at the region of the threaded part which may fracture the threaded portion because of its small length.

- If the shank of the bolt is turned down to a diameter equal or even slightly less than the core diameter of the thread (Dc) as shown in Fig. (b), then shank of the bolt will undergo a higher stress. - This means that a shank will absorb a large portion of the energy, thus relieving the material at the sections near the thread. The bolt, in this way, becomes stronger and lighter and it increases the shock absorbing capacity of the bolt because of an increased modulus of resilience. This gives us bolts of uniform strength.

- A second alternative method of obtaining the bolts of uniform strength is shown in Fig. (c). In this method, an axial hole is drilled through the head as far as the thread portion such that the area of the shank becomes equal to the root area of the thread. Here, D = Diameter of the hole.

Mathematically the size of hole is calculated as,