Page 98 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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However, reducing the stiffness of the bolt or increasing the stiffness of the clamped
members under a constant preload will inevitably reduce the resultant load in the
clamped members and consequently weaken the sealing effect. Therefore, it is prefer-
able to increase the preload simultaneously so that the resultant load in the clamped
members does not change too much. While increasing the preload, the preload must
be controlled to prevent twisting off the bolt while tightening.
2. Reduce stress concentration
Stress concentration is an important factor affecting fatigue strength. Stress concen-
tration often happens where geometrical discontinuity appears. For a bolt, the screw
end, the connection of bolt head and shank and the area where the cross-section
changes are the places stresses concentrate. Proper modification of standard bolts,
like increased fillet radius under the head, can effectively reduce stress concentration.
3. Adopt proper manufacturing methods
Rolled threads are preferred compared with cut or grounded threads, as rolling pro-
cesses cause work hardening of materials and generate a favourable grain structure
and compressive residual stresses, which benefit the improvement of fatigue strength.
Furthermore, proper heat treatment like nitriding and shot peening will also increase
fatigue strength of bolts.

3.6 Design of Bolted Joints

3.6.1 Introduction

More often than not, multiply bolted joints involve several bolts placed in a specified
pattern to improve strength and stability of a connection. The design of multiply bolted
joints consists of two tasks: one is to decide the number of bolts in a pattern and their
layout; the other is to specify the dimension and material of the fasteners. The former
requires structural design, while the latter needs force and strength analysis.

3.6.2 Materials and Allowable Stresses
A wide variety of materials are used for threaded fasteners. The selection of materials
for threaded fasteners is normally based on the requirements of strength, weight, cor-
rosion resistance, magnetic properties, life expectancy and costs. The most widely used
materials are carbon steels and alloy steels. Stainless steels and nickel-based superal-
loys, such as Inconel and Hastelloy, are used for fasteners working in corrosive or high
temperature environments. Aluminium, bronze and brass threaded fasteners are used
for applications where corrosion resistance and good thermal and electrical conductivity
are required. Nylon and plastics are both suitable for applications with more economical
considerations than strength requirements.
The strength of steels for bolts and screws is used to determine property classes. Three
strength ratings are involved; namely, tensile strength, yield strength and proof strength.
The proof strength is defined as the stress at which the bolt or the screw would undergo
permanent deformation, usually 90–95% of the yield strength [4]. The value depends on
the material, heat treatment and other factors.
Recommended materials for property class 4.6–5.8 are low or medium carbon steels;
for property class 8.8–9.8, low carbon alloy steel and medium carbon steel, heat treated