Design of Machine Elements: Threaded and Welded Joints, Summaries of Mechanical Systems Design

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Design of Machine Elements

Unit – IV

Threaded and Welded Joints

Threaded Joints

• Threaded joint is defined as a separable joint of

two or more machine parts that are held together

by means of a threaded fastening such as a bolt

and a nut.

• Threaded joints are extensively used in

mechanical assemblies. It has been observed that

over 60% of the parts have threads.

Disadvantages of Threaded Joints

• Threaded joints require holes in the machine parts that are to be clamped. This results in stress concentration near the threaded portion of the parts. Such areas are vulnerable to fatigue failure.
• Threaded joints loosen when subjected to vibrations.
• Threaded fasteners are considered as a major obstacle for efficient assembly.

BASIC TYPES OF SCREW FASTENING

• There are three parts of a threaded fastening, viz.,a bolt or

screw, a nut and a washer.

• A bolt is a fastener with a head and straight threaded shank
• and intended to be used with a nut to clamp two or more

parts. The same bolt can be called screw when it is

threaded into a tapped hole in one of the parts and not into

the nut. Although bolt and screw are similar, there is a

fundamental difference in their assembly.

• A bolt is held stationary, while torque is applied to the nut

to make threaded joint, whereas the torque is applied to

the screw to turn it into matching threads in one of the

parts.

BASIC TYPES OF SCREW FASTENING

A fastener when assembled consists of a bolt (or a screw), a

nut, a washer, and sometimes a collar for increasing friction.

The different terms for fasteners are bolts, screws, studs, cap

screws, setscrews.

BOLT OF UNIFORM STRENGTH

• Bolts are subjected to shock and impact loads in certain

applications. The bolts of cylinder head of an internal

combustion engine or the bolts of connecting rod are the

examples of such applications. In such cases, resilience of

the bolt is important design consideration to prevent

breakage at the threads.

• Resilience is defined as the ability of the material to absorb

energy when deformed elastically and to release this

energy when unloaded. A resilient bolt absorbs energy

within elastic range without any permanent deformation

and releases this energy when unloaded.

• It can be called spring property of the bolt. A resilient bolt

absorbs shocks and vibrations like leaf springs of the

vehicle. In other words, the bolt acts like a spring.

ISO metric screw threads

Basic Definitions (i)Major Diameter : The major diameter is the diameter of an imaginary cylinder that bounds the crest of an external thread ( d). (ii) Minor Diameter : The minor diameter is the diameter of an imaginary cylinder that bounds the roots of an external thread ( dc) or the crest of an internal thread ( Dc). The minor diameter is the smallest diameter of the screw thread. It is also called core or root diameter of the thread. (iii) Pitch Diameter : The pitch diameter is the diameter of an imaginary cylinder, the surface of which would pass through the threads at such points as to make the width of the threads equal to the width of spaces cut by the surface of the cylinder. It is also called the effective diameter of the thread. (iv) Pitch : Pitch is the distance between two similar points on adjacent threads measured parallel to the axis of the thread. It is denoted by the letter p. (v) Lead : Lead is the distance that the nut moves parallel to the axis of the screw, when the nut is given one turn. (vi) Thread Angle: Thread angle is the angle included between the sides of the thread measured in an axial plane. Thread angle is 60o for ISO metric threads.

ISO metric screw threads

ISO metric screw threads

Examples Example 1: An electric motor weighing 10 kN is lifted by means of an eye bolt as shown in Fig. 7.13. The eye bolt is screwed into the frame of the motor. The eye bolt has coarse threads. It is made of plain carbon steel 30C8 (Syt = 400 N/mm2) and the factor of safety is 6. Determine the size of the bolt. Example 7.2 Two plates are fastened by means of two bolts as shown in Fig. 7.14. The bolts are made of plain carbon steel 30C8 (Syt = 400 N/mm2) and the factor of safety is 5. Determine the size of the bolts if,

Welded Joints

 Welding can be defined as a process of joining

metallic parts by heating to a suitable temperature

with or without the application of pressure. Welding

is an economical and efficient method for obtaining

a permanent joint of metallic parts.

 There are two distinct applications of welded

joints—a welded joint can be used as a substitute

for a riveted joint and a welded structure as an

alternative method for casting or forging.

Welded Joints Welded joints have the following disadvantages: (i) As compared with cast iron structures, the capacity of welded structure to damp vibrations is poor. (ii) Welding results in a thermal distortion of the parts, thereby inducing residual stresses. In many cases, stress-relieving heat treatment is required to relieve residual stresses. Riveted or cast structures do not require such stress relieving treatment. (iii) The quality and the strength of the welded joint depend upon the skill of the welder. It is diffi cult to control the quality when a number of welders are involved. (iv) The inspection of the welded joint is more specialised and costly compared with the inspection of riveted or cast structures.

WELDING PROCESSES

Welding processes are broadly classified into the

following two groups:

(i) Welding processes that use heat alone to join the two parts.

(ii) Welding processes that use a combination of heat and pressure to join the

two parts.

• The welding process that uses heat alone is called the fusion welding

process.

• Fusion welding is further classified into the following three groups:

(i) Thermit welding

(ii) Gas welding

(iii) Electric arc welding

• Welding processes that use a combination of heat and pressure to join the

two parts are classified into the following two groups:

(i) Forge welding

(ii) Electric resistance welding