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Chapter: Mechanical - Design of Machine Elements - Design of Temporary and Permanent Joints

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Design of Temporary and Permanent Joints

A welded joint is a permanent joint which is obtained by the fusion of the edges of the two parts to be joined together, with or without the application of pressure and a filler material.

DESIGN OF TEMPORARY AND PERMANENT JOINTS

 

Welded Joints

 

A welded joint is a permanent joint which is obtained by the fusion of the edges of the two parts to be joined together, with or without the application of pressure and a filler material. The heat required for the fusion of the material may be obtained by burning of gas (in case of gas welding) or by an electric arc (in case of electric arc welding). The latter method is extensively used because of greater speed of welding.

 

Welding is extensively used in fabrication as an alternative method for casting or forging and as a replacement for bolted and riveted joints. It is also used as a repair medium e.g. to reunite metal at a crack, to build up a small part that has broken off such as gear tooth or to repair a worn surface such as a bearing surface.

 

Advantages and Disadvantages of Welded Joints over Riveted Joints

 

Following are the advantages and disadvantages of welded joints over riveted joints.

 

Advantages

 

The welded structures are usually lighter than riveted structures. This is due to the reason, that in welding, gussets or other connecting components are not used.

 

The welded joints provide maximum efficiency (may be 100%) which is not possible in case of riveted joints.

Alterations and additions can be easily made in the existing structures.

As the welded structure is smooth in appearance, therefore it looks pleasing.

 

In welded connections, the tension members are not weakened as in the case of riveted joints.

 

A welded joint has a great strength. Often a welded joint has the strength of the parent metal itself.

 

Sometimes, the members are of such a shape (i.e. circular steel pipes) that they afford difficulty for riveting. But they can be easily welded.

 

The welding provides very rigid joints. This is in line with the modern trend of providing rigid frames.

 

Disadvantages

 

Since there is an uneven heating and cooling during fabrication, therefore the members may get distorted or additional stresses may develop.

It requires a highly skilled labour and supervision.

 

Since no provision is kept for expansion and contraction in the frame, therefore there is a possibility of cracks developing in it.

The inspection of welding work is more difficult than riveting work.

 

Types of Welded Joints

Following two types of welded joints are important from the subject point of view:

Lap joint or fillet joint, and

Butt joint.

 

Lap Joint

 

The lap joint or the fillet joint is obtained by overlapping the plates and then welding the edges of the plates. The cross-section of the fillet is approximately triangular. The fillet joints may be

 

Single transverse fillet,

Double transverse fillet, and

Parallel fillet joints.

 

The fillet joints are shown in Fig. A single transverse fillet joint has the disadvantage that the edge of the plate which is not welded can buckle or warp out of shape.

 

Butt Joint

 

The butt joint is obtained by placing the plates edge to edge as shown in Fig. 10.3. In butt welds, the plate edges do not require bevelling if the thickness of plate is less than 5 mm. On the other hand, if the plate thickness is 5 mm to 12.5 mm, the edges should be bevelled to V or U-groove on both sides.

 


 

The butt joints may be

Square butt joint,

Single V-butt joint

Single U-butt joint,

Double V-butt joint, and

Double U-butt joint.



Riveted Joints

 

A rivet is a short cylindrical bar with a head integral to it. The cylindrical portion of the rivet is called shank or body and lower portion of shank is known as tail, as shown in Fig. The rivets are used to make permanent fastening between the plates such as in structural work, ship building, bridges, tanks and boiler shells. The riveted joints are widely used for joining light metals. The fastenings (i.e. joints) may be classified into the following two groups:

Permanent fastenings, and

Temporary or detachable fastenings.

 

 

Types of Riveted Joints

 

Following are the two types of riveted joints, depending upon the way in which the plates are connected.

Lap joint, and

Butt joint.

 

Lap Joint

 

A lap joint is that in which one plate overlaps the other and the two plates are then riveted together.

Butt Joint

 

A butt joint is that in which the main plates are kept in alignment butting (i.e. touching) each other and a cover plate (i.e. strap) is placed either on one side or on both sides of the main plates. The cover plate is then riveted together with the main plates. Butt joints are of the following two types:

Single strap butt joint, and

Double strap butt joint.

 

In a single strap butt joint, the edges of the main plates butt against each other and only one cover plate is placed on one side of the main plates and then riveted together.

 

In a double strap butt joint, the edges of the main plates butt against each other and two cover plates are placed on both sides of the main plates and then riveted together.

 

In addition to the above, following are the types of riveted joints depending upon the number of rows of the rivets.

Single riveted joint, and

Double riveted joint.


 

A single riveted joint is that in which there is a single row of rivets in a lap joint as shown in Fig. (a) and there is a single row of rivets on each side in a butt joint as shown in Fig.

 

A double riveted joint is that in which there are two rows of rivets in a lap joint as shown in Fig. (b) and (c) and there are two rows of rivets on each side in a butt joint as shown in Fig.

 


 

Important Terms Used in Riveted Joints

 

The following terms in connection with the riveted joints are important from the subject point of view :

 

1. Pitch. It is the distance from the centre of one rivet to the centre of the next rivet measured parallel to the seam as shown in Fig. 9.6. It is usually denoted by p.

Back pitch. It is the perpendicular distance between the centre lines of the successive rows as shown in Fig. It is usually denoted by pb.

 

Diagonal pitch. It is the distance between the centres of the rivets in adjacent rows of zig-zag riveted joint as shown in Fig. 9.6. It is usually denoted by pd.

 

Margin or marginal pitch. It is the distance between the centre of rivet hole to the nearest edge of the plate as shown in Fig. 9.6. It is usually denoted by m.

 

 

Efficiency of a Riveted Joint

 

The efficiency of a riveted joint is defined as the ratio of the strength of riveted joint to the strength of the un-riveted or solid plate.

We have already discussed that

Strength of the riveted joint = Least of Pt, Ps and Pc

 

Strength of the un-riveted or solid plate per pitch length,

 

P = p × t × σt

 

Efficiency of the riveted joint,

 

η = (Least of Pt , Ps and Pc) / (p × t × σt)

 

where  p = Pitch of the rivets,

t  = Thickness of the plate, and

 

σt = Permissible tensile stress of the plate material.

 

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