Table of Contents
Mechanical joints are broadly classified into two classes, viz.,
Non-permanent joints and permanent joints.
Non-permanent joints can be assembled and disassembled without damaging the components. Examples of such joints are threaded fasteners (like screw-joints), keys, couplings, etc.
Permanent joints cannot be disassembled without damaging the components. These joints can be of two kinds, depending on the nature of the force that holds the two parts. The force can be of mechanical origin, for example, riveted joints, joints formed by press or interference fit, etc., where two components are joined by applying mechanical force.
The components can also be joined by molecular force, for example, welded joints, brazed joints, joints with adhesives, etc.
Not until long ago, riveted joints were very often used to permanently join structural members. However, significant improvements in welding and bolted joints have curtailed the use of these joints. Even then, rivets are used in structures, ship bodies, bridges, tanks, and shells, where high joint strength is required.
A rivet is a short cylindrical rod having a head and a tapered tail. The main body of the rivet is called the shank. According to Indian standard specifications, rivet heads are of various types.
Rivet heads for general purposes are specified by Indian standards IS: 2155-1982 (below 12 mm diameter) and IS: 1929-1982 (from 12 mm to 48 mm diameter). Rivet heads used for boiler works are specified by Indian standard IS: 1928–1978.
To get the dimensions of the heads, we may use any machine design handbook.
Riveting is an operation whereby two plates are joined with the help of a rivet. Adequate mechanical force is applied to make the joint strong and leak proof. Smooth holes are drilled (or punched and reamed) in two plates to be joined, and the rivet is inserted. holding, and then the head is formed by means of a backing up bar. The necessary force is applied at the tail end with a die until the tail deforms plastically to the required shape.
There are two types of series Basic Number Series and Derived Number Series and derived series are further divided into Method 1 and Method 2.
A die is placed on the protruding end of the shank, and blows are applied by a hammer.
Here the die is a part of the hammer, which is operated by pneumatic, hydraulic, or steam pressure.
Depending upon whether the rivet is initially heated or not, the riveting operation can also be of two types:
a) cold riveting – riveting is done at ambient temperature and
b) hot riveting – rivets are initially heated before applying force.
After riveting is done, the joint is heat-treated by quenching and tempering.
In order that Head, Shank, and Tail ensure leak-proofness of the joints, additional operations like caulking are done.
Riveted joints are mainly of two types: 1. lap joints and 2. butt joints.
The plates that are to be joined are brought face-to-face so that an overlap exists. Rivets are inserted in the overlapping portion. Single or multiple rows of rivets are used to give strength to the joint.
Depending upon the number of rows, the riveted joints may be classified as single riveted lap joint, double or triple riveted lap joints, etc. When multiple joints are used, the arrangement of rivets between two neighboring rows may be of two kinds. In chain riveting, the adjacent rows have rivets in the same transverse line. In zig-zag riveting, on the other hand, the adjacent rows of rivets are staggered.
In this type of joint, the plates are brought to each other without forming any overlap. Riveted joints are formed between each of the plates and one or two cover plates. Depending upon the number of cover plates, the butt joints may be single- or double-strap butt joints. Like lap joints, the arrangement of the rivets may be of various kinds, namely, single row, double or triple chain, or zigzag.
The strength of a riveted joint is measured by its efficiency.
The efficiency of a joint is defined as the ratio between the strength of a riveted joint and the strength of an unriveted joint or a solid plate. Obviously, the efficiency of the riveted joint depends not only on the size and strength of the individual rivets but also on the overall arrangement and the type of joints. The usual range of the efficiencies, expressed in percentiles of the commercial boiler joints, is given in the table below.
Commonly used range of the efficiencies of the commercial boiler joints.
The following are the important parameters that are required to specify the arrangement of rivets in a riveted joint.
The strength of a riveted joint is evaluated, taking all possible failure paths in the joint into account. There are four possible ways a single rivet joint may fail.
(i) Tearing of the plate:
If the force is too large, the plate may fail in tension along the row.
The maximum force allowed in this case is [math] {P_t}= {(p-d)}{t}{\sigma_t} [/math]
where [math] {σ_t}[/math]= allowable tensile stress of the plate material,
p= pitch,
d= diameter of the rivet hole
t= thickness of the plate Failure
(ii) Shearing of the rivet:
The rivet may shear, as shown in the figure. The maximum force withstood by the joint to prevent this failure is
[math] {P_s}= {n}{(\frac{\pi}{4})}{d^2}{\tau} [/math] for lap joint, single strap butt joint
[math] {P_s}= {n}{2}{(\frac{\pi}{4})}{d^2}{\tau} [/math] for double strap butt joint
where [math] {\tau} [/math] = allowable shear stress of the rivet material.
(iii) Crushing of rivets:
If the bearing stress on the rivet is too large, the contact surface between the rivet and the plate may get damaged.
With a simple assumption of uniform contact stress, the maximum force allowed is [math] {P_c}= {n}{d}{t}{\sigma_c} [/math]
where [math] {\sigma_c} [/math] =allowable bearing stress between the rivet and plate material.
(iv) Tearing of the plate at edge:
If the margin is too small, the plate may fail, as shown in the figure. To prevent failure, a minimum margin of m = 1.5d is usually provided.
Efficiency of the single riveted joint can be obtained as ratio between the minimum [math] {P_t}, {P_s} and {P_c} [/math] and the load carried by a solid plate which is [math] {p}{t}{\sigma_t} [/math]
Thus efficiency [math] {(η)} = \frac{min ({P_t}, {P_s}, {P_c})}{ {p}{t}{σ_t}} [/math]
In a double or triple riveted joint the failure mechanisms may be more than those discussed above.
The failure of plate along the outer row may occur in the same way as above. However, in addition the inner rows may fail.
For example, in a double riveted joint, the plate may fail along the second row. But in order to do that the rivets in the first row must fail either by shear or by crushing. Thus the maximum allowable load such that the plate does not tear in the second row is [math] {P_2ndrow} = {(p-d)}{t}{σ_t} + min ({P_s}, {P_c}) [/math]
Further, the joint may fail by
(i) Shearing of rivets in both rows
(ii) Crushing of rivets in both rows
(iii) Shearing of rivet in one row and crushing in the other row.
The efficiency should be calculated taking all possible failure mechanism into consideration.
The design parameters in a riveted joint are d, p and m
Diameter of the hole (d): When thickness of the plate (t) is more than 8 mm, Unwin’s formula is used, d =6√t mm. Otherwise d is obtained by equating crushing strength to the shear strength of the joint.
In a double riveted zigzag joint, this implies [math] {d}{t}{σ_c}= \frac{\pi}{4}{d^2}{\tau} [/math] (valid for t<8mm)
However, d should not be less than t, in any case.
The standard size of d is tabulated in code IS: 1928-1961.
Pitch (p): Pitch is designed by equating the tearing strength of the plate to the minimum of shear strength or crushing strength of the rivets.
In a double riveted lap joint, this takes the following form. [math] {(p-d)}{t}{\sigma_t}= min (2\frac{\pi}{4}{d^2}{\tau}, {2}{d}{t}{\sigma_c}) [/math]
But 2p≥d in order to accommodate heads of the rivets.
Margin (m): m=1.5d
In order to design boiler joints, a designer must also comply with Indian Boiler Regulations (I.B.R.).
(pb: usually 0.33p + 0.67d mm)
The material of the rivets must be tough and ductile.
They are usually made of steel (low carbon steel or nickel steel), brass, aluminium or copper, but when strength and a fluid tight joint is the main considerations, then steel rivets are used.
The rivets for general purposes shall be manufactured from steel conforming to the following Indian Standards :
(a) IS : 1148–1982 (Reaffirmed 1992) – Specification for hot rolled rivet bars (up to 40 mm diameter) for structural purposes; or (b) IS : 1149–1982 (Reaffirmed 1992) – Specification for high tensile steel rivet bars for structural purposes.
The rivets for boiler work shall be manufactured from material conforming to IS : 1990 – 1973 (Reaffirmed 1992) – Specification for steel rivets and stay bars for boilers.
According to Indian standard specifications, the rivet heads are classified into the following three types :
The snap heads are usually employed for structural work and machine riveting.
The counter sunk heads are mainly used for ship building where flush surfaces are necessary.
The conical heads (also known as conoidal heads) are mainly used in case of hand hammering.
The pan heads have maximum strength, but these are difficult to shape.
Caulking:
In order to make the joints leak proof or fluid tight in pressure vessels like steam boilers, air receivers, tanks, etc., a process known as caulking is employed.
In this process, a narrow, blunt tool called a caulking tool, about 5 mm thick and 38 mm in breadth, is used.
The edge of the tool is ground to an angle of 80°. The tool is moved after each blow along the edge of the plate, which is planed to a bevel of 75° to 80° to facilitate the forcing down of the edge.
It is seen that the tool burrs down the plate at A in Figure A, forming a metal-to-metal joint. In actual practice, both the edges at A and B are caulked. 
The head of the rivets, as shown at C, are also turned down with a caulking tool to make the joint steam tight.
A great deal of care is taken to prevent injury to the plate below the tool.
Fullering:
A more satisfactory way of making the joints staunch is known as fullering which has largely superseded caulking.
In this case, a fullering tool with a thickness at the end equal to that of the plate is used in such a way that the greatest pressure due to the blows occur near the joint, giving a clean finish, with less risk of damaging the plate.
Q. 1. A double riveted lap joint is made between 15 mm thick plates. The rivet diameter and pitch are 25 mm and 75 mm respectively. If the ultimate stresses are 400 MPa in tension, 320 MPa in shear and 640 MPa in crushing, find the minimum force per pitch which will rupture the joint. If the above joint is subjected to a load such that the factor of safety is 4, find out the actual stresses developed in the plates and the rivets.
Q. 2. Find the efficiency of the following riveted joints :
Assume: Permissible tensile stress in plate = 120 MPa, Permissible shearing stress in rivets = 90 MPa, Permissible crushing stress in rivets = 180 MPa
Q. 3. A double riveted double cover butt joint in plates 20 mm thick is made with 25 mm diameter rivets at 100 mm pitch. The permissible stresses are : σt = 120 MPa; τ = 100 MPa; σc = 150 MPa
Find the efficiency of joint, taking the strength of the rivet in double shear as twice than that of single shear.
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