Machine Design Lec3

Machine Design

UET, Taxila

Lecture: 3 “riveting”

Rivets and Riveting:

A Rivet is a short cylindrical rod having a head and a tapered tail. The main body of the rivet is called shank (see next figure).

According to the standard specifications, rivet heads are of various types.

Rivets heads for general purposes are specified by the standards as follows: (below 12.7 mm diameter) (from 12.7 mm to 48 mm diameter).

Rivet heads used for boiler works are specified by special specifications. Dimensions of the heads are in special tables in any machine design handbook.

Riveting Operation

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 the head by means of a backing up bar

Necessary force is applied at the tail end with a die until the tail deforms plastically to the required shape.

This process is shown in next figure.

Riveting Operation Schematic Representation

Depending upon whether the rivet is initially heated or not, the riveting operation can be of two types:

(a) cold riveting: riveting is done at ambient temperature

and (b) hot riveting where rivets are initially

heated before applying force. After riveting is done, the joint is heat-

treated by quenching and tempering.

Types of riveted joints and joint efficiency:

Riveted joints are mainly of two types: 1. Lap joints 2. Butt joints

Lap Joints:

The plates that are to be joined are brought face to face such that an overlap exists, as shown in next figure. Rivets are inserted on the overlapping portion.

Lap Joint

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 a- single riveted lap joint,

b- double or triple riveted lap joint etc.

When multiple joints are used, the arrangement of rivets between two neighbouring rows may be of two kinds:

1- Chain Riveting 2- Zigzag Riveting

1- In chain riveting (chain arrangement) the adjacent rows

have rivets in the same transverse line. 2- In zigzag riveting, on the other hand,

the adjacent rows of rivets are staggered.

Different types of lap joints are sketched

in the following figures.

Single rivet lap joint

Double Riveted Lab Joint (Chain)

Zigzag Riveting

Butt Joints

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 strap or double strap butt joints.

A single strap butt joint is shown in next figure.

Butt joint with single strap

Like lap joints, the arrangement of the rivets may be of various kinds, namely, single row (for each plate), double or triple chain or zigzag.

A few types of joints are shown in next figures

Single riveted butt joint with single and double strap

Double riveted butt joint with single and double straps

Zigzag double riveted butt joint with single and double straps

Efficiency of riveted joints

Efficiencies of riveted joints (in %)

An efficiency is usually expressed as a per cent. To express it thus, multiply the ratio strength of joint ÷ strength of solid plate, by 100.

Important terms used in riveted joints:

Few parameters, which are required to specify arrangement of rivets in a riveted joint are as follows:

a- Pitch. b- Back Pitch c- Diagonal pitch d- Margin of marginal pitch.

a- Pitch

Pitch is the distance between two similar points (e.g centers) of the consecutive rivets in a single row.

(usual symbol p)

b- Back Pitch

Back Pitch: This is the shortest distance between two successive rows in a multiple riveted joint.

(usual symbol pt or pb )

C- Diagonal Pitch

Diagonal pitch: This is the distance between the centers of rivets in adjacent rows of zigzag riveted joint.

(usual symbol pd)

d- Margin or marginal pitch

Margin or marginal pitch: This is the distance between the centre of the rivet hole to the nearest edge of the plate.

(usual symbol m) m = 1.5 d

Important design parameters in riveting

Strength of riveted joint

Strength of riveted joint

Strength of a riveted joint is evaluated taking all possible failure paths in the joint into account. Since rivets are arranged in a periodic manner, the strength of joint is usually calculated considering one pitch length of the plate. There are four possible ways a single rivet joint may fail as follows:

possible ways a single rivet joint may fail

a) Tearing of the plate B) Shearing of the rivet c) Crushing of rivet d) Tearing of the plate at edge

a) Tearing of the plate:

If the force is too large, the plate may fail in tension along the row (see figure 4).

Fig. (4) Failure of plate in tension

The maximum force allowed in this case is:

b) Shearing of the rivet:

The rivet may shear as shown in figure 5.

Fig. (5): Failure of a rivet by shearing

The maximum force withstood by the joint to prevent shear failure is

c) Crushing of rivet:

If the bearing stress on the rivet is too large the contact surface between the rivet and the plate may get damaged. (see figure 6).

Fig. (6): Failure of the rivet by crushing

With a simple assumption of uniform contact stress the maximum force allowed is:

d) Tearing of the plate at edge:

If the margin is too small, the plate may fail as shown in figure 7.

Fig. (7): Tearing of the plate at the edge.

To prevent the failure a minimum margin of (1.5 d ) is usually provided.

2. Efficiency:

Efficiency of the single riveted joint can be obtained as ratio between the minimum of P1, P2 & P3 and the load carried by a solid plate which is

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 :

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.

3. Design of rivet jointsa- Diameter (d) if (t) is more than 8 mm

b- Diameter (d) if (t) is less than 8 mm

d constraint and Pitch

p, m & pb constraint

Multi-rivets considerations

If (t) denotes the thickness of the plates joined ; d, the diameter of the holes; n1, the number of rivets in a row ; and w. the width of the plate or bar; then the net section = (w - n1d) t.

Let St denote the tensile working strength of the plate ; then the Force for the un-riveted plate is (wtSt) , and the reduced tensile Force is

{w - n1d) t St.

n1, n2 and n3

n1, the number of rivets in a row n2 denoting the total number of rivets

in the joint; and n3 denoting the total number of rivets

in a lap joint, and one-half the number of rivets in a butt joint.

Distance between raws

Examples:

Example No: (1),

Answer:

Answer: hole diameter from table = 34.5

All failure mechanisms has to be considered separately

Diamond Riveting in structural joint

Example: 4