2.1 RIVETING
Rivets of various shapes are made from mild steel bars as shown in fig.2.1.The rivet head is generally round and called a button head.The size of the rivets is the diameter of the shank.
fig:2.1
A flat head rivet may be used if the space is limited.Countersunk rivets are used to get a flatter surface.
Holes are drilled or punched in the plates to be riveted.The size of holes are 1.5mm larger than the size of rivets upto 25mm.For rivets of diameter more than 25mm,the diameter of the holes should be 2mm larger than the diameter of the rivets.For riveting,a rivet is heated uniformly to light red colour and then placed in the hole.The head is kept pressed on the plate,while the projected shank is hammered to form another head.On cooling the rivets contracts and grips the plate tightly.
Rivets may be driven by hydraulic or by pneumatic power,The rivets may br classified according to the method of driving them as follows:
(i)Power-driven shop rivets are power-driven in the fabrication shop.
(ii)Power-driven field rivets are power driven at site.Their strenght is less than the strength of the power driven shop rivets.
(iii)Hand-driven rivets have less strength than the power-driven rivets.
(iv)Cold-driven rivets of diameter more than 10mm are not permitted by specifications.
2.2 BOLTED JOINTS
Bolts may be used in place of rivets for structures not subjected to vibrations.the cost of bolts is more but it takes less time to fabricate structure with bolted connections.The fabrication work with bolts is noiseless and less skilled labours can also handle it.The stress concentration at the bolts can also be reduced by using high strength friction bolts.
The following types of bolts are used in the structures:
(i)Black Bolts
Hexagonal black bolts are commomly used in steel works.They are made from low or mediu
carbon steels and should conform to IS:1363-1984.In metric sizes,black bolts are produced in sizes of 5mm to 36mm diameter and of lengths 3 to 10 times the diameter.They are designated as black bolts Md*L-IS:1363,where d=diameter and L=length of the bolts.
(ii)Precision and Semi Precision Bolts
They are also known as Close tolerance bolts conforming to IS:1364-1967.Sometimes to prevent excessive slip,close tolereance bolts are provided in holes of 0.15 to 0.12mm oversize,Thios may cause difficulty in alignment and delay in the progess of work.
(iii)High-strenth friction grip bolts
HSFG bolts have a high yield strength.They should conform to IS:3757-1985.They are tightened by
torque wrenches and require hardened washers to induce initial tension,which causes friction between the plate surfaces.Due to friction,there is no slip in the joint and therefore the joints with HSFG Bolts are called friction type or non-slip joints.
2.3 TYPES OF RIVETED AND BOLTED JOINTS
There are two types of riveted or bolted joints:
(i)Lap joint
The first is the lap joint in which the plates to be connected over lap each other.The lap joint may have single-row,staggered or overlap each other. The lap joint may have single-row, staggered or chain riveting.
(ii)Butt joint
The second is the butt joint in which the plates to be connected butt against each other and the connection is made by providing a cover plate on one or both sides of joint.The butt joint may have a single row or staggered or chain riveting.
2.4 Some Important Definition used for Riveted and Bolted joints
(i)Nominal diameter
The diameter of the shank of rivet before riveting,is called the nominal diameter.For a bolt,the diameter of the unthreaded portion os the shank is calles its nominal diameter.
(ii)Effective diameter or gross diameter
The effective or gross diameter of rivet is equal to the diameter of the hole it fills after riveting.For a bolt,the nominal diameter is same as the gross diameter.
(iii)Gross area
The gross area of a rivet or bolt is given by its gross diameter.
(iv)Net area
The net area of a bolt is the area at the root of the thread.
(v)Pitch
The distance between centres of any two adjacent rivets is called pitch.
Diagonal pitch : The distance between two centres of any two adjacent rivets in the diagonal direction is called diagonal pitch.
Staggered pitch : The distance between any two consecutive rivets in a zig-zag riveting,measured parallel to the direction of stressnin the member is called staggered pitch.
(vi)Gauge
A row of rivets parallel to the direction of force is called a gauge line.The normal distance between two adjacent gauge lines is called the gauge distance.
(vii)Edge distance
It is the distance between the edge of a member or cover plate of the centre of the earest rivet hole.
(viii)Proof load
Initial tension in HSFG bolts is known as proof load of the bolt.
(ix)Slip factor
Coefficient of friction in friction type joint is known as slip factor.
(x)Property class
Bolts are grouped under different grades or property classes depending upon their strength.For example,property class 4.6 indicates that the nominal ultimate tensile strength is 400 Mpa and the nominal yield stress is 0.6*400 = 240Mpa
TABLE:2.1 Rivet diameter,Pitch and Edge Distance
= actual edge distance 2 x effective dia of rivet or bolt | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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