Riveted joints classified into two types
1. Lap joint, and 2. Butt joint.
1. Lap Joint
A lap joint is that in which one plate overlaps the other and the two plates are then riveted together.
2. 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.
Further, types of joints as shown in below
1. Lap joints
a) Single lap joint
b) Double lap joint.
i) Chain riveting.
ii) Zig-zag riveting.
c) Triple lap joint.
i) Chain riveting.
ii) Zig-zag riveting.
2. Butt joints
a) Double strap butt joint.
i) Chain riveting.
ii) Zig-zag riveting.
b) Triple strap butt joint.
i) Chain riveting.
ii) Zig-zag riveting.
1. Lap joint, and 2. Butt joint.
1. Lap Joint
A lap joint is that in which one plate overlaps the other and the two plates are then riveted together.
2. 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.
Further, types of joints as shown in below
1. Lap joints
a) Single lap joint
b) Double lap joint.
i) Chain riveting.
ii) Zig-zag riveting.
c) Triple lap joint.
i) Chain riveting.
ii) Zig-zag riveting.
2. Butt joints
a) Double strap butt joint.
i) Chain riveting.
ii) Zig-zag riveting.
b) Triple strap butt joint.
i) Chain riveting.
ii) Zig-zag riveting.
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 It is
usually denoted by p.
2. Back pitch. It is the
perpendicular distance between the centre lines of the successive rows. It is
usually denoted by Pb.
3. Diagonal pitch. It is the
distance between the centres of the rivets in adjacent rows of zig-zag riveted
joint. It is usually denoted by Pd.
4. Margin or marginal pitch. It is the
distance between the centre of rivet hole to the nearest edge of the
plate. It is usually denoted by m.
Caulking and Fullering:
In order to make the joints leak proof or fluid tight
in pressure vessels like steam boilers, air receivers and tanks etc. a process known
as caulking is employed. In this process, a narrow blunt tool
called 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 edge. It is seen that the tool burrs down the plate at A
in
Fig. (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 a joint steam tight. A great care
is taken to prevent injury to the plate below the tool. 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. A fullering process is shown in Fig.
FAILURES OF A RIVETED JOINT
1. Tearing of the plate across a row of
rivets.
2. Shearing of the rivets
3. Crushing of the plate or rivets
4. Tearing of the plate at an edge
1. Tearing of the plate across a row of rivets.
Due to the tensile stresses
in the main plates, the main plate or cover plates may tear off across a row of
rivets as shown in Fig. In such cases, we consider only one pitch length of the
plate, since every rivet is responsible for that much length of the plate only.
The resistance offered by
the plate against tearing is known as tearing resistance or tearing
strength or tearing value of the plate.
Let p = Pitch of the rivets,
d = Diameter
of the rivet hole,
t = Thickness
of the plate, and
σt
= Permissible tensile stress for the plate material.
We know that tearing area
per pitch length,
At = (p –
d ) t
∴ Tearing
resistance or pull required to tear off the plate per pitch length,
Pt = At.σt
= (p – d)t.σt
When the tearing resistance
(Pt) is greater than the applied load (P) per pitch length, then
this
type of failure will not
occur.
3. Crushing
of the plate or rivets. Sometimes,
the rivets do not actually shear off under the tensile stress, but are crushed
as shown in Fig. Due to this, the rivet hole becomes of an oval shape and hence
the joint becomes loose. The failure of rivets in such a manner is also known
as bearing failure. The area which resists this action is the
projected area of the hole or rivet on diametral plane.
The resistance offered by a
rivet to be crushed is known as crushing resistance or crushing
strength or bearing value of the rivet.
Let d = Diameter of the rivet hole,
t = Thickness
of the plate,
σc
= Safe permissible crushing stress for the rivet or plate material, and
n = Number of
rivets per pitch length under crushing.
We know that crushing area
per rivet (i.e. projected area per rivet),
Ac = d.t
∴ Total
crushing area = n.d.t
and crushing resistance or
pull required to crush the rivet per pitch length,
Pc = n.d.t.σc
When the crushing resistance
(Pc) is greater than the applied load (P) per pitch length, then
this type of
failure will occur.
Note : The number
of rivets under shear shall be equal to the number of rivets under crushing.
4.
Tearing of the plate at an edge. A joint may
fail due to tearing of the plate at an edge as shown in Fig. This can be
avoided by keeping the margin, m = 1.5d, where d is the
diameter of the rivet hole.
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
Rivets can be used for Attaching Fastening Wall, Ceiling Decorations, Hinges, Door Knobs, Drawer Pulls, Handles, Woodworking Applications,Industrial Applications.
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