Clutch

CLUTCH:

          A clutch is a machine member used to connect a driving shaft to a driven shaft so that the driven shaft may be started or stopped at will, without stopping the driving shaft. The use of a clutch is mostly found in automobiles. A little consideration will show that in order to change gears or to stop the vehicle, it is required that the driven shaft should stop, but the engine should continue to run. It is, therefore, necessary that the driven shaft should be disengaged from the driving shaft. The engagement and disengagement of the shafts is obtained by means of a clutch which is operated by a lever.

Failures of a Riveted Joint

Failures of a Riveted Joint:

A riveted joint may fail in the following ways :
1. 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.

2. 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. Shearing of the rivets. The plates which are connected by the rivets exert tensile stress on the rivets, and if the rivets are unable to resist the stress, they are sheared off as shown in Fig. 9.15.

It may be noted that the rivets are in *single shear in a lap joint and in a single cover butt joint, as shown in Fig. 9.15. But the rivets are in double shear in a double cover butt joint as shown in Fig.9.16. The resistance offered by a rivet to be sheared off is known as shearing resistance or shearing strength or shearing value of the rivet.

Let d = Diameter of the rivet hole,
τ = Safe permissible shear stress for the rivet material, and
n = Number of rivets per pitch length.

We know that shearing area,

∴ Shearing resistance or pull required to shear off the rivet per pitch length,

4. 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. 9.17. 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.

Types of Rivet Heads

Types of Rivet Heads:

          According to Indian standard specifications, the rivet heads are classified into the following
three types :

1. Rivet heads for general purposes (below 12 mm diameter) as shown in Fig. 9.3, according to IS : 2155 – 1982 (Reaffirmed 1996).

2. Rivet heads for general purposes (From 12 mm to 48 mm diameter) as shown in Fig. 9.4, according to IS : 1929 – 1982 (Reaffirmed 1996).

 

3. Rivet heads for boiler work (from 12 mm to 48 mm diameter, as shown in Fig. 9.5, according to IS : 1928 – 1961 (Reaffirmed 1996).

 

Material of Rivets

Material of Rivets:

          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 consideration, then the 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.

Methods of riveting

Methods of riveting:

First the plates are punched and reamed or drilled. The size of the hole made is usually 1.5mm larger than the nominal diameter of the rivets as shown in the fig.

 

The drilled plates rae seperated for a while to remove any burrs or chips, and then kept together. A cold rivet is introduced into the plates in case of structural joints and a red-hot rivet, in case of leak-proof joints such as pressure vessels. Then with the help of the backup bar and die, hammer blows, are applied and riveting is done as shown in fig. when the rivet cools, the longitudinal tension caused due to thermal conduction holds the plates firmly together.

When we use cold rivet for joining it is called as Cold riveting, and when hot rivets used  it is known as hot riveting.