Types of clutches

Types of Clutches:
Following are the two main types of clutches commonly used in engineering practice :
1. Positive clutches                2. Friction clutches.

1. Positive Clutches:
The positive clutches are used when a positive drive is required. The simplest type of a positive clutch is a jaw or claw clutch. The jaw clutch permits one shaft to drive another through a direct contact of interlocking jaws. It consists of two halves, one of which is permanently fastened to the driving shaft by a sunk key. The other half of the clutch is movable and it is free to slide axially on the driven shaft, but it is prevented from turning relatively to its shaft by means of feather key. The jaws of the clutch may be of square type as shown in Fig. or of spiral type as shown in Fig.


A square jaw type is used where engagement and disengagement in motion and under load is not necessary. This type of clutch will transmit power in either direction of rotation. The spiral jaws may be left-hand or right-hand, because power transmitted by them is in one direction only. This type of clutch is occasionally used where the clutch must be engaged and disengaged while in motion. The use of jaw clutches are frequently applied to sprocket wheels, gears and pulleys. In such a case, the non-sliding part is made integral with the hub.

2.Friction Clutches:
A friction clutch has its principal application in the transmission of power of shafts and machines which must be started and stopped frequently. Its application is also found in cases in which power is to be delivered to machines partially or fully loaded. The force of friction is used to start the driven shaft from rest and gradually brings it up to the proper speed without excessive slipping of the friction surfaces. In automobiles, friction clutch is used to connect the engine to the drive shaft. In operating such a clutch, care should be taken so that the friction surfaces engage easily and gradually bring the driven shaft up to proper speed. The proper alignment of the bearing must be maintained and it should be located as close to the clutch as possible. It may be noted that :
1. The contact surfaces should develop a frictional force that may pick up and hold the load with reasonably low pressure between the contact surfaces.
2. The heat of friction should be rapidly *dissipated and tendency to grab should be at a minimum.
3. The surfaces should be backed by a material stiff enough to ensure a reasonably uniform distribution of pressure.

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.