KEY& COUPLINGS
Unit-2
LECTURE-5
KEY
A key is a piece of mild steel inserted between the shaft and hub or
boss of the pulley to connect these together in order to prevent relative
motion between them.
It is always inserted parallel to the axis of the shaft.
Keys are used as temporary fastenings and are subjected to considerable
crushing and shearing stresses.
A keyway is a slot or recess in a shaft and hub of the pulley to
accommodate a key.
Types of Keys
1. Sunk keys, 2. Saddle keys, 3. Tangent
keys, 4. Round keys, and 5. Splines.
Sunk Keys
1.
2.
3.
4.
5.
Rectangular sunk key
Square sunk key.
Parallel sunk key.
Gib-head key.
Feather key
What is the effect of key way cut into the shaft?
Placing of keyway the cross section area of shaft is slightly reduced and due
to sharp edges and corners, the stress concentration is also occurred which in turn
reduce the strength and power transmitting capacity of the shaft.
Splines
Sometimes, keys are made integral with the shaft
which fits in the keyways broached in the hub. Such
shafts are known as splined shafts as shown in Fig.
These shafts usually have four, six, ten or sixteen
splines.
The splined shafts are relatively stronger than
shafts having a single keyway.
The splined shafts are used when the force to be transmitted is
large in proportion to the size of the shaft as in automobile transmission
and sliding gear transmissions. By using splined shafts, we obtain axial
movement as well as positive drive is obtained.
http://www.youtube.com/watch?v=IKywZ730JFs
http://www.youtube.com/watch?v=2ra-CapAQeg
COUPLING
Requirements of a Good Shaft Coupling
Shafts are usually available up to 7 m 1. It should be easy to connect or disconnect.
length due to inconvenience in transport. In 2. It should transmit the full power from one
shaft to the other shaft without losses.
order to have a greater length, it becomes
necessary to join two or more pieces of the 3. It should hold the shafts in perfect
shaft by means of a coupling.
alignment.
Purposes
4. It should reduce the transmission of shock
loads from one shaft to another shaft.
1. To provide for the connection of shafts of
5. It should have no projecting parts.
units that are manufactured separately
such as a motor and generator and to
Types of couplings
provide for disconnection for repairs or
alternations
Rigid coupling
Muff or sleeve coupling.
2. To provide for misalignment of the shafts or
Split muff or clamp coupling.
to introduce mechanical flexibility.
Rigid flange coupling.
3. To reduce the transmission of shock loads
from one shaft to another.
4. To introduce protection against overloads.
5. It should have no projecting parts.
Flexible coupling
Bushed pin type Flexiable coupling
Oldham coupling
Universal coupling
antages of flexible coupling over rigid coupling T = Torque to be transmitted by the
coupling, and
c = Permissible shear stress for
the material of the sleeve which is cast
iron.
The safe value of shear stress for cast
iron
may be taken as 14 MPa.
http://www.youtube.com/watch?v=Uddhk4e3BPU
leeve or Muff-coupling
We know that torque transmitted by a
hollow section
2. Design for key
The length of the coupling key is
atleast equal to the length of the
sleeve (i.e. 3.5 d). The coupling key is
usually made into two parts so that
the length of the key in each shaft
Outer diameter of the sleeve,D = 2d + 13 mm
Length of the sleeve,L = 3.5 d
where d is the diameter of the shaft.
Procedure
1. Design for sleeve The sleeve is
designed by considering it as a
