38177922 design of keys and couplings
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INDUS INSTITUTE OF
TECHNOLOGY & ENGINEERING
DEPARTMENT OF
MECHANICAL ENGINEERING
Name : Keval Patel
Subject Code : 131902
Subject Name : Machine Design and Industrial
Drafting
B.E Mechanical Engineering , 3rd Semester
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Design of keys and
couplings
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Keys
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.
Keys are used as temporary fastenings and are subjectedto consider able crushing and shearing stresses.
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Types of keys
Sunk keys
Saddle keys
Tangent keys Round keys
Splines
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1. Sunk keys
Rectangular sunk key
w = d / 4 ; and thickness of key, t = 2w / 3 = d / 6
Square sunk key
w = t = d / 4
Gib-head key
w = d / 4 ;
t = 2w / 3 = d / 6
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2. Saddle keys
1. Flat saddle key: Aflat saddle key is a taper key which fits in a
keyway in the hub and is flat on the shaft as shown in Fig.
2. Hollow saddle key: A hollow saddle key is a taper key which
fits in a keyway in the hub and the bottom of the keyis shaped
to fit the curved surface of the shaft.
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3. Tangent keys
The tangent keys are fitted in pair at right angles as shown in
Fig.
Each key is to withstand torsion in one direction only.
These are used in large heavy duty shafts
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4. Round Keys
Round keys are circular in section and fit into holes drilled partly
in the shaft and partly in the hub.
Round keys are usually considered to be most appropriate for
low power drives.
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Strength of a Sunk Key
T= Torque transmitted by the shaft,
F= Tangential force acting at the circumference of the shaft,
d= Diameter of shaft,
l= Length of key,
w= Width of key.
t= Thickness of key, and
and c = Shear and crushing stresses for the material of key.
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Considering shearing of the key
F = Area resisting shearing Shear stress = l w
Torque transmitted
Considering crushing of the key, the tangential crushing
force acting at the circumference of the shaft,
F = Area resisting crushing Crushing stress =
Torque transmitted by the shaft,
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The key is equally strong in shearing and crushing, if
In order to find the length of the key to transmit full power of
the shaft, the shearing strength of the key is equal to the
torsional shear strength of the shaft.
We know that the shearing strength of key,
(i)
and torsional shear strength of the shaft,
(ii)
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From equations (i) and (ii), we have
When the key material is same as that of the shaft , then =1
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SHAFT
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Shaft coupling
Shafts are usually available up to 7 meters length due to
inconvenience in transport. In order to have a greater length, it becomes necessary to join
two or more pieces of the shaft by means of a coupling.
Shaft couplings are used in machinery for:
1. Connection of shafts of such as a motor and generator
and for disconnection for repairs or alternations.
2. For misalignment of the shafts or mechanical flexibility.3. To reduce the transmission of shock loads
4. To introduce protection against overloads.
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Requirements of a Good Shaft Coupling
1. It should be easy to connect or disconnect.
2. It should transmit the full power from one shaft to the
other shaft without losses.
3. It should hold the shafts in perfect alignment.
4. It should reduce the transmission of shock loads from
one shaft to another shaft.
5. It should have no projecting parts.
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Types of Shafts Couplings
Shaft couplings are divided into two main groups as follows:
1. Rigid coupling: It is used to connect two shafts which are
perfectly aligned.
(a) Sleeve or muff coupling
(b) Clamp orsplit-mufforcompression coupling
(c) Flange coupling.
2. Flexible coupling: It is used to connect two shafts having bothlateral and angular misalignment.
(a) Bushed pin type coupling
(b) Universal coupling
(c) Oldham coupling 17PREPARED BY: KEVAL PATELMECHANICAL ENGG. DEPT.
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Sleeve or Muff-coupling
It is the simplest type of rigid coupling, made of cast iron.
It consists of a hollow cylinder whose inner diameter is thesame as that of the shaft.
It is fitted over the ends of the two shafts by means of a gib
head key, as shown in Fig.
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Outer diameter of the sleeve, D = 2d + 13 mm
length of the sleeve, L = 3.5 d,
where d= diameter of the shaft.
T= Torque to be transmitted by the coupling,
c = Permissible shear stress for the material of the sleeve which
is cast iron.
Design for key:
The length of key is at least equal to the length of the sleeve
The coupling key is usually made into two parts so that the
length of the key in each shaft
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After fixing the length of key in each shaft, the induced shearing
and crushing stresses may be checked.
We know that torque transmitted,
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Clamp or Compression Coupling
It is also known as split muff coupling.
In this case, the muff or sleeve is made into two halves and arebolted together.
Number of bolts may be two, four or six.
Used for heavy duty and moderate speeds.
Advantage of this coupling is that the position of the shaftsneed not be changed for assembling or disassembling of the
coupling.
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Design of split muff coupling
The usual proportions of the muff for the clamp or compression
coupling are : Diameter of the muff or sleeve, D = 2d + 13 mm
Length of the muff or sleeve, L = 3.5 d
where d= Diameter of the shaft.22PREPARED BY: KEVAL PATEL
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Design of muff and key:
The muff and key are designed
in the similar way as discussed
in muff coupling
Design of clamping bolts:
Let T= Torque transmited by
the shaft,
d= Diameter of shaft,
db = Root or effective diameter
of bolt,
n = Number of bolts, t= Permissible tensile stress
for bolt material,
= Coefficient of friction
between the muff and shaft,
L = Length of muff.
We know that the force
exerted by each bolt
Force exerted by the bolts
on each side of the shaft
Letp be the pressure on the
shaft and the muff surfacedue to the force
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Frictional force between each shaft and muff,
F = pressure area =
Torque that can be transmitted by the coupling,
From this equation (root diameter of bolt) db cab be evaluated.24PREPARED BY: KEVAL PATELMECHANICAL ENGG. DEPT.
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Flange coupling
A flange coupling usually applies to a coupling having two
separate cast iron flanges. Each flange is mounted on the shaft end and keyed to it.
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Design of flange coupling
d= Diameter of shaft or inner diameter of hub,
D = Outer diameter of hub,
d1 = Nominal or outside diameter of bolt,
D1 = Diameter of bolt circle,
n = Number of bolts, tf= Thickness of flange,
s, b andk= Allowable shear stress for shaft, bolt and key
material respectively
c = Allowable shear stress for the flange material i.e. cast iron,
cb, andck= Allowable crushing stress for bolt and key
material respectively.
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1. Design forhub:
The hub is designed by considering it as a hollow shaft
The outer diameter of hub is usually taken as twice the
diameter of shaft.
Therefore from the above relation, the induced shearing stressin the hub may be checked.
The length of hub (L) is taken as 1.5 d
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2.Design for key:
The key is designed with usual proportions and then checked for
shearing and crushing stresses.
Length and width of key are related with the diameter of shaft.
From above equation shear stress and crushing stress of key can be
evaluated.
The material of key is usually the same as that of shaft. The length of
key is taken equal to the length of hub.29PREPARED BY: KEVAL PATEL
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3.Design for flange:
The flange at the junction of the hub is under shear while
transmitting the torque.
T= Circumference of hub Thickness of flange Shear stress of
flange Radius of hub
The thickness of flange is usually taken as half the diameter of
shaft.
Therefore from the above relation, the induced shearing stress
in the flange may be checked.
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4.Design forbolts:
The bolts are subjected to shear stress due to the torque
transmitted. The number of bolts (n)
Depends upon the diameter of shaft and the pitch circle
diameter of bolts (D1) is taken as 3d
Load on each bolt
Total load on all the bolts
Torque transmitted,
From this equation, the diameter of bolt (d1) may be
obtained
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Now the diameter of bolt may be checked in crushing.
We know that area resisting crushing of all the bolts
Crushing strength of all the bolts
Torque,
From this equation, the induced crushing stress in the bolts may
be checked
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Ex:1
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THANK YOU
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