Couplings by wishall

MACHINE DESIGN
MODULE-5
COUPLINGS

COUPLING

A coupling is defined as a mechanical device that
permanently joins two rotating shafts to each
other.

Example: Driver & driven shaft of pump.

Difference between coupling & clutch

Coupling is a permanent connection where as clutch
can diconnect and connect as and when required.

Types of coulpings
1. Oldham coupling:

Used for two parallel shafts with smaller distance
between their axes

2. Hookes Coupling= Universal coupling:

Used when two shafts having intersecting axes

Types of misalignment
Lateral (Parallel)
Angular
Axial

Misalignment exists because:
1. Delflection of shaft due to lateral forces
2. error in shaft mounting due to manufacturing
tolerances
3. Use of two separately manufactured units-
example- elctric motor or worm gear box
4. Thermal expansion of parts.

A good coupling should satisfy following
requirements:
1. Should be capable of transmitting torque from driving
shaft to driven shaft
2. Should keep two shafts in proper alignment
3. should be easy to assemble and disassemble for
repairs & alterartions.
4. Should be protected by guard or by providing suitable
shape to projecting parts
As, the failure of revolving bolt head, nuts,keyheads and
other projecting part may cause accidents.

Rigid Couplings & Flexible Coplings
Rigid couling Flexible coupling
Can not tolerate misalignment Can tolerate 1.5 º of angular
misalignment & 0.5 mm of axial
displacement.
Do not absorbs shocks Absorbs shocks & vibrations
Simple & inexpensive Costlier due to additional parts.
More popular

Bushed -Pin Flexible coupling

Similar to rigid coupling. Instaed of bolts, Pins &
rubber bush are used

Advantages of Bushed-Pin type flexible coupling
1. It can tolerate Lateral (Axial) misalignment upto 0.5
mm & Angular misalignment of 1.5 º.
2. It prevents transmission of shocks from one shaft to
another and absorbs vibrations.
3. It can be used for transmitting high toques.
1. Cost is more due to additional parts
2. It requires more radial space compared with other
types of couplings.
Disadvantages of Bushed-Pin type flexible coupling

Empirical relations

Shaft diameter =d
 Outside diameter of Hub= dh
= 2d
 Length of Hub/ effective length of key = lh
=1.5d

Thickness of output flange = t = 0.5 d

Pitch circle diameter =D = 3d to 4 d

Thickness of protective rim = t1= 0.25 d

Diameter of pin = d1 = 0.5 d /[( N)^(1/2)]

Number of pins=N

Analysis of Bushed-Pin coupling : (1/3)
Mt= Torque
transmitted
P= force acting on
each rubber or pin
D= Pitch circle
diameter of Pins
(mm)
N = Number of
bushes or pins.
Mt = P x Radius
Mt = P x (D/2) x N ---- (1)
P= Projected area x Intensity of pressure
P= (Db
x lb
) x Pm
----- (2)

Db= Outer diameter of bush
(mm)
lb= Length of bush (mm)
Pm = Permissible intensity of
pressure (N/mm2) = 1 for
rubber bush and cast iron
flange.
Ration of lb to Db is usually=1
.: lb=Db
Projected area= Db x lb ----(2)
Putting this value in eqn (1)
Mt= (Db
x lb
x Pm
) x D/2 x N -----(3)
Mt= ½ x Db
2
DN -----(4)---- Outer diameter of rubber bush
is obtained by this equation

Shear stress :
T = P/( 3.14 d1
2
) ---(5)
P = 2 Mt
/ ( D x N)
.: T = 8 Mt
/( 3.14 d2
DN) -----(6)
According to Indian Standerds:
Allowable Shear Stress for ins is 35 N/mm2
Component Material
Flanges Grey cast iron Grade FG 200
Pins Carbon Steel

More about Bushed-pin Flexible coupling
1. Pin is also aubjected to bending moment
2. Recommended fit for shft & hub is H7
-j7
3. Maximum allowabale peripheral speed of coupling is 30 m/s
4. Features of flexible coupling:
a. Can accommodate gap between driving & driven flanges
b. Permissible stress is Pm =1 N/mm2
which is very less than rigid
type of coupling. Hence corresponding diameter of pin and PCD
is larger.
5. Location of coupling:
Should be as near as possible towards bearings to avoid Bending
moment due to coupling forces.

1.Shaft Diameter
2.Dimension of flanges
3.Diameter of Pins
4.Diameter of Bushes
5.Dimension of Keys
Design Procedure for Flexible coupling: (1/3)

Muff Coupling= Sleeve coupling
=Box coupling

Design Procedure for Flexible coupling: (1/3)
I Shaft Diameter
a Mt= [60 x 106 (kW) ]/ [ 2 x3.14 x n]
b T = [16 Mt] / [ 3.14 x d3]
II Dimensions of flanges by Emperical relations
Shaft diameter =d
Outside diameter of Hub= dh = 2d
Length of Hub/ effective length of key = lh =1.5d
Thickness of output flange = t = 0.5 d
Pitch circle diameter =D = 3d to 4 d
Thickness of protective rim = t1= 0.25 d
Diameter of pin = d1 = 0.5 d /[( N)^(1/2)]
Number of pins=N
Torsional Shear Stress:
T = [Mt r ]/ J
J

Muff Coupling
Construction:
It consists of sleeve
or hollow cylinder
which is fitted over
the ends of input and
output shafts by
means of sunk key
Power Transmission in Muff Coupling:

Advantages of Muff coupling:
1. Simplest Form of coupling.
Only two Parts: Sleeve & Key
Simple to design & manufacture.
2. No projecting parts except key head. External
Surface is Smooth & safe.
3. Compact construction with small radial
dimensions
4. Cheaper than other types of couplings.
Muff Coupling

Disadvantages of Muff coupling:
1.Difficult to assemble or disassemble (dismantle).
2.Rigid type of coupling & requires accurate
alignment of shafts
3. No flexible element hence do not absorb shocks &
vibrations.
4. It requires more axial space compared with flange
coupling
Less popular and can be used for shaft diameter
upto 70 mm.
Muff Coupling

Emperical Relations:
D= (2d +13)
L = 3.5 d
D= Outer diameter of sleeve
L= Length of sleeve
D= Diameter of shaft
Muff Coupling

Design procedure for Muff coupling:
1. Calculate diameter of each shaft:
T= [60x 106
x (kW)]/2πn , T =[16 T ]/πd3
2. Calculate dimensions of sleeve using ratios:
D= (2d+13) , L= 3.5 d
3. Check for torsional shear stress induced in the sleeve
T =[ T.D]/ [(π/16) (D4
-d4
)
4.Determine standard dimensions of key
Table
5. Check shear & compressive stress in key.
T = [2T ]/dbl , σ =[4 T ]/dhl
Muff Coupling

Clamp Coupling= Split Muff Coupling=
Compression coupling

Construction:
1.Coupling sleeve is made up of two halves that are
clamed together with bolts.
2.Number of bolts =2,4,8 etc. (multiples of four)
3.Bolts are placed in the recesses formed in the
sleeve halves.

Advantages of clamp coupling:
1. Easy to assemble & dismantle
2. Easy to remove without shifting the shaft in axial
direction unlike solid muff coupling.
3. As compared to flange coupling, small diametral
dimensions.
Application of clamp coupling:
Line shaft in power transmission which has become
obsolete nowadays.

Disadvantages of clamp coupling:
1. Difficult in dynamic balancing. So cannot be used
for high speed applications
2. unsuitable for shock loads
3. It is necessary to provide a guard for coupling to
comply with Factory Regulation Act.

Muff coupling Vs. Clamp Coupling
Muff coupling Clamp Coupling
Torque is transmitted by
shear resistance of keys Torque is transmitted by
1] Friction between sleeve
halves & shaft
2]shear resistance of keys

Ratios:
1. D=2.5 d
2. L = 3.5 d
Where,
D= Outer diameter of Sleeve halves (mm)
L= Length of Sleeve (mm)
D= Diameter of shaft (mm)
3. Diametr of clamping bolt:
D1
= 0.2d + 10 …... When d< 55 mm
D1
= 0.15d + 15 …...When d > 55 mm

Ratios:
4. Clamping force on each bolt:
P1
= π/4. D1
2
.σt
5. Claming force on each shaft:
N =P1
n/2
6. Mt
= [ f.d. P1
.n] /2
Or
P1 =[2 Mt]/ [ f.d.N]

Since rigid type of coupling, it requires precise location.
Spigot & recess are provided for precise location.
Spigot=cylindrical projection provided on driving flange.
Recess= Corresponding hollow part on driven flange.
Spigot & recess are machined with more accuracy.
Rigid flange coupling

Construction:
Two flanges each key to one shaft.
Both flanges are connected by means of bolts arranged on
PCD.
Power transmission:
Rigid flange coupling

Couplings by wishall

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