shaft coupling
What is the shaft?
Shafts are usually available up to 7 meters in length due to
inconvenience in transport. To have a greater length, it becomes necessary to
join two or more pieces of the shaft using a coupling.
Manufacturing of the Shaft :
Shafts are generally manufactured by hot rolling and
finished to size by cold drawing or turning and grinding. The cold-rolled
shafts are stronger than hot-rolled shafts but with higher residual stresses.
The residual stresses may cause distortion of the shaft when it is machined,
especially when slots or keyways are cut. Shafts of larger diameter are usually
forged and turned to size in a lathe.
What is the purpose of the shaft?
Shaft couplings are used in machinery for several purposes,
the most common of which are the following :
1. To provide for the
connection of shafts of units that are manufactured separately such as a motor
and generator and to provide for disconnection for repairs or alternations.
2. To provide for the misalignment of the shafts or to
introduce mechanical flexibility.
3. To reduce the
transmission of shock loads from one shaft to another.
4. To introduce
protection against overloads.
What is the requirements of a good shaft coupling?
Requirements of a Good Shaft Coupling:
A good shaft coupling should have the following requirements
:
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
Describe the types of shafts?
Types of Shafts Couplings
Shaft couplings are
divided into two main groups as follows :
1.
Rigid coupling:
It is used to connect two shafts
that are perfectly aligned.
Types of Rigid coupling:
The following types of rigid coupling are
important from the subject point of view :
(a) Sleeve or muff coupling.
(b) Clamp or split-muff or
compression coupling
(c) Flange coupling.
What is 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 the same as that of the shaft. It is fitted over the ends of the
two shafts using a gib head key. The power is transmitted from one shaft to the
other shaft using a key and a sleeve. It is, therefore, necessary that all the
elements must be strong enough to transmit the torque.
The usual proportions of a cast
iron sleeve coupling are as follows:
Outer diameter of the sleeve, D =
2d + 13 mm
length of the sleeve, L = 3.5 d
d = is the diameter of the shaft.
Designing of sleeve or muff
coupling:
In designing a sleeve or
muff-coupling, the following procedure may be adopted.
1.
Design for sleeve the sleeve is designed by
considering it as a hollow shaft.
Let T = Torque be transmitted by the coupling
and Ï„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.
The torque transmitted by a hollow
section,
T = 
.
What is Clamp or
spilled muff or compression coupling:
It is also known as
split muff coupling. In this case, the muff or sleeve is made into two halves
and is bolted together. The halves of the muff are made of cast iron. The
shaft ends are made to about each other and a single key is fitted directly in
the keyways of both the shafts. One-half of the muff is fixed from below and
the other half is placed from above. Both the halves are held together by means
of mild steel studs or bolts and nuts. The number of bolts may be two, four, or
six. The nuts are recessed into the bodies of the muff castings. This coupling
may be used for heavy-duty and moderate speeds. The advantage of this coupling
is that the position of the shafts need not be changed for assembling or
disassembling the coupling
What is Flange
coupling?
Flange
Coupling is a driving coupling between rotating shafts that
consists of flanges one of which is fixed at the end of each shaft,
the two Flanges being bolted together with a ring of bolts to
complete the drive. A flange coupling is meant to bring two tube ends
together in a flush, sealed manner.
Describe the types of flange couplings?
Types of flange coupling:
The flange couplings are of the following
three types :
1.
Unprotected type flange coupling
2.
Protected type flange coupling.
3. Marine type flange coupling
unprotected type flange coupling:
A type of
coupling, in which each shaft is keyed to the boss of a flange with a countersunk key and the flanges are coupled together by means of bolts is called
unprotected type flange coupling. Mainly, three, four, or six bolts are used.
Protected
type flange coupling:
A type of coupling in which the protruding bolts and nuts are
protected by flanges on the two halves of the coupling, in order to avoid
danger to the workman is called protected type flange coupling.
Marine type flange coupling:
A type of flange coupling, in which the flanges are forged integral
with the shafts is called Marine type flange coupling. The flanges are held
together by means of tapered headless bolts, numbering from four to twelve
depending upon the diameter of the shaft.
2. Flexible coupling.
It is used to connect two shafts having both
lateral and angular misalignment.
The following types of flexible coupling are
important from the subject point of view:
(a) Bushed pin-type coupling,
(b) Universal coupling
(c) Oldham coupling.
What is Bushed pin-type coupling:
A
bushed-pin flexible coupling is a modification of the rigid type of flange
coupling. The coupling bolts are known as pins. The rubber or leather bushes
are used over the pins. The two halves of the coupling are dissimilar in construction.
A clearance of 5 mm is left between the face of the two halves of the coupling.
There is no rigid connection between them and the drive takes place through the
medium of the compressible rubber or leather bushes. In designing the
bushed-pin flexible coupling, the proportions of the rigid type flange coupling
are modified.
The main modification is to reduce the bearing pressure on
the rubber or leather bushes and it should not exceed 0.5 N/mm2. In order to
keep the low bearing pressure, the pitch circle diameter and the pin size is
increased.
Let l = Length of
bush in the flange,
d2 = Diameter of
bush,
pb = Bearing pressure on the bush or pin,
n = Number of pins
D1 = Diameter of the pitch circle of the pins.
We know that bearing load acting on each pin, W = pb × d2 ×
l
∴ Total bearing load on the bush or pins = W × n = pb ×
d2 × l × n
and the torque transmitted by the coupling,
The threaded portion of the pin in the right-hand flange
should be a tapping fit in the coupling hole to avoid bending stresses.
The threaded length
of the pin should be as small as possible so that the direct shear stress can
be taken by the unthreaded neck.
Direct shear stress due to pure torsion in the coupling
halves
Since the pin and the rubber or leather bush are not rigidly
held in the left-hand flange, therefore the tangential load (W) at the enlarged
portion will exert a bending action on the pin. The bush portion of the pin
acts as a cantilever beam of length l. Assuming a uniform distribution of the
load W along with the bush, the maximum bending moment on the pin,
We know that bending stress,
Since the pin is subjected to
bending and shear stresses, therefore the design must be checked either for the
maximum principal stress or maximum shear stress by the following relations :
Maximum principal stress = 
and the maximum shear stress on the pin = 
The value of maximum principal
stress varies from 28 to 42 MPa.
What is Universal (or Hooke’s) Coupling:
A universal or Hooke’s coupling is
used to connect two shafts whose axes intersect at a small angle. The
inclination of the two shafts may be constant, but in actual practice, it
varies when the motion is transmitted from one shaft to another. The main
application of the universal or Hooke’s coupling is found in the transmission
from the gearbox to the differential or back axle of the automobiles. In such
a case, we use two Hooke’s coupling, one at each end of the propeller shaft,
connecting the gearbox at one end and the differential on the other end. A
Hooke’s coupling is also used for transmission of power to different spindles
of multiple drilling machines. It is used as a knee joint in milling machines.
In designing a universal coupling, the shaft diameter and the pin diameter are
obtained as discussed below. The other dimensions of the coupling are fixed by
proportions.
Let d = Diameter of the shaft,
Dp = Diameter of a pin, and Ï„ and Ï„1 = Allowable shear stress for the material of the shaft and pin respectively.
We know that torque transmitted by
the shafts,
From this relation, the diameter
of shafts may be determined. Since the pin is in double shear, therefore the
torque transmitted
When a single Hooke's coupling is
used, the ratio of the driving and driven shaft speeds is given by
N = Speed of the driving shaft in
r.p.m.
N1 = Speed of the driven shaft in r.p.m
α = Angle of inclination of the shafts, and
θ = Angle of the driving shaft from the
position where the pins of the driving shaft fork are in the plane of the two
shafts.
We know the maximum speed of the driven
shaft,
And the minimum speed of the driven
shaft,
From above we see that for a single
Hooke’s coupling, the speed of the driven shaft is not constant but varies from
maximum to minimum. In order to have a constant velocity ratio of the driving and
driven shafts, an intermediate shaft with a Hooke’s coupling at each end (known
as double Hooke’s coupling) is used.
What is Oldham
Coupling?
It is used to join two shafts that have a lateral misalignment. It consists of two flanges A and B with slots and a
central floating part E with two tongues T1 and T2 at right angles. The central
floating part is held by means of a pin passing through the flanges and the
floating part. The tongue T1 fits into the slot of flange A and allows for the ‘to
and fro’ relative motion of the shafts, while the tongue T2 fits into the slot
of the flange B and allows for vertical relative motion of the parts. The
resultant of these two components of motion will accommodate lateral
misalignment of the shaft as they rotate
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