A shaft coupling is a mechanical
device used to connect rotating shafts and absorb misalignments between
them. A coupling is a mechanical device, which is used to connect driver
and driven shaft permanently or semi permanently.
Couplings can be rigid or flexible depending
on the alignment accuracies of the system and torque requirements. Shaft
couplings are used for power and torque transmission between two rotating
shafts such as on motors and pumps, compressors, and generators.
Functions of couplings:
- Connects the shafts of two units, which are manufactured separately.
- Transfers motion, power and torque
- To reduce transmission shock loads from one shaft to another
- Protection against overloads.
- Introduces mechanical flexibility.
Design considerations for
couplings:
- Type of drive & driven equipments
- Alignment accuracy
- Operating & surrounding temperatures
- Type operation (Intermediate, continuous, frequent ON/OFF etc)
- Shafts diameters to be connected
- Maximum & minimum bores size
- Operating & design power
- Maximum & peak loads
- Space available
- Operation & Maintenance cost
- Service factor (Generally, for medium duty use a service factor of 1.5. For heavy duty use a factor of 2 and for extra heavy duty a factor of 3 should be used)
Types of couplings:
Main types of couplings: Rigid coupling and
Flexible coupling
Regid couplings:
Rigid coupling is used to connect two shafts
which are perfectly aligned. Most of the rigid couplings are made of aluminum, steel,
or stainless steel.
Types of Regid couplings:
- Sleeve or muff coupling
- Clamp/compression
- Flange coupling
Considerations for regid coupling
selection:
- Angular misalignment tolerance
- Parallel misalignment tolerance
- Axial motion allowed
- Dimensions like Bore diameter,Coupling diameter,Coupling length & Design units
Flexible
Couplings:
- Pin bush coupling (Protected and unprotected type)
- Jaw/spider/love joy coupling
- Gear coupling
- Bibby/grid coupling
- Metaflex/flexible disc coupling
- Tyre coupling
- Fluid coupling
- Oldham’s coupling
- Coupling size
- Coupling flange diameter
- Hub diameter
- Coupling gap
- Coupling maximum & minimum bore diameters
- No.of pins or bolts
- Maximum speed
- Torque
Gear couplings also transmit high
torques. They have misalignment capabilities generally about 0.01-0.02 inch in
parallel and 2 degrees in angular. Gear couplings are often used in pairs with
spacer shafts to span the distance between the driving and driven equipment.
They generally require lubricant although some designs intended for lighter
duty use lubricant free nylons or other polymers for the center sleeve.
Grid Couplings:
Grid couplings employ spring-like
connecting elements that weave between slots machined in the coupling hubs.
They are capable of high torque transmission with an added bonus of shock
absorption and torsional vibration dampening. They operate without lubricant.
They are appropriate for power transmission and capable of handling parallel
misalignment up to 0.30 inch and angular misalignment of about ¼ degree.
Disc Couplings:
Disc couplings use single or multiple
discs and single or double stages which bolt to the shaft hubs. They are used
for power transmission and rely on the flexibility of their thin metal discs to
transmit torque and accommodate angular misalignment. They are not especially
good at managing parallel misalignment. They are capable of transmitting high
torques and are often used to couple high horsepower motors, gas turbines, etc.
to loads.
Oldham Couplings:
Oldham couplings handle high degrees
of parallel misalignment owing to their sliding element design. Use of an
elastomer center element instead of metal is popular in modern versions. Some
manufacturers claim an ability to tolerate up to 5-degree angular misalignment
through the use of cylindrical, rather than rectangular, sliders.
Fluid Couplings:
Fluid
couplings or hydraulic couplings work on the hydrodynamic principle. In drives
consisting fluid couplings, there is no mechanical contact between the driver
and the driven machine and power is transmitted by means of a fluid. Due to the
mechanical separation between the driver and the driven machine, a fluid
coupling enables to achieve two separate value of acceleration in the drive,
the fast value of acceleration for the driver and simultaneously the slow value
of acceleration for the driven machine.
Fluid couplings are often used to drive large
inertia machines in combination with squirrel cage motors. They permit a load
free acceleration of the motor and consequently with increasing oil fill,
provide a soft/gentle quasi steady state start-up of the machine. The maximum
torque occurring during the start-up process is restricted to lowest possible
level. As fluid coupling allows quick acceleration of the motor and short
duration of high value starting current, it results into economical design for
electrical system. In addition, systems that use multiple motors can be
switched on in a staggered sequence to limit the current demanded during the
motor acceleration. This avoids grid overloading caused by simultaneous motor
starts.
Fluid
couplings are used in drives for conveyor systems such as belt conveyors, bucket
elevators and chain conveyors. The smooth application of fluid coupling torque
provides a smooth start-up of belt conveyor to protect the belt from damaging
stresses. In heavy industry, they are used for applications such as crushers,
roller presses, mixers, large ventilators, boiler feed pumps, large
compressors, centrifuges, etc
Types of fluid couplings:
Constant fill type:
Constant-fill
Couplings Couplings of this type are mainly used for start-up (to limit torque)
and to cushion the torsional vibration of the drive chain. In this type of
couplings, various designs mainly differ through adjoining chambers, who’s
automatically controlled filling and emptying have a significant influence on
the start-up behavior. Constant-fill couplings are sealed to the outside.
Filling of the operating fluid in a coupling is carried out before its
commissioning.
Drive
requirements determine the design and filling quantity. The ratio of the
operating fluid volume filled to the overall volume of the coupling is called
the fill level.
Variable-speed
Couplings:
Couplings
of this type are used to control or regulate the speed of the driven machine
over a wide range below the drive speed. These couplings have devices that
seamlessly change the transmission behavior during operation. This mainly
occurs by changing the fill level. The fill level can be changed during
operation either via a radially movable scoop tube or by controlling the
operating fluid inlet and outlet via valves and nozzles. These couplings always
have an external fluid circuit for filling changes that can also aid cooling.
Missalignment tolerances for angular & parallel alignments
Speed (RPM)
|
Angular
misalignment in Mills/inch of coupling diameter
|
Parallel misalignment Mills
|
||
Good
|
Acceptable
|
Good
|
Acceptable
|
|
600
|
1
|
1.5
|
5
|
9
|
900
|
0.7
|
1
|
3
|
6
|
1200
|
0.5
|
0.8
|
2.5
|
4
|
1800
|
0.3
|
0.5
|
2
|
3
|
3600
|
0.2
|
0.3
|
1
|
1.5
|
7200
|
0.1
|
0.2
|
0.5
|
1
|
Calculations:
Example-1:What is the size of muff coupling, which is required to
fit on 50 mm shaft Outer diameter of muff or sleeve = 2 X shaft diameter + 13 mm
=
2 X 50 + 13 = 113 mm
Length
of sleeve = 3.5 X shaft diameter =3.5 X 50 =175mm
Example-2: Calculate the
flange coupling dimensions required to fit on a shaft of 65 mm diameter.
Based on shaft diameter we can calculate the following dimensions
of flange coupling.
Outside diameter of hub = 2 X shaft diameter (d) ==2 X 65 = 130 mm
Length of the hub = 1.5 X d = 1.5 X 65 = 97.5 mm
Pitch circle diameter (PCD) of the bolts = 3 X d = 3 X 65 = 195 mm
Outside diameter of the flange = 4 X d = 4 X 65 = 260 mm
Thickness of flanges =
0.5 X d = 0.5 X 65 = 32.5 mm
Example-3:What is the
maximum torque developed on a gear coupling mounted for pump & motors of
power rating 525 KW & speed 3000 RPM
Torque = 9550 X
Power/Speed
T = 9550 X 525/3000
T = 1671.25 Nm
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