Shaft couplings & selection guide

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
No.of coupling bolts for flange couplings

  • 3 if shaft size is up to 40 mm
  • 4 if shaft size is 40–80 mm
  •  6 if shaft size is 80–180 mm
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
Dimensions available Pin bush coupling catalogues:
  • Coupling size
  • Coupling flange diameter
  • Hub diameter
  • Coupling gap
  • Coupling maximum & minimum bore diameters
  • No.of pins or bolts
  • Maximum speed
  • Torque
Gear Couplings:

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


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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