Rigid coupling | Flange rigid coupling | It’s important facts and 5+FAQs


Coupling Definition:

A Coupling is a connecting device, which connects two rotating shafts.
A coupling is used for power transmission and torque transmission.
Connected at the ends of the shafts, there is the possibility of failure or slippage depending on the torque limit of the shafts.

rigid coupling
Image credit: Occupational Safety and Health Administration part of the U.S. Department of Labor, Rotating coupling, marked as public domain, more details on Wikimedia Commons.


⦁ The purpose of coupling is to transmit power and torque.

⦁ Transportation of shafts becomes easier by dismantle and assembling the shafts by the use of a coupling.

⦁ Connect the driving part to the driven element.

⦁ To reduce transmission shocks.

⦁ Protects the system.

There are some shafts that are manufactured separately and still can be joined together by the use of coupling:

Motor and generator

Electric motor

Centrifugal pump

Types of couplings | Rigid coupling types

⦁ Clamped or compression rigid

⦁ Rigid coupling

⦁ Beam coupling

⦁ Bellows coupling

⦁ Bushed pin coupling

⦁ Bush pin type flange coupling

⦁ Based on Constant velocity: Rzeppa joint, Double Cardan joint, and Thompson coupling.

⦁ Clamp or split-muff coupling

⦁ Diaphragm coupling

⦁ Disc coupling

⦁ Donut coupling

⦁ Elastomeric coupling:

⦁ Flexible,

⦁ Geislinger coupling,

⦁ Grid coupling,

⦁ Hydrodynamic coupling (fluid coupling),

⦁ Jaw coupling,

⦁ Magnetic coupling,

⦁ Schmidt coupling-Oldham

⦁ Sleeve, box, or muff coupling

⦁ Tapered shaft lock

⦁ Twin spring coupling

Main two types:
I) Rigid coupling ii) Flexible coupling

I)Rigid couplings:

  • Sleeve couplings
  • Sleeve with taper pins
  • Clamp coupling
  • Ring compression type
  • Flange coupling

II)Flexible Coupling:

  • Elastomeric coupling
  • Mechanically flexible coupling
  • Metallic membrane coupling

Rigid Couplings Definition:

A rigid coupling is a coupling device used to connect shafts that are perfectly aligned, or there is no misalignment in the shafts in all directions.

These type of couplings are mostly used in vertical actions in the system.
Rigid couplings transmit rotational as well as an axial motion to the two connected shafts rotating at certain rpm.
Rigid couplings transmit power and torque between the shafts and between the two systems only if the shafts are appropriately aligned.
Example: Vertical pump.
Example: electric motor.

A rigid coupling is connected from the equipment shaft to the motor shaft.

The coupling shafts transmit axial thrust.

Split configuration: Split along the centerline.
Flanged configuration: The two couplings and the flanges are bolted together.
Flanged rigid couplings use adjusting plates that are utilized to set up proper vertical position.

Rigid couplings types:

The flanged type rigid coupling
The clamp-type rigid coupler
The sleeve type rigid coupling

Flange rigid coupling:

Flange coupling is the device that is used to connect to shafts if both the machine shafts are properly aligned to each other. Flange coupling is used where free access is available for both the shafts.

Flange rigid coupling
Image credit:https://www.growmechanical.com/2021/04/what-is-rigid-coupling.html

It is mostly used coupling, and the couplings flanges and the shafts are bolted together.

Advantages of flange coupling:

Flange coupling is less expensive as compared to the other type.
Less space is required for installation.

Material used :

Flanged couplings are constructed using various materials, including grey cast iron, malleable iron, carbon steels and carbon steels series ranging from 1035 to 1050.

Rigid couplings can be manufactured from most metal materials.
This allows Rigid couplings to be used in many applications and variable conditions.
It can transmit more power.


The manufacturing process used to manufacture flanged coupling is the casting process, as it contains recess and projection.
The flange coupling is commonly made from grey cast iron those which are characterized by graphite microstructure, causing a fracture to the material to appear as grey.

Cast iron is the most commonly used material due to its casting properties having less tensile strength than compressive strength.
Alloys of iron contain carbon and silicon 2.5-4% and 1.3%, respectively.

Cast iron experiences less solidification shrinkage.

Silicon is corrosion resistant, and in the casting process, it leads to an increase in fluidity and offers good weldability.

Advantages of rigid coupling:

⦁ Rigid couplings can be used in complex motion systems.

⦁ Rigid coupling provides more torque between the shafts.

⦁ It is also useful for better positioning as it has High torsional stiffness.

⦁ Easy availability.

⦁ Cost-efficient.

⦁ It has precision with zero backlashes.

⦁ Rigid couplings are used for the proper alignment and rigid connection.

⦁ Easy to assemble and disassemble.

⦁ Easy for the maintenance operations

Specifications: Rigid flange couplings

⦁ Rigid couplings are stiff connected, and it do not absorb vibrations leading to the possibility of replacement of the coupling due to wear on the parts are not properly aligned.
⦁ It requires routine check-up for wear and alignment check.
⦁ Apply lubrication regularly.

Difference between rigid and flexible couplings:

Rigid coupling is the coupling device used to connect shafts, and the connection between the shafts is the rigid connection where the two shafts are closely connected, whereas, in the case of flexible coupling, the connection between the two shafts is the flexible connection.

Flexible coupling provides the connection between the shaft components, which assemble the fixed coupling with some amount of loose connection. This gives some misalignment between the shafts.

Rigid coupling gives the smooth transmission of torque between both the shafts and the components, whereas in the flexible coupling, only the metallic type flexible coupling has a large capacity than other flexible couplings, and there is the possibility of some torque loss during the operation.

Flange coupling adapter:

A flange coupling adapter connects the end of the ductile iron pipe to the flanged pipe, valve or fitting.

Design procedure for flange coupling:

Assembly of muff coupling:

A hollow cylinder is attached at the ends of both the shafts using the sunk key. The hollow cylinder is called the sleeve.
Torque and power are transmitted through shafts using these hollow cylinders.

First, it is transmitted from the first shaft to the sleeve; From the sleeve, it is transmitted to the key.
Then from the key, it is again transmitted to the hollow cylinder(sleeve).

It is easy to manufacture and design and difficult to assemble and disassemble.
The material used: Cast iron
The factor of safety =6-8 (on the ultimate strength)
It is required to have more axial space and less radial space dimensions.

Sleeve standards :

Sleeve outer dia. D = 2d + 13
Length of the sleeve, L = 3.5d
d= diameter of the shaft.

Design of Shafts:

Shaft design is based on the torsional shear stress.

For torque transmission,
shear stress T is given by,
\tau =\frac{Tr}{J}<=[\tau ]

T = Torque acting on shafts,
J = shaft polar moment of inertia,
r = d/2

Allowable shear stress=[τ] determines the dimensions of the shaft.

Sleeve Design:

D = 2d + 13 L = 3.5d,

Consider a hollow shaft,
The torsional shear stress in the sleeve is calculated,

\tau =\frac{Tr}{J}<=[\tau ]

Design of Key:

Cross-section of the key selected corresponding to the shaft dia and key dimensions.
cross-sections of the keys: Square and rectangular
Length of the key in each shaft,

Shear and crushing stresses,

shear stress,\tau =\frac{P}{wl}<=[\tau ]
\sigma crushing=\frac{P}{lh/2}<=[\sigma c]
w= Width of the key.
h= height of the key.

Clamp coupling:

Clamp coupling is compression coupling or also called split muff coupling.
Split coupling is coupling in whose sleeves are split into two halves along the plane passing through the shaft axis.

These split sleeves are attached using bolts and placed in the recesses.
Assembling and disassembling is easy for the clamp coupling.
Clamp coupling balancing is difficult for high speeds and shock loads.

Design of Bolts:

Bolts design is based on torque transmission.

Let [σt] = permissible tensile stress,

dc = diameters of bolts,
n = number of bolts

Clamping force of each bolt,

clamp force is applied equally on each shaft.

Pb=\frac{\pi }{4}dc^{2}[\sigma t]

N=\frac{\pi }{4}dc^{2}[\sigma t]*\frac{n}{2}

Frictional Torque,
Tf=\mu Nd

Flange Coupling:

Flange coupling is the coupling device consisting of two flanges that are keyed to the shafts. The flanges are joined together using the bolts on a circle concentric to the shaft.

Power transmission is from the driving shaft to the flange on the driving shaft with the help of the key and from the flange on the driving shaft to the flange on the driven shaft using the key again.

For the proper alignment, projection and recess is used with the flanges
Inner hub, flanges and protective circumferential flanges – Protected type flanges
Flange coupling design dimensional proportions:

Flange coupling:
The outer diameter of hub, D = 2
Bolts diameter, D1 = 3 d
Flange diameter, D2 = 4 d
Hub length L = 1.5 d
tf = 0.5 d
tp = 0.25 d

Design of Hub:

A hollow shaft is considered, with the inner diameter = diameter of the shafts,
Outer diameter= 2* inner diameter.
For torsional shear stress.
\tau =\frac{Tr}{J}<=[\tau ]

T = In designing Hub required Twisting moment (or torque)
J = shaft’s Polar moment of inertia ( axis of rotation)
r = D/2

Design of Flange:

The hub is for the toque transmission through the bolts,
The flange is subjected to the shear.

Tangential force,
Shear stress,

\tau =\frac{F}{\pi D*tf}\leq [\tau ]

Design of Bolts:

Let n be the total number of bolts.
Force acting on each bolt,Fb=\frac{T}{nD1/2}
where D1 is the pitch circle diameter of bolts.
Area resisting shear,

A=\frac{\pi }{4}dc^{2}

where, dc = core diameter of bolts Shear stress,

\tau =\frac{Fb}{\frac{\pi }{4}dc^{2}}\leq [\tau ]

Area under crushing
Crushing stress,

\sigma crushing=\frac{Fb}{dctf}\leq [\sigma c]
Bolts are subjected to both shear and crushing stress,
Due to the transmission of torque, the force acts perpendicular to the bolt axes.

Types of flange coupling as follows:
⦁ Protected type flange coupling
⦁ Marine flange coupling.
⦁ Unprotected type flange coupling.

Unprotected type flange coupling:

In unprotected type flange coupling,
No of bolts used= 3-6
The keys are attached at the right angle along the circumference of the shafts dividing the keyways.

Unprotected flange coupling and cast iron flange coupling dimensions:

d= diameter of the shaft,
then D = 2 d
Hub length, L = 1.5 d,
flange, D2 = D1 + (D1 – D) = 2D
The thickness of flange, tf = 0.5 d
Number of bolts = 3,

Flanges are attached using the bolts.

Protected type flange coupling:

A protective circumferential rim is used.
The rim covers the nut and the bolt.

It consists of the the following protective procedures:

Perform visual inspections,
Check signs of wear or fatigue,
Clean couplings regularly and change the lubricants regularly.
Maintenance is required in operating conditions and adverse situations.

Advantages of the protective type flange coupling:

  1. It can transmit high torque.
  2. It is simple to construct.
  3. Easy to assemble and disassemble

Marine flange coupling:

This is a type of coupling where the flanges are attached to the shafts using the tapered headless bolts.
thickness, t=d/3,


  1. It is cheap.
  2. It is simple in structure.
  3. More efficient.
  4. Maintainance is not required.


1.It cannot be de-engaged in motion.
2.This type of coupling cannot transmit the torque between the shafts that are not linear.

Checking the coupling balance:

Balancing requires cost and it is difficult to balance.
The amount of the coupling unbalance can be tolerated by the system.
The analysis gives the detailed functions and the characteristics of the system and the connected machines.

Rigid flange coupling applications | Coupling applications

Rigid flange couplings are less expensive than the flexible couplings.
Rigid type couplings have rigid connections so they are torsional stiff and does not give access to any misalignment between the shafts. Due to the thermal effect, parts have misalignment during the operation, and both the shafts are physically aligned.

Rid couplings are couplings having rigid connections. It does not absorb vibrations leading to the replacement of the parts. Due to wear on the parts, misalignment occurs.
The operators require routine maintenance and checking of the parts for wear and alignment.

Flanged pin bush couplings:

Flanged pin bush coupling is also called as bush pin type coupling.

This coupling works as protective type flange coupling with better modifications.

This coupling device has pins, and it is used to work with coupling bolts.

The material used: Rubber
The rubber bushing can absorb vibrations and shocks during its operations.

Flange compression coupling:

The flange compression coupling of the coupling device.

Flange compression couplings have two cones which is used to place over the shafts.

The shafts should be coupling shafts.

The hollow cylinder is a sleeve that is used to fit over the cones.

Sleeve coupling flange:

Sleeves are attached to the shafts.

To lock the coupling in position, two threaded holes are provided.

Split flange coupling:

Split flange coupling is the coupling device the sleeves are split into two halves made up of the cast iron.
These split parts are connected using mild steel bolts.

Advantages of the split flange coupling:

Easy assembling and dismantling without changing the position of the shafts.
It can be used to connect two shafts of heavy transmission at moderate speed.


Flange coupling is what type of coupling:

Rigid type coupling.

Flange coupling specifications. Explain.

⦁ It should be easy to assemble or disassemble.
⦁ Flange coupling should transmit torque and power.
⦁ Maintain proper alignment.
⦁ Minimize the shock loads transmission.

Requirements to ensuring of the shaft alignment before attaching the fixing bolts:

⦁ If it is easy to connect or disconnect the coupling.
⦁ No projecting parts
There should be less misalignment in running operation, leading to maximum power transmission.

Why is the key used in protective type flange coupling?

Keys are used to preventing rotational motion.
The surfaces of the shaft and hubs parts provide cut to mount the keys, joints.

Why did recess provide in flange coupling?

To provide the clearance in the flanges, recess is provided. The flanges are tightly fitted with the use of bolts using the torque to be transmitted.

The minimum number of bolts required in flange coupling:

Four, six, or up to 12 bolt assemblies.

What is the grade of cast iron used to make rigid type flange coupling?

Grade 1- Grey cast iron.

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About Sulochana Dorve

I am Sulochana. I am a Mechanical Design Engineer—M.tech in design Engineering, B.tech in Mechanical Engineering. I have worked as an intern at Hindustan Aeronautics limited in the design of the armament department. I have experience working in R&D and design. I am skilled in CAD/CAM/CAE: CATIA | CREO | ANSYS Apdl | ANSYS Workbench | HYPER MESH | Nastran Patran as well as in Programming languages Python, MATLAB and SQL.
I have expertise on Finite Element Analysis, Design for Manufacturing and Assembly(DFMEA), Optimization, Advanced Vibrations, Mechanics of Composite Materials, Computer-Aided Design.
I am passionate about work and a keen learner. My purpose in life is to get a life of purpose, and I believe in hard work. I am here to excel in the field of Engineering by working in a challenging, enjoyable & professionally bright environment where I can fully utilize my technical and logical skills, constantly upgrade myself & benchmark against the best.
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