What is flexible coupling and An overview of flexible coupling

What is flexible coupling

Flexible coupling

Flexible coupling is a type of coupling which is use for transmit torque, power and connect two different rotating machine shafts. Which are compensated for end movement of the shaft and rotate at the same speed.

It will also compensate little amount of misalignment (Angular and parallel) between both machine shaft if this small misalignment is not compensated cause wear, tear and premature failure of equipment and its parts. In many causes it is use as a preventing device from damping vibration too safe bearing and mechanical seal life.

Mostly this type of shaft coupling is use if both machine (Mean both machine shaft will be in horizontal to each other or axially connected).

This type coupling will provide more parallel misalignment than other types and Easy to maintenance and replacement.

In a flexible coupling need more than one flexible element to accommodate angular offset.

but some elastomeric element couplings that can be accommodated both parallel and angular misalignments of both machine shaft. To save from internal rubbing, so sleeve bearing is using some pump instead of thrust bearing.

There are some factors that can be disturbed the proper alignment for both machine shaft like as thermal expansion,

Transfer hot or cold service liquid. But by used of flexible coupling is not a solution of all problems. it has their own limitations.

The two major misconception between coupling misalignment capacity and equipment misalignment tolerance with selector that should be clear before selecting suitable coupling. that will cover another chapter.


What is the Functions of Flexible Couplings?

Functions of Flexible Couplings

There are some of the basic Functions of Flexible Couplings that is describe below and some of them have already explained in details in the above section. 

1. Dampen vibration, reduce peak and shock loads 

2. Protect both machine from overload 

3. Measure the output torque of driven machinery 

4. Isolate the driver from the driven machine 

5. Position a rotor of a motor or generator and another machine 

6. Be used to tune a machinery system out of a torsional critical mode


How many types of flexible coupling?

Type of flexible coupling

It is categorized depends on the purpose of uses, these are followings.

  1. Flexible coupling 
  2. General purpose
  3. spacial purpose
  4. mechanically flexible type
  5. General purpose flexible type 
  6. Gear type (Straight teeth gear)
  7. Chain type (chain type steel), chain type nylon
  8. Grid type coupling (Verticall split grid type and horizontally split grid type)


Gear coupling are use for high speed and high torque applications.

How to select appropriate flexible couping?

Selection procedure

first, we have to Identify the requirements for equipment operation and select the type of coupling which will be best suits for your system, 

Horsepower requirement for operate, Operating speed RPM, Interface connect information, fitting of hub, Torques required to operate and first, minimum and greatest torque produce by driver, Angular misalignment, Offset misalignment, Axial movement, Ambient temperature of equipment during operation, Potential excitation or critical frequencies (Torsional, Axial, Lateral) , Limitation on that coupling-generated forces (Axial, Moments,  Unbalance) Any other unusual condition or requirements, and Space limitations (drawing of system showing coupling, envelope), eristics-weight, torsional stiffness.

Standard for balancing specification of coupling

AGMA 9000 flexible coupling potential and balance classification

API 671 limitations

Where to classes comment to API that is following low speed coupling in this criteria the equipment RPM is approx 1800.

AGMA  9 High speed coupling over 1800

Speed class specification accordingly low speed is less than 2300 intermediate or medium speed consider as a greater than 2300 rpm but for high speed is always greater than 3200.

How many types of clearance required to fit coupling in shaft.

There are three types of clearance required to fix flexible coupling in shaft. thease are following 

Clearance fit

Clearance fit or loose fit in this type of fit the clearance between inner bore of coupling and outer bore of shaft is in plus and is provided by vendor this type of clearance fit is used with approx 150 HP. drivers only and the AGMA 9002 is used for or balancing purpose .

 Interference fit

Interference fit this type of fixing coupling is also come in AGMA 9002 accordingly it is used for more than 150 HP equipment.

Set screw type coupling fix the key fitted in shaft and coupling.

How to install the coupling in shaft of equipment ?

Installation of coupling

Before the installation you should have to check physically of cracks are any other damage in coupling then after you should have to check ID, OD and roughness of internals bore and shaft of equipment. If any roughness observes in any of the equipment shaft, clean the roughness. Last if you have a manufacture installation manual you should have to follow the same. Or you have also required installation manual supplied by manufacturer.

The coupling is stored for a long time as a spare as we put the installation check erosion and corrosion for the same and before the store any equipment should have to apply in oil, grease or wax type material to save from the corrosion.

Before installation clean the shaft, the coupling has to be fixed and coupling inside the boss as well the key parameters

That has been to measure before installation required bore dimensions, key thickness, length of the coupling, distance between shaft end (DBSE) of equipment’s, diameter of and other dimension prescribed by manufacturers.


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