Coupling Basics

7/28/2019 Coupling Basics

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What is a Coupling?

A coupling, as we all know , is an element in a power transmission system,that connects a drive such as an electric motor or an engine, to a driven

machine such as a conveyor,an excavator, etc. for transmitting torque and

speed ie., power.

Classification of Couplings

Couplings can be broadly classified as follows.

1) Rigid Couplings2) Flexible Couplings

3) Hydrodynamic or Fluid Couplings

A simple Rigid coupling can be a Flange coupling connecting a motor flange

to a driven machine flange with the help of bolts and nuts.

A Flexible coupling on the other hand can be a Universal coupling or more

popularly known as a cardan shaft.

What then is a Hydrodynamic or Fluid or Turbo Coupling ?



What is a Hydrodynamic or Fluid or Turbo

Coupling?

A Hydrodynamic Coupling is a coupling which transmits power, making

use of Fluid Power. It is essentially a combination of a Hydraulic Pump and

a Hydraulic Turbine.

Basic Principle of operation

A centrifugal pump and a Francis turbine with guide vanes are connected by

piping and an oil reservoir, which contains operating fluid. The pump

absorbs the mechanical energy supplied by motor or engine and produces

kinetic energy which is absorbed by turbine and in turn converted back into

mechanical energy.

By eliminating all avoidable sources of loss such as draft tubes, spiral casingand the piping itself, a compact Hydrodynamic Coupling results.



Advantages of Hydrodynamic couplings

• Smooth acceleration of the heaviest masses with an inexpensive squirrel

cage motor.

• Wear-free power transmission; no metal-metal contact between

transmitting elements.

• Non-stressed motor start-up even when the machine is loaded or has

stalled; quick reduction of the starting current.

• Torque limitation at starting (important eg., at belt conveyor plant)

• Effective cushioning of shocks, load vibrations and torsional vibrations.

• Safe protection for motor, driven machine and goods handled in case of

overload or stall because of limitaion on maximum torque that can be

transmitted.

• Simple control of maximum torque by altering the filling.

• Load balance in the case of multi motor drives; individual motors can be

started one after another thus avoiding the need for simultaneous

switching arrangement.

• Explosion-proof design is possible.

• Very high efficiency due to low slip at rated duty.



Salient features of a Hydrodynamic Coupling

A Hydrodynamic coupling when used in combination with a simple,

inexpensive and almost maintenance –free squirrel cage motors, finds a

wide field of applications.

It enables a quick start-up without high initial starting currents.

The coupling behaves according to the Propeller Law ie., torque

transmitted increases with square of the input speed and power

transmitted increases with cube of input speed.

Hydrodynamic couplings can be designed to be protected against

excessive temperature rise . This can be done by incorporating Fusible

plugs in which the solder melts at high temperature, allowing oil to

escape out. If oil escape is undesirable, then an element can be fitted

which senses temperature and at excess temperature (which is normally

set below the melting temperature of solder) an actuator moves to cut-off

the Power supply.



Major Applications of Hydrodynamic couplings

Material Handling and Conveying

♦ Belt conveyors

♦ Bridge cranes

♦ Port loading plants

Mining

♦ Chain and armored conveyors

♦ Bucket wheel excavators

♦ Crushers, Pumps, Fans

Chemical Industry

♦ Centrifuges

♦

Fans/Blowers♦ Stirrers , Mixers

♦ Compressors

Mechanical Engineering

♦ Textile Machines

♦ Wire drawing machines

♦ Extruders

♦ Machine tools

♦ Fans, Cranes

♦ Balancing machines

Ship building



♦ Marine propulsion

Why Fluid couplings ?

Problems with rigid coupling

On a variety of machines such as Mixers, tube mills and particularly Belt-

conveyor drives, motors are often fitted to a single drive for technical and

availability reasons.

With such drive arrangements, if there is a rigid connection between the

motor and the gear unit, it is impossible to avoid constraining forces arisingin the drive. These constraining forces and the resulting varying loads of the

electric motors in multi-motor drives arise as a result of the following

reasons.

1. Different motor characteristics in the various motors

This is due to different characteristics of squirrel cage induction

motors manufactured by different manufacturers. Even if it is the

single manufacturer, characteristics vary because of manufacturingtolerances.

2. Different Gear transmission ratios

Different makes of gear units can vary by +/- 3% from the nominal

transmission ratio.

3. Different drive pulley diameters

This can be due to manufacturing tolerances or due to wear.

4. Loads that differ between the various drive stations

Various motors are subjected to different loads by the machine.



What’s the solution?

In multi-motor drives, if we install a Hydrodynamic coupling , then

• the motors are no more connected to one another by positive connection

by way of the conveyor belt.

• The load can be evenly shared between the various motors.

• By varying the filling in each coupling, each motor can be run at a speedthat all motors share the load uniformly.

1. Different motor characteristics in the various motors

By referring Fig.1 , we can see that the difference between the torques

transmitted, in case of Direct coupled drive Mrigid is substantially

higher than the torque difference when the drive is connected with

Hydrodynamic coupling, MTC

Now by adapting the oil filling, slip of each coupling can be

so adjusted that both motors deliver a similar torque i.e., P5 and P6

Hence, by adjusting the oil fillings both motors can be loaded equally

in spite of their different speeds.

2. Different gear transmission ratios

This can be explained using Fig.2.

With rigid connection, the difference in torques transmitted by two

motors with different gear transmission ratios , Mrigid is considerably

higher than, when a Hydrodynamic coupling is installed ie., MTC



3. Different drive pulley diameters

Different Pulley diameters have a similar effect as different gear

transmission ratios and the explanation which applies to different gear

transmission ratios, also applies to different pulley diameters.

Here again, adjusting the fillings of the couplings makes it possible to

equalize the motor loads.

4. Loads that differ between the various drive stations

Refer Fig.3

In case of rigid couplings, the motors are forced to run at the same

speed (point P1) and thus to deliver similar torques though the loads

(Mload1 and Mload2) differ. The result is stresses in the coupling

elements.

In case of Hydrodynamic coupling, by varying the oil fillings, the

required different loads(Point P2 and P3) can be achieved. The n2

speed curves differ in slope. They intersect the motor curves at the

required differing operating points and thus, in spite of different loads,

internal constraining forces within the system are avoided.



ConclusionWe have had an introduction to Hydrodynamic Coupling which is definitely

a better alternative to Rigid couplings in terms of adjustability to different

speeds, different load conditions and ultimately different torque

requirements.

However, attention must be drawn to the fact that in theory, load sharing can

be achieved with the installation of fluid couplings for any speed and thus

for any torque within the operating range of the plant. In practice , however ,

for different motor characteristics or gear transmission ratios, to keep the

efficiency of the drive systems high, load sharing is kept within narrow

limits.



Properties of Recommended mineral oil

Viscosity at 40 degC : 29-33

Air release property at 50 degC : 5 minutes

Viscosity Index : 100

Pour point : -21 degC

Flash point : 200 degC



FLUID COUPLINGS IN INDUSTRY

• INTRODUCTION

• SALIENT FEATURES OF FLUID COUPLING

• CONSTRUCTION

• PROPERTIES OF OIL

• NEED FOR FLUID COUPLINGS

• ADVANTAGES

• APPLICATIONS

• CONCLUSION



What’s Inside...

1. What is a Fluid coupling?

2. Salient features of Fluid coupling

3. Assembly of Fluid coupling

4. Properties of recommended oil

5. Why Fluid couplings?

6. Advantages of Fluid coupling

7. Major applications

8. Calculation of important parameters

9. Conclusion

Coupling Basics

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