A Seminar Report On

A Seminar Report on

FLUID COUPLINGS

Submitted in the partial fulfillment of the requirement for the 7th Semester course 11 ME7DC SEM

BACHELOR OF ENGINEERING

In

MECHANICAL ENGINEERING

Submitted by: VINEET KHANDELWAL

1BM08ME128

Guided by:

Dr. L.K JAYASHANKAR

Professor

Department of Mechanical Engineering

B.M.S College of Engineering

Autonomous College under VTU

Accredited by NBA, Approved by AICTE

BANGALORE 560 019 November – 2011

FLUID COUPLINGS 2011

Mechanical Dept. i B.M.S.C.E

DEPARTMENT OF MECHANICAL ENGINEERING

BMS COLLEGE OF ENGINEERING

BANGALORE-560019

CERTIFICATE

This is to certify that the seminar entitled

FLUID COUPLINGS

Is submitted in the partial fulfillment of the requirement for the 7th Semester course 11 ME7DC SEM

BACHELOR OF ENGINEERING In

MECHANICAL ENGINEERING

Submitted by: VINEET KHANDELWAL

1BM08ME128

Guided by:

DR. L.K JAYASHANKAR Dr. K. GURUPRASAD Professor Prof. and Head of the Department

Department of Mechanical Engineering

BMSCE, Bangalore

Semester End Examination

Name Signature

Examiner 1

Examiner 2


Mechanical Dept. ii B.M.S.C.E

DECLARATION

I, VINEET KHANDELWAL bearing USN 1BM08ME128, of VII semester B.E., Department of Mechanical

Engineering do hereby declare that seminar report entitled "FLUID COUPLINGS" has been compiled by me

under the esteemed guidance and supervision of Dr. L.K Jayashankar , Professor, BMSCE, Bangalore. This

work and any part of this work have not been submitted anywhere for the award of any degree.

Place: Bangalore

Date

Place: Bangalore

Date

Signature

Signature

VINEET KHANDELWAL


Mechanical Dept. iii B.M.S.C.E

ACKNOWLEDGEMENT

The satisfaction and euphoria that accomplished the successful completion of any task would be incomplete

without the people who made it possible, whose constant guidance and encouragement crowned out effort

with success.

I take this opportunity to express my deep sense of gratitude and respect to my guide

Dr. L.K Jayashankar, Professor, Department of Mechanical Engineering, BMSCE, for his valuable

guidance. I am greatly indebted to his help, which has been of immense value and has played a major role in

bringing this to a successful completion.

I would also like to thank Dr. G .Giridhar, Seminar coordinator, BMS College of Engineering for his help

and encouragement.

I express heartfelt thanks to Dr. K. Guruprasad, Professor and Head of Department of Mechanical

Engineering, BMS college of Engineering for the help and encouragement.

Heartfelt thanks to Dr. K. Mallikarjun Babu, Principal, BMS College of Engineering for the facilities

provided and encouragement.

I wish to express sincere thanks to all the teaching and non-teaching staff, Department of Mechanical

Engineering, B.M.S. College of engineering, for extending necessary assistance without which this seminar

would not have been possible.

I would like to thank my family and friend for their constant support and encouragement throughout this

work.


Mechanical Dept. iv B.M.S.C.E

ABSTRACT

Fluid couplings have been used extensively in separate several applications for decades, showing their

simplicity and naturally beneficial torque transmission characteristics. Various designs of constant-fill

(passively controlled) fluid couplings were developed to meet the ever changing drive requirements and

they continued to increase in power and complexity.

The technology continued to advance through the 1990's. In an effort to remain competitive, many mines

had to increase their net clean coal capacity and/or reduce their cost per ton. These challenges resulted in the

need for increasing the capacity and reliability of the mine's systems, which forced operators to consider

actively controlled and cooled drive systems. Various drive technologies were being developed or used to

meet the demands of complex systems, each with their unique set of advantages and disadvantages.

This paper starts with the background and history of the fluid couplings. In the net section we deal with the

working of a basic fluid coupling, the parts of a fluid coupling and the categories of the fluid couplings.

In the later part we proceed with the hydraulic analysis of one of the variant of fluid couplings namely

“Reversible Fluid Couplings”. In this section we express the equations of the pump, turbine and the turning

vanes.

We then summarize the power transmission and losses to different parts of the fluid coupling, and hence

define the efficiency of the fluid coupling.


Mechanical Dept. v B.M.S.C.E

LIST OF FIGURES

Fig 1 Parts of a Fluid Coupling .............................................................................................. 5

Fig 2 Fluid Coupling at Rest .................................................................................................. 5

Fig 3 Fluid Coupling at Start ................................................................................................. 6

Fig 4 Fluid Coupling in Running ........................................................................................... 6

Fig 5 Fluid Coupling in Normal Running Condition.............................................................. 7

Fig 6 Fluid Coupling in Overload Condition ......................................................................... 7

Fig 7 Fluid Coupling Characteristic Graphs........................................................................... 8

Fig 8 Soft Cushioned Starts ................................................................................................... 9

Fig 9 Increased Starting Torque............................................................................................. 9

Fig 10 Overload Protection.................................................................................................. 10

Fig 11 Reduced Current Drawn ........................................................................................... 10

Fig 12 Load Balancing for Multiple Drives ......................................................................... 10

Fig 13 Power Transmitting Components of a Hydrokinetic Fluid Coupling ........................ 13

Fig 14 Components of Hydrodynamic Fluid Coupling ........................................................ 13

Fig 15 Components of Hydroviscous Fluid Couplings ........................................................ 14

Fig 16 Typical Package Hydrostatic Fluid Coupling ........................................................... 14

Fig 17 Cross Section of a Reversible Fluid Coupling Showing Key Locations in the Fluid

Cavity .................................................................................................................................. 15

Fig 18 Sketch showing subdivision of flow into stream tubes.............................................. 16

Fig 19 Velocity Triangle at the Turbine-Pump Transition station ........................................ 17

Fig 20 Geometry of a Turning Vane .................................................................................... 20

LIST OF TABLES

Table 1 Comparison of Fluid Couplings against Mechanical Clutches ................................ 12

Table 2 Nomenclature Used ................................................................................................ 17

Table 3 Power Transmission and Losses ............................................................................. 21


Mechanical Dept. vi B.M.S.C.E

CONTENTS

CERTIFICATE .................................................................................................... i

DECLARATION ................................................................................................. ii

ACKNOWLEDGEMENT .................................................................................. iii

ABSTRACT ....................................................................................................... iv

LIST OF FIGURES .............................................................................................. v

LIST OF TABLES ............................................................................................... v

CONTENTS ....................................................................................................... vi

CHAPTER 1: PREAMBLE .................................................................................. 1

CHAPTER 2: LITERATURE REVIEW .............................................................. 3

2.1 HISTORY ............................................................................................................. 3

2.2 BACKGROUND ................................................................................................... 3

2.3 DEFINITION ........................................................................................................ 4

2.4 BASIC CONSTRUCTION .................................................................................... 4

2.5 PRINCIPLE OF OPERATION.............................................................................. 4

2.5.1 STARTING............................................................................................................ 5

2.5.2 RUNNING ............................................................................................................. 6

2.5.3 OVERLOAD- STALL ........................................................................................... 7

2.6 CHARACTERISTIC GRAPHS OF FLUID COUPLING ...................................... 8

2.7 BENEFITS OF A FLUID COUPLING ................................................................. 9

2.8 PURPOSE OF FLUID COUPLING .................................................................... 11

2.9 EFFICIENCY OF A FLUID COUPLING ........................................................... 11

2.10 COMPARISION BETWEEN FLUID COUPLING AND MECHANICAL

CLUTCH .................................................................................................................. 12

2.11 TYPES OF FLUID COUPLINGS ..................................................................... 12

2.11.1 HYDROKINETIC FLUID COUPLINGS ........................................................... 12


Mechanical Dept. vii B.M.S.C.E

2.11.2 HYDRODYNAMIC FLUID COUPLINGS ........................................................ 13

2.11.3 HYDROVISCOUS FLUID COUPLINGS .......................................................... 13

2.11.4 HYDROSTATIC FLUID COUPLINGS ............................................................. 14

CHAPTER 3: REVERSIBLE FLUID COUPLING............................................ 15

3.1 BASIC EQUATIONS ......................................................................................... 17

3.1.1 PUMP .................................................................................................................. 18

3.1.2 TURBINE ............................................................................................................ 19

3.1.3 TURNING VANES ............................................................................................. 20

3.2 POWER TRANSMISSION SUMMARY ............................................................ 21

CHAPTER 4: CONCLUSION ........................................................................... 22

CHAPTER 5: BIBLIOGRAPHY ....................................................................... 24


Mechanical Dept. - 1 - B.M.S.C.E

PREAMBLE

In an automobile people review its engine, ergonomics and aesthetics, but what goes unnoticed is the clutch of

the automobile. It has to be understood that whatever power the engine develops is useless unless it is not

properly coupled with the output. For example, let us consider an engine can generate 3000 horse power, but

if the power loss between the input and output is high, we might end up getting only around 500 horse power

at the output. Such a major loss can be avoided by efficiently designing the clutch that transmits power from

input to output.

In the automobile, we can say that, the clutch is the least talked about. This is majorly due the fact that it is

taken for granted that the power transmission from engine to wheels has to take place. Thus the clutch plays a

major role in the making of an automobile. The new technologies evolving in the automobile field are majorly

based on the types of clutch.

In the older automobiles, the transmission was manual type and hence human effort was required while

changing the gears. As the power used to be transmitted by mechanical contact a lot of power was lost as

friction and other linkage losses. With the increase in the demand of efficiency, speed and comfort manual

transmissions have been slowly replaced by automatic transmission.

Today the big players in the automobile field are shifting their focus, apart from designing a powerful engine,

to designing an efficient clutch. If the power of the engine has to be used, it has to be efficiently transmitted to

the output. This is not possible by manual transmissions as they come with inherent frictional and linkage

losses.

Gone are the days when only mechanical contact was considered as a means of power transmission. Today in

the most advanced cars we find an automatic transmission, which existed because of the invention of fluid

couplings.



Fluid couplings and torque converters are now commonly used in a wide variety of applications requiring

smooth torque transmission, most notably in automobiles. They usually consist of an input shaft that drives a

pump impeller which is closely coupled to a turbine impeller that transmits the torque of an input shaft coaxial

with the output shaft. The fluid is usually hydraulic oil and the device is normally equipped with a cooling

system to dissipate the heat generated.

In a typical fluid coupling used, for example, in a ship propulsion system, the pump and turbine are mounted

back to back with little separation between the leading and trailing edges of the two impellers. It is common to

use simple radial blades and a higher solidity (the present pump rotor has 30 vanes) than would be utilized in

most conventional pumps or turbines.

In a fluid coupling the input side is known as the Impeller and the output side is known as the Rotor. There is

no mechanical contact between the impeller and the rotor (i.e. the driving and driven units) and the power is

transmitted by virtue of the fluid filled in the coupling. The impeller when rotated by the prime mover

imparts velocity and energy to the fluid, which is converted into mechanical energy in the rotor thus rotating

it.

Not only automobiles but fluid couplings find their applications wherever there is a need for an efficient

power transmission. From underground & overland belt conveyors to crusher and mixing applications,

equipment professionals are constantly seeking better technology to protect critical production systems

against the effects of damaging shock loads. And when it comes to eliminating sudden/jarring starts, or

preventing system failure/deterioration due to overloads, nothing outperforms the system-saving capability of

the fluid couplings.



LITERATURE REVIEW

2.1 HISTORY

The fluid coupling originates from the work of Dr. Hermann Föttinger, who was the chief designer at the AG

Vulcan Works in Stettin. His patents from 1905 covered both fluid couplings and torque converters.

In 1930 Harold Sinclair, working with the Daimler Company, devised a transmission system using a fluid

coupling and planetary gearing for buses in an attempt to mitigate the lurching he had experienced while

riding on London buses during the 1920s.

In 1939 General Motors Corporation introduced Hydramatic drive, the first fully automatic automotive

transmission system installed in a mass produced automobile. The Hydramatic employed a fluid coupling.

The first Diesel locomotives using fluid couplings were also produced in the 1930s.

2.2 BACKGROUND

Widespread interest in hydraulic drives was created when hydraulic couplings were incorporated in several

popular automobiles. This was the outgrowth of the inventions of Dr. Hermann Foettinger in Hamburg,

Germany over 70 years ago. Dr. Foettinger developed both the hydraulic coupling and the hydraulic torque

converter for use in Diesel-powered vessels having up to 20000 horse-powers available for driving the

propeller.

The use and development of hydraulic drives spread to Sweden, where the Ljurigstrorn works further

developed the hydraulic torque converter under Lysholm-Smith patents and to England, where Vulcan-

Sinclair developed the hydraulic coupling. The Swedish applications were made largely to rail cars and the

English applications to trucks and buses.

It has been in the last four or five decades that widespread industrial development has occurred in these two

hydro-kinetic drives. The American Blower Corporation was sub-licensed by Vulcan-Sinclair to manufacture

hydraulic couplings. The Twin Disc Clutch Company licensed under Lysholm-Smith patents to manufacture

industrial hydraulic torque converters and hydraulic couplings and Spicer Manufacturing Corporation to

manufacture torque converters for automotive uses.



2.3 DEFINITION

A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power or device that transfers

power through a fluid between its inputs and outputs. A fluid coupling basically consists of two fans in a

sealed, oil-filled housing. The input fan churns the oil, and the churning oil in turn twirls the output fan. Such

a coupling allows some speed difference between its input and output shafts.

2.4 BASIC CONSTRUCTION

The internal appearance of a fluid coupling has traditionally been likened to the two valves of a grapefruit,

each facing the other with the pulp scooped out and the cell dividers left intact.

Translated into technical language, the fluid coupling may be described as consisting of two toroidally

grooved discs facing one another with a small clearance between them. Radial blades are formed across the

grooves to divide them into curved cells. This blade also supports the hollow semicircular cores for guide

rings, which reduce turbulence in the coupling.

The guide rings are offset within their torodal cavities so as to equalize flow area in the cells. One disc is

mounted from the engine flywheel via a tore’s cover but connects to the input shaft of the gear box and is

termed the turbine. Fluid coupling are either produced from aluminum die casting or as in later practice

fabricated from steel pressing.

2.5 PRINCIPLE OF OPERATION

There are three essential parts to a fluid coupling: the driving (input) section known as the impeller the driven

(output) section known as the runner and the casing which bolts to the impeller enclosing the runner providing

an oil tight reservoir. Both impeller and runner each represents half of a hollow torus with flat radial

vanes. At the inner circumference a conical baffle is attached to the impeller and a flat baffle is attached to

the runner. These components comprise the working circuit.



Fig 1 Parts of a Fluid Coupling

The operation of the fluid coupling requires mechanical input energy, normally provided by a standard

NEMA B electric motor which is connected to the impeller and casing. The runner, which has no mechanical

connection with the impeller, is directly connected to the driven load. A variety of mechanical connections;

couplings, sheaves, and hollow shaft mountings, are available to provide the mounting configuration best

suited to the application. Finally the fluid coupling must be initially charged by removing the fill

(fusible) plug and adding the recommended amount of oil based on the required torque.

2.5.1 STARTING

Standard NEMA B motors are recommended when using fluid couplings and will start virtually unloaded.

Since the motor is mechanically connected to the impeller and casing, the low inertia of these components and

the oil are the only loads imposed. As the electric motor accelerates to running speed, the impeller begins to

centrifugally pump oil to the stationary runner.

Fig 2 Fluid Coupling at Rest



Transmission of oil is diffused by the conical impeller baffle, producing a gradual increase in torque, allowing

the motor to accelerate rapidly to full running speed. When all the oil is pumped into the working circuit,

continuous circulation of oil will occur between the impeller and runner forming a flow path like a helical

spring formed in a ring.

Fig 3 Fluid Coupling at Start

As soon as the transmitted torque reaches the value of the resisting torque, the runner starts rotating and

accelerates the driven load. The time required to reach full speed is dependent on the inertia of the driven

load, the resistive torque, and the torque being transmitted by the fluid coupling.

2.5.2 RUNNING

The operation of a fluid coupling is based on the hydrokinetic principles and requires that the output speed be

less than the input. This difference in speed is called slip. Further this principle provides that the output

torque is equivalent to the input torque, since windage and oil circulation losses are negligible. Therefore,

efficiency equals 100% minus the percent of slip.

Fig 4 Fluid Coupling in Running



At full running speed fluid couplings will normally slip between 1% and 4%. The oil circulation between the

impeller and runner has formed a vortex at the outside circumference of the working circuit and is no longer

deflected by the conical baffle.

2.5.3 OVERLOAD- STALL

Should the load torque increase, the slip will increase, which causes the runner to drop in speed. The vortex

of oil circulating between the impeller and runner will expand to provide additional torque. The extent to

which this vortex can expand is limited by the flat baffle on the runner. Consequently fluid couplings provide

inherent overload protection.

Fig 5 Fluid Coupling in Normal Running Condition

If the increase in torque causes the oil in the working circuit to expand to the point of contacting the baffle,

the coupling will stall and slip will be 100%. This continuous high slip generates heat and the oil temperature

will rise unless the overload is removed.

When the temperature rises to the temperature limit of the fusible plug, the core of the plug will melt, release

oil from the coupling and effectively disconnect power to the output shaft. To prevent the loss of oil the use of

a proximity cutout switch or thermal trip plug and limit switch is recommended.

Fig 6 Fluid Coupling in Overload Condition



2.6 CHARACTERISTIC GRAPHS OF FLUID COUPLING

Fluid coupling has centrifugal characteristics during starting, thus enabling no load start-up of prime mover,

which is of great importance.

The slipping characteristics of fluid coupling provides a wide range of choice of power transmission

characteristics which also result in speed variation, smooth & controlled acceleration, clutching and

declutching operations and other characteristics of load limiting shock & peak load absorption and dampening

etc. By varying the quantity of oil filled in the fluid coupling, the normal torque transmitting capacity

can be varied. The maximum torque of the fluid coupling can also be set to a predetermined safe value by

adjusting the oil filling.

The fluid coupling has the same characteristics in both directions of rotation.

Fig 7 Fluid Coupling Characteristic Graphs



2.7 BENEFITS OF A FLUID COUPLING

1. Soft cushioned starts Without the fluid coupling, the motor instantly transmits its locked

rotor torque (starting torque) to the driven machine (point A at right).

Then, it quickly reaches and applies the breakdown torque (Point B).

For common applications, locked rotor and breakdown torque often

approximates 200% or more, of the motor nameplate rating.

Damage can result if the required break-away starting torque of the

driven machine is significantly less, the machine will be abruptly

accelerated to rated speed.

With a fluid coupling installed, however, the torque to the driven machine starts at zero (point C) and

gradually increases as the coupling impeller accelerates to point D.

When the output torque of the fluid coupling exceeds the break-away starting torque of the driven machine

(point D), the driven machinery gradually accelerates (right half of chart). As the machine comes up to rated

speed, the slip of the fluid coupling decreases to (point E) and uniform power is transmitted at maximum

efficiency.

2. Increased starting torque

With a fluid coupling, the breakdown torque of a standard NEMA

B squirrel cage motor can be used to provide additional torque to start

the machinery.

In this example of a high horsepower application, the NEMA B motor

will initially exert only 80% of its rated torque (point A).

Only if the motor can accelerate to 85% of its synchronous speed can it

take advantage of its 175% breakdown torque (point B).

However, with a properly selected fluid coupling, the motor can start

under no load (point C), and reach its breakdown torque in only a few

shaft revolutions.

If a fluid coupling is not utilized, an oversized motor or a special high starting torque motor may be required.

These solutions are costly, and introduce undesirable variables into the system. A properly selected and filled

fluid coupling can provide an initial starting torque ideally matched to the needs of the driven machine.

Attainable initial starting torque values range from 40% of normal running load minimum, up to a maximum

initial starting torque value that equals the full breakdown torque of the motor.

Fig 8 Soft Cushioned Starts

Fig 9 Increased Starting Torque



3. Overload protection

When a machine jams, the life of individual components may

be drastically reduced. Without overload protection, the

stored energy of the machine is absorbed in the first second

following the jam, increasing stresses on components to many

times their normal running values.

The fluid coupling slip characteristics spreads the absorption

of the impact over a period of time, reducing stress on the

components and therefore, protecting your machinery.

4. Reduced current draw when starting

The electric current draw at startup is dramatically reduced when a fluid coupling is present. The motor starts

without any meaningful load yet applied by the driven machinery

(point A, Figure 1). In the absence of load, the motor quickly

accelerates to approximately 90% of its full load speed (point B,

Figure 1).

The result is that low current draw (point C, Figure 1) is achieved

within several shaft revolutions. At this point the inertia of the

motor has been accelerated, and the application of load from the

driven machine will not cause subsequent periods of high inrush

current.

With a fluid coupling, the duration of high current draw is reduced substantially as shown in the shaded area

of Figure 2. This results in electrical savings and extended service life of the motor.

5. Load balancing for multiple drives

Loads can be easily balanced on multiple drive systems when fluid couplings are installed at each motor.

Refer to the illustration at the right, two motors, each equipped with a fluid coupling. Load sharing between

the two motors has been achieved by appropriately adding

fluid to one coupling, or withdrawing fluid from the other

coupling.

As an added benefit, motors on multiple drive systems can be individually started because of the ability of one coupling to temporarily operate at 100% slip. Starting one motor at a time provides extended and even softer starts on multiple drive conveyors. Since the motors are started as separate events, the system maximum inrush current will be approximately half the system maximum inrush current experienced when both motors are started simultaneously.

Fig 10 Overload Protection

Fig 11 Reduced Current Drawn

Fig 12 Load Balancing for Multiple Drives



2.8 PURPOSE OF FLUID COUPLING

The purpose of using fluid coupling are listed below according to their application

1. In vehicle transmission system it is generally used to secure the following:

a. Absence of direct mechanical contact between the driving and driven members minimizes the

transmission of shock and torsional vibration between the engine and the drive line.

b. No positive disengagement or engagement of drive allows a smoother starting characteristic

this being particularly advantageous when restarting up a steep hill

c. Protect against harmful laboring of the engine at low speeds, since the fluid coupling will

merely slip and allow the engine to increase speed when overloaded.

2. In Aviation transmission system it is used in the engine's exhaust gases and then, using three fluid

couplings and gearing, converted low torque high-speed turbine rotation to low-speed, high-torque output

to drive the propeller.

2.9 EFFICIENCY OF A FLUID COUPLING The power is transmitted hydraulically from the driving shaft to driven shaft and the driven shaft is free from

engine vibrations. The speed of the driven shaft B is always less than the speed of the driving shaft A, by

about 2%. The efficiency of the power transmission by fluid coupling is about 98%. This is derived as given

below



2.10 COMPARISION BETWEEN FLUID COUPLING AND MECHANICAL CLUTCH

MECHANICAL CLUTCHES FLUID COUPLINGS

The power transmission is via mechanical contact

The power transmission is via fluid movement

The efficiency of power transmission is low

The efficiency of power transmission is as high as 98%

As the clutch wears the cable needs to be adjusted

The absence of cables makes it self adjusting

Initial cost of installation is low Initial installation cost is comparatively higher

Requires human effort Does not require human effort

Requires a clutch paddle to be actuated hence an added extra part

No extra components are required as it shifts automatically

Suitable for manual transmissions Suitable for automatic transmissions

Table 1 Comparison of Fluid Couplings against Mechanical Clutches

2.11 TYPES OF FLUID COUPLINGS

All fluid couplings may be broken into 4 types

1. Hydrokinetic

2. Hydrodynamic

3. Hydroviscous

4. Hydrostatic

2.11.1 HYDROKINETIC FLUID COUPLINGS

Basic Principle

In the hydrokinetic drive, commonly known as a fluid drive or hydraulic coupling, oil fluid particles are

accelerated in the impeller (driving member) and then decelerated as they impinge on the blades of the runner

(driven member). Thus, power is delivered in accordance with the basic law of kinetic energy:

� =1

2�(��

� − ��)

Where E represents energy, M is the mass of the working fluid, V1 is the velocity of the oil particles before

impingement, and V2 is the velocity after impingement on the runner blades.



Components

1. Housing

2. Impeller

3. Vortex

4. Rotor

2.11.2 HYDRODYNAMIC FLUID COUPLINGS

Basic Principle

In the most common forms of hydrodynamic drives, planetary gear trains utilize some components as oil

pumps. Throttling the discharge of these pumps creates

back pressure and increases drive torque.

Components

The input shaft, supported by a bearing either on an

independent bearing pedestal or on a packaged sub base

assembly, drives the housing, endplate, manifold and

planetary gear shafts. These planet gears are partially

surrounded by the manifold, which forms a pump cavity.

The sun gear drives the output shaft. The control yoke

moves the internal valve.

2.11.3 HYDROVISCOUS FLUID COUPLINGS

Basic Principle

Hydroviscous drives operate on the basic principle that oil has viscosity and energy is required to shear it.

More energy is required to shear a thin film than a thick one. The Hydroviscous drive varies its torque

capability by varying the film thickness between driving and driven members.

Components

1. Housing

2. Rotor

3. Disks

4. Piston

Fig 13 Power Transmitting Components of a Hydrokinetic Fluid Coupling

Fig 14 Components of Hydrodynamic Fluid Coupling



Fig 15 Components of Hydroviscous Fluid Couplings

2.11.4 HYDROSTATIC FLUID COUPLINGS

Basic Principle

There are many variations of hydrostatic variable-speed drives, but in one form or another they invariably use

positive displacement hydraulic pumps in conjunction with positive displacement hydraulic motors.

In some cases, varying amounts of fluid are bypassed from the pump discharge back to the pump suction. This

provides a controllable variable flow to the positive displacement motor and therefore a variable output speed.

This system has no particular advantages over the more common variable-speed drives. The higher-than-

average first costs and above-average maintenance required explain why this type of hydrostatic system is

seldom used.

Components

Fig 16 Typical Package Hydrostatic Fluid Coupling



REVERSIBLE FLUID COUPLING

The present section presents the hydraulic analysis of a variant of fluid couplings namely a “Reversible Fluid

Couplings”. The device was developed and built by Franco Tosi in Italy in conjunction with SSS Gears Ltd.

in the U.K. Tests on the device conducted by the US Navy (NSWC Philadelphia) and are documented in

Nufrio et al.

This section presents a method of analysis of the performance of such devices and uses one of the Franco Tosi

designs tested by NSWC as an example. As shown diagrammatically in figure 17, the reversible fluid coupling

has an added feature, namely a set of guide vanes.

Fig 17 Cross Section of a Reversible Fluid Coupling Showing Key Locations in the Fluid Cavity

With the vanes retracted the device operates as a conventional fluid coupling and the direction of rotation of

the output shaft is the same as the input shaft. When the vanes are inserted, the direction of rotation of the

output shaft is reversed. In traditional terms, the reversible fluid coupling can, in theory, operate over a range

of slip values from S = 0 to S = 2. In this section, we utilize overall coupling performance data obtained by

NSWC and several investigations of flow details carried out by WesTech Gear Corporation



A number of recent papers have demonstrated how complex and unsteady the flow is in torque converters.

Due to the need to operate the machines over wide ranges of slip values, the incidence angles on the impeller

blades tend to be very large thus generating substantial flow separation at the leading edges as well as much

unsteadiness and high turbulence levels. To accommodate these violent flows and to force the flow to follow

the vanes at impeller discharge, the solidity of the impellers is usually much larger than would be optimal in

other turbo machines.

Though several efforts have been made to compute these flows from first principles such complex, unsteady

and turbulent flows with intense secondary flows are very difficult to calculate because of the lack of

understanding of unsteady turbulent flows.

In the present paper we begin with a simple one-dimensional analysis of the flow in a reversible fluid

coupling. This one-dimensional analysis may be used as a first order estimate of the coupling performance.

Alternatively it can be applied to a series of streamtubes into which the coupling flow is divided. Such a

multiple streamtube (or two-dimensional flow) analysis allows accommodation of the large variations in flow

velocity and inclination which occur between the core and the shell of the machine.

In the multiple streamtube analysis the flow is subdivided into streamtubes as shown in figure 18; all the data

presented here used ten streamtubes of roughly similar cross-sectional area. The flow in each streamtube is

characterized by meridional and tangential components of fluid velocity, �� and ��, at each of the transition

stations, i =1, 2, 3, between the turbine and the pump (i =1), between the pump and the turning vanes (I =2)

and between the turning vanes and the turbine (i =3).

Fig 18 Sketch showing subdivision of flow into stream tubes



A typical velocity triangle, in this case for the transition station i = 1, is included in figure 19; the velocity

triangles for the other transition stations are similar. Flow is from the right to left, the direction of rotation is

upward and the angles are shown as they are when they are positive.

Fig 19 Velocity Triangle at the Turbine-Pump Transition station

3.1 BASIC EQUATIONS The nomenclature used is as shown in Table 2

Table 2 Nomenclature Used



The process of power transmission through the coupling (operating under steady state conditions) will now be

delineated. In the process, several loss mechanisms will be identified and quantified so that a realistic model

for the actual interactions between the mechanical and fluid mechanical aspects of coupling results.

3.1.1 PUMP

The power input to the shaft is clearly NPTP. some of this is consumed by windage losses in the fluid annulus

between the pump shell and the stationery housing. This is denoted by a pump winding torque, Tpw, which

will be proportional to ��. Included in this loss will be the shaft seal loss as it has the same functional

dependence on pump speed. It is convenient to denote this combined windage and seal torque, Tpw, by a

dimensionless coefficient, Cpw, where

Tpw = CpwρR5�� ........................(1)

Furthermore the labyrinth seal in the core between the pump and turbine rotors causes direct transmission of

torque from the pump shaft to the turbine shaft. This torque which is proportional to (Np- Nt)2 will be denoted

by Ts and is represented by a seal windage torque coefficient, Csw, defined as

�� = ��(��

� − ��)(��− ��)�........................(2)

Experimentally the value of �� is found to be 0.014. In referring, to this labyrinth seal, we should also

observe that the leakage through this seal has been neglected in the present analysis.

It follows that the power available for transmission to the main flow through the pump is Np (Tp - Tw – Ts)

and this manifests itself as an increase in the total pressure of the flow as it passes through the pump.

The power balance between the mechanical input, the losses and the ideal fluid power applied to the pump

yields

Np (Tp - Tw – Ts) = QHpi..........................(3)

Where, from the application of angular momentum considerations in the steady flow between pump inlet (i=1)

and pump outlet (i=2) the pump head rise, Hpi, is given by

Hpi = ρNp (r2v2 – r1v1).......................(4)

More specifically, Hpi, will be referred to as ideal pump total pressure rise in the absence of fluid viscosity

when the pump would be 100% efficient. However, in a real viscous flow, the actual total pressure rise

produced, Hp, is less than Hpi; the deficit is denoted by Hpl where

Hp = Hpi - Hpl.....................(5)



This total pressure loss, Hpl, is difficult to evaluate accurately and is a function, among other things, of the

angle of attack on the leading edges of the vanes.

The angle of attack, αp, on the pump blades is given by

�� = ��

�� − ��....................(6)

3.1.2 TURBINE

We now jump to the turbine output shaft . the power delivered to the turbine shaft is NtTt. As in the pump

there are windage losses, NtTtw, where the windage torque, Ttw, is described by a dimensionless

coefficient,�� = ��/(��). Then the power delivered to the turbine rotor, Nt (Tt + Ttw – Ts), by the main

flow through the turbine is related to the ideal total pressure drop through the turbine, Hti, by

��(�� + �� − ��) = �� .................(7)

Where again from angular momentum considerations

�� = ��(�� − ��).....................(8)

With an inviscid fluid, �� would be the actual total pressure drop across the turbine. But in a real turbine the

actual total pressure drop is greater by an amount, ��, which represents the total pressure loss in the turbine,

an hence

�� = �� + ��.....................(9)

The angle of attack, ��, on the turbine blades is given by

�� = ��

�� − ��......................(10)



3.1.3 TURNING VANES

The geometry of a turning vane used in the coupling discussed here is shown in figure 20.

Fig 20 Geometry of a Turning Vane

The total pressure rise produced by the pump, Hp, is equal to the pressure drop across the turbine, Ht, plus the

total pressure drop across the turning vanes, Hv, so that

�� = �� + �� with the turning vanes inserted

Hv = 0 with the turning vanes retracted

It is this balance which essentially determines the flow rate, Q, and the meridional velocities, ��. The total

pressure drop across the vanes Hv, is described by a loss coefficient defined by

�� = ��/(�� + ��

��)..................................(11)

Though both Hv and Cv will vary with the angle of attack of the flow on the vanes, ��, we have not exercised

that option here since there is no independent information on the turning vane performance. Estimates suggest

that 0.3 < Cv < 1.0



3.2 POWER TRANSMISSION SUMMARY

This completes the description of the power transmission through the coupling which is summarized in Table

3.

Table 3 Power Transmission and Losses

The overall efficiency of the coupling, η, is given by

� =��

=�� − �� + ��

�� + �� + ��.

Or substituting from equations (5) and (9)

� =��(�� − ��) − (�� + ��)/�

��(�� − ��) + (�� + ��)/�



CONCLUSION

After a thorough survey of fluid couplings we can conclude that the fluid couplings come with their own

advantages and disadvantages.

The advantages being:

1. A fluid coupling is smoother in operation compared to the mechanical clutches

2. Fluid couplings give the following benefits

2.1 Soft cushioned starts

2.2 Increased starting torque

2.3 Overload protection

2.4 Reduced current drawn when starting

2.5 Load balancing for multiple drives

3. The efficiency of power transmission is very high

4. No mechanical linkages and hence less wear and tear

5. The characteristics of a fluid coupling remains the same in both the directions

6. A reverse fluid coupling is advancement in the fluid couplings where in the power can be transmitted

by the introduction of a turning vane.

7. Fluid coupling technology has a lot of scope for further research and hence is a competitive field.

8. Torque converters are the other form of a fluid coupling which are rigorously used in the automatic

transmissions.

9. Fluid couplings not only find their application in the automobile field but also in the conveyors, and

anywhere else where power transmission is required.



However apart from these advantages the fluid couplings also have certain disadvantages they are listed below

1. The installation cost of fluid couplings is higher when compared to mechanical

counterpart.

2. You need to check clutch fluid level and top up when necessary.

3. Wear is less prominent to feel in fluid couplings and hence one might be completely

unaware of the wear.

4. There might be a risk of leakage across the seals and the housing.

5. The fluid in the coupling plays an important role. The fluid may change its properties

along with the changing ambience. Hence a fluid coupling which is 98% efficient at one

place might just be 80% efficient in the other depending on the topology.



BIBLIOGRAPHY

TECHNICAL PAPERS:

1. Charles N Mckinnon, Danamichele Brennen and Christopher E Brennen, Hydraulic Analysis of a

Reversible Fluid Coupling

2. Klaus Maier, modern drive technology with hydrodynamic coupling for coal mine applications, for

VOITH turbo.

3. Nicholas A D’arcy Jr., hydro-kinetic drives, Hydraulic Torque converters and Hydraulic Couplings,

Engineering and science monthly, 10-11.

4. Conrad L Arnold, Fluid Couplings, 1-15

BOOKS AND OTHER DOCUMENTS:

1. A text book of fluid mechanics and fluid machinery by R K Bhansal

2. Term paper on fluid coupling Yaba College of technology, Yaba, Lagos.

3. True torque Fluid Couplings by FALK TRUE COUPLINGS.

INTERNET SOURCES

1. http://en.wikipedia.org/wiki/Fluid_coupling

2. http://members.fortunecity.com/nedians2/fluid_coupling.htm

3. http://www.twindisc.com/IndustrialProducts/IndFC.html?pid=154

4. http://www.ehow.com/about_4596360_hydraulic-clutches.html

5. http://www.voithturbo.com/startup-components.htm

6. http://www.fluidomat.com

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