Fluid Coupling 581356

Fluid coupling T.M.JOARDAR

• Principle of fluid coupling

Impeller

Runner

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Fluid coupling is a combination of pump and turbine acting and reacting simultaneously to give birth a new hybrid element for torque transmission. As we everybody aware that pump is transforming kinetic energy of impeller into pressure head in fluid and turbine in turn absorb the pressure head and produce kinetic energy of runner. So the kinetic energy transfers from one shaft to other one without any mechanical in between them. Now the point comes into play on the selection of working fluid for smooth running of pump and turbine placed in close vicinity in a common chamber. Like in all cases of transmission and energy transformation, lost energy handling is very big problem in this case. Considering heat generated due to loss and lubricating requirement of the internal components, lub oil of low viscosity and high flash point is selected as working fluid Usually mineral oil is used as working fluid for the reason Universally available Relatively low in cost Lubricates the fluid coupling internals when running, and protects them when stationary. Non-toxic, requiring only simple precautions in use.

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The toque transmitted by the coupling is proportional the difference in moment of momentum of the fluid as it enters and leaves each member. The speed difference or ‘slip’, creates the net difference in opposing centrifugal heads of impeller and runner to circulate the fluid against the friction & shock loss with in the vaned space. So speed of the primary shaft i.e input is always greater than that of secondary i.e output shaft . slip is defined as 100 X (primary speed –secondary speed)/primary speed. This slip characteristics is deciding factor for the selection of working fluid and the necessity of cooling circuit of working fluid and provision of heat dissipating fins as change in viscosity at elevated temperature may deteriorate the performance of coupling.

• Classification FLUID COUPLING

Constant Fill

Stepped Circuit with Antechamber

Controllable Fill

Scoop control

Modified Stepped Circuit with delayed filling chamber.

Scoop trim Multi Vane Circuit with Baffle and Reservoir Volume

• Before going to the main points of elaborate discussion of different class of fluid couplings about their constructional design and advantage and utility applications, one thing is to be very clear that position of impeller and runner w.r.t input shaft is designed to suit the sealing system and delayed chamber location i.e some coupling have the orientation having impeller adjacent to input shaft and extended part of resilient plate covers the runner and sealing with output shaft and in some coupling impeller is on the output side and connected with resilient driving plate through circumferential bolting. • Although in each cases the operating principal is same as described in Dr. Harmann Fottinger, an electrical engineer. This change in orientation of impeller and runner and delay chamber is modified in course of time to suit the requirement of handling ,heat dissipation, loading pattern and weight of the FCU. Some technologist regarded Fluid coupling as the hydraulic analog of the AC squirrel cage induction motor as the motor torque is developed by interaction between the magnetic field at synchronous speed created by the stator current, and the field created by the current it generates in the rotor cage, which in turn is slightly lower speed equivalent to the slip.

Multi Vane Circuit with Baffle and Reservoir Volume Stepped Circuit with Antechamber

• Mounting arrangement is of carden shaft design i.e FCU load is distributed in 2:1 M:L • Multi vane impeller & runner are of similar shaped vanes. disc shaped baffle on the hub of the runner. The casing or shell covering Iplr. Rnr is acting as reservoir of working fluid. • Torque/output speed characteristics can be varied by selecting appropriate size of baffle disc & adjusting fluid filling accordingly upto 120% to 250% of FLT at4.5 4.7% slip and helps field commission trouble. • Aluminium casting enables to incorporate fin and other protuberances to improve self thermal dissipation .

• Stepped Circuit with Antechamber •

During rapid acceleration of motor up to full speed some fluid is held back in the antechamber & inert in torque transmission but comes in to action when full speed attained. • As no baffle inside FCU, optimization in field can only be done by varying impeller configuration in a Ltd zone & have low top speed slip. • Impeller & runner are of different profile and antechamber within the inner profile of the impeller of all steel construction generally used in conveyor and in coal mine .water is working fluid. • Operator safety is ensured at all times by provisioning three o/l protection features operating in sequence namely fusible plug ,responsive to temperature , a bursting disc which will rupture at slightly higher pressure of fusible temp. and finally floating ring type shaft gland will operate if previous 2 fails.

Modified stepped circuit with delayed filling chember

• In the version shown, the delayed filling chamber rotates with the primary ports and is located directly behind the impeller. By conducting special test to view in a stroboscopic light the fluid escape through the nozzles into the working circuit at motor start-up, and its re-entry into the chamber through the appropriate ports, either under severe overload conditions or shutdown, this was found to be a preferable location to one in which the chamber, whilst rotating with the primary parts, was located behind the runner. • In the mid-filling regions of the coupling a motor unloading effect more pronounced than that with an antechamber can be achieved. As may be expected, with very low and very high fillings, there is virtually no change in the nett fluid coupling of the working circuit, and thus an additional softening effect is not obtained • Probably the most important contribution of all made by the delayed filling chamber, is to enable the good characteristics conferred by the designs having runner shafts of diameter proportioned only to their duties of torque transmission and load carrying also to be available on the hollow shaft mounted versions. The latter have substantially greater shaft diameters to accommodate the large hollow bores necessary to receive the gearbox (or motor) shaft. Tests show that the space in the center of the toroid plays an important part in achieving low top speed slips in combination with low acceleration torque levels. Where space is of necessity obstructed or filled by a large diameter runner shaft, then it is found that the provision of a delayed filling chamber volume rotating with the primary parts has a compensatory effect.

System resistance curve ( valve throttle )

speed n1 H e a d

2* System resistance curve speed n2 1

H 2 speed n1 speed n2

P

Power saving due to speed regulation

Q capacity

• . The working circuit is contained within an inner casing which, with the surrounding reservoir casing, rotates at motor speed. A sliding scoop tube with open mouth facing into the motor rotation is carried within the rotating reservoir casing from a stationary bracket assembly bolted to the drivehead framework. Calibrated nozzles at the periphery of the inner casing allow a continuous controlled escape flow from the working circuit space in the reservoir casing. • Movement of the sliding scoop tube by an external pivoted lever to which a suitable actuator can be connected, will control the amount of fluid remaining within the reservoir casing and not returned to the working circuit. Hence, scoop lever position determines the net filling of the working circuit at any time, and the dynamic head generates as the scoop tip circulates the fluid (through an external cooler if need be) back into the working circuit. The scoop tube is double ended to cater for both directions of motor rotation. The calibrated nozzles (three in number) are each drilled into a plug screwed into a threaded seating in the inner casing. When the application so requires, these plugs can be replaced, in the field if need be, by input speed sensitive centrifugal valves, or diaphragm quick emptying valves

P Pump impeller T Turbine wheel S Scoop chamber 1. Main lube oil pump 2. Input shaft 3. Output shaft 4. Gear 5. Working oil pump 6. Scoop tube (adjustable) 7. Scoop tube control (VEHS)

Scoop trim coupling

Basic scheme of fluid coupling Pump Runner

Ws Primary shaft

Wp

Secondary shaft

Turbine Runner Flow of oil

• To better understand what benefit a fluid coupling provides when connected between an electric motor and gear train, the speed and torque profile of the electric motor must be considered. During start-up, an across-the-line started motor transmits torque to the drive system components. As shown in the graph, these values can range anywhere from 180 percent starting torque to 250 percent breakdown torque based on full load. • Severe damage may result to the connected equipment if less than 180 percent of full-load torque for starting is required because components in the drive train must absorb the additional load. Any number of components, from belts on a conveyor to bearings or rotating shafts and more, could fail as a result of "over-torquing." If the driven equipment requires more than 180 percent breakaway torque, the motor will fail to start. This is when the benefits of a fluid coupling become evident. The fluid coupling controls the motor's output characteristics to match load requirements. When the electric motor is started, no load is demanded since fluid has not flowed between the impeller and runner. The only load imposed on the motor is the inertia of the casing and impeller. As the motor accelerates, the impeller begins to pump oil to the runner and torque gradually builds following the square of the motor speed. Therefore, torque build-up is smooth and gradual. Once the torque build-up has matched the required breakaway value, the runner will begin to rotate and accelerate the driven load. The electric motor is now running at full-load speed and "flow" in the coupling is fixed. The torque developed by the fluid coupling is directly related to the amount of oil circulating between the impeller and runner. Adjustment of the coupling's fill can provide a wide range of torque values. More oil in a fluid coupling provides higher starting

TM=motor torque NM = motor speed

TK = coupling trq NL =Load speed

TL = Load torque TN= rated toprque U =transition point

• The operating characteristics of a coupling type T

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with two different machine characteristics (constant torque, parabolic torque characteristic). From the secondary coupling characteristics, the primary characteristic as a function of motor loading can be derived, and the associated time characteristics can be derived from the operating data of the machine being driven. These system characteristics clearly show the almost parabolic torque build-up as the motor runs up to speed, after which the coupling characteristic depends only on its design. The torque transmitted by the coupling to each machine differs only with respect to the initial breakaway torque, and the transition point Ü in the coupling characteristic depending on the moment of inertia of the machine in question.

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Fluid media

• Power is transmitted in hydrodynamic couplings as specific kinetic energy of fluid flow. This depends primarily on the physical properties of the fluid – density and viscosity– and also demands the efficient removal of heat due to losses. The fluid characteristics required in practice vary widely, depending on the kind of coupling and how it is integrated in the drive system. As far as energy transmission is concerned, water is even better than mineral oil or synthetic fluids. With regard to safety and availability, water is excellent. However, river or seawater in particular is unsuitable with regard to abrasion, bearing lubrication, corrosion and cavitation. • Although Föttinger carried out tests with seawater, it turned out to be unsuitable in practice. The fluids mainly used comprise mineral oils, which can be modified according to drive system demands. Apart from their good long-term operating characteristics, mineral oils also meet control and lubrication requirements. The majority of these oils comprise paraffin-based solvates with excellent ageing resistance and additional properties as required. Mineral oils with low viscosity are preferable, since they reduce flow friction losses through the blading channels and thus increase power transmission efficiency. • If hydraulic oil is also used for gear lubrication, properties must be carefully balanced.In order to keep oil supply aggregates as compact as possible, good delivery capacity is also important.

• let us assume wp is input speed ws be secondary speed i.e angular velocity so power input to the primary wheel & power output in of torque(T) and angular velocity(w) • Pin = Tp.wp Pout = Ts.ws • Therefore efficiency of transmission h = Ts.ws / Tp.wp and as there is no parts is in between to provide torque reaction so the input and output torque is considered to be same ignoring the minor loses. Tp=Ts so h=ws/wp • h = 1-s • [s = (wp -- wp)/ wp •

U3

Vr3 U2

Vr2

U2 > U3

V2

Vf

Vw2=Vw3

Wp

Vw4 Vf=constant

Vw1 1

Vf

Vw3 Turbine half

Vw2 pump half

V3

V1 U1

Vr1

4

Vf

V4

2

Vw4=Vw1 U1 >U4

U4

Vr4

5

Vf

• Let us draw velocity triangle Assumption fluid enter & leaves impeller& runner with same tangential velocity component i.e zero whirl slip Vw2=Vw3 , Vw4=Vw1 Now By Euler’s eq. the work done by the primary per unit mass Ep/m=Yp=(Vw1.U2-Vw2.U1)/g As U2= wp.ra U1=wp.ri from velocity triangle Vw2=U2= wp.ra Vw1=Vw4=U4=ws.ri So we get Ep/m=Yp=(wp.ra2 -wp.ws.ri2) similarly, Es/m=Ys=(wp.ws.ra2-ws2.ri2) So the energy dissipation may be obtained dY=Yp-Ys=(wp-ws)(wp.ra2-ws.ri2)/g it is also assumed that dy=kQ2

• The power transmission characteristic of a hydrodynamic coupling can be shown graphically by plotting the relation = f (). The entire characteristic field is described by a series of curves as a function of the filling level V. • The basic characteristic of hydrodynamic couplings exhibits a steadily reducing power factor with rising speed n (decreasing slip), beginning at the startup point A. • Hydrodynamic couplings are selected according to their power transmission characteristic so that the rated torque (N) is transmitted at lowest possible nominal slip sN. The other characteristics can then be developed according to requirements of a particular driven machine.For startup and safety couplings, the flattest possible characteristic is usually required over the entire startup range, with relatively low torque peak max. • For variable-speed and fill-controlled couplings, consistently reducing power factor characteristics are requiredfrom = 0. This ensures stable operating points with various kinds of speed-regulated machinery.

• Dimensional analysis on law of geometric similarity • Let mass flow m fluid density r  filling volume Vol profile Dia Dp • power factor l press diff acr. implr dP speed characteristic Vn = ws/wp Kinetic viscosity n Reynolds no. Re=w.D2/n • Geometrical similarity • Euler number Eu= dP/(r.c2)

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Parameter Streaming pressure Streaming force Volumetric flow Mass flow Hydraulic torque Hydraulic power

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Basic dimensions: Length r, l D Surface area A D2 Velocities u, w, c D · Hydraulic similarity Reynolds number Re= w· D2/n

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Equation Eu = const. F =d p · dA V=c·A m=r·V T=F·r P = T.w

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Model law p ~ r.D2 · w2 F ~r.D4 · w2 V ~ D3 · w m ~r. D3 ·w T ~r. D5 ·w 2 P ~r. D5 · w3

• Important Do’s & don’ts • Never do painting on the FCU surface as heat dissipation will be hampered as well as performance • Always check the recommended level of filling

• In case of scoop control coupling after stopping motor always put the scoop in 100% position to have extended time & quantity of lub oil supply to the internal component of coupling . • Generally FCU are bi-rotational and of straight radial blading. So reversal of motor rotation causes server o/L due to output gear box

• Only one VFD was supplied experimentally to a major power plant in India for the drive of BFP and this was never repeated • For the drive of ID fans, the business is almost equally shared between Voith hydraulic couplings and VFDs depending on the experience of the end s. • There are of course a few customers, who have opted for VFDs for ID fan drives • But, there are also customers, who have reverted to hydraulic couplings in their extension units after using VFDs in the earlier ones.

Break-away torque

Pull up torque

Starting torque

TORQUE

e u q r o t r o t Mo

e u q or t g n e i t v a r r u e l c e e c u q Ac r to p m Pu

SPEED

Pull out torque

Full load torque

• Torque curve

e v r cu

•Thank you