Construction, Working, Operation And Maintenance Of Fluid .
Construction, Working, Operation and MaintenanceofFluid CouplingsByK. P. ShahEmail: kpshah123[at]gmail.com (Please replace [at] with @)Committed to improve the Quality of LifeFor more articles on mechanical maintenance, visit www.practicalmaintenance.netThe information contained in this article represents a significant collection of technicalinformation about construction, working, installation, operation and maintenance of fluidcouplings. This information will help to achieve increased reliability at a decreased cost.Assemblage of this information will provide a single point of reference that might otherwisebe time consuming to obtain. Most of information given in this article is mainly derived fromliterature on the subject from sources as per the references given at the end of this article.For more information, please refer them. All information contained in this article has beenassembled with great care. However, the information is given for guidance purposes only.The ultimate responsibility for its use and any subsequent liability rests with the end user.Please view the disclaimer uploaded on http://www.practicalmaintenance.net.(Edition: May 2018)1Construction, Working, Operation and Maintenance of Fluid Couplingswww.practicalmaintenance.net
Fluid CouplingsFluid couplings or hydraulic couplings work on the hydrodynamic principle. In drivesconsisting fluid couplings, there is no mechanical contact between the driver and the drivenmachine and power is transmitted by means of a fluid. Due to the mechanical separationbetween the driver and the driven machine, a fluid coupling enables to achieve two separatevalue of acceleration in the drive, the fast value of acceleration for the driver andsimultaneously the slow value of acceleration for the driven machine. Fluid couplings areoften used to drive large inertia machines in combination with squirrel cage motors. Theypermit a load free acceleration of the motor and consequently with increasing oil fill, providea soft/gentle quasi steady state start-up of the machine. The maximum torque occurringduring the start-up process is restricted to lowest possible level. As fluid coupling allowsquick acceleration of the motor and short duration of high value starting current, it results intoeconomical design for electrical system. In addition, systems that use multiple motors can beswitched on in a staggered sequence to limit the current demanded during the motoracceleration. This avoids grid overloading caused by simultaneous motor starts.Fluid couplings are used in drives for conveyor systems such as belt conveyors, bucketelevators and chain conveyors. The smooth application of fluid coupling torque provides asmooth start-up of belt conveyor to protect the belt from damaging stresses. In heavyindustry, they are used for applications such as crushers, roller presses, mixers, largeventilators, boiler feed pumps, large compressors, centrifuges, etc. In view of this,information about starting of a machine with a squirrel cage motor, working of a fluidcoupling, construction of different type of fluid couplings and their maintenance is given inthis article.Starting of Machine with Squirrel Cage MotorThe squirrel cage motors are probably the most widely used motors in industry today. Simpleand rugged design, low-cost and low maintenance are their main advantages.Above figure shows typical torque versus speed curve for NEMA Design B motor.Locked Rotor Torque (Breakaway Torque or Starting Torque)The locked rotor torque of a motor is the minimum torque, which it will develop at rest. Thiscapability is true with rated voltage and frequency applied.2Construction, Working, Operation and Maintenance of Fluid Couplingswww.practicalmaintenance.net
Pull-up TorqueThe pull-up torque of a motor is the minimum torque developed by the motor whenaccelerating from rest to the breakdown torque point. For motors that do not have a definitebreakdown torque, the pull-up torque is the minimum torque developed up to rated speed.Breakdown Torque (Peak Torque)The maximum torque developed by the motor during the period of acceleration between thespeed corresponding to pull-up torque and the full-load speed.Rated Load Torque (Full-load Torque)The rated load torque of a motor is the torque necessary to produce the motor’s ratedhorsepower at rated-load speed. (Note: Rated load speed is normally considered basespeed. Base speed means actual rotor speed when rated voltage, frequency, and load areapplied to the motor.)NEMA classifies low voltage AC induction motors into several classes with respect to lockedrotor, pull-up, breakdown torques, and running slip.Above figure shows typical torque versus speed curve for various NEMA Design classes.In general, NEMA Design A motors have low to moderate locked rotor torque, low pull-uptorque, high breakdown torque, high starting currents, and small running slip (slip 5%).NEMA Design B motors have moderate locked rotor torque, moderate pull-up torque,medium to high breakdown torque, moderate starting current, and moderate running slip ( 5%). Design B motors are the most popular motor design, commonly referred to as generalpurpose motors, and used in most applications.NEMA Design C motors have moderate to high locked rotor torque, high pull-up torque,medium breakdown torque, medium to low starting current, and medium running slip ( 5%).NEMA Design D motors have high locked rotor torque, high pull-up torque, moderate to lowbreakdown torque, low starting current, and high running slip (5-13%). The high slip valuesmake this motor suitable for applications with changing loads and subsequent sharpchanges in the motor speed, such as in machinery with energy storage flywheels, punchpresses, shears, elevators, extractors, winches, hoists, oil-well pumping, wiredrawing, etc.This motor design is usually considered a "special order" item.3Construction, Working, Operation and Maintenance of Fluid Couplingswww.practicalmaintenance.net
NEMA Design E motor is similar to Design B, but has a higher efficiency, high startingcurrent and lower full-load running current.It may be noted that slip of the motor increases with the classes (low slip with A & B classesand high slip for C & D classes) which would result in lower running speeds and higherlosses. NEMA Design B and Design C motors are most commonly applied to belt conveyors.NEMA Design C motors are usually chosen for direct coupled loads; the high pull up torquegiving more available torque for acceleration of the load through starting.A fluid coupling allows the AC motor to start unloaded. The load accelerates slowly based onthe transmitted torque from the coupling, while the coupling absorbs slip energy.As shown in above figure, when the motor is started, the torque transmitted is zero and thenincreases as parabolic curve. The motor torque ‘Tm’ is available to accelerate the motor andthe primary parts of the fluid coupling. The motor attains the full speed ‘Nm’, resultingreduction in current drawn ‘Im’ quickly.The machine starts when the torque transmitted ‘Ts’ exceeds the nominal torque ‘Tr’ tobreak the machine away. The slip across the coupling reduces as the speed of the machine‘Ns’ reaches its normal operating speed.The curve ‘Ts’ indicates the torque transmitted to the driven machine. It shows the smoothintroduction of torque during starting. The Torque ‘Ts’ is less than the maximum torque of themotor. The coupling limits the torque transmitted. In the event of machine stalling, the motoris protected by the coupling by increasing the slip. Thus motor protection and machineprotection are built in characteristics of fluid couplings. As the motor starting current is low,motor reaches full speed quickly and motor current normalizes very rapidly, motor cantherefore be smaller for a given duty and sized only for demand, not for starting torque.Working of Fluid CouplingsIn a conventional machine construction, transmitting of power from the driver to the drivenmachine occurs predominantly according to the direct operating principle, for examplethrough shafts, mechanical couplings or gear units.4Construction, Working, Operation and Maintenance of Fluid Couplingswww.practicalmaintenance.net
As shown in above figure, power can be also transmitted from a motor (a driver) to a drivenmachine without direct operating principle using a pump, a turbine and associated piping. Itcan be seen that the power is transferred from the pump to the turbine without any contact(indirectly). Similarly, in a fluid coupling, transmission of power occurs indirectly based on anindirect operating principle, principle of hydrodynamic power transmission.As shown in above figure, in a fluid coupling, pump impeller and turbine wheel are enclosedin a leak proof housing (also called casing or shell) for hydrodynamic power transmission.The pump impeller is connected to the primary driver (motor) and the turbine wheel to thedriven machine. The pump impeller transfers the introduced mechanical energy to kineticenergy in fluid flow. The higher energy fluid flows centrifically from the pump impeller to theturbine wheel, where a reconversion to mechanical energy (force) takes place. Thus thepower is transferred from the pump to the turbine without any contact and thus without wear.Only the bearings and sealing elements are subject to natural wear and tear. Because of thestraight radially arranged vanes/blades, the torque is transmitted independently of thedirection of rotation and solely by the amount/quantity of oil filling in the coupling.To generate circulation of the operating fluid for torque/power transmission, a difference inspeed is necessary between the pump impeller and turbine wheel. A centrifugal forcepressure field is set up that is greater in the faster rotating pump impeller than in the turbinewheel. The difference in speed, usually termed “slip”, at the continuous operating point of thecoupling is from 2% to 6%, depending on application and coupling size. Immediately afterdrive motor start-up, slip is 100%, i.e. the pump impeller is driven at the speed of the motor,but the turbine impeller remains stationary.5Construction, Working, Operation and Maintenance of Fluid Couplingswww.practicalmaintenance.net
Types of Fluid Coupling DesignsFluid couplings are designed in two types: constant-fill couplings and variable-speed (fillcontrolled) couplings.Constant-fill CouplingsCouplings of this type are mainly used for start-up (to limit torque) and to cushion thetorsional vibration of the drive chain. In this type of couplings, various designs mainly differthrough adjoining chambers, who’s automatically controlled filling and emptying have asignificant influence on the start-up behavior.Constant-fill couplings are sealed to the outside. Filling of the operating fluid in a coupling iscarried out before its commissioning. Drive requirements determine the design and fillingquantity. The ratio of the operating fluid volume filled to the overall volume of the coupling iscalled the fill level. The coupling obtains a specific characteristic curve from the fill level.Manually adding or removing operating fluid (at a standstill) enables altering thecharacteristic curve.Variable-speed CouplingsCouplings of this type are used to control or regulate the speed of the driven machine over awide range below the drive speed.These couplings have devices that seamlessly change the transmission behavior duringoperation. This mainly occurs by changing the fill level. The fill level can be changed duringoperation either via a radially movable scoop tube or by controlling the operating fluid inletand outlet via valves and nozzles. These couplings always have an external fluid circuit forfilling changes that can also aid cooling.Construction and Working of Fluid CouplingsConstruction and working of commonly used couplings as per designs of Voith Turbo GmbH& Co. KG, Germany is given in this section.6Construction, Working, Operation and Maintenance of Fluid Couplingswww.practicalmaintenance.net
The Voith Turbo fluid coupling Type T (basic design), shown in above figure, consists of afew main components without additional adjoining storage chambers. The shell and thevanned outer wheel forms the working chamber. This working chamber is housed relative tothe hub and sealed to the outside. The inner part consists of the hub and the inner (turbine)wheel, which are fixed together.A fluid coupling is installed between co-axial shafts. Normally a fluid coupling is connected tothe shaft of a motor (driving machine). As it is a rigid unit, it is connected to the motor by aflexible connection coupling which absorbs any small assembly misalignment. If necessaryor for constructional advantage, a fluid coupling can also be mounted on the motor shaft.In Type T fluid coupling, the full quantity of operating fluid (usually oil) remains in the workingchamber (power transmitting chamber) all the time.This type of coupling is suitable for conveyors of less inertia. In general, Type T coupling issuitable for conveyor up to 200 m length, and carrying material of medium bulk density.Above figure shows design features of constant-filled fluid couplings. They fulfill a variety ofrequirements for the start-up and operating behavior as needed. The use of adjoiningchambers and the volume exchange capability via fixed bores or bores closed withcentrifugal force valves enable a great variation of characteristic curves in relation to speed,time and slip.Voith TypeDesignationTTVTVVTV S7Design FeaturesBasic designNormal delaychamberEnlarged delaychamberDelay chamber andannular chamberFunction DescriptionThe working chamber filling determines the start-up andoperating behavior.The delay chamber takes in part of the operating fluid at astandstill.During start-up, the delay chamber empties into the workingchamber via nozzle bores. The delay chamber and annular chamber take in part ofthe operating fluid at a standstill. During motor run-up, the annular chamber also takes partof the operating fluid from the working chamber. During start-up, the delay chamber empties into theworking chamber via nozzle bores.Construction, Working, Operation and Maintenance of Fluid Couplingswww.practicalmaintenance.net
Note: For information on Types TV Y and TV F, please see “Hydrodynamic Couplings - Principles Features Benefits” by Voith Turbo (www.voithturbo.com/startupcomponents).Above table shows description of function for fluid couplings having various design features(for Voith type designation).Type TV fluid coupling with normal delay chamber is having an integral delay filling chamber.The delay filling chamber acts as a temporary store for the working fluid. Above figure showsdistribution of the operating fluid in a Type TV fluid coupling. As shown in the figure, whenthe coupling is standstill (stationary), the operating fluid spreads due to gravity; with theresult part quantity of operating fluid is in working chamber and part quantity of operatingfluid is in the delay chamber. During start-up the delay chamber retains part of the operatingfluid from the coupling circuit and thus decreases the transmitted torque. Only after run-up iscompleted, this retained fluid flows back into the coupling’s working chamber. The flow rateis governed by the in-built size of apertures connecting the delay chamber with the workingchamber. This ensures more (compared to Type V coupling) effective relief for the electricmotor during start-up.In general, Type TV coupling is suitable for conveyor length from 200 to 500 m whereasType TVV coupling is suitable for conveyor length from 400 to 1000 m (it may be noted thatconveyor length is influenced by various factors like bulk density, conveyor inclination, etc.).Type TVVS fluid coupling with enlarged delay chamber is having an enlarged delay chamberand an annular chamber. Above figure shows distribution of the operating fluid in a TypeTVVS fluid coupling.8Construction, Working, Operation and Maintenance of Fluid Couplingswww.practicalmaintenance.net
When the motor is started, the portion of operating fluid quantity which is in the workingchamber, immediately flows (thrown) into the annular space due to centrifugal action. Thusmotor and primary side of the coupling begins the run-up almost without presence ofoperating fluid in the working chamber. This results into still better performance compared tocoupling with enlarged delay chamber.The machines/conveyor with high inertia can accelerate at ease with this type of coupling.This type of coupling has been used for conveyors up to 2.0 km length. This type of couplingis also used for lesser length high capacity (high power) conveyors when superiorperformance is needed.Comparison of start-up behavior of a driven machine with constant load torque and massmoment of inertia while using various Voith coupling types is shown in above figure. Thedrive motor is a squirrel cage asynchronous motor.9Construction, Working, Operation and Maintenance of Fluid Couplingswww.practicalmaintenance.net
Above figure shows schematic view of a scoop tube type variable-speed fluid coupling. Inthis type of coupling, the fill level can be changed during operation via a radially movablescoop tube.When the coupling is stationary, most of the oil is in sump. This allows the motor to run uppractically without load, because in this condition motor is practically not connected to load.The motor side of the coupling drives an inbuilt oil pump, which becomes operative with therotation of the motor. This oil pump injects the oil into working chamber, by means of oil jet.Subsequently, the quantity of oil in working chamber depends upon the position of scooptube. The stationary scoop tube mouth intercepts the rotating oil, and thereby empty out theoil which is within the radius formed by scoop tube mouth. Thus, oil quantity in workingchamber is governed by scoop tube radial position. During start-up phase, the scoop tubeposition gets automatically adjusted (retracted), which gradually increases the quantity of oilin working chamber. The gradual increase of oil quantity in working chamber results inoptimum starting torque steadily for longer time. This allows the design of a machine (e.g.conveyor) with low starting forces.Above figure shows construction of fill-controlled fluid coupling, Type TPKL. This coupling’sworking principle rests on the metered filling of the rotating working chamber. In thiscoupling, operating fluid exits the working chamber via nozzles into an annular shellconnected to the pump impeller. A dynamic pressure pump (fixed scoop tube) siphons offthe operating fluid from this annular shell and then conducts it through a heat exchanger viaa catching ring back into the working chamber. This results in an outer, enclosed operatingfluid circuit. Working chamber filling changes are carried out indirectly through the addition orremoval of operating fluid from this outer circuit. The flow rate of the filling pump depends onthe desired fill time of the working chamber.Variable-speed fluid couplings are used principally in conveyor systems with higher powersand long starting times.Note:For figures showing various operating conditions (at motor start, acceleration of drivenmachine, nominal operation and operation at part-load speed), please see “Fill-controlledFluid Couplings” by Voith tion, Working, Operation and Maintenance of Fluid Couplingswww.practicalmaintenance.net
Special Design Considerations for Constant Fill CouplingsAs fluid couplings of constant fill type are sealed to the outside and the exchange of gases orfluids with the environment is not easily possible, measures to limit the thermal innerpressure must be implemented.To generate the operating fluid circulation, a difference in speed, usually termed “slip”, isnecessary between the pump impeller and turbine wheel. Slip mult
Variable-speed Couplings Couplings of this type are used to control or regulate the speed of the driven machine over a wide range below the drive speed. These couplings have devices that seamlessly change the transmission behavior during operation. This mainly occurs by changing the fill level. The fill level can be changed during
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