Hydraulic Coupling

HYDRAULIC HYDRAU LIC COUPLING

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Fluid coupling is a combination of pump and turbine acting and reacting simultaneously to give a new element for torque transmission. As we know 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 contact in between them. Now the point comes into play on the selection of working fluid for smooth running of pump and turbine placed in closed vicinity in a common chamber. Like in all cases of transmission and energy transformation, lost energy handle 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 following reason:a) Universally available. b) Relatively low in cost. c) Lubricates the fluid coupling internals when running, and protects them from stationary.













SLIP is defined as 100 x (primary speed-secondary speed)/primary speed. This slip characteristic is deciding factor for the selection of working fluid and the necessary 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 OF FLUID COUPLING (A). Constant fill fluid coupling & (B) Controllable fill fluid coupling.

FIXED OIL TYPE FLUID COUPLING This is a PEMBRIL FLUIDRIVE fluid coupling which is a simple power transmitting unit to couple an electric motor to a machine. It consists of two rotating assemblies













CONSTRUCTION

























The main components are: INPUT SIDE- Impeller and casing. OUTPUT SIDE- Runner and shaft. The impeller, runner and casing are high tensile aluminium alloy castings, and the impeller and runner both have a large number of straight radial vanes. The runner shaft is carried in ball and roller bearings in the casing and the runner. There is no mechanical connection between the runner and the impeller. The amount of oil in the coupling may be varied over a wide range, to give adjustment of acceleration and torques. A gland sealing is fitted fitted between the casing casing and the runner shaft. How the fluid coupling works:















Characteristics Typical Typical characteristics of the fluid f luid coupling (when used with direct connection of motor) are shown in the following figures;













Starting At the motor switch on, the fluid coupling has no torque capacity. capacity. As the motor accelerates , the coupling torque remains low. low. The motor thus starts under light load and runs up to speed quickly, quickly, while the torque of the fluid coupling increases smoothly to start the machine.

Running Typical Typical coupling torque/output speed characteristics available for bringing the machine up to speed smoothly and rapidly. High torques available for starting and accelerating machine, also accelerating torque













Operation The torque /speed and the stalling characteristics of the fluid coupling can be adjusted by increasing or decreasing the quantity of oil initially filled into the coupling.













CONTROLLABLE FILL FLUID COUPLING:i)PEMBRILvariable speed control fluid coupling & ii) Variable speed turbo coupling (make VOITH)

PEMBRILL FLUIDRIVE FLUID COUPLING:The fluidrive fluid coupling is an adjustable speed fluid coupling designed for mounting between an electric motor and a driven machine( such as a pump or a







































CONSTRUCTION

























OPERATION Power is transmitted from the impeller (driven by the motor) to the runner (coupled to the













A continuous circulation of oil is maintained, as oil escapes from the working circuit through small leak-off holes into the reservoir casing all the time that the coupling is







































































































































VOITH GEARED VARIABLE SPEED TURBO COUPLING FOR B.F.P VARIABLE SPEED GEARED COUPLING AS BOILER FEED PUMP DRIVE Power requirements of driven machine: 3000 kw Motor speed: 1483 rpm Gear ratio: 139/39 (=3.56) Primary speed: 5285 rpm Full load slip: 2.0% Max. output speed of the variable speed geared Turbo coupling: 5178 rpm Regulating range: 4: 1 downwards downwards Oil tank filling: 700 litres (approx) Filling & lub oil together driven

























DESIGN & OPERATION The geared variable speed turbo coupling is used for infinitely variable speed control of high speed machines. The variable speed turbo coupling and step up gear are incorporated in a mutual housing, while the lower section s ection op the housing serves as an oil tank. Flexible connecting couplings are applied for power transmission from the motor the to the geared variable speed turbo coupling and from there to the machine. The speed is reduced between input and primary shaft by means of a gear stage. Torque is transmitted hydrodynamically from the primary wheel to the secondary wheel by the working oil. The torque generated by the driving machine is provided to accelerate the fluid in the primary wheel (impeller). This is delayed in the secondary wheel (turbine) to produce an equal torque. The fluid can only circulate by a pressure drop between the primary and secondary wheel. This requires that the secondary wheel speed is lower than that of the primary wheel. Thus a slip is required for power transmission. The coupling size is selected so that the full power can be transmitted with a low slip, the “full load slip” (at max. filling). The output speed can be regulated infinitely by changing













Operating Oil Circuit As the type of regulation supplied is a slip regulation, hydraulic losses unavoidably occur in the variable speed turbo coupling. These losses manifest themselves in heating up of the coupling oil which is then cooled in a heat exchanger in order to dissipate the heat. The amount of output losses depends on the load characteristics and the slip required for attaining the desired output speed.













Starting The centrifugal pump 1 delivers through a restrictor 63 into the circulation control valve casing 57. The coupling is filled according to the resistances via the cooler II and the scoop tube 41 as well as via valve III. The pressure holding valve IV is closed.

Normal Operation (without regulation ) The amount of circulating oil required for dissipation of heat is a product of the pressure (1.2-2.4 bars) preset in the circulation Control valve housing 57 by means of the pressure holding valve IV and the cross sectional area uncovered by the valve III control piston. The amount of oil approaching the coupling is collected at the scooping edge of the scoop tube after flowing through the impeller parts and pumped back from the scoop tube 41 via the cooler II into the circulation control valve casing 57 against the pressure prevailing in this valve casing. The filling pump I delivers a differential amount into the circulating















Step-up speed regulation . The scoop tube moves away from the coupling oil level, the delivery decreases, the pressure inside the circulating control valve casing 57 drops, the pressure holding valve IV closes, the filling pump delivery is used for filling the impeller parts.

Step-down speed regulation . The scoop tube moves into the oil level, the scoop tube delivery increases, the pressure inside the circulating control valve casing 57 rises. The pressure holding













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 % starting torque to 250 % breakdown torque based on full load. Severe damage may result to the connected equipment if less than 180 percent of fullload torque for starting is required because components in the drive train must absorb the additional load. load. Any number of components, components, from belts on a conveyor conveyor to bearings or rotating shafts and more, could fail as a result of “over of “over -torquing”. torquing”. If the driven equipment requires more than 180 percent breakaway torque, Motor will fail













Selection criteria . Due to the operating principle of hydrodynamic couplings, they affect the Power train in several ways. Various selection criteria therefore apply. Hydrodynamic couplings are generally selected according to the requirements on torque transmission and power train characteristics.

The following conditions apply to hydrodynamic couplings with constant filling level filling level : 1.The drive motor power depends on the coupling rather than being driven startup relief relief low-cost low-cost drives).

The machine













The following criteria apply to hydrodynamic coupling size and type selection : 1. Characteristics. 2. Low rated slip. 3. Installation conditions. 4. Thermal storage capacity capacity (switching (switching frequencies). 5. Heat transfer and disposal. 6. Temperature emperatur e limitation limita tion (explosion (explosio n risk). 7. Operating medium. 8. Secondary wear wear and tear (bearings, (bearings, seals). 9. noise development.













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.







































Hydraulic Coupling

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