What is fluid coupling and advantage of fluid coupling

What is fluid coupling?

Fluid coupling

A fluid coupling or hydraulic coupling is a hydrodynamic or hydrokinetic equipment that can transmit power and torque with the help of liquid (hydraulic oil). It belongs to the category of flexible coupling power transmission. This a based on hydrodynamic torque transmission device. Fluid coupling is the assembly of pump impeller, casing gaskets, hydraulic oil, casing, turbine impeller, input shaft, and output shaft. The diagram of fluid coupling is given below. Nowadays Fluid couplings are used in many applications, i.e., belt Conveyors, Crushers, Ball and rod mills, Agitators and Mixers, Pumps, Fans, Boiler Feed Pumps, gear box, Industrial drives, to provide smooth torque and power transmission. And connect two different equipment shafts to each other for proper operation.

The key feature of this coupling is Power Transmission by the help of hydraulic fluid without mechanical connection between driver and driven machine shaft.

     To learn about πŸ‘‰πŸ‘‰πŸ‘‰        ——  Rigid coupling ——–

Diagram of fluid coupling


Diagram of fluid coupling

Why do we need fluid couplings? 

For example, an Electric motor is used to rotate a belt conveyor for transporting raw material if the belt conveyor stuck somewhere and that will produce a shock load on the drive assembly.

There should be a device or system which isolate or disengage to each other and prevent form such type of shock load to the motor, but there is not possible by mechanically (physically) connection. Contacted system which can efficiently isolate the driven machine from the drive one like an automobile automatic transmission system when the driver wants to stop the vehicle. He starts applies break the transmission stops its rotation but engine shaft rotation will continue.

That will be possible only by fluid coupling, which allows a controlled amount of slip and mechanically isolate and disengaging the engine from end point of transmission.

This coupling has the biggest advantages is that power transmission cannot start during the fluid coupling the starting performance of motor, without the fluid coupling the motor instantly transmits its starting torque to the driven machine. It will quickly reach upper limit and apply the breakdown torque for applications locked rotor and breakdown torque, often approximates to a hundred percent or more of the motor nameplate rating. 

Damage can be a result if the required starting torque of the driven machine is significantly less. The machine will be abruptly accelerated to rated speed with a fluid coupling installed the torque to the driven machine starts at 0 and gradually increases as the coupling impeller accelerates when the output torque of the fluid coupling exceeds the Breakaway starting torque of the driven machine The Driven Machinery gradually accelerates too.


As the machine comes up to rated speed, the slip of the fluid coupling decreases and uniform power is transmitted reached at maximum efficiency. The motor actually starts in a no-load condition. So, the motor draws less current as compared to other couplings.

What is the working principle of fluid coupling?

Working principle of fluid coupling

An example beautifully explained the working principal fluid coupling, a table fan is to operating with an electric current which make the air to flow directly hits the other fan blades in front of first one, further other fan blades also stats rotating.

Electric motor the impeller motion imparts, both outwards linear rotational motion, high pressure and velocity that impact on the blades of the runner and makes the runner run at the same rotational direction as impeller during this process. Same principle applies on close casing of fluid coupling.

For fluid coupling when the electric motor starts rotating the coupling impeller, then mechanical energy of the prime mover converts into the kinetic energy and potential energy of hydraulic oil, and then the runner changes that energies of oil into mechanical energy of the output shaft.

Therefore, the transmission of power flexibly can realize this is the basic working theory of the fluid coupling. Other hand The input drive shaft through which the input power enters the device is connected to the pump impeller which drives the oil circulation within the device. That oil flow then drives the turbine which is connected to the output drive shaft through which the power emerges from the device. 

What is the torque converter?

Torque converter

As we called a torque converter in that system, the pump typically connected to the flywheel of the engine crankshaft. The turbine is connected to the input shaft of the transmission the introduction of stator in between the pump and turbine makes it a little different from the industrial grade fluid coupling as engine speed increases torque is transferred from the engine to the input shaft by the motion of the fluid propelling the vehicle, but the efficiency is never reached at 100%. In this regard a clutch is used which lock the impeller and turbine at high speed the behaviour of the fluid coupling becomes that of a mechanical clutch driving a manual transmission. 

How many types of fluid couplings?

Types of fluid coupling

There are two types of fluid coupling used to connect two different equipment shafts, depends on application their name and description as below.

  1. Fixed speed or constant fill type fluid coupling
  2. Variable speed or scoop type fluid coupling

Fixed speed or constant fill fluid coupling

 Fluid couplings constant filling fluid coupling where the oil filling is fixed. In close casing are used to contain oil and other assembly parts. Its operation speed and torque to be provided is fixed. A plug is used for safety purpose if operating torque goes beyond its upper limits, oil temperature will Goes up, and used plug melt, through which hydraulic oil drain, then output shaft stop rotating.

Variable speed or scoop fluid coupling

The scoop coupling provides less speed variation in a wide range when connected with fixed speed driver. The speed variation got only by the help varying the oil filling rate through the sliding type scoop tube during operation. Variable filling fluid coupling also known as variable speed fluid coupling where the quantity of oil in the working circuit can be varied whilst running to give control over the amount of slip between the impeller and Runner that will give speed regulation. The variable filling fluid coupling also gives a clutching a machine from its drive. It is used extensively use in conveyor. for Drives to control appropriatly the torque applied on equipment during during acceleration and control easly load balancing using external controls.

Scoop type fluid coupling

What part used to assemble the fluid coupling?

Parts of fluid coupling

There are many parts are used to construct the fluid coupling, name is given below, find from below list.

Fusible plug, snap ring, washer, impeller, coupling hydraulic seals, bearing, baffle plate, shaft runner, runner, casing chamber, Heli coil and another name as per manufacturer.

Construction of fluid coupling

In the construction of this fluid coupling the pump impeller is closely coupled with turbine impeller within the coupling casing that only transmits the torque to output shaft. which is Co Axial with driver shaft as shown in figure.

Preventive maintenance check point

Following point need to be check during planned preventive maintenance for fluid coupling.

  1. Check oil quantity and quality of oil
  2. Checking alignment between both equipment shafts
  3. Check the seal leakage or failure
  4. Check parts and plug condition.
  5. Physical coupling condition

How to check hydraulic oil in the fluid coupling.

Check hydraulic oil in the fluid coupling

Procedure of hydraulic oil checking in fluid coupling.

First rotate the driver shaft until oil just start come out from oil filling point. Measure the hydraulic oil inside, if found an excess extract from it, and oil level has gone down, top up the oil in fluid coupling. Then after tighten the cap or replace accordingly if found damaged. But for safety purpose the plug use in coupling is fusible, for industrial application set temperature to melts plug is approx. 140 degree Celsius. After the melting the plug it will allow the oil to drain.

What types of oil use in fluid coupling?

Fluid coupling hydraulic oil

The fluid is used in fluid coupling will directly impact on performance and torque transmission. Usually, hydraulic oil used in this type of coupling and it is equipped with a self-cooling system to remove the heat, generated by mechanical part. The selection of fluid and quantity of fluid (Lube oil) has to be filled into fluid coupling depends upon the following factor that is motor power, torque to be transferred, coupling slip and motor current. For normal operating condition mostly ISO VG 22 used. The thumb rule is to fill up hydraulic oil approx. 80% of total capacity of fluid coupling. The level of should be maintained below the overflow, but if cause of over level may be causes increase in oil pressure and the result will be seal failure.

Fluid coupling hydraulic oil

Properties of fluid coupling hydraulic oil

The working oil must be advantaged in low viscosity, high density, high flash point, low condensation point, High aging resistance, less corrosive and good flow ability.

What are advantages of fluid coupling?

Advantages of fluid of coupling

  1. Motor could be starting with no overload, and Smooth acceleration of the load
  2. Driver machine, driven machine and coupling protection during overload and limited to the maximum torque transmitted.
  3. Motor Selection through the operating torque, and Low motor power consumption.
  4. High efficiency because of the low sliding features
  5. Fluid coupling can transmit Maximum torque capacity can be achieved between 80 up to 270% of the operating torque.
  6. Available with a variety of designs and features.
  7. If the shaft jam, it will impact increasing stresses on components too many times their normal running values the fluid coupling slip characteristics spreads the absorption of the impact over after some time, reducing stress on the components and therefore protecting rotating equipment. If a jam occurs without a fluid coupling, the motor will start and draw over five hundred percent of full load current. So, it not only can effectively prevent the machine from being broken but also can considerably prolong. 

There are some key data to select appropriate fluid coupling for your services Power Range describe as below Fixed speed for up to 1850 KW, for Variable speed approx. up to 11000 KW. Speed Range for Fixed speed- 720 RPM–3600 RPM, and Variable speed 490 RPMβ€”3600 RPM, Starting torque range for Fixed speed: 80%β€”280%.

What is rigid coupling and An overview of rigid coupling

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