What is Fluid Coupling?- Definition, Types, And Uses

The principles of fluid dynamics are present in nearly all modern inventions. From modern toilets to rocket propulsion, it’s hard to find any revolutionary technology that doesn’t use it. Fluid dynamics is one of the most important products in engineering.

Like most engineering concepts, fluid coupling can be a bit of a puzzle. Luckily, we’re here to make things simple. Read on for a simple and intuitive way to understand fluid coupling.

Fluid coupling, also called hydraulic coupling, is a hydrodynamic device that uses transmission fluid to transfer mechanical power.

German scientist Dr. Hermann Fottinger invented fluid couplings. His invention was patented posthumously in 1950 and has been used for various industrial applications since then.

What is a Fluid Coupling?

A fluid coupling is a mechanism that makes a controlled amount of slip between the input and output, between the drive and the buffer, without any mechanical interaction.

Due to this, the fluid couplings give the engineer real benefits in several different forms of power transfer.

These advantages include start-up control, smooth acceleration, overload safety, energy saving, speed control, nearly frictionless clutch, starting from zero engine load and thus reducing engine size, torsional insulation (important for example between diesel engine and its load) and long-term stability of the drive, which has no mechanical contact between the power-transmitting element.

The fluid coupling, also known as hydraulic coupling, is a hydrodynamic mechanism used to transfer rotational force from one shaft to another by means of transmission fluid.

It is used in vehicle transmission systems, marine propulsion systems and power transmission industries. It is used as an alternative to electric clutches, or gear couplings.

fluid coupling diagram

How Does It Work?

The principles of fluid coupling are based on hydrokinetics and fluid dynamics. This makes it distinctly different from hydrostatic systems like hydraulic pumps.

A fluid coupling consists of a pair of bladed wheels that face each other but make no physical contact. One wheel, called the impeller, gets connected to a power source. The other, called the turbine, receives power from the impeller that transfers directly to the transmission.

The impeller, also known as the input, spins faster than the turbine, which is the output. Fluid flowing from the input causes the output to spin and transfers energy throughout the system. Both wheels are housed in an oil-filled casing that keeps them in place.

Specifications

Fluid couplings consist of two-bladed wheels that face each other, but that is not in physical contact. The pump wheel, or driving turbine, is connected to the driving machine. The turbine wheel is connected to the driven machine.

Power is transmitted hydraulically in a wear-free manner. The higher the input speed, the greater the amount of mechanical energy transmitted from the blades on the pump wheel to the blades on the turbine wheel.

With fluid couplings, the only connecting element between the two-bladed wheels is the fluid in the working circuit.

During machine startup, the amount of fluid in the coupling can be varied to control the machine startup behavior and the amount of power transmitted. By dampening torsional vibration and driveline shock, hydrodynamic devices can help extend equipment life.

Changing the fill level in fluid couplings also allows the speed of the driven machine to be controlled. Additionally, hydrodynamic couplings protect the drive and the machine against damaging torque spikes. If necessary, the slip can reach 100%, meaning that the motor can continue to run in a stable machine range.

What are the types of fluid couplings?

In practice there are two types of fluid couplings; Constant Filling, where the oil filling is fixed and variable filling also known as variable speed 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 and so give speed regulation.

The variable filling fluid coupling also gives a means of declutching a machine from its drive and is used extensively in conveyor drives to control accurately the torque applied during acceleration and give easy load balancing using external controls.

What Are The Advantages Of Fluid Couplings?

Hydrodynamic couplings transmit mechanical power from an electric motor to a gearbox with practically no wear. Here are some additional benefits of using fluid couplings over gear couplings:

Load-free Motor Run-up

Asynchronous (induction) electric motors are commonly used, particularly in stationary drives such as conveyors. However, the drawback of such engines is the high starting current, which can easily be five times the rated current.

High starting current creates a decrease in voltage and engine torque and, in particular, in poor power systems, may strain the electrical grid or cause the power supply to fail.

The fluid coupling in the drive chain greatly relieves the engine loads during the run-up, so that the engine travels rapidly across the high-current spectrum and minimises the impact of the voltage decrease on the drive mechanism. It also makes use of cost-effective standard engines that do not need to be oversized.

These couplings absolutely decouple the engine and the powered unit, so that the conveyor start-up is independent of the engine run-up. The coupling stays empty until the engine is fired up, then slowly fills and raises the torque.

In combination with electrical sensors, the coupling progressively adds stress to the belt and efficiently eliminates shock and vibration. When assembled, the coupling can work with an efficiency of up to 98.5% and automatically compensates for minor torque and load variations.

Load Sharing

Many belt conveyor systems depend on multi-motor drives. Fluid couplings allow sequential engine start-up—avoiding peak current and helping to secure the power grid.

The couplings remain empty until both cylinders are enabled, so the torque builds up steadily and the friction on the belt is exerted softly in a controlled fashion.

The torque can be kept under close limits, depending on the load. Some fluid couplings also have effective load sharing, which prevents individual motors from overload and handles various load conditions for different types of drives.

Torque Limiting

The fluid coupling restricts the maximum torque of the drive train in relation to the speed. This offers protection against pressure and longer life, and the drive-system modules do not need to be oversized to accommodate the higher torque in break-away conditions to propel the belt.

Inherent Damping

The effect on unequal torque conditions is another benefit of this power transmission system. These couplings greatly minimise low-frequency torque variability, and higher-frequency differences are rarely transferable at all.

They boost the dynamic characteristics of the drive and secure all components of the mechanism. Torsional frequency conflicts in the drive train are practically avoided when the fluid coupling is part of the drive mechanism.

Low Maintenance

Fluid divides the input and output sides of the coupling so that the pump and turbine have no wear and need limited maintenance. In addition, durable units are engineered and constructed so as not to be impacted by environmental impacts.

They run consistently and under rough operating conditions. This results in high machine availability, improved efficiency and reduced downtime.

Applications of Fluid Coupling

  • It is used in the automobile industry in place of the clutch to transmit energy from the motor to the wheels.
  • It is a propellant material used in maritime propulsion systems.
  • It is utilized to transmit energy in a variety of Industrial sectors for applications like Centrifugal Fans, centrifugal pumps, Conveyors, Crushers, Ball mills, mixers,etc