Hydraulic Coupling

Hydraulics- Hydraulics is a topic in applied science and engineering  dealing with the mechanical properties of liquids.

Fig-hydraulic coupling Assemblies

Coupling

A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power.

Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.

The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both. By careful selection, installation and maintenance of couplings, substantial savings can be made in reduced maintenance costs and downtime. 

What is a Hydraulic Coupling ?

Definition : Hydraulic (or fluid) coupling  is a device used for transmitting power from driving shaft to driven shaft with the help of fluid (generally oil).

     
Working fluid used in hydraulic coupling 

Oil is used as the working fluid because of its stability , non-corrosive nature and lubricating  properties.
There is no mechanical connection b/w the two shafts .It has no mechanical connection or face to face contact. The magnitudes of input and output torques are equal.

Construction
Hydraulic Coupling consists of the following two rotating elements:
   (a) Pump impeller:- it is attached to a driving shaft of the prime mover which may be an I.C. engine, a steam engine or an electric motor.
   (b) Turbine runner:-  It is attached to driven shaft.

Fig-Runner, impeller, and cover of hydraulic coupling.

Both impeller and runner are identical in shape and together form a casing which is completely enclosed and filled with oil. This oil serves to transmit torque from the pump impeller to the turbine runner . There is no direct contact b/w the driving and driven parts.

Working of hydraulic coupling

As soon as the prime mover starts rotating, the pump impeller also starts rotating and throws the oil outwards by centrifugal action. The oil then enters the turbine runner and exerts a force on the runner blades. The magnitude of the torque increases with an increase in the speed of the driving shaft and eventually when this torque overcomes the inertia effects, the turbine runner and the the driven shaft begin to rotate. The oil from the runner then flows back into the pump impeller, thus a complete hydraulic (oil) circuit is established.

Uses of hydraulic coupling

• Although a hydraulic or fluid coupling has a low value of tranmission efficiency when compared to mechanical coupling ,yet it is widely used in the following fields:
• Automobile
• Marine engine
• Ropeway cable drive units and such other applications where driven shaft is required to run at a speed close to that of the driving shaft
• These couplings are used particularly where large initial loads are involved and smooth shock-free operations are required
• Fluid couplings were used in a variety of early semi-automatic transmissions and automatic transmissions like General Motors' Hydramatic.

Advantages of hydraulic coupling

• Fluid couplings cushion the shock from equipment overloads, machinery jamming, reversing operations or sudden speed changes by increasing slip During prolong startups, the fluid coupling allows the motor to accelerate to full speed before the load resulting in lower current draw and preventing overheating.

• Wear-free power transmission because of absence of mechanical connection between the input and output elements.

• Simple control of maximum or limiting torque by easy variation of oil filling

• Smooth and controlled acceleration of driven machines. Important for machines such as conveyors, wire drawing and textile machines.

• Fluid couplings are used in many applications including conveying systems, crane travel drives, processing equipment, and line systems such as filling and packaging.

• They have largely been replaced for automotive use by the torque converter, a fluid coupling with an additional component (or components) called a stator that increases the efficiency of the coupling and provides torque multiplication as well as torque transmission

• Effective dampening of shocks, load fluctuations and torsional vibrations. Helpful in improving the drive duty conditions in machines subjected to shock loads, frequent load changes, startups and reversals.