What is a Fluid Coupling?

A Fluid coupling allows for a soft start or overload protection via the use of hydraulic oil or water running through a fully enclosed circuit. Fluid couplings allow for no load start-up of prime mover. This is a great advantage, allowing smaller motors or engines to be used and offers great savings on both set-up and running costs.

Fluidomat fluid couplings work on the hydrodynamic principle. It consists of a pump generally known as impeller and a turbine generally known as rotor, both enclosed in a casing. The impeller and rotor are bowl shaped and have large number of radial vanes. They face each other with an air gap. The impeller is connected to the prime mover while the rotor has a shaft bolted to it. This shaft is further connected to the driven machine.

Oil is filled in the fluid coupling from the filling plug provided on its body. A fusible plug is provided on the fluid coupling which blows off and drains out oil from the coupling in case of sustained overloading.

There is no mechanical internal connection between the impeller and the rotor (i.e. driving and driven units) and the power is transmitted by virtue of the fluid in the fluid coupling. The impeller when rotated by the prime mover imparts the velocity and energy to the fluid which is converted into mechanical energy in the rotor thus rotating it. The fluid follows a closed circuit of flow from the impeller to rotor through the air gap at the outer periphery and from the rotor to impeller again through the air gap at the inner periphery. To enable the fluid to flow from the impeller to rotor it is essential that there is a difference in "Head" between the two and thus it is essential that there is a difference in RPM known as Slip, between the two.

Slip is an important and inherent characteristic of a fluid coupling resulting in several desired advantage. As the slip increases, more fluid can be transferred from the impeller to the rotor and more torque can be transmitted. However when the rotor is at standstill, maximum fluid is transmitted from the impeller to rotor and maximum torque is transmitted from the coupling. The fluid coupling also acts a torque limiter.

Fluidomat fluid couplings have centrifugal characteristics during starting, thus enabling no-load start-up of prime mover which is of great importance.

The slipping characteristic of a fluid coupling provides a wide range of advantages such as speed variation, smooth & controlled acceleration, clutching and declutching operations as well as load limiting, shock loads & peak load dampening. By varying the amount of oil in the fluid coupling, the normal torque transmitting capacity can be varied. The maximum torque of the fluid coupling can also be set to a pre-determined value by adjusting the oil level.

Fluid couplings have the same characteristics in both directions of rotation.

Scoop Control Variable Speed Couplings

These coupling have a sliding scoop tube which enters the coupling rotating casing. The oil level in the coupling can be varied during operation by changing the position of the scoop tube. This change of oil level shifts the torque characteristic of the coupling thus enabling stepless speed control.

Advantages of using Fluid Couplings

  • Wear-free power transmission because of absence of mechanical connection between the input and output elements.
  • No-load start-up of motor, irrespective of machine load and motor peak torque utilized for machine acceleration.
  • Motor selection to operating kW rather than starting loads. Saving on energy cost and capital cost.
  • Simple control of maximum or limiting torque by easy variation of oil level.
  • Added protection for motor and driven machine by limiting torque to a predetermined safe value
  • Automatic disconnection of prime mover in case of any sustained overload, by blowing-off of a fusible plug on the coupling, thus draining out oil.
  • Automatic synchronization and load sharing in case of multi-motor drives.
  • 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. Reduced fatigue on transmission components/elements.
  • Smooth and controlled acceleration of driven machines. Important for machines such as conveyors, wire drawing and textile machines.
  • Stepless speed variation in wide range obtained in scoop type fluid couplings.
  • Economy on machine and drive designs.