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Hitachi Nico Transmission Co.,Ltd.

Image: Fixed-charge hydraulic coupling

Our fluid couplings are of the fixed-charge type, in which the case is filled with a fixed volume of oil.
In a fluid coupling, a pair of radial vanes face each other, with the impeller directly connected to the engine side and the runner directly connected to the driven side. When the impeller rotates, it acts as a pump, forcing oil outward, so that the power from the engine is temporarily converted into kinetic energy in the oil.
That energy is conveyed to the runner on the driven side, where it is converted again to mechanical energy and transmitted to the driven device.


  • Power is transmitted through the oil, so it makes a cushioned start for smooth acceleration. The coupling also functions as a torque limiter against sudden overloads while the system is operating at its rated level.
  • The motor has already reached peak torque when the load starts to turn, so there is no need to use the low torque range when the engine starts, and the full torque of the engine can be used on starting. This is particularly effective when linked to an induction motor.
  • Even if impact loads or vibrations (torsional vibration) act on the coupling while it is in operation, they are absorbed by the fluid coupling, preventing damage to the machine and extending its life.
  • When multiple engines are running, it is easy to distribute loads between the engines.
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