Monday, November 21, 2011

Hydrualic Pumps

To supply a flow of fluid to the actuator, a pump must be provided.  It is important to realize that the pump does not deliver pressure.  Hydraulic pressure is created only when an attempt is made to compress a fluid.  Fluid pumped through an open-ended pipe will have no pressure, but, if the pipe is connected to an actuator, the resistance to the flow will create pressure.  The rate at which a single hand pump can deliver fluid is very slow.  In the aircraft hydraulic system an engine driven pump is therefore installed in addition to the hand pump.  The purpose of the hand pump would then be for emergency use and for ground testing of the system.

Hand pumps
These are usually of double acting type, delivering fluid on each stroke.  As the piston moves upward in the cylinder, fluid is drawn in through the inlet non return valve (NRV) into the cylinder.  At the same time fluid above the piston is discharged through the outlet NRV.  As the piston moves downward, the inlet NRV closes and the transfer NRV opens, allowing fluid to flow through the piston.  Since the volume above the piston is smaller than below the piston, part of this fluid is discharged through the outlet NRV.

Friday, November 11, 2011

HYD Reservoirs

The reservoir’s purpose is to store, receive and supply hydraulic fluid. Some years ago reservoirs simply consisted of a tank with several connections, a filler and sight glass assembly and possibly for some form of filter or strainer for initial filtration of fluid as it being filled up.  Indeed, there are still many of this type of reservoir in common use on small or older aircraft,these type of reservoirs would usually incorporate a stack(stand) pipe to ensure a reserve of fluid for use in an emergency.

However these type always had few of drawbacks during its operation as a reservoirs. these are required to fulfill following conditions in order to work effectively.

  • The necessity to maintain a head of pressure, under all that the tank has to be placed at higher point than the EDP (Engine Driven Pump)
  •  The possibility of the oil becoming aerated during manoeuvrings.
  • Cavitation in the supply pipeline to the pump during banking.
  • A lowering of hydraulic fluid boiling point at high altitude.