Cessna Hydraulic Pumps

Operating Characteristics of Pressure- Flow Compensated Piston Pump in Load Sensing Systems:

• Variable pressure and variable flow.
• Pump is in low-pressure standby mode when system is not actuated.
• Pump automatically adjusts output flow and pressure to meet system requirements.
• Allows for simultaneous operation of multiple functions.
• Fast response to system pressure and flow requirements
• One pump may be used instead of multiple pumps.

The Cessna Pressure-Flow Compensated Piston Pump senses pressure and flow requirements and provides only flow and pressure required, plus 14 bar ( 200 PSI ), to operate the operate the compensator. The low pressure standby, 14 bar ( 200 PSI ), prevents excessive heat buildup in the pump and eliminates unnecessary horsepower drain. The pressure-flow compensated load sensing system is noted for its high efficiency and controllability. The following two systems demonstrate the general load sensing concept.

Pressure-Flow Compensated--Load Sensing System.
When used with a closed center load sensing control valve, and the hydraulic system is not actuated, the pressure-flow compensated piston pump will remain in the low pressure standby mode at only 14 bar ( 200 PSI ).

When a hydraulic function is actuated, the pressure-flow compensated piston pump senses the demand for flow and adjusts the pump displacement to increase the flow required at only the pressure required to operate the circuit, plus the 14 bar ( 200 PSI ) to operate the compensator. The pump will strive to maintain the flow required, at only 14 bar ( 200 PSI )over the pressure required, under all working conditions of the system. A load sensing line is connected from the load sensing control valve to the pump to feed all the circuit requirements to the pump compensator. The pump will respond to the highest pressure of the circuits and provide the total flow required to operate multiple circuits. A load sensing line bleed orifice is preferred in the load sensing control valve to prevent high pressure compensation of the piston pump when the control valve is returned to neutral.

If the load on the system produces a hydraulic pressure that is equal to the high pressure setting of the compensator, the pump will go into high pressure standby mode and destroke until the load is overcome or the control valve is returned to neutral. This prevents a large volume of pump flow from passing over the system relief valve as compared to the conventional open center hydraulic system.

To achieve true simultaneous operation of individual circuits, a flow compensator should be incorporated in each circuit, otherwise the system will send the most flow to the circuit with the least resistance.


REPAIR INFORMATION

Cleanliness is extremely important when repairing these pumps. Work in a clean area. Before disconnecting the lines clean port area of pump. Disconnect hydraulic lines and remove pump assembly from vehicle and plug ports. Thoroughly clean the outside of the pump. After cleaning, remove port plugs and drain oil.

STARTUP PROCEDURE

When initially starting a rebuild load sensing system, it is extremely important that the startup procedure be followed. It prevents the chance of damaging the pump which might occur if the system was not properly purged with oil before startup.

1. After the pump has been properly installed onto the machine and all the hydraulic connections have been made, check all the fittings to make sure that they are tight.
2. Fill the pump housing at least 1/2 full with system oil that has been filtered through a 10 micron filter
3. Fill the reservoir with an approved oil that has been filtered through a 10 micron filter. Leave the filter cap loose as a means of air that is trapped in the system.
4. Note on gasoline or L.P. engines: remove the coil wire and turn the engine over for 15 seconds. Diesel engines: shut off the fuel flow to the injectors and turn the engine over for 15 seconds. This procedure enables the pump to pickup the oil before startup. At this time disconnect the sensor line from the pump compensator and pull one of the valve spools while the engine is being turned over. This allows oil to flow through the sensor line, thus, removing any air in the sensor line. Reconnect the sensor line after a steady flow of oil is coming from the line.
5. Replace the coil wire or return the fuel flow to the injectors and start the engine. Run at low idle
   speed for one minute. The pump should immediately pick up oil and go into low pressure standby. If
   there is no indication of fill in 30 seconds, stop the engine and determine the cause.
6. After the pump has stabilized in low pressure standby, operate the control valve and steering,
   if the system is equipped with one to purge the system of air and to fill the cylinders with oil.
   Continue operating the system slowly with no load until it responds fully.
7. Check fluid level at the reservoir and refill if necessary to the proper level with an approved filtered oil.
8. Check all line connections for leaks, tighten if necessary.
9. The machine is now ready to be put into operation.
10. Short hour filter changes are recommended for the first two changes after placing the machine back into operation. The first filter would be changed in 3-5 hours and the second at approximately 50 hours. Routine scheduled filter changes are recommended for maximum life of hydraulic system.