While motion control (motors, cylinders) for travel, swing, and attachment functions are produced by hydraulic flow, the control levers that the operator moves are electrical.
How does the initial electrical function of an operator lever (or pedal) control a large hydraulic flow?
That is the question we set out to explore and answer in the first part of the lesson.
A key electro-hydraulic control valve is called the EHC valve, and there are many of them on each Hitachi shovel model. For example, there are 14 EHC valves in use on an EX3600-6 Shovel.
An EHC valve assembly has eight separate EHC valves, all grouped up into one aluminum manifold. The manifold has a filter screen on the master inlet port (labeled PB).
The symbol on a Hitachi schematic shows an electrically operated, 3 port, 2 position valve.
But really it’s a pressure valve which might look more like this one.
The EHC valve is a pressure reducing valve, and when no electrical current passes through the solenoid, no pressure is made available on the valve outlet.
When the current is increased, the outlet pressure will rise.
In a non-electric valve, the pressure setting changes would be made by tensioning a spring using an hand adjustment screw.
Pressure from EHC valves are only used to move valve spools or poppets, inside of pump controllers, or other larger pilot valves such as a DQR (Dual Quick Response) valve.
Pressure on the outlet of an EHC valve increases gradually while moving a control lever, to move another control spool smoothly, only because of springs being compressed. Where are these springs? In pump controllers, or in DQR pilot valves.
So in reality, the EHC valve, just a pressure reducing valve, sets a pressure that’s needed to operate another pressure reducing valve (DQR, or pump controller).
The desired pressure on the output port of an EHC valve is not mechanically adjusted like it would be on a simple pressure reducing valve, but rather, is chosen via a variable electrical signal sent from the ELU (Electric Lever Control Unit), to the solenoid coil in the EHC valve.
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