Why use a three-way directional control valve in automatic machine control?


It is useful to many pull-behind scraper applications.

The three-way spool - circled in red in the above graphic - powers the cylinder only in the extend direction. Gravity retracts the cylinder rod.


Center position of the spool is "B" blocked/"A" open to tank. The spool will prevent the ground from retracting the rod, but if the ground level "falls away," the rod can "float out" or extend.


When a "lower" correction is called for by the controller, the cylinder raises the wheels, which makes the cutting edge go down. If the four-way contol valve remains shifted too long, the wheels raise off the ground. This is self-defeating because:


1. The blade is already on the ground so raising the wheels father cannot move

    the blade down any farther.


2. When a "raise-the-blade" correction is sent from the controller, the wheels

    start to move down, but it will take some time for them to get to the ground.

    The cutting blade cannot begin to raise until the wheels touch the ground, so

    the blade position correction is delayed because the wheels had been raised

    farther than necessary.



What is the significance of a three-way valve in this situation?

The valve is used to power the blade up - that is, to power the wheels down, which raises the blade - but gravity is used to push the bucket and cutting edge down. The wheels never leave the ground because they are not powered in the "wheels up" direction.


The cylinders in the photographs above will be powered in the "extend" direction, but not in the "retract" direction.


One valve port - "A" or "B" - is permanently open to tank. When the spool is centered, the cylinder will be held in its loaded direction, but it can "float" in the other direction.


Cylinder speed can be proportional in both directions of travel, if a proportional solenoid is used.



  • If it is necessary to raise the wheels off the ground - for maintenance, for example - disconnect the scraper's cylinder from the grade control valve and plug into the QD set on the back of the tractor. The grade control valve is a three-way valve, which means it will power the cylinder hydraulically in only one direction. The tractor's valve is a four-way valve, meaning it will power the cylinder hydraulically in both directions.
  • Plumb the un-powered port of the cylinder to the un-powered port of the PVG valve rather than allowing  the cylinder to "breathe" atmospheric air. There is too much dust in the air in a dirt-forming application. Even with a filtered air breather in the port, the ingestion of solid contamination will eventually damage the hydraulic system. Plumbing to the un-powered port of the valve allows the un-powered port of the cylinder to "breathe" oil from the tank core of the valve. NEVER plug the un-powered port of the cylinder.
  • The only force available to make the cutting blade descend is the weight of the scraper. If the scraper is very light weight, it might be necessary to add weight to it in order for it to descend as rapidly as needed - especially when the three-way PVG valve has a proportional solenoid, and when a slight "down" correction is called for by the GPS system.
  • If the outside temperature is cold the descent speed might not be as quick as it was when the weather was hot.



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4099 B.F. Goodrich Blvd

Memphis, TN USA 38118

Phone:  1.901.794.4884

Fax:  1.901.794.1618

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