Topic: Re: Hydrostatic vs Hydraulic drive? Re: Case 222
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The flow from the Case pump is directed to what is called a Travel/Lift valve.
That is a cast iron control valve with two moveable "spools" in it that are moved in and out of the bores in the casting. These spools are connected to levers situated on either side of the steering column. When the lever is moved up or down, the appropriate spool moves and directs oil to the drive motor. The further the lever is moved in one direction, the greater the quantity of oil directed to the drive motor, thus causing it to increase its speed. Moving the lever the opposite way will cause the drive motor to spin the opposite way. There fore, direction and speed are managed by this single lever.
The lift lever controls a hydraulic cylinder that raises or lowers attachments. The lift lever also has a "float" position that allows attachments to follow the ground they are sitting on. Each spool has its own relief port that prevents the oil pressure from exceeding certain limits.
This travel/lift valve is an "open center" style that allows the full flow of the pump to pass through it continuously while allowing the spools to redirect whatever percentage of that flow needed for the task at hand. Oil from those tasks is returned to the travel/lift valve and directed by the spool to the outlet port of the valve so that it can continue to journey to the oil cooler and ultimately the oil reservoir.
This is like sitting next to a fast moving river. If you get thirsty, you just dip your cup into the flow. The larger the cup, the more water you get. However, the water never stops flowing past you. It is always there to meet your needs. big or small.
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