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| Illustration 1 | g00798082 |
When the machine is equipped with a bulldozer blade, the bulldozer tilt control valve is the second valve in the valve stack. The bulldozer tilt control valve controls the tilting of the bulldozer blade to the right and to the left. The control valve is equipped with resolver (12), flow control valve (17), load check valve (20), and control valve spool (23) .
The bulldozer tilt control valve is a spring centered valve. Control valve spool (23) is a closed center spool and a pilot operated spool with three positions: TILT RIGHT, HOLD and TILT LEFT. Spring (2) keeps the spool in HOLD position when the bulldozer tilt circuit is not active. Movement of the spool is controlled by the pilot control system. The pilot control system connects the bulldozer control lever to valve spool (23) .
Resolver (12) is a double check valve. Flow control valve (17) limits the maximum flow of oil to the tilt circuit. The functions of the resolver and of the control valve spool are described in the sections on positions of the control valve. The description of the function of load check valve (20) is in a separate section.
Load Check Valve
Load check valve (20) prevents reverse oil flow in the tilt cylinder, which can cause cylinder drift or load loss. The load check valve does not open until pump oil pressure in chamber (19) becomes greater than the force of the spring for the load check valve and the cylinder oil pressure in passage (21) .
HOLD Position
When the control valve spools are in the HOLD position, pump oil flows to inlet passage (16). Oil flows around the flow control valve and oil flows in metering slots (18). Oil through metering slots (18) opens the flow control valve against spring (14) .
Since all the valve spools are in the HOLD position, pump oil fills the parallel oil passages of the control valves. The pump maintains oil pressure at 2950 kPa (410 psi). Passage (3), chamber (8), and resolver (12) are drained. Passages (4), (5) and (21) are blocked.
TILT RIGHT Position
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| Illustration 2 | g00843228 |
When pilot control pressure causes control valve spool (23) to move to the TILT RIGHT position, passage (10) is blocked from outlet passage (7). Passage (3) is opened to passage (5). Cylinder oil from passage (5) flows through passage (3) to chamber (8). The oil functions as signal oil. Part of the signal oil flows through passage (10) and orifice (11) to the spring chamber behind flow control valve (17). The signal oil opens flow control valve (17). The rest of the signal oil flows through resolver (12) .
The increase of pump oil flow to inlet passage (16) flows through metering slots (18) into chamber (19). The pump oil opens load check valve (20). The oil flows through load check valve (20). The oil then flows through passage (21) around control valve spool (23) and out passage (5) to the rod end of the tilt cylinder. The tilt cylinder then tilts the bulldozer blade to the right.
Return oil from the head end of the tilt cylinder flows through passage (4), around control valve spool (23) and through outlet passage (7) to the hydraulic tank.
TILT LEFT Position
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| Illustration 3 | g00843236 |
When pilot control pressure causes control valve spool (23) to move to the TILT LEFT position, passage (10) is blocked from outlet passage (7). Passage (3) is opened to passage (4). Cylinder oil from passage (4) flows through passage (3) to chamber (8). The oil functions as signal oil. Part of the signal oil flows through passage (10) and orifice (11) to the spring chamber behind flow control valve (17). The signal oil opens flow control valve (17). The rest of the signal oil flows through resolver (12) .
The increase of pump oil flow to inlet passage (16) flows through metering slots (18) into chamber (19). The pump oil opens load check valve (20). The oil flows through load check valve (20). The oil then flows through passage (21) around control valve spool (23) and out passage (4) to the head end of the tilt cylinder. The tilt cylinder then tilts the bulldozer blade to the left.
Return oil from the rod end of the tilt cylinder flows through passage (5), around control valve spool (23) and through outlet passage (22) to the hydraulic tank.