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| Illustration 1 | g00794574 |
When the machine is equipped with a bulldozer blade, the bulldozer lift control valve is the first valve in the valve stack. The bulldozer lift control valve controls the raising and lowering of the bulldozer blade. The control valve is equipped with these components: makeup valve (7), resolver (13), flow control valve (18), load check valve (21) and control valve spool (24) .
The bulldozer lift control valve is a spring centered valve. Control valve spool (24) is a closed center spool and a pilot operated spool with four positions: RAISE, HOLD, LOWER and FLOAT. Spring (2) keeps the valve spool in the HOLD position when the bulldozer lift control valve 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 (24) .
Resolver (13) is a double check valve. Flow control valve (18) limits the maximum flow of oil to the lift circuit. The functions of resolver (13) and of the control valve spool are described in the sections on positions of the control valve. Descriptions for load check valve (21) and makeup valve (7) are in the following sections.
Load Check Valve
Load check valve (21) prevents reverse oil flow in the lift cylinder, which can cause cylinder drift or load loss. The load check valve does not open until pump oil pressure in chamber (20) becomes greater than the force of the spring for the load check valve and the cylinder oil pressure in passage (22) .
Makeup Valve
Makeup valve (7) is in the head end of the lift circuit. The makeup valve opens when cylinder oil pressure in head end passage (5) drops 14 kPa (2 psi) below the return oil pressure in outlet passage (6). Makeup valve (7) adds return oil from outlet passage (6) to cylinder oil in head end passage (5). The additional oil prevents cavitation (vacuum) in the lift cylinders.
HOLD Position
When the control valve spools are in the HOLD position, pump oil flows to inlet passage (17). This passage is common to all sections of the valve stack and this passage has no outlet. 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), passage (6), and chamber (9) are drained. Passages (4), (5), and (22) are blocked.
RAISE Position
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| Illustration 2 | g00794623 |
When pilot control pressure causes valve spool (24) to move to the RAISE position, cylinder oil from passage (9) flows through orifice (12) to chamber (16). The oil functions as signal oil. The signal oil opens the resolver and flows to chamber (16). The oil in chamber (16) moves flow control valve (18) to the left. The signal oil flows to the next control valve, to the inlet manifold, and to the compensator valve on the pump.
The increase of pump oil flow to inlet passage (17) opens load check valve (21). The oil flows through load check valve (21). The oil then flows through passage (22) around valve spool (24) and out passage (4) to the rod ends of the lift cylinders. The oil flow to the rod ends of the lift cylinders raises the bulldozer blade.
Return oil from the head ends of the lift cylinders flows through passage (5), around valve spool (24) and through outlet passage (8) to the oil tank.
LOWER Position
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| Illustration 3 | g00794737 |
When pilot control pressure causes valve spool (24) to move to the LOWER position, cylinder oil from passage (5) flows through passage (3) to chamber (9). The oil functions as signal oil. The signal oil opens the resolver and flows to chamber (16). The oil in chamber (16) moves flow control valve (18) to the left. The signal oil flows to the next control valve, to the inlet manifold, and to the compensator valve on the pump.
The increase of pump oil flow to inlet passage (17) opens load check valve (21). The oil flows through load check valve (21). The oil then flows around valve spool (24) and out passage (5) to the head ends of the lift cylinders. The oil flow to the head ends of the lift cylinders lowers the bulldozer blade.
Return oil from the rod ends of the lift cylinders flows through passage (4), around valve spool (24), and through outlet passage (23) to the oil tank.
FLOAT Position
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| Illustration 4 | g00794756 |
When pilot control pressure causes valve spool (24) to move to the FLOAT position, there is no signal pressure in signal oil passage (3). Chamber (9) is open to outlet passage (8). Pilot control pressure enters the signal resolver network. This causes the pump to maintain margin pressure over pilot control pressure. Margin pressure over pilot control pressure is required in order to shift control valve spool (24) to the FLOAT position.
In the FLOAT position, both passage (4) and passage (5) are open to outlet passages (8) and (23). Since the ends of the lift cylinders are open to the tank, the cylinder rods can move freely in either direction. The amount and direction of the force on the blade controls the movement of the blade.