General Information
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| Illustration 1 | g00857124 |
The bulldozer lift control valve is located between the steering control valve and the bulldozer tilt control valve in the valve stack.
Pilot supply oil from the pilot control valve is used to control the position of control valve spool (26). When pilot supply oil with sufficient pressure enters the bulldozer lift control valve at port (9) control valve spool (26) is shifted to the left. This action causes pilot supply oil at the opposing end of control valve spool to flow out of port (27). This oil flows to the pilot control valve. Then, the oil flows to the tank. When the spool is shifted to the left, the valve is in the RAISE position.
Similarly, when pilot supply oil with sufficient pressure enters the bulldozer lift control valve at port (27) control valve spool (26) is shifted to the right. This action causes pilot supply oil at the opposing end of control valve spool to flow out of port (9). This oil flows to the pilot control valve. Then, the oil flows to the tank. When the spool is shifted to the right, the valve is in the LOWER position.
The lift valve has four positions.
- RAISE
- HOLD
- LOWER
- FLOAT
The bulldozer lift control valve is spring centered. The springs that are located at the left end of control valve spool (26) keep the spool in the HOLD position when the lift circuit is not active.
Resolver (11) is a two-way check valve. Signal oil from the previous resolver enters signal oil passage (12). Resolver (11) compares the pressure signal that is between the bulldozer lift control valve and the following control valve. The pressure signal from the bulldozer lift control valve enters the resolver through passage (10). The higher pressure of signal oil exits signal oil passage (13). Refer to Systems Operation, "Signal Resolver Network" for detailed information about the resolver network.
Flow Control Valve
Flow control valve (19) limits the maximum flow of the oil to the lift circuit. As the oil flows from passage (22) to either rod end passage (3) or head end passage (5), the design of control valve spool (26) causes a pressure drop. The left end of flow control valve (19) is at the same pressure as passage (22). The right end of the flow control valve is at the same pressure as either rod end passage (3) or head end passage (5). As the flow increases from passage (22) through control valve spool (26) to the cylinders, the pressure differential between the left and the right ends of flow control valve (19) increases. Flow control valve (19) moves to the right against spring (16), closing off metering slots (20) and reducing pump oil flow into passage (22) through load check valve (23). The flow control valve limits the flow of oil to the lift cylinders at 190 L/min (50 US gpm).
Note: Different designs of the control valve stem that are used in the various valves change the limits of the maximum flow into the various cylinders.
Pressure Limiter Valve Operation
Pressure limiter valve (15) is used in the bulldozer lift control valve in order to limit the pressure to the lift circuit. The pressure setting of the pressure limiter valve is 22750 ± 300 kPa (3300 ± 45 psi).
When the bulldozer lift control valve is in an operating position, cylinder oil flows through passage (2) and the oil becomes signal oil. Signal oil flows to chamber (8). Then, the oil flows to resolver (11) and through passage (14) to spring chamber (17) .
When the signal oil pressure in spring chamber (17) reaches approximately 22750 kPa (3300 psi), the spool in pressure limiter valve (15) moves from the valve seat. Oil is sent through outlet passage (7) to the tank.
The signal oil causes flow control valve (19) to move to the right. The flow of pump oil to the lift circuit decreases until the pressure of the cylinder oil decreases.
Load Check Valve
Load check valve (23) prevents reverse oil flow from the lift cylinders which can cause cylinder drift or load loss. The load check valve will not open until pump oil pressure in chamber (21) creates a force against load check valve (23) that is higher than the opposing force against the other side of the check valve. The opposing force is the sum of the force of spring (24) and the force that is created by the oil pressure in passage (22) .
Makeup Valve
Makeup valve (6) is in the head end of the lift circuit. The makeup valve opens when cylinder oil pressure in passage (5) drops approximately 14 kPa (2 psi) below the return oil pressure in outlet passage (4). Makeup valve (6) adds return oil in outlet passage (4) to cylinder oil in passage (5). The additional oil prevents cavitation (vacuum) in the lift cylinders.
Basic Valve Operation
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| Illustration 2 | g01239780 |
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| Illustration 3 | g00857124 |
When the control valve spool, all of the implement valve spools, and the steering valve spools are in the HOLD position, pump oil through the steering control valve flows to inlet passage (18). Then, oil flows around the flow control valve and into metering slots (20). Oil through metering slots (20) moves the flow control valve against spring (16). This valve movement allows all the oil flow through inlet passage (18) to flow to the next control valve. Because all the valve spools are in the HOLD position, pump oil fills the parallel oil passage of the control valves. The pump maintains the pressure at approximately 3600 kPa (525 psi). Passage (2), chamber (8), passage (10), resolver (11), and passage (13) are drained. The oil that is in passages (3), (5), and (22) is blocked.
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| Illustration 4 | g01240669 |
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| Illustration 5 | g00857231 |
When pilot control pressure causes control valve spool (26) to move to the RAISE position, cylinder oil from passage (3) flows through the passage (2) to chamber (8). The oil now becomes signal oil. Some of the signal oil flows through an orifice to spring chamber (17) that is behind flow control valve (19). Then, flow control valve (19) moves to the left. The remaining signal oil moves to resolver (11) and the signal oil flows through passage (13) to the previous control valve. Then, the signal oil flows to the inlet manifold. The inlet manifold sends signal oil to the compensator valve in order to upstroke the pump.
The increased pump oil flows to inlet passage (18) and through metering slots (20) to chamber (21) and load check valve (23) opens. The oil through load check valve (23) flows to passage (22), around control valve spool (26), and out of passage (3) to the rod end of the lift cylinders. The bulldozer blade raises.
Return oil from the head end of the lift cylinders comes through passage (5), around control valve spool (26), and through outlet passage (7) to the tank.
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| Illustration 6 | g01241044 |
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| Illustration 7 | g01241059 |
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| Illustration 8 | g01241080 |
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| Illustration 9 | g00857311 |
Note: Refer to the hydraulic schematic that is for ACCUGRADE READY machines if the machine that is being worked on is equipped with AccuGrade.
When pilot control pressure causes control valve spool (26) to move to the LOWER position, the same pilot oil flows into the resolver network at passage (12). This causes the pump to upstroke to pilot control pressure plus margin pressure. The oil now becomes signal oil. Some of the signal oil flows through the orifice to spring chamber (17) that is behind flow control valve (19). The remaining signal oil moves resolver (11) to the right, if the signal oil is the highest resolved pressure. The signal oil flows through passage (13) to the previous control valve and to the inlet manifold. The inlet manifold sends the signal oil to the compensator valve in order to upstroke the pump. System pressure is approximately 2100 kPa (305 psi) above the pressure of the signal oil.
The increased pump oil flows to inlet passage (18) and through metering slots (20) to chamber (21). The pump oil opens load check valve (23). The oil flows to passage (22), around control valve spool (26), and through passage (5) to the head end of the lift cylinders. The bulldozer blade lowers.
Return oil comes from the rod end of the lift cylinders, through passage (3), around control valve spool (26), and through outlet passage (25) to the tank.
Machines that are equipped with AccuGrade use an electrohydraulic control group to operate the pilot hydraulic system. Refer to Systems Operation, "Signal Resolver Network" and Systems Operation, "Pilot Hydraulic System" that are in this manual for more information on machines that are equipped with AccuGrade.
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| Illustration 10 | g01241061 |
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| Illustration 11 | g00857333 |
When pilot control pressure causes the control valve spool to move to the FLOAT position, there is no signal oil pressure. Chamber (8) is open to outlet passage (7). Pilot control pressure enters the signal resolver network at passage (12). The pressure at passage (12) is the same pressure that is supplied by the pilot signal pressure at pilot port (27) that is causing the control valve spool to move to the FLOAT position. 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 (26) to the FLOAT position.
Because of the position of control valve spool (26), passages (3) and (5) are open to outlet passages (25) and (7). Because both ends of the lift cylinders are opened to the tank, the cylinder rods move freely in either direction. The amount and direction of the force on the blade controls the movement of the blade.
Machines that are equipped with AccuGrade use an electrohydraulic control group to operate the pilot hydraulic system. Refer to Systems Operation, "Signal Resolver Network" and Systems Operation, "Pilot Hydraulic System" that are in this manual for more information on machines that are equipped with AccuGrade.