General Information
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| Illustration 1 | g01241014 |
Flow Control Valve
Flow control valve (18) limits the maximum flow of the oil to the tilt circuit. As the oil flows from passage (21) to either rod end passage (5) or head end passage (3), the design of control valve spool (25) causes a pressure drop. The left end of flow control valve (18) is at the same pressure as passage (21). The right end of the flow control valve is at the same pressure as either rod end passage (5) or head end passage (3). As the flow increases from passage (21) through control valve spool (25) to the cylinders, the pressure differential between the left and the right ends of flow control valve (18) increases. Flow control valve (18) moves to the right against spring (15), closing off metering slots (19) and reducing pump oil flow into passage (21) through load check valve (22). The flow control valve limits the flow of oil to the tilt cylinders at 80 L/min (21 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 (14) is used in the bulldozer tilt control valve in order to limit the pressure to the tilt circuit. The pressure setting of the pressure limiter valve is 17225 ± 300 kPa (2500 ± 45 psi).
When the bulldozer tilt control valve is in an operating position, cylinder oil flows through passage (2) and the oil becomes signal oil. Signal oil flows to chamber (7). Then, the oil flows to resolver (10) .
When the signal oil pressure in spring chamber (16) reaches approximately 17225 kPa (2500 psi), the spool in pressure limiter valve (14) moves from the valve seat. Oil is sent through outlet passage (6) to the tank.
The system oil causes flow control valve (18) to move to the right. The flow of pump oil to the tilt circuit decreases until the pressure of the cylinder oil decreases.
Load Check Valve
Load check valve (22) prevents reverse oil flow in the tilt cylinder which can cause cylinder drift or load loss. The load check valve will not open until system pressure in chamber (20) 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 (21) .
Tilt Left
When the control valve spools are in the HOLD position, pump oil flows to inlet passage (17). Oil flows around the flow control valve (18) and oil flows in metering slots (19). Oil through metering slots (19) moves the flow control valve against spring (15). Pump oil also flows through inlet passage (17) to the next control valve.
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 3600 kPa (525 psi). Passage (2), chamber (7), and resolver (10) are drained. Passages (3), (5) and (21) are blocked.
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| Illustration 2 | g01240865 |
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| Illustration 3 | g01241018 |
When pilot control pressure causes control valve spool (25) to move to the TILT LEFT position, passage (2) is opened to passage (3). Cylinder oil from passage (3) flows through passage (2) to chamber (7). The oil functions as signal oil. Part of the signal oil flows to the spring chamber (16) behind flow control valve (18). The rest of the signal oil flows through resolver (10) to the inlet manifold and to the compensator valve in order to upstroke the pump.
The increase of pump oil flow to inlet passage (17) flows through metering slots (19) into chamber (20). The pump oil opens load check valve (22). The oil flows through load check valve (22). The oil then flows through passage (21) around control valve spool (25) and out passage (3) 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 (25) and through outlet passage (6) to the oil tank.
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| Illustration 4 | g01240877 |
Machines that are equipped with AccuGrade use an electrohydraulic control group to operate the pilot hydraulic system. The use of the electrohydraulic control group does not change the operation of the bulldozer tilt control valve. Refer to Systems Operation, "Pilot Hydraulic System" that is located in this module for more information about the electrohydraulic control group.
Tilt Right
The operation of the tilt control valve in the TILT RIGHT position is similar to the operation of the tilt control valve in the TILT LEFT position.
When pilot control pressure causes control valve spool (25) to move to the TILT RIGHT position, passage (2) is opened to passage (5). Cylinder oil from passage (5) flows through passage (2) to chamber (7). The oil functions as signal oil. Part of the signal oil flows to spring chamber (16) behind flow control valve (18). The rest of the signal oil flows through resolver (10) to the inlet manifold and to the compensator valve in order to upstroke the pump.
The increase of pump oil flow to inlet passage (17) flows through metering slots (19) into chamber (20). The pump oil opens load check valve (22). The oil flows through load check valve (22). The oil then flows through passage (21) around control valve spool (25) 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 (3), around control valve spool (25) and through outlet passage (24) to the oil tank.