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| Illustration 1 | g00497820 |
Schematic for quick-drop valve (A) Line to control valve (B) Quick-drop valve (C) Bulldozer lift cylinders (D) Line to rod ends of cylinders (E) Line to head ends of cylinders (F) Bypass valve (G) Bulldozer tilt cylinder | |
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| Illustration 2 | g00579888 |
Location of the quick-drop valve | |
A single quick-drop valve is installed at the center of the top of the radiator guard. The valve allows the bulldozer blade to drop quickly to the ground.
When the operator lowers the blade quickly, the oil pressure in the head ends of the lift cylinders drops. The hydraulic circuit from the pump to the head ends of the lift cylinders cannot supply sufficient oil flow in order to maintain the pressure. The valve diverts oil from the rod end of the lift cylinders to the head end of the lift cylinders.
The quick-drop valve minimizes pause time. The valve permits full down pressure in a minimum amount of time after the blade hits the ground.
The bypass valves (F) in the bulldozer lift cylinders are built into the cylinder pistons. When the piston reaches the end of the stroke, the end of the bypass valve touches the end of the cylinder barrel. As the piston moves closer to the end of the cylinder barrel, the bypass valve is pushed open. When the bypass valve is open, oil passes through the piston to the low-pressure side of the cylinder. The oil flow through the piston is important so that excessive pressure does not build up on either side of the piston when the cylinder is at the end of the stroke.
Bulldozer Control Lever in the HOLD Position
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| Illustration 3 | g00795358 |
Quick-drop valve with the bulldozer control lever in the HOLD position (1) Cover (2) Passage to the head ends of the lift cylinders (3) Passage (4) Passage (5) Passage to the LOWER port of the control valve (6) Valve spool (7) Spring (8) Cover (9) Plunger (10) Orifice sleeve (11) Passage to RAISE port of the control valve (12) Passage to the rod ends of the lift cylinders | |
When the bulldozer control lever is moved to HOLD position, both the head end and the rod end passages of the bulldozer lift control valve are blocked. The oil that is in passages (2) and (12) is blocked.
Bulldozer Control Lever in the RAISE Position
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| Illustration 4 | g00854955 |
Quick-drop valve with the bulldozer control lever in the RAISE position (1) Cover (2) Passage to the head ends of the lift cylinders (3) Passage (4) Passage (5) Passage to the LOWER port of the control valve (6) Valve spool (7) Spring (8) Cover (9) Plunger (10) Orifice sleeve (11) Passage to RAISE port of the control valve (12) Passage to the rod ends of the lift cylinders | |
When the bulldozer control lever is moved to RAISE position, pump oil enters the quick-drop valve at passage (11) . The oil flow moves orifice sleeve (10) to the right. The oil flows past orifice sleeve (10) and out passage (12) to the rod ends of the bulldozer lift cylinders. Return oil from the head ends of the cylinders enters the valve at passage (2) . The oil flows past spool (6) and out passage (5) to the control valve.
The circuit pressure on the left end of plunger (9) keeps the plunger shifted to the right. Hydraulic circuit pressure also increases in passage (4) to the right end of spool (6) . The pressure on the right end of the spool is equal to the pressure on the left end. Therefore, spring (7) keeps spool (6) shifted to the right.
Bulldozer Control Lever in the Slow LOWER Position
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| Illustration 5 | g00854962 |
Quick-drop valve with the bulldozer control lever in the slow LOWER position (1) Cover (2) Passage to the head ends of the lift cylinders (3) Passage (4) Passage (5) Passage to the LOWER port of the control valve (6) Valve spool (7) Spring (8) Cover (9) Plunger (10) Orifice sleeve (11) Passage to RAISE port of the control valve (12) Passage to the rod ends of the lift cylinders | |
The bulldozer control lever is moved to the LOWER position below 75 percent of the maximum travel. The blade lowers at a slow speed. Return oil from the rod end of the bulldozer lift cylinders enters the quick-drop valve at passage (12) . The return oil flows past orifice sleeve (10) and out passage (11) to the control valve. The return oil flow moves orifice sleeve (10) to the left against a retaining ring. A pressure drop in the return oil is set up across orifice sleeve (10) .
Pump oil enters the valve at passage (5) . The pump oil flows past spool (6) and out passage (2) to the head ends of the cylinders. Pump oil flow enters passage (3) in spool (6) , which raises the pressure on the right end of plunger (9) . However, the return oil pressure on the left side of plunger (9) is greater. This pressure keeps plunger (9) shifted to the right.
Return oil pressure comes from the rod end of the lift cylinders. The return oil pressure goes through passage (4) to the right end of spool (6) . The pressure also goes to the outer diameter of the left end of spool (6) before passing orifice sleeve (10) .
Return oil pressure after the pressure drop across orifice sleeve (10) goes to the minor diameter of the left end of spool (6) . The pressure drop across orifice sleeve (10) produces a pressure differential between the right and the left ends of spool (6) . However, the higher pressure on the right end of spool (6) is exceeded by the combined force of spring (7) with the pressure on the left end of the spool. This keeps spool (6) shifted to the right.
Bulldozer Control Lever in the Fast LOWER Position
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| Illustration 6 | g00854967 |
Quick-drop valve with the bulldozer control lever in the fast LOWER position (1) Cover (2) Passage to the head ends of the lift cylinders (3) Passage (4) Passage (5) Passage to the LOWER port of the control valve (6) Valve spool (7) Spring (8) Cover (9) Plunger (10) Orifice sleeve (11) Passage to RAISE port of the control valve (12) Passage to the rod ends of the lift cylinders | |
The bulldozer control lever is moved to the LOWER position above 75 percent of the maximum travel. The blade lowers at a rapid speed. The return oil flow from the rod ends of the lift cylinders increases. A higher pressure drop in the return oil is set up across orifice sleeve (10) .
Return oil pressure after the pressure drop across orifice sleeve (10) goes to the minor diameter of the left end of spool (6) . The pressure drop across orifice sleeve (10) produces a pressure differential between the right and the left ends of spool (6) . However, the force of spring (7) is exceeded by the higher pressure on the right end of spool (6) , which moves spool (6) to the left.
75 percent of travel of the bulldozer control lever toward the LOWER position is called the trigger point. This operating position produces movement of spool (6) . When spool (6) moves, the area between orifice sleeve (10) and the opening in the valve body decreases. Since the pressure drop increases, spool (6) moves farther. Therefore, once spool (6) begins movement, the spool moves all the way to the left. The rod ends of the lift cylinders are now connected to the head ends of the lift cylinders across the slots in spool (6) . Since resistance to return oil flow is now lower, return oil flow from the rod ends of the lift cylinders increases. The blade drops faster.
This line connection through the quick-drop valve also keeps the hydraulic circuit full. As a result, the pause time at ground level is minimum. Part of the return oil from the rod ends of the lift cylinders flows across orifice sleeve (10) . This flow produces a pressure drop, which holds spool (6) to the left.
Bulldozer Control Lever in the Fast LOWER Position With Down Pressure
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| Illustration 7 | g00854973 |
Quick-drop valve with the bulldozer control lever in the fast LOWER position with down pressure (1) Cover (2) Passage to the head ends of the lift cylinders (3) Passage (4) Passage (5) Passage to the LOWER port of the control valve (6) Valve spool (7) Spring (8) Cover (9) Plunger (10) Orifice sleeve (11) Passage to RAISE port of the control valve (12) Passage to the rod ends of the lift cylinders | |
When the blade hits the ground and stops, return oil flow from the rod ends of the lift cylinders ceases. With no pressure drop across the orifice, spring (7) shifts spool (6) back to the right. The implement hydraulic pump fills the head ends of the lift cylinders. The fill cycle is called pause time. When the pressure in the head ends of the lift cylinders increases, the blade can move again.
Pump oil flow enters passage (3) in spool (6) , which raises the pressure on the right end of plunger (9) . Return oil pressure after the pressure drop across orifice sleeve (10) goes to the left side of plunger (9) . The pump oil pressure to the hydraulic circuit exceeds the return oil pressure, which moves plunger (9) to the left. The pressure drop of return oil across orifice sleeve (10) pushes spool (6) toward the left. However, the combined force of spring (7) with the pressure of the oil against the plug at the right end of the plunger cavity overcomes the pressure differential on the ends of spool (6) . The spool stays to the right.