The load check valve performs the following two functions.
- The load check valve prevents unexpected movement of an implement when a joystick is initially activated at a low pump delivery pressure.
- The load check valve prevents oil loss from a high pressure circuit to a lower pressure circuit.
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| Illustration 1 | g01332712 |
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Boom I control valve (partial shift) (1) Port (boom cylinder head end) (2) Passage (3) Spring (4) Load check valve (5) Port (boom cylinder rod end) (6) Return passage (7) Parallel feeder passage (8) Spring (9) Pilot port (10) Spool (11) Center bypass passage |
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When the joystick for the boom is in the NEUTRAL position, spring (8) positions spool (10) in the center position. The drive pump is at a destroked position. The drive pump is delivering standby pressure to the boom I control valve. The pump delivery pressure in center bypass passage (11) and parallel feeder passage (7) is lower than the pressure in the boom cylinder head end at port (1) . Load check valve (4) is in the CLOSED position.
Slight movement of the joystick for the boom toward the BOOM RAISE position causes low pilot oil pressure to enter port (9) . Spool (10) shifts slightly to the right. The drive pump begins to move to an upstroke position. A passage partially opens allowing the oil from the rod end of the boom cylinders in port (5) to flow to return passage (6) . A passage partially opens allowing the oil from the head end of the boom cylinders in port (1) to flow through passage (2) . The work load pressure from the head end of the boom cylinders and the force of spring (3) now acts on load check valve (4) . Since the pump delivery pressure is lower than the work load pressure in passage (2) , load check valve (4) remains in the closed position. The oil in the boom cylinder head end is blocked.
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| Illustration 2 | g01523562 |
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Boom I control valve (full shift) (1) Port (boom cylinder head end) (2) Passage (3) Spring (4) Load check valve (5) Port (boom cylinder rod end) (6) Return passage (7) Parallel feeder passage (8) Spring (9) Pilot port (10) Spool (11) Center bypass passage |
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As the joystick for the boom is moved farther from the NEUTRAL position, the pilot oil pressure at pilot port (9) increases. Spool (10) shifts farther to the right. The drive pump upstrokes farther. The pump delivery pressure in center bypass passage (11) and parallel feeder passage (7) increases. Load check valve (4) will not open until the pump delivery pressure becomes greater than the combined force of the work load pressure in passage (2) and the force of spring (3) . Unexpected downward movement of the boom during a BOOM RAISE operation is prevented.
Load check valve (4) also prevents oil loss from a high pressure circuit to a lower pressure circuit. For example, the work tool is moved under a light load, and the boom cylinders are raised at the same time. The high pressure oil of the boom cylinders wants to flow toward the low pressure side of the work tool. The load check valve prevents the boom from lowering.