Pneumatic And Hydraulic Equipment Page 1127

When cylinder motion must be sequenced (such as extending cylinder 1 before cylinder 2 and retracting cylinder 2 before cylinder 1), consider: 1) Direct acting sequence valves with built in reverse free flow checks. (This drawing is not a real circuit, but is intended for description only.) SUN pilot operated and direct acting sequence valves can be used: To regulate pressure more accurately than relief valves, especially with long or restricted return lines to tank from port 2 (outlet). To assure that one circuit is pressurized before a second circuit (such as releasing a mechanical brake before operating a winch motor). To control the sequence of two or more cylinders or motors while maintaining pressure on the actuator that moves first. SUN three port counterbalance cartridges (with pilot-to-open assist) are modulating devices that allow free flow from port 2 (inlet) to port 1 (load) and then block reverse flow until a pilot pressure inversely proportional to the load pressure is sensed at port 3 (pilot). They typically employ a check valve for free flow in one direction, a relief valve to control flow in the other direction, and a pilot that lowers the relief valve setting with pressure. Thermal relief protection is provided in most systems. Deceleration control can be provided when used with open center directional control valves. COUNTER BALANCE VALVES SEQUENCE VALVES Counterbalance valve on the blind end of the cylinder with a load retracting the cylinder rod. PILOT OPERATED CHECK VALVES Pilot check on the blind end of the cylinder with a load retracting the cylinder rod. SUN three port pilot-to-open check cartridges are non-modulating, on/off devices that allow free flow through a check valve from port 2 (inlet) to port 1 (load) and then block reverse flow until a pilot pressure directly proportional to the load pressure is sensed at port 3 (pilot) so that a pilot piston can push the check poppet off its seat. NOTE: When new, pilot-to-open check valves can lock loads with zero or near zero leakage for extended periods (typically 30 minutes or more). Absent nicks in the seating area, silting (even with "clean" oil) will normally produce a zero-leak seal within seconds after closing. Therefore, they are well suited for many clamp holding applications. Care should be taken to see that trapped oil does not contract due to cooling, or expand due to heating, which will lower or raise clamping force. NOTE: The on/off action of pilot-to-open check valves makes them unsuitable for use with overrunning loads that will cause a loss of pilot pressure. Without pilot pressure the check valve will close until adequate pilot pressure is again restored, causing a ratcheting motion. NOTE: Three port pilot-to-open check valves should never be used with paired cylinders. Pilot pressure will open the valve with the least load first, transferring the combined load to the second valve at double the pressure. NOTE: Pilot-to-open check valves are not suited for use with hydraulic motors that leak internally. NOTE: Counterbalance valves improve motion control on most systems by guar- anteeing that the directional valve always sees a positive load pressure, even under overrunning load situations. Because of the inverse pilot ratio, low loads require more pilot pressure and high loads require less pilot pressure. This tends to provide fine motion control and good stability in load moving systems. NOTE: Counterbalance valves (with pilot-to-open assist) will lock with very low (approaching zero) leakage. Absent nicks in the seating area, silting (even with "clean" oil) will normally produce a zero leak seal within minutes after closing. Counterbalance valves are well suited for load positioning applications. NOTE: Counterbalance valves (with pilot- to-open assist) can be used with rotary hy- draulic motors to control overunning loads. ( NOTE: Anti-cavitation protection must be provided and mechanical brakes should be used to hold static loads. NOTE: The setting of counterbalance valves should be at least 1.3 times the load induced pressure. Valves should be selected accordingly. HYDRAULICS PILOT OPERATED CHECKS, COUNTERBALANCE AND SEQUENCE VALVES S U N H Y D R A U L I C S Sun Hydraulics cartridge valves JHF Catalog [ Volume 7 ] (314) 427-0600 800-444-0522 (FAX) 314-427-3502 www. j hf.com 1127 Prices Subject to Change Without Notice John Henry Foster pneumatic and hydraulic equipment

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