A lockout or load-holding valve associated with a hydraulic cylinder is used to restrict unintended cylinder movement when flow is not commanded or when a hose or circuit loses pressure. Its exact function depends on the circuit: it may isolate flow, hold a load, control lowering or provide maintenance isolation. It is not a substitute for mechanical support or formal lockout\/tagout.<\/p>\n\n\n\n
This article provides general engineering guidance. Machine-specific safety procedures, controlled drawings and qualified hydraulic personnel take precedence, especially where a cylinder supports or controls a load.<\/p>\n\n\n\n
Terms such as lockout valve, holding valve, pilot-operated check valve and counterbalance valve are sometimes used loosely. These components behave differently.<\/p>\n\n\n\n
Use the hydraulic schematic, valve model and machine documentation to determine whether the device isolates, locks, meters or protects the load.<\/p>\n\n\n\n
A correctly selected valve can block flow from a cylinder port until a pilot signal or controlled condition opens it. This can reduce movement caused by valve leakage or line failure.<\/p>\n\n\n\n
Holding performance depends on valve condition, settings, pilot ratio, leakage, cylinder condition and machine geometry.<\/p>\n\n\n\n
Overrunning loads can drive the cylinder faster than incoming oil can control. A counterbalance arrangement can maintain back pressure and meter movement.<\/p>\n\n\n\n
Incorrect adjustment can cause instability, heat, slow movement or unsafe lowering. Settings should follow the engineered circuit and qualified service procedure.<\/p>\n\n\n\n
A valve position or closed hydraulic circuit must not be treated as the only protection for work beneath a raised load. Leakage, contamination, thermal change or incorrect operation can allow movement.<\/p>\n\n\n\n
Use the equipment manufacturer’s mechanical supports, blocking and energy-control procedure before maintenance.<\/p>\n\n\n\n
Drift or failure to hold may come from piston-seal bypass, the directional valve, the load-control valve, plumbing or thermal effects.<\/p>\n\n\n\n
An approved isolation test should separate these paths without defeating the safety function.<\/p>\n\n\n\n
| Device or function<\/th> | Typical purpose<\/th><\/tr><\/thead> |
|---|---|
| Shutoff\/isolation valve<\/td> | Manually blocks a defined flow path<\/td><\/tr> |
| Pilot-operated check valve<\/td> | Holds flow until pilot pressure opens the valve<\/td><\/tr> |
| Counterbalance valve<\/td> | Controls an overrunning load and supports holding<\/td><\/tr> |
| Directional valve<\/td> | Commands cylinder direction and neutral behavior<\/td><\/tr> |
| Mechanical support<\/td> | Physically supports a load during service<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nInformation to include in an engineering review<\/h2>\n\n\n\nFrequently asked questions<\/h2>\n\n\n\n |