A 4–20 mA proportional valve controls commanded motion, but it does not automatically provide positive load holding. Follow the complete path: the PLC sends current through the output wiring, the valve converts current into spool movement, hydraulic flow moves the cylinder, and the suspended load applies force back through the cylinder. If oil can escape through the return path, proportional-valve clearance, blocking valve, or cylinder seals, gravity can move the load with every solenoid de-energized.
Where does the command path stop?
Layer one first. Identify the command and hydraulic paths before changing PLC logic. The electrical path ends at the proportional-valve actuator; the load path continues through the hydraulic circuit after the command reaches zero.
| Path point | Function | Commissioning check |
|---|---|---|
| PLC analog output | Sends the 4–20 mA motion command | Measure loop current at commanded up, down, and stop conditions |
| Output wiring | Carries the current to the proportional valve | Check polarity, continuity, shielding, terminals, and unintended shared returns |
| Proportional valve | Meters flow according to its current and spool position | Confirm that the neutral command produces the intended valve state |
| Blocking valve | Isolates the load-holding oil path when closed | Verify coil voltage, installed flow direction, and actual seat sealing |
| Cylinder and load | Convert pressure into force and position | Observe whether position changes with both valves de-energized |
| Position transducer | Reports cylinder position to the PLC | Compare its indication with an independent physical position measurement |
Do not infer hydraulic isolation from a de-energized coil. A valve can be electrically off while its hydraulic path remains open or leaks. The proof for this stage is a measured stop command at the proportional valve and no voltage at the blocking-valve coil while the cylinder is drifting.
Is the movement inertia or load-induced drift?
Inertia produces motion immediately after deceleration and then dissipates. A cylinder that continues downward while stopped, or reaches the bottom after the machine remains powered off overnight, has a sustained energy source: the load acting through gravity. The position changes only if displaced oil has an escape path or oil transfers across the piston.
| Observed symptom | Primary mechanism to test | Deciding check |
|---|---|---|
| Brief overtravel after a fast stop | Motion profile, valve response, or hydraulic compliance | Reduce approach speed and compare stopping distance |
| Steady downward movement at an intermediate position | External valve leakage, open return path, or internal cylinder leakage | Monitor position with all motion commands removed |
| Downward movement with machine power off | Gravity forcing oil through a non-isolating path | Test the load-holding circuit independently of PLC output |
| Same behavior on several similar cylinders | Shared circuit design, valve selection, plumbing, or commissioning error | Compare schematics, valve markings, port connections, and neutral states |
The position transducer reports the consequence; it cannot restrain the load. A pressure switch that indicates oil presence also does not prove that a trapped cylinder chamber remains sealed. The check is whether downward motion continues after residual motion should have stopped.
Does the blocking valve actually isolate the load?
Trace the oil that must leave the load-supporting cylinder chamber for the rod to move. Every possible route matters: through the blocking valve, through the proportional valve to return, across the cylinder piston seals, or through external plumbing leakage. Valve names are not functional proof. Confirm the symbol, normal state, port orientation, pilot arrangement, and rated leakage behavior from the installed component documentation.
- Secure the mechanism against uncontrolled descent before opening lines or changing the circuit.
- Identify which cylinder chamber supports the load in the raised position.
- Trace that chamber to every connected valve and return path on the hydraulic schematic.
- Remove the motion command and verify the blocking-valve coil is de-energized.
- Confirm that the installed valve’s de-energized state blocks flow in the required direction.
- Check whether pilot pressure, reversed ports, contamination, seat damage, or a manual override can keep the valve open.
- Inspect fittings and lines for external leakage, then test for pressure decay or position change under a stationary load.
If the cylinder still moves, the load path is not isolated. The proof for this stage is stable position when the supporting chamber is positively isolated by a correctly applied load-holding device.
Can the proportional valve hold the cylinder by itself?
A proportional directional valve meters motion. Its spool requires operating clearance, and its neutral configuration determines which ports are blocked, connected, or relieved. A zero or neutral electrical command therefore does not establish zero hydraulic leakage. If the neutral condition connects the loaded chamber to return, gravity can drive the cylinder directly. If the ports are nominally blocked, internal leakage may still produce slow drift.
Measure the actual 4–20 mA loop rather than relying only on the PLC display. A scaling error, residual command, failed analog output, or valve-electronics offset can prevent the spool from reaching its intended neutral position. Then remove electrical power and repeat the position test. Drift that remains without electrical power shifts the diagnosis from PLC control toward the hydraulic load-holding path.
The check is a comparison of drift with the commanded neutral current present and with valve power removed. If both tests produce similar descent, changing the position-control program will not create positive hydraulic isolation.
How should the load-holding function be corrected?
Use a valve arrangement intended to hold an overrunning load, such as a correctly selected and installed load-control valve or pilot-operated check valve. Selection depends on cylinder configuration, load direction, required controlled lowering, available pilot pressure, allowable leakage, pressure, flow, and the required response during power loss. Read those values from the hydraulic design and component datasheets; a generic valve substitution cannot be sized from the 4–20 mA command alone.
- Define which cylinder port must remain blocked when electrical power is absent.
- Select the load-holding function from the hydraulic requirements, including whether the load must remain locked or lower under controlled pilot pressure.
- Install the device close to the cylinder so a hose or downstream valve path does not bypass the holding function.
- Connect flow and pilot ports according to the component symbol and manufacturer documentation.
- Configure the PLC sequence so the holding valve releases only when the proportional valve is ready to command controlled motion.
- During stopping, remove the motion command in the required sequence and confirm the holding valve closes without producing uncontrolled movement.
Do not mask leakage by continuously correcting position with the proportional valve. That approach depends on power, feedback, valve authority, and available pressure; it does not address overnight descent after power removal. The check is controlled movement while commanded and stationary load retention when commands are removed.
How is the repaired system verified end to end?
Commission the electrical and hydraulic paths together. Record position from the transducer and an independent reference so sensor drift cannot be mistaken for cylinder motion.
- Command upward and downward motion and verify correct direction, stable 4–20 mA scaling, and smooth proportional response.
- Approach an intermediate position at a controlled speed, command stop, and verify the analog output reaches its intended neutral value.
- Confirm the blocking or load-holding valve enters its holding state and the cylinder remains stationary under load.
- Remove valve-control power while maintaining safe mechanical protection. Verify that loss of power does not open a gravity-driven return path.
- Repeat the test at the raised position for the required unattended holding interval, then compare initial and final physical position and transducer readings.
The final acceptance check is unchanged cylinder position after the defined power-off holding test, with no energized electro-valve and no uncontrolled path from the load-supporting chamber to return.
FAQ
How do I stop a proportional-valve cylinder from drifting down?
Trace the loaded cylinder chamber to return, verify the proportional valve’s neutral path, and test the blocking valve for actual hydraulic isolation. Apply a correctly selected load-control valve or pilot-operated check valve when positive load holding is required.
How do I tell whether cylinder overtravel is inertia or leakage?
Inertial overtravel occurs directly after deceleration and stops; leakage-driven drift continues under the stationary load. Repeat the position test with the motion command removed and then with valve power removed.
How do I test a blocking valve that is de-energized?
Verify zero coil voltage, confirm the valve’s normal state and port direction from its documentation, and monitor cylinder position or supporting-chamber pressure under load. A de-energized coil does not prove that the hydraulic seat is closed.
How do I verify the cylinder will hold after power loss?
Raise the load, place the load-holding circuit in its closed state, remove control power under safe mechanical protection, and compare the initial and final physical positions after the required holding interval. The final verification passes only when the cylinder remains stationary without energized valves.