MOOG D633 and D634 Power Loss: Where Will the Cylinder Stop?
The power-off configuration determines where the D633 or D634 spool returns when electrical power is lost. M returns to centre; F and D leave an opening in a specified direction. Whether the cylinder stops also depends on the remaining oil supply, the A/B connections and the load-holding circuit.
The MOOG D633 and D634 use a linear force motor to move the spool. With power available, the internal electronics regulate spool position using position feedback. Without power, springs determine its position. A mechanism that remains stationary at zero command can therefore move when valve power is disconnected.
What M, F and D connect
Item 6 in the D633/D634 Rev. D ordering code specifies the power-off position. Spool overlap is a separate item: the same letter in another part of the code can have a different meaning.
| Power-off code | Spool position after loss of power | Connections |
|---|---|---|
| M | Centre position | Port closure and leakage depend on the centre-spool configuration |
| F | Approximately 10% opening | P→B and A→T |
| D | Approximately 10% opening | P→A and B→T |
The 10% figure describes valve opening. Cylinder speed depends on opening, pressure drop, effective piston area and load.
Consider a cylinder with A connected to the cap end and B to the rod end. If hydraulic supply remains available and no other valve isolates the circuit, D provides an extension path and F a retraction path. Swapping A and B reverses these directions.
A replacement selected only by flow rating and mounting dimensions may work normally under power yet behave differently at shutdown if this code has been overlooked.
Zero command and loss of 24 V are different states
At zero command, the spool-position loop continues to operate while the 24 V supply remains on. Removing 24 V disables electronic control and allows the springs to position the spool. F and D then return to their specified power-off openings.
The command midpoint also matters. A ±10 V interface has its midpoint at 0 V. For a bidirectional 4–20 mA command, the midpoint is 12 mA; 4 mA is full command in one direction. Using 4 mA as a stop command sends the wrong signal.
| Action | Valve state | What to check |
|---|---|---|
| Zero command, 24 V retained | Position control remains active | Actual command, null and feedback |
| 24 V removed | Specified power-off position | M/F/D configuration and circuit |
| Command wire disconnected | Abnormal input signal | Interface and wire-break response |
| Emergency stop operated | Pump stop, power removal, unloading or load holding | The actions actually performed by the machine |
Record that valve voltage fell from 24 V to 0 V, or that the command changed from a measured value to zero. A note saying only “disconnected” or “stopped” does not establish the valve state.
A horizontal pusher retracts after a valve replacement
Suppose a horizontal pusher has A connected to the cap end and originally uses an M configuration. An F replacement connects P to B and A to tank after valve power is removed. If the hydraulic unit is still supplying oil, the pusher may retract.
When the old and new power-off codes differ, correct the replacement configuration and shutdown circuit. Adjusting controller null changes powered operation; it cannot alter a spring-defined power-off position.
If the configuration has not changed, observe the movement. Brief travel followed by a stable stop calls for checking inertia, switching and pressure release. Continuing slow movement calls for checking load forces, leakage paths and load-holding components.
A position-versus-time record distinguishes them: the first movement ends, while displacement continues to accumulate in the second.
A vertical load remains after the spool centres
Gravity continues to act after power is removed. Even with an M spool at centre, spool clearances and other circuit leakage paths can allow oil to leave the loaded chamber.
For example, a 500 kg load supported by an effective piston area of 20 cm² requires approximately 24.5 bar just to balance gravity. That load remains on the chamber after the pump stops.
If an assumed leakage path removes 5 mL/min from this chamber, the cylinder moves approximately 2.5 mm/min. Such leakage may have little visible effect during a fast stroke but can produce noticeable drift while holding position.
Vertical loads therefore need appropriate cylinder locking, counterbalance circuitry or mechanical support. If a machine that previously held position starts drifting, check these components and cylinder seals rather than only adjusting servo-valve null.
Useful measurements on the machine
Record valve-terminal voltage, actual command, A/B chamber pressures and cylinder position on one time axis. This identifies whether movement starts before power removal, during switching or after pressures have stabilised.
Electronic valve feedback may also disappear when power is removed. Use independent cylinder-position measurement rather than interpreting an unpowered feedback signal as proof of spool position.
Use the machine's prescribed test mode for horizontal mechanisms. Secure vertical loads with suitable load-holding equipment or mechanical support before electrical and hydraulic personnel verify shutdown behaviour. Do not unplug a loaded valve as an improvised test.
Movement during normal powered operation at zero command is covered in servo-valve null shift and cylinder drift. Flow and mounting differences are covered in the D633 versus D634 comparison.
Conclusion
Retraction that begins after a replacement calls for checking the old and new M/F/D codes and A/B connections. Persistent slow movement with unchanged configuration calls for checking leakage and load holding. A vertical mechanism's ability to remain stationary depends on its complete load-supporting circuit.