What a valve does when the power is removed is one of the last things to get specified. Flow rates, response times, leakage, contamination class and mounting pattern are all pinned down early. The power-off position often stays open until late in the design, at the point someone asks what the ram, the clamp or the platen actually does when the power drops out.
That is usually the moment someone asks whether the valve is fail-safe. We call it the power-off position, for reasons set out later, but the question behind both terms is the same. The answer depends on how the valve reacts once the plug is pulled, and on what is connected to it hydraulically.
The Moment The Signal Stops
A servo or proportional valve holds its spool where the control loop puts it. Remove the electrical supply, or turn the enable signal off, and control stops. What governs the spool from that moment onwards is whatever mechanical means remain: an active mechanism such as a spring or a set of magnets, or nothing at all, in which case flow forces decide.
Where nothing is specified, the valve is passive. The spool sits wherever the pressures acting on it take it. On a well designed valve those flow forces tend towards closing, which is useful, but it is not the same as a defined position you can name at the design review.
The consequences follow the circuit rather than the valve. A load that was being held can creep or drop. A cylinder mid-stroke can drift. Pressure can stay trapped where the next person expects it to be vented. A machine can come to rest in a state that makes restart slower.
Where a defined position is offered on a servo or proportional valve, it is usually one condition, with the spool driven to centre and the service ports blocked. That suits an actuator holding load. Anything else has conventionally been built around the valve, using pilot-operated check valves to hold the load, counterbalance valves on over-running loads, or an unloading valve to release trapped pressure back to tank. Choosing the position inside the valve removes the need for some of those components, along with the plumbing, cost, envelope and leakage paths that come with them.
Positions Now Available
Power-off has been part of the Domin Valves part number for some time. Passive was the standard selection, and anything else was handled as a special enquiry.
That choice has now been expanded. Defined power-off positions are available as standard on the S6 Pro and P6 Pro on NG06, the S10 Pro and P10 Pro on NG10, and the S12 Pro and P12 Pro on NG16. You choose the one that suits the application, it is stated against the part number, and it arrives built into the valve.
Code | Connection | What The Valve Does |
B | Float | P blocked, A/B connected to Return |
D | Blocked | All ports blocked |
PA | P to A | P connected to A, B connected to T |
PB | P to B | P connected to B, A connected to T |
Power-off position lets you specify the outcome rather than the mechanism. If the system needs to hold pressure with every port blocked, that is the blocked position. If it needs to relieve pressure, or allow movement during recovery, that points to float. If it needs to travel to a known end of stroke, that points to P to A or P to B. Passive remains the default where no other position is ordered, and for plenty of circuits that is the right choice.
Why We Say Power-Off Position And Not Fail-Safe
Most engineers looking for this capability search for fail-safe valve, spring return valve or spring bias valve. When a customer opens the conversation that way, we know exactly what they mean.
We use power-off position in our own documentation because it is the more precise term. Fail-safe carries no shared definition across the hydraulics market. Some suppliers tie it to pressure-dependent conditions, some to spool design, one or two to formal certification, and several use the word and then immediately qualify it with a warning about what it does not guarantee. Used loosely, it promises a system-level outcome that no valve can deliver on its own.
A defined hydraulic position is something we can specify, test and stand behind. Safety is a property of a machine, established through its hazard analysis, its control architecture and the standards it is built to.
We define the power-off position and state the conditions it holds under. Machine safety is established by whoever designs and validates the machine, and a defined position is an exact, repeatable input to that work.
The Conditions Attached
When electrical power is removed, or the enable signal drives the valve off, the valve moves to the defined spool position shown in the hydraulic symbol, provided it is operated within its specified conditions, including fluid cleanliness and the supply and pilot pressure range for the selected position. It is not, by itself, a certified safety function, and suitability has to be assessed and validated at machine and system level.
Four things are worth agreeing at the design stage:
• The power-off position itself, chosen from the standard options above
• Pressure conditions within the range specified for that position
• Fluid type, filtration and contamination control to the datasheet
• Expected leakage for the chosen position, where that matters to the circuit
Used within those conditions, the position is repeatable. Outside them, it cannot be guaranteed.
Common Questions
Is a power-off position the same as a fail-safe valve?
It is the capability most engineers mean by the term. We describe it as a power-off position because we can state precisely which position the valve adopts and under which conditions, and because the safety case belongs to the machine.
Is this the same as a spring return valve?
Spring return describes the mechanism. Power-off position describes the outcome, and the mechanism that delivers it depends on the position selected. Where no position is ordered, the valve is passive and the spool is governed by flow forces designed to close it.
Specify A Power-Off Position
Send us the system, what it has to do when the power is removed, and the pressures involved, and we will confirm the position and the part number to specify.
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