Quick Overview
- An overcenter valve is a load-holding valve mounted directly on a hydraulic cylinder to stop the cylinder from moving unless commanded — it is also called a counterbalance valve or motion control valve.
- On truck-mounted cranes, a single overcenter valve is fitted to each single-acting outrigger cylinder to prevent the leg from settling or drifting once it is set and loaded.
- The valve also provides hose-burst protection: if a supply hose fails, the valve blocks reverse flow out of the cylinder, so the outrigger leg cannot collapse.
- HFD Hydraulic’s VBCD-SE-A Type A single overcenter valve is built for exactly this circuit: in-line mounting, a 4.5:1 pilot ratio, and pressure ratings up to 350 bar.
Every truck-mounted crane lift starts the same way: before the boom moves, the outriggers go down. Those extendable legs lift the truck’s wheels off the ground and spread the load across a wider footprint, and the entire lift’s stability depends on those legs holding their set position under a shifting, rotating load. The component doing that job quietly at each cylinder is an overcenter valve.
This article looks at how an overcenter valve functions specifically in a truck-mounted crane outrigger circuit — why it’s needed, how it holds a single-acting outrigger cylinder in place, and what to check for when specifying one. For a broader technical breakdown of how these valves work across applications, see our guide on how a counterbalance valve works, and for terminology, our comparison of overcenter valves vs. counterbalance valves.
What Is an Overcenter Valve?
An overcenter valve is a load-holding cartridge or in-line valve mounted at (or very close to) a hydraulic cylinder port. It allows free flow into the cylinder, but blocks flow out of the cylinder until a pilot signal — proportional to the pressure on the opposite side of the cylinder — confirms that the directional valve is actively commanding movement. Manufacturers commonly list “over-center valve” and “motion control valve” as alternate names for the same device, alongside “counterbalance valve.” Other names for this valve include motion control valve and over-center valve.
Because the valve only opens when it receives a genuine pilot signal, it does two things at once: it holds an overrunning load in a fixed position when the operator isn’t actively commanding motion, and it prevents the actuator from running away with the load — for example, if a hose bursts or a spool fails to center.
Why Load Holding Matters on Truck-Mounted Crane Outriggers
Truck-mounted, wheel-mounted, and boom-truck cranes fall within the scope of OSHA’s construction crane standard, 29 CFR 1926 Subpart CC, which explicitly covers “commercial truck-mounted” and “boom truck” cranes. This standard applies to power-operated equipment, when used in construction, that can hoist, lower and horizontally move a suspended load, including mobile cranes such as wheel-mounted, rough-terrain, all-terrain, commercial truck-mounted, and boom truck cranes. Within that standard, outrigger and stabilizer deployment is directly regulated: once outriggers are required for a lift, they must be fully extended (or deployed exactly as the load chart specifies) and locked before the crane operates. The outriggers or stabilizers must be either fully extended or, if manufacturer procedures permit, deployed as specified in the load chart, and equipment with outriggers or stabilizers must have them all extended and locked.
The requirement is straightforward in practice: an outrigger leg is only doing its job if it stays exactly where it was set. If a single-acting outrigger cylinder is allowed to creep — even slightly — under the shifting load of a rotating boom, the crane’s effective support footprint and level change without the operator commanding it, which is the condition this OSHA requirement is written to prevent.
This is precisely the failure mode an overcenter valve is designed to close off. It is a mechanical, always-on safeguard that holds the cylinder position independent of hydraulic pump pressure, hose integrity, or a momentary loss of pilot supply.
How an Overcenter Valve Works in an Outrigger Cylinder Circuit
Outrigger legs on truck-mounted cranes are typically actuated by single-acting cylinders — pressure extends the leg, and the leg’s own weight (plus the load transferred through it) is what would retract it if nothing held it back. This is exactly the load-holding scenario an overcenter valve is built for, and it’s why these valves are commonly mounted straight onto the cylinder port rather than back at a remote manifold, on booms, cranes, and outriggers alike. Locking counterbalance valves are usually mounted directly to a cylinder to lock it in position when the directional valve is in neutral, providing smooth, positive motion control of a cylinder with a high load-generated pressure tending to cavitate it, such as on booms, cranes, or outriggers.
The valve’s check section allows unrestricted flow into the cylinder as the leg extends. Once the directional valve returns to neutral, the valve’s relief section closes and blocks reverse flow out of the cylinder — the leg stays exactly where it was set. It only reopens when a pilot signal from the opposite line, scaled by the valve’s pilot ratio, confirms the operator is genuinely commanding retraction. The pilot ratio describes how pilot pressure scales against the relief setting of the valve. Because that pilot signal has to come from active system pressure, a burst or disconnected hose on the supply side simply removes the signal rather than releasing the load — the valve stays closed and the leg does not collapse. Valves mounted this way are used on booms and manlifts to protect against damaged hoses, since the valve will not allow the cylinder to move until there is positive pressure at the opposite port.
To keep the leg from drifting under normal operating pressure, the valve’s relief setting also needs enough margin above the load-induced pressure it’s holding back. Manufacturer guidance for this class of valve generally calls for a setting with meaningful headroom over the maximum pressure the load itself generates, rather than a setting close to that pressure. Internal relief is set to at least 1.3 times load-induced pressure as a minimum.
Application Example: Setting Up on Uneven Ground
Consider a boom truck crane arriving at a site with a mildly sloped, unpaved lay-down yard — a common scenario, not an edge case. The operator extends all four outrigger legs to the beam positions specified in the load chart and lowers each single-acting cylinder onto its pad. As the crane begins to rotate and lift, load is transferred unevenly across the four legs — it is normal for one outrigger to carry a disproportionate share of the total load at any given boom position during a pick.
Without a load-holding valve at each cylinder, that shifting pressure would tend to push oil back through the directional valve’s neutral position and let the loaded leg settle, even by a small amount. With a single overcenter valve mounted directly on each outrigger cylinder’s port, that reverse flow path is blocked at the cylinder itself. The leg holds its set height and load distribution regardless of what’s happening elsewhere in the circuit — including a hydraulic line elsewhere in the system losing pressure — because each valve’s holding function doesn’t depend on the rest of the system staying pressurized.
This is also why a single (rather than dual) overcenter valve is the correct fit for this circuit: an outrigger cylinder only needs to be held against one direction of load-induced pressure — the retract direction — since extension is only ever done under active, commanded pump pressure. A single overcenter valve, sized to the cylinder’s bore and the leg’s maximum load-induced pressure, does the job without adding unneeded restriction on the extend side.
Selecting an Overcenter Valve for Outrigger Circuits
A few application-specific factors matter most when specifying an overcenter valve for a single-acting outrigger cylinder:
| Factor | Why it matters for outrigger circuits |
| Pilot ratio | Determines how much pilot pressure is needed to reopen the valve; too high a ratio can cause instability, too low can make the leg feel sluggish to retract |
| Maximum working pressure | Must exceed the highest load-induced pressure the leg will see, including transient spikes as the boom swings |
| Flow rating | Should match the cylinder’s rod-side/cap-side flow at normal retract speed without excessive pressure drop |
| Port size | Needs to match the cylinder port and hose fitting size used on the outrigger beam |
| Mounting style | In-line, cylinder-mounted valves keep the holding function as close to the actuator as possible, minimizing the length of hose between the valve and any potential leak point |
| Operating temperature range | Outriggers are exposed to ambient conditions on the job site, so the valve’s rated range should cover expected seasonal extremes |
For a full walkthrough of these selection criteria across valve types, see our guide on how to select a counterbalance valve.
Featured Product: HFD VBCD-SE-A Single Overcenter Valve
For single-acting outrigger cylinder circuits like the one described above, HFD Hydraulic’s Type A Counterbalance Single Overcenter Valve — VBCD-SE-A is built specifically for in-line mounting directly at the cylinder port. Key specifications:
- Body construction: zinc-plated steel body with hardened internal parts and NBR (BUNA-N) seals
- Pilot ratio: 4.5:1
- Maximum pressure: up to 350 bar (steel body) / 210 bar (aluminum body)
- Flow rating: up to 60 L/min, depending on port size
- Port sizes: G1/4, G3/8, G1/2
- Operating temperature: -20°C to +90°C (ideal range +40°C to +70°C)
Its single-direction holding function is a direct match for a single-acting outrigger cylinder: it blocks reverse flow out of the cylinder while allowing free flow in during extension, with enough pressure margin and pilot ratio to hold a loaded leg steady without adding unnecessary restriction to the circuit.
An overcenter valve is a small component doing a job that outrigger stability depends on entirely: keeping a single-acting cylinder exactly where it was set, regardless of shifting load or a compromised hose elsewhere in the circuit. For truck-mounted crane builders and hydraulic system integrators specifying outrigger circuits, matching the valve’s pilot ratio, pressure rating, and port size to the cylinder is what determines whether that holding function performs the way the standard governing the equipment assumes it will.
HFD Hydraulic manufactures the VBCD-SE-A and a full range of single and double overcenter/counterbalance valves for mobile hydraulic applications. Contact our team to discuss valve sizing for your outrigger or other load-holding circuit.
