A hydraulic lock valve is a pilot operated check valve fitted between the control valve and a hydraulic cylinder that traps the oil inside the cylinder. The cylinder then holds its position when the pump stops, the lever returns to neutral or a hose fails. On a truck mounted crane it is the part that keeps each outrigger leg firmly on the ground while you lift. The same idea, in a different valve, holds the boom.
We have built truck mounted cranes in Xuzhou since 2003, and the question we hear most about this valve is simple: “Why does my crane lean after a while?”
This guide explains how the lock works, how it differs from a counterbalance valve, why outrigger legs creep, and how a technician can tell a leaking lock from a leaking cylinder seal without putting anyone at risk.
Key takeaways
- A hydraulic lock is a pilot operated check valve. It lets oil into the cylinder freely and stops it flowing out until pilot pressure from the other line opens it.
- Outrigger jacks usually carry a double pilot operated check valve, often called a two-way hydraulic lock. US rules require hydraulic outrigger jacks to have “an integral holding device/check valve”.
- Boom cylinders on loader cranes usually use counterbalance (holding) valves instead, because they must also control the speed of a lowering load.
- A creeping leg has two usual suspects: oil passing the lock, or oil leaking across the piston seal. A simple port check separates them, but only a competent technician should do it.
- Never loosen a fitting between the lock and the cylinder while that cylinder holds weight. The oil there is under full load pressure.
What is a hydraulic lock valve?
The hydraulic lock goes by many different names: hydraulic engineers usually call it a pilot operated check valve, a PO check valve or a pilot operated non return valve. Crane mechanics say hydraulic lock, cylinder lock valve or blocking valve. When two of these checks share one body, one for each side of the cylinder, it is a double pilot operated check valve, also sold as a two-way or bidirectional hydraulic lock. Whatever the name, they all perform the same function, holding oil in a cylinder until the operator deliberately commands it to move.
On cranes, mining machines, port equipment and access platforms, the lock sits on any cylinder that must carry weight while standing still. On a truck crane that mainly means the outrigger jacks. Without it, a loaded leg would gradually give way, the truck would tilt, and the load chart you rely on would no longer describe the actual stability of the machine.
How a pilot operated check valve works
A plain check valve works like a one-way door, in which a spring presses a poppet or ball firmly onto a machined seat. Oil flowing in the free direction lifts the poppet and passes. Oil trying to flow back pushes the poppet harder onto its seat, so the reverse flow stops completely. Because the seal is metal on metal with no sliding clearance, a good check valve holds with virtually no leakage.
A pilot operated check valve adds a small piston, the pilot piston. When pilot pressure reaches this piston, it pushes the poppet off its seat and lets oil flow in the reverse direction too. The valve therefore has three states:
- Free flow: oil from the control valve lifts the poppet and enters the cylinder.
- Locked: without pilot pressure the poppet remains seated, so the cylinder cannot move.
- Piloted open: pressure in the opposite line lifts the poppet, allowing oil to leave the cylinder.
In a double pilot operated check valve the two checks are cross-piloted. Pressure in line A opens the check in line B, and pressure in line B opens the check in line A. When you push the lever to lower a leg, oil enters one side and, at the same moment, opens the return path on the other side. When the lever is released, both checks close and lock the oil on both sides of the piston. That is why the lock is often described as “two-way”.
Why the control valve cannot hold the leg on its own
The levers on a crane move spools inside the control valve bank. A spool must slide, so there is always a fine clearance between the spool and its bore. When the pump stops or the spool sits in neutral, a loaded cylinder pushes its oil back toward the valve, and some of that oil seeps through the clearance. The old version of this article put it well: the oil in the cylinder “is not hard to ooze out” through the gap of the reversing valve.
Seepage of this kind is normal for a spool valve but unacceptable for a leg supporting a crane. A seated check valve is therefore installed between the control valve and the cylinder, converting a slow leak into a positive hold. This is the core reason every truck hydraulic lock exists.
Where the hydraulic lock sits on a truck mounted crane
On most truck cranes the lock is mounted on or next to each vertical outrigger jack. Fitting it directly on the cylinder, with rigid tubes to the cylinder ports, is good practice. If a hose between the control valve and the lock bursts, the trapped oil stays in the cylinder and the leg does not collapse. Some cranes also lock other cylinders, such as the horizontal beam extension cylinders, and some combine several functions in a single valve block. Your crane’s hydraulic schematic shows exactly which cylinders carry a lock.
The US construction crane rule makes this holding function compulsory. Under 29 CFR 1926.1415(a)(5), “hydraulic outrigger jacks and hydraulic stabilizer jacks must have an integral holding device/check valve.” In other words, the lock is a required safety device, not an optional extra. Set-up rules depend on it as well: 29 CFR 1926.1404(q) says the outriggers “must be set to remove the equipment weight from the wheels”. A leg that creeps slowly hands that weight back to the tyres.
Pilot operated check valve vs counterbalance valve
Both valves hold a load, and both are opened by pilot pressure, so they are frequently confused; the essential difference is what happens while the load is moving. A pilot operated check valve is either shut or open. A counterbalance valve, also called a holding valve, overcentre valve or balance valve, opens only as far as needed and keeps resisting the load while it lowers.
| Feature | Pilot operated check valve (hydraulic lock) | Counterbalance valve (holding valve) |
|---|---|---|
| Main job | Hold a cylinder still with near-zero leakage | Hold the load and control its speed while lowering |
| How it opens | Pilot pressure lifts the poppet fully off its seat | Load pressure plus pilot pressure act against an adjustable spring |
| During lowering | Open; the load is not braked by the valve | Partly open; it meters the outflow so the load cannot run away |
| Thermal and shock relief | None built in | The spring setting also acts as a relief against pressure spikes |
| Typical place on a loader crane | Outrigger jacks, beam extension cylinders | Boom lift, knuckle (fold) and telescope cylinders, winch motors |
| Adjustment | None | Factory setting; changed only by a qualified technician per the manual |
Why not use a simple lock on the boom? A boom carrying a load is an overrunning load: gravity wants to pull it down faster than the pump supplies oil. With a plain PO check, pilot pressure collapses as soon as the load starts to run, the check snaps shut, pressure builds, it opens again, and the boom judders down in steps. A counterbalance valve avoids this by remaining partly open and restricting the flow progressively, whereas outrigger legs are lowered without a suspended load, so the simpler, leak-tight lock is the appropriate choice.
Loader cranes usually follow this split, but designs vary between models and options. Check the hydraulic schematic in your crane manual before you order a valve or change a setting. If you are comparing systems, our guide to the hydraulic crane for a truck explains how pump flow, pressure and the valve bank fit together.
Why outrigger legs creep
Many operators describe the same experience: the truck is level when the legs are deployed, yet after some time it leans noticeably. A tilted crane is a concealed hazard, because the side loads on the boom and slewing ring increase, and a lift close to the chart limit can become an overturn. These are the common causes:
- Contamination on the lock seat. A particle trapped under the poppet prevents it from seating, so oil gradually returns to the control valve.
- A worn or damaged piston seal. Oil leaks across the piston inside the cylinder, with no visible evidence on the outside.
- Trapped pilot pressure. A double PO check needs both cylinder lines open to tank when the lever is in neutral. If pressure stays in a line, a check may not close fully.
- Cooling oil. Oil shrinks as it cools. For example, a leg set while the oil is hot after a long working session can shorten slightly as the system gradually cools, even when every seal is in perfect condition.
- The ground, not the hydraulics. A pad sinking into soft ground looks exactly like a creeping cylinder. Our guide to crane outriggers and ground bearing pressure explains how to size pads and mats.
- External leaks. Problems such as wet fittings, a weeping rod seal or a chafed hose are usually easy to spot during a careful walk-around inspection.
Eliminate the simple causes first: inspect the pads, wipe the fittings clean and note whether the leg still moves when the oil is cold. If the leg still sinks, the fault is almost always either the lock or the cylinder, and the next section separates the two.
Leaking lock or leaking cylinder seal? A safe check
The idea is the one from our original article, and it still works. The lock sits between the control valve and the cylinder. If oil keeps coming out of the lock’s port on the control valve side while the leg carries weight, the check is passing oil, so the lock is at fault. If that port stays dry but the leg still sinks, the oil is leaking across the piston, so the cylinder seal is at fault. US rules list exactly this cylinder defect for the annual inspection: 29 CFR 1926.1412(f)(2)(xiii)(A) names “drifting caused by fluid leaking across the piston”.
This is work for a competent hydraulic technician who follows the crane manual. Oil at crane working pressure can pierce skin, and a leg that drops suddenly can crush. A safe sequence looks like this:
- Position the truck on firm, level ground, stow the boom, remove any load and keep all personnel clear of the outrigger beams.
- Extend the beam and set the suspect leg on a solid pad. Place timber cribbing under the beam so that it cannot drop far if the jack gives way.
- Stop the engine, disengage the power take-off, and operate the outrigger lever in both directions several times to relieve residual pressure in the lines between the control valve and the lock.
- Wear safety glasses and protective gloves, and position a clean container under the port.
- Slowly loosen only the line on the control valve side of the lock, at the port that feeds the bottom (rodless) chamber of the jack. Never loosen a fitting between the lock and the cylinder.
- Observe the port for several minutes. A continuous trickle indicates that the lock is passing oil, whereas a few drops that quickly stop indicate a sound lock, and a leg that still sinks then points to the piston seal.
- Retighten the connection, restart the engine, cycle the leg and confirm there are no leaks before any lifting operation.
Example scenario (an illustration, not a customer record): an operator finds the rear right leg of a knuckle boom crane a few millimetres shorter after a morning of work. The pad is on concrete and the fittings are dry. The technician runs the port check: oil keeps trickling from the valve-side line, so the lock is cleaned and its seat inspected rather than the cylinder being stripped. The leg is re-tested before the crane goes back to work.
Repairing a hydraulic lock
When the lock is at fault, the usual cause is contamination, so the technician removes the valve with the leg fully supported and the system depressurised, then cleans the poppet and seat and checks them for scoring. A strained or bent valve stem, a scored seat or a weak spring means the valve or cartridge must be replaced. If the cylinder seal is at fault, the cylinder is resealed, and a cylinder that is damaged inside is replaced.
Any replacement valve must suit the crane’s working pressure, which varies widely between models. Our technical drawings list these hydraulic system pressures:
| Model (BOB-LIFT drawing) | Crane type | Hydraulic system pressure |
|---|---|---|
| SQ2SA3 | Telescopic | 16 MPa |
| SQ3.2SA3 | Telescopic | 20 MPa |
| SQ12SA4 | Telescopic | 26 MPa |
| SQ12ZA6 | Knuckle boom | 28 MPa |
| SQ16SA5 | Telescopic | 30 MPa |
| SQZ700 | Knuckle boom | 35 MPa |
Fit a genuine part, or one with the same function, port size and pressure rating, and flush the circuit if dirt caused the fault. Do not adjust counterbalance valves or relief valves by guesswork. US rules expect the manufacturer’s “hydraulic relief valve settings” to be available in the cab, under 29 CFR 1926.1433(d)(1)(xvi). Our guide to truck mounted crane parts shows where the valves sit among the other components.
Inspection and maintenance
Clean oil is the most effective protection for any hydraulic lock, so change filters on schedule, use the oil grade specified in the manual and prevent contamination whenever the system is opened. Beyond that, make the lock part of your routine checks:
- Every shift: look at hoses, tubes and fittings around each jack, and check the oil level. 29 CFR 1926.1412(d)(1) asks for a check of “hydraulic, and other pressurized lines for deterioration or leakage” and of the “hydraulic system for proper fluid level”.
- During set-up: level the truck, then watch the level indicator and the legs during the first lifts. Any change means stop and investigate.
- Annually: a qualified person inspects valves for leaks and cylinders for drifting, as listed in 1926.1412(f)(2)(xii) and (xiii).
- In the UK: LOLER regulation 9 requires a thorough examination of lifting equipment “at least every 12 months”, or under an examination scheme. HSE guidance on thorough examination of lifting equipment says it covers “the equipment and safety-critical parts”, and the hydraulic lock is one of them.
Our truck mounted crane inspection checklist includes the outrigger and hydraulic checks in its daily routine. A leg that holds level throughout a lift is also what makes the numbers in a load chart valid; see how to read a crane load chart and the guide to the lifting moment of a crane.
Standards that cover crane holding valves
In the United States, the consensus standard for loader cranes is ASME B30.22, Articulating Boom Cranes, published in a 2023 edition. Its scope covers cranes, including commercial truck-mounted ones, “that are articulated by hydraulic cylinders”. OSHA’s construction rules quoted above make the outrigger holding device mandatory and set the inspection items. In the UK, LOLER covers thorough examination. Your crane manual adds the exact valve types and settings for your machine.
Every BOB-LIFT crane is mounted on our test bay before shipping and run through lifting, slewing and outrigger functions under load, as shown on our quality and testing page. A buyer may ask for a test video of their own crane with the serial number visible. The crane load test guide explains what the standards require after delivery.
Frequently asked questions
What does a hydraulic lock do on a truck crane?
It traps oil in the outrigger jack cylinders so the legs cannot retract when the lever is in neutral, the engine stops or a hose fails, keeping the crane level and stable throughout the lifting operation.
Is a pilot operated check valve the same as a counterbalance valve?
No. Both hold a load, but a pilot operated check valve is simply closed or open, whereas a counterbalance valve meters the flow while a load is lowered and therefore controls its speed. Loader cranes usually use locks on the outriggers and counterbalance valves on the boom.
What is a double pilot operated check valve?
It consists of two pilot operated check valves in one body, one for each cylinder line, piloted crosswise so that pressure in one line opens the check in the other. It locks a double acting cylinder in both directions and is often called a two-way hydraulic lock.
Why do my crane outriggers slowly retract?
The usual causes are dirt on the lock seat, a worn piston seal, oil cooling down, or a pad sinking into soft ground. A technician can separate a leaking lock from a leaking cylinder with a port check on the control valve side of the lock.
Can I adjust a holding valve myself?
A pilot operated check valve has no adjustment. A counterbalance valve does, but its setting is specified by the crane manufacturer and directly affects safety. Leave any change to a qualified technician who works from the manual and the hydraulic schematic.
How we compiled this guide
We compiled this update from our original article on hydraulic locks and from service questions we answer about BOB-LIFT cranes. The hydraulic system pressures come from the BOB-LIFT technical drawings for the six models listed. The photos show an SQ5ZA3TJ valve block, an SQ10ZA4 control valve bank and a BLZ90K5 support leg cylinder, all taken at our Xuzhou factory in 2026. The quotations come from the official eCFR text of 29 CFR 1926.1404, 1926.1412, 1926.1415 and 1926.1433, from legislation.gov.uk, from hse.gov.uk and from the ASME website. Achord Chan, Product Operations Director, reviewed the guide in November 2026. It does not replace your crane manual or the law where you work.
Need a hydraulic lock or a new crane?
If you own a BOB-LIFT crane and a leg creeps, send us the model, the serial number and a short video of the leg, and our service team will help you find the cause. Every crane carries a 12-month warranty on the whole machine, common spare parts leave the factory within 48 hours, and we reply within 12 hours. Planning a new crane? Browse our knuckle boom truck cranes and telescopic boom truck cranes, then contact us for a quotation with the drawing and load chart.
