The Whole Idea, in One Diagram
Every hydraulic system on a backhoe loader can be drawn the same way: something makes flow, something decides where the flow goes, something turns the flow into force. The pump makes the flow, the valve block decides, the cylinder does the work. The question that defines the whole system is what the oil does when the valve has not been asked to do anything.
Simplified, and drawn for the idea rather than for a parts catalogue. The top half is the state that defines the system: the pump runs, the oil circulates, and nothing is being done with it.
That is the whole concept. In an open-centre system, the middle position of the control valve has a free passage through it. The pump's oil arrives, walks straight through the valve, and goes back to the tank. The pump never stops pumping, because a gear pump bolted to an engine that is running cannot stop — it has no way to make less oil. It is a fixed-displacement pump: every revolution moves the same amount of oil, always.
"Open" Is Two Different Words, and Both Apply
Buyers say "open-centre" and mean one thing, and the Chinese term 开式液压系统 may mean either of two things. It is worth separating them, because they are answers to different questions.
Open loop (开式回路) — where the oil lives
In an open loop, the pump draws from a tank that is vented to atmosphere and the used oil returns to that same tank, through a filter, before it is drawn again. The oil is, at one point in its journey, sitting in a tank being breathed on. This is how the working hydraulics of essentially every backhoe loader are arranged — the loader arms, the backhoe, the stabilisers, the steering. The opposite is a closed loop (闭式回路), where a pump and a motor are connected to each other in a sealed circle with a small charge pump topping it up; that arrangement is what people mean by a hydrostatic drive. Our smallest model, the BL 15-06 in its diesel version, uses hydrostatic travel — and that is a separate circuit from the working hydraulics, not a replacement for them.
Open centre (开芯式) — what the valve does at rest
In an open-centre valve, the spool at its middle position leaves a passage through the block, so the pump always has somewhere to put its oil. This is the sense that matters for the rest of this article, because it dictates what kind of pump has to be fitted, and it dictates everything you feel afterwards. The opposite is a closed-centre arrangement, in which the valve blocks the passage at rest and the pump is a variable-displacement piston pump that simply stops delivering when nothing is asked of it — the family that load-sensing systems belong to.
Our machines from the BL 70-25 through the BL 90-25 are both: an open loop, and an open centre. If someone tells you a machine has "an open system", ask which of the two they mean. Half the confusion in quotations comes from that sentence.
What Happens When You Let Go of the Lever
Take our mid-size machines as the worked example. The working pump on the BL 90-25 and the BL 105-25 is listed on the parameter sheet at a displacement of 63 ml per revolution, and the system pressure for both the loading side and the excavation side is 20 MPa. A figure like 63 ml/r means that at rated engine speed the pump is moving something on the order of 63 × 2,200 ≈ 139 litres of oil every minute. That oil does not stop when you release a lever.
So where does it go? In the moment between two functions, it goes through the open centre of the valve, down the return line, through the filter, and back into the tank. On the way it passes through plumbing that resists it slightly — the passage, the spool lands, the hose, the filter, the cooler. That resistance is only a few bar, not the 200 bar of the working pressure, but a few bar at 139 litres a minute is real power, and physics only has one place to put spent power: into the oil, as heat.
When you are actually working, the story gets slightly better rather than worse. Pull a lever and the spool shifts; the free passage closes and the oil is offered to the cylinder instead. If the cylinder can take all of it — a light load, a fast movement — most of the flow does useful work. If the cylinder cannot, because the bucket is against a rock or the function has reached the end of its travel, the pressure climbs until the relief valve opens and dumps the surplus straight to the tank. The relief valve is a safety device, not a control device, and every minute it is open is a minute the engine is burning fuel to heat the oil.
You can put a number on it if you want to, with a formula I use when a buyer asks whether a machine will run hot in their climate:
Heat put into the oil (kW) ≈ bypassed flow (L/min) × pressure drop (bar) ÷ 600. The pressure drop is small at rest and large when a relief valve is open — which is why holding a lever at the end of travel is the most expensive thing an operator can do with a machine, and why it does not cost anything at all on a load-sensing machine.
Why It Has to Be a Gear Pump
Open centre and a fixed-displacement gear pump are a matched pair — you cannot really have one without the other, for a simple reason. A gear pump delivers the same volume every revolution no matter what, so it always needs an escape route when nothing is being asked for. An open-centre valve supplies that escape route. A variable-displacement piston pump, on the other hand, can swash itself to near zero delivery and hold pressure instead, so it can sit behind a closed-centre valve and do nothing at all when you are not working.
That pairing is not a ranking. It is two coherent designs, and each one is the right answer for a different kind of owner:
| Open centre + gear pump | Closed centre + variable piston pump (load sensing) | |
|---|---|---|
| At rest | full flow circulating at low pressure, constantly | almost no flow; pressure held, waiting |
| What the lever commands | a share of whatever flow exists | a demanded flow, metered to the need |
| Movement speed | follows engine speed — throttle up to go faster | largely independent of engine speed |
| Two functions at once | they share the available flow; the lighter load wins | flow is apportioned, speeds stay similar |
| Heat at idle or at the end of travel | yes, continuously | little |
| Tolerance of dirty oil | high — gear pumps survive what piston pumps do not | low — tight clearances demand clean oil |
| Field repair | rebuildable, and the parts are generic and cheap | usually a specialist, and usually a clean room |
Our range sits on both sides of that table, and it is worth being plain about where. The BL 70-25, BL 80-25, BL 85-25 and BL 90-25 run a working gear pump and open-centre valve blocks. The current BL 105-25 configuration, as its own spec sheet describes it, uses a variable-displacement piston pump with a dedicated nine-way valve and a full load-sensing system, and lists an attachment line capable of 160 litres a minute. Those are different machines for different kinds of work, picked for what the owner does — not a ladder with a cheap rung and a good rung.
What You Feel in the Seat
Every one of the rows in that table shows up as something an operator notices within the first hour.
- Speed follows the throttle. On an open-centre machine, raising engine rpm raises flow, and raising flow raises speed. Operators learn this within a day and it becomes second nature. It also means the machine cannot be both quiet and fast, and in a residential area that is a real constraint.
- Two levers at once is a negotiation. With a single pump feeding every function, two simultaneous movements split one flow, and the oil takes the easier path — so the lightly loaded function moves and the heavily loaded one crawls. This is why our BL 70-25 is built with twin-pump confluence and three separate valve groups: two pumps feeding separate circuits means the boom and the bucket stop competing with each other. It is a plumbing decision, not a power one.
- Steering gets first call. On an open-centre machine the steering has to be protected from the working hydraulics, or a heavy loader lift would starve it at exactly the moment you needed to steer. That job belongs to a priority valve, which sends flow to the steering circuit first and lets the working circuit have what is left. When you see a priority valve on a configuration list, that is a machine built with an open-centre circuit in mind.
- Metering is coarser but honest. Fine grading at the edge of a bucket is easier on a load-sensing machine. But there is a corresponding virtue in the open-centre machine: what the lever commands is what you get, with no electronics, no signal lines, and no controller deciding on your behalf. On a machine that will spend its life in dust with a mechanic who has a spanner and a multimeter, that is not a small thing.
- It hisses. A working open-centre machine makes a continuous background noise from oil passing restrictions. It is normal and it is not a fault — but a buyer who has only run load-sensing machines will ask about it, and now you know the answer.
I wrote through the wider picture of these systems — the components, the oil, the symptoms of each failure — in the full hydraulic system guide. This article is the narrower question underneath it: what kind of circuit it is.
The Three Things People Confuse It With
This is the part that causes the most expensive misunderstandings, because all four of these words get used as if they were interchangeable. They are three separate axes, and a machine can sit anywhere on each of them independently.
| Question | The answers | What it actually changes |
|---|---|---|
| How is the spool moved? | mechanical levers and linkages · pilot (先导) low-pressure hydraulic signal | how heavy the lever feels and how precisely it can be held. Nothing else. |
| What happens at rest? | open centre · closed centre | whether the pump needs an escape route — and therefore which pump is fitted |
| How much oil does the pump make? | fixed (gear) · variable (piston), often load-sensing (负载敏感) | heat, fuel, simultaneity, and whether speed depends on engine rpm |
| Is the travel drive a loop? | open loop · closed loop (hydrostatic, 静液压) | how the machine moves itself; separate from the working hydraulics |
The confusion is understandable, because the words arrive together in brochures. Our BL 80-25 is a clean example of a machine that gets mislabelled: it has hydraulic pilot control on both the loader and the backhoe, and it also has an open-centre gear-pump circuit. A buyer reading "pilot" and hearing "load-sensing" will expect variable-pump behaviour from it, and will be disappointed by something that is not a fault — the machine will be slower at idle, it will get warm when held at the end of travel, and it will need the throttle to make it quick. None of that is a defect. It is what that circuit does.
Where an Open-Centre Machine Is the Right Answer
I do not sell this as the entry-level option, because for a lot of buyers it is the better machine outright. What I do is put the buyer's situation next to the circuit and see which one fits.
| If you are this buyer | I would point you at | And I would leave off |
|---|---|---|
| Remote site, no dealer within a day's drive, a mechanic with hand tools | Open centre with a gear pump, mechanical or pilot valve blocks, and a spare-parts list you can order by name | Electronics you cannot diagnose, and any pump that needs a clean bench to be opened |
| Dusty, sandy, or hot; oil changes happen late more often than not | Gear pump — its clearances forgive what a piston pump will not | The assumption that filtration can be casual if the pump is expensive. It cannot. |
| Municipal, farm, general contracting: one function at a time, hours not days, budget-driven | Open centre, with twin-pump confluence if you do run two functions together | Paying for metering precision you will not use in a trench |
| Many attachments — breakers, augers, thumbs — and long shifts with fuel bought at retail | The load-sensing end of the range, with the attachment line flow to match | A small open-centre machine doing continuous high-flow attachment work; it will run hot and it will be slow |
| Finish grading, pipelaying, tight tolerance work | Load sensing — the fine control is the job | The idea that a skilled operator can compensate for a coarse valve. They can partly, and it costs them a whole day of concentration. |
None of those rows says "and therefore buy the expensive one". The first three are the majority of the buyers I talk to, and for them an open-centre machine is not a compromise — it is the machine that can be fixed in their town, by their mechanic, with parts they can name.
What It Costs You, and How to Live With It
The price of an open-centre circuit is paid in three currencies: heat, fuel, and noise. None of them is a reason to avoid the machine, and all three can be managed.
- Heat. The oil gets hot because the circuit is always bypassing something. Give it somewhere to go: keep the oil cooler clean — not visually clean, actually clean, with fins you can see through — and treat a rising temperature gauge as information rather than a nuisance. Our BL 105-25 lists oil-cooling circulation cooling for its hydraulic circuit; on every machine in the range, the cooler matters in a hot climate in the way a radiator does.
- Fuel. Oil that is bypassed is engine power that was paid for and not used. The practical habit is the one that costs nothing: do not hold a lever against a relief valve. At the end of travel, release it. It is the single cheapest fuel saving available to an operator, and it is the one nobody enforces.
- Noise and oil condition. Continuous circulation means the oil works harder for its hours, so viscosity grade and cleanliness matter more than they would on a variable-pump machine. I have written about the oil itself, and about what cold does to it, in the cold-weather guide — the same discipline applies at the other end of the thermometer.
There is a fourth cost that is not really a cost, but buyers should know it: the pump and valve are genuinely wear parts with a warranty clock. On our machines the working pump, the multi-way valve, the steering unit, the priority valve and the pilot handle carry the same 12 months or 2,000 hours as the rest of the hydraulic group. That is not an admission that they are fragile. It is an acknowledgement that they are the parts doing the work, and I would rather say so in writing than argue about it later.
Why It Is Also Why the Machine Can Still Be Repaired
The other side of this is the part I care about most when a buyer lives three countries away from a dealer. Open the spare-parts list for one of our mid-size machines and the hydraulic section reads like a hardware catalogue: a twin gear pump, a travel pump, a mechanical three-spool valve with float, a mechanical eight-spool valve, a pilot three-spool, a pilot eight-spool. Those are named, catalogued, interchangeable parts, and the mechanical three-spool costs roughly a third of what the pilot eight-spool does. A local hydraulics shop anywhere in the world can take one apart.
A variable-displacement piston pump with a load-sensing compensator is a different proposition. It is a better piece of engineering for the job it was designed for, and it is also a part that usually travels to a specialist rather than to a bench. For a buyer with a clean workshop, a parts account and a technician who reads schematics, that is fine. For a buyer whose nearest hydraulic specialist is a two-day drive and a border crossing, it is not fine at all — and saying so plainly in the quotation is cheaper than discovering it in year two.
That is the honest trade. An open-centre machine trades some fuel and some finesse for a hydraulic system that tolerates neglect, can be diagnosed with a gauge and an ear, and can be rebuilt by someone who has never seen the factory it came from.
Six Checks Before You Accept One
Whether you are inspecting a machine at my factory or one that has just been delivered to you, these six take twenty minutes and they are the ones that matter on this kind of circuit.
- Start it cold and listen. A gear pump should be a steady whine, not a knock. A rattle that changes with engine speed is cavitation or aeration — usually a suction-side leak or a blocked strainer, and it destroys a pump quietly.
- Cycle every function to the end of its travel and hold it for two seconds. You should hear the relief valve open. Then stop holding it. If the noise continues at rest, someone has set the relief too low or a spool is not centring.
- Try two functions at once. On a single-pump machine, expect one to slow. On a twin-pump machine, expect them to hold. Either is correct for its circuit; what you are really checking is that the confluence is actually working.
- Steer while you lift. If the steering stiffens when you raise the loader, the priority valve is not doing its job, and that is a fault worth fixing before the machine leaves.
- Watch the temperature after half an hour of real work. Not idling — working. Oil that climbs steadily on a normal day will climb into trouble on a hot one.
- Check the oil and the filter, then ask what was in it before. Metal in the filter element tells you which part is next, and it tells you before the part fails.
The wider pre-purchase routine — cold start, cycling, load-hold, temperature, listening, oil — is in the hydraulic system guide, and the fuel side of the same behaviour is in the fuel consumption article.
What It Does Not Change
The circuit type is one decision inside a machine, and it leaves a lot alone. It does not change the engine, the transmission, the axles, the tyres, the frame, the cab, or the service intervals on anything that is not hydraulic. It does not change the warranty on the machine, only on the hydraulic group. It does not change how the machine ships, how it fits in a container, or what the parts cost to send. And it does not change the two things that actually decide whether a hydraulic system lives a long life, which are the same on every circuit everywhere: clean oil, changed on schedule, and an operator who does not hold levers against the relief valve.
It also does not decide whether the machine is any good. I have watched load-sensing machines ruined by dirty oil and gear-pump machines outlive their engines, and the difference was never the circuit. It was the person holding the grease gun and the filter wrench.
The Short Version
An open-centre hydraulic system is one in which the control valve has a free passage through it at rest, so the oil a fixed-displacement gear pump is making all the time has somewhere to go when nobody is working. That single arrangement explains why the functions speed up with the throttle, why two levers at once compete for flow, why the steering needs a priority valve in front of it, why the oil runs warm, and why the pump and valve can be rebuilt in a town with no dealer in it. It is not the cheap option and it is not the old option — it is the option that matches a buyer whose mechanic has a spanner and not a laptop, and whose oil does not always get changed on the day it should be.
It is also three different things from the words it gets confused with. Pilot control is about how the lever moves the spool. Load-sensing is about how much oil the pump makes. A closed loop is about how the machine drives itself. A machine can be any combination of those, and ours are — which is why the useful question is never "is it open or closed", but "what does this machine do when I let go of the lever".
Tell Me the Work, and I Will Tell You Which Circuit
What the machine will be doing, how many hours a day, what attachments it will carry, how far the nearest hydraulic specialist is, and how hot it gets where you are. I will tell you which hydraulic circuit I would put on it, which model in our range that points to, and what I would leave off the quotation.
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