What happens to a constant-displacement hydraulic pump's output when fluid is diverted to the reservoir?

A simple rule about constant-displacement pumps and what happens when you return oil to the tank

If you’ve spent time around hydraulic systems, you’ve probably run into a constant-displacement pump. Think of it like a bicycle tire pump that always pushes the same amount of air with each stroke, no matter how hard you push. In hydraulics, that "same amount" is the fixed volume the pump moves per revolution. The motor might be turning faster or slower, but the pump’s displacement—the amount of fluid it moves per cycle—stays constant.

Let me explain what that means in everyday terms. When a system is running, the pump delivers a fixed flow rate based on its displacement and speed. The pressure in the circuit, on the other hand, is determined by the load the hydraulic components are applying to that flow. If you take fluid out of the circuit and send it straight back to the reservoir, you’re changing the path of least resistance, but not the pump’s habit of delivering a fixed volume per turn.

What happens when you divert fluid to the reservoir?

Here’s the thing: diverting some of the pump’s output to the reservoir reduces the resistance in the route that fluid would normally take through the work circuit. The return line to the tank often has lower resistance than the path through a heavy load, a valve, or actuator. With less resistance ahead of the pump, the system pressure doesn’t have to push as hard to move the same amount of fluid.

In practical terms, the pump keeps delivering the same volume per revolution. That volume has to go somewhere, and when part of it is routed back to the reservoir, the pressure in the main circuit—the pressure that the load is seeing—drops. It’s a bit like water trying to fill a tank when you’ve opened a side channel that drains water away: the stream is still constant, but the pressure behind the water pushing into the main line is reduced because part of the flow isn’t contributing to lifting or moving the load.

So the core idea is straightforward: the output pressure decreases as you divert fluid away from the work circuit, but the output volume remains the same, provided the pump continues to run at the same speed and the motor isn’t stalling or slipping.

A quick mental image helps, too. Imagine a garden hose with a sprinkler attached. If you pinch the hose, the water backs up a little, pressure rises, and the sprinkler doesn’t spin as freely. Now, instead, imagine a second path that takes some water back into a bucket nearby. The overall amount of water the hose pushes per second doesn’t change—the pump is still pushing the same amount—yet the pressure and the effectiveness of the sprinkler at the main outlet can drop because part of the flow isn’t going where you want it.

Key terms in plain English

• Displacement: the fixed volume the pump moves with each turn. For a constant-displacement pump, this value doesn’t change with the system pressure.

• Flow rate (volume per unit time): the actual amount of fluid moved by the pump each second. If the rpm stays the same, this tends to stay the same because the displacement is fixed.

• Pressure: the force the fluid exerts in the circuit. It depends on the load and how easy or hard it is for the fluid to move through the circuit.

• Reservoir return: the path back to the tank. If this path offers less resistance than the load path, more fluid may take that route, lowering circuit pressure.

Why the pressure drops, not the volume

Pumps that don’t change how much fluid they push per rotation behave differently from variable-displacement pumps. A fixed-displacement unit doesn’t “ride” on the load; it simply delivers a set quantity per cycle. So if you route some of that fluid back to the tank, you’re effectively reducing the amount of fluid that’s doing useful work in the circuit at any given moment. The remaining flow still moves the same amount of fluid per second, but with less of it being used by the load, the system pressure relaxes.

That distinction matters because it explains a common observation: the pressure gauge on a hydraulic line can show a noticeable dip when you divert flow away from the actuators or the main line, while the pump’s output remains stubbornly constant in volume. It’s not magic; it’s the geometry of the circuit and the fixed heart of the pump at work.

Consider the broader circuit

A well-designed hydraulic system accounts for this behavior. You’ll see relief valves, pressure-compensating valves, and carefully chosen routing to ensure that the main load receives the right flow at the right pressure under different operating conditions. If the goal is to keep a steady working pressure while allowing some flow to return to the reservoir, designers use specific valve configurations and sometimes a separate bypass loop. In other words, the system can manage the trade-off between constant flow and the demand for pressure.

For students and curious minds, a few practical takeaways pop out

• If you observe a drop in system pressure when you divert some flow to the reservoir, don’t panic. The pump is still delivering the same volume per rotation; the pressure change comes from how much load the circuit is presenting to that fixed flow.

• If you need steady pressure at the work points, you’ll want to look at how the circuit is loaded and whether you can isolate the reservoir return path or add a pressure-compensating element to keep the pressure from sagging.

• In real machines, friction, leakage, and mechanical efficiency all nudge the numbers. A little wear can change the effective displacement a bit, but with a true constant-displacement pump, the fundamental rule holds: fixed volume per cycle, variable pressure depending on the load.

• The same principle helps explain other hydraulic behaviors. For example, when a valve is fully opened and the load is light, pressure drops because the fluid has an easier path to move, while the pump still provides its standard flow. When the load gets heavier, pressure climbs to meet the demand, but the pump continues to push its fixed volume per cycle (barring slipping or stall conditions).

A quick real-world aside

Let’s connect this to something tangible: a hydraulic press you might see in a shop. If you divert a portion of the fluid back to the reservoir—say, through a bypass path while you still want some movement in the work piece—the main press cylinder will feel less resistance. The motor may keep turning at the same speed, the pump still moves the same volume, but the cylinder doesn’t see the same pressure it did before. It’s a reminder that hydraulic systems are about balance: fixed mechanical inputs (displacement) meet variable conditions (load and resistance) to create the results you expect.

A succinct recap

• Constant-displacement pumps push a fixed volume per revolution.

• Diverting flow to the reservoir reduces the pressure in the main circuit because less fluid goes through the load path.

• The output volume stays the same as long as the pump continues to run at the same speed; the pump isn’t changing how much it pushes each cycle.

• The exact pressure you measure depends on how the rest of the circuit is loaded and what paths the fluid can take.

If you enjoy these little mental experiments, you’re in good company. Hydraulic systems blend a touch of physics with practical engineering. It’s not just about numbers on a page; it’s about understanding how flow, pressure, and resistance mingle in real machines. The next time you see a hydraulic schematic or walk past a machine in action, try tracing where the fluid is going. Notice where it stays in the circuit and where it sprints back to the tank. You’ll start to picture the “why” behind the “what” you observe.

Bottom line, in plain terms: with a constant-displacement pump, diverting fluid to the reservoir keeps the volume steady but lowers the pressure in the main circuit. The pump keeps delivering its fixed amount per turn; the system just rebalances as some of that flow takes a detour. And that little balance is what makes hydraulic systems both predictable and, yes, a bit fascinating.