Variable piston pumps run most of the heavy hydraulics we see: offshore cranes, excavators, steering gear, hatch cover systems and large power packs. They make high pressure efficiently and, unlike a gear pump, they can change how much oil they deliver while turning at a constant speed. Tilt the swashplate in the animation and watch the pistons’ stroke change.
In simple terms
Inside the pump is a round block called the barrel, with nine holes bored along its length. Each hole has a piston in it. The barrel is turned by the drive shaft.
The feet of the pistons rest against a plate called the swashplate. If the swashplate is flat, square to the shaft, the pistons go round without moving in or out, and the pump delivers nothing, even though it is spinning.
Tilt the swashplate and everything changes. As the barrel turns, each piston’s foot slides round the tilted plate, so the piston is pulled out of its hole for half a turn (drawing oil in) and pushed back in for the other half (pushing oil out). The more you tilt, the longer each piston’s stroke, and the more oil per turn.
At the back of the barrel is the valve plate, which you can see end-on in the animation. It has two curved slots, called kidneys. Each piston’s hole passes over the suction kidney while it is filling, and over the delivery kidney while it is emptying. That is how oil is let in and pushed out without any valves opening and closing.
Some oil always leaks past the pistons and the valve plate. That is deliberate: it lubricates them. The leaked oil collects in the pump case and returns to the tank through a separate pipe, the case drain. As the pump wears, more oil leaks, and the case drain flow goes up. Measuring it is the quickest way to check a piston pump’s health.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free account.
For engineers
| Quantity | Formula | Example (this pump at 15°, 1,500 rpm, 210 bar) |
|---|---|---|
| Piston stroke | s = 2R tan α | 2 × 35 × tan 15° = 18.8 mm |
| Displacement | V = z × A × s | 9 × 346 mm² × 18.8 mm = 58.5 cm³ |
| Theoretical flow | Q = V × n / 1000 | 58.5 × 1.5 = 87.7 L/min |
| Delivered flow | Q − Q(drain) | 87.7 − 1.8 = 85.9 L/min |
| Hydraulic power | p × Q / 600 | 210 × 85.9 / 600 = 30 kW |
| Shaft torque | V × p / (20π × η(m)) | 58.5 × 210 / 62.8 / 0.92 = 213 Nm |
Displacement is proportional to tan α, so at small angles flow is close to proportional to the angle. Maximum angles are typically 15 to 21 degrees.
In a variable pump, the swashplate is held by a spring and moved by a control piston. The control decides what the pump regulates:
- A pressure compensator reduces the angle as pressure reaches a setting, so the pump delivers only the flow the system will take at that pressure.
- A load-sensing control sets the angle so the pump pressure stays a fixed margin above the highest load pressure (lesson 16).
- Power and electro-proportional controls limit the input power or follow an electrical signal.
Case drain is the health test. Internal leakage goes to the case: past the pistons, under the slippers (which ride on a hydrostatic film of oil), and across the valve plate. Leakage is roughly proportional to pressure, so the test is done at full working pressure and normal oil temperature, with the flow measured in the case drain line. A healthy pump typically drains 1 to 3% of its maximum flow. Rising case drain flow over successive tests, or a value above about 5%, means the running surfaces are worn. Most makers publish an expected figure for each size.
Deep dive
Why the case pressure matters. The case is protected by a shaft seal that is only rated for low pressure, commonly around 2 bar absolute for many industrial pumps (check the data sheet). The case drain must therefore run to the tank with its own line, sized generously, returning below the oil level, and never through a filter or cooler that can block it or teed into a busy return line. A blocked, kinked or undersized case drain pushes the shaft seal out. On a vessel that often shows up as oil around the pump flange after a drain line was rerouted during other work.
Filling the case. A piston pump must be filled with clean oil through the case drain port before its first start, or after any repair. The case oil lubricates the bearings and slippers during the first seconds. Starting a dry pump can ruin it within a minute.
Wear modes. What we find most often when we strip piston pumps:
- Valve plate and barrel face scoring: fine contamination being dragged across the sealing land. It raises case drain flow and lowers volumetric efficiency.
- Slipper wear or lift-off: from contamination, low oil viscosity (hot oil in summer) or overspeeding. Badly worn slippers can break away from the piston balls.
- Swashplate saddle and control piston wear: the pump hunts, can’t hold its setting, or won’t come off stroke.
- Bearing failure: often from contaminated oil or high case pressure.
Inlet conditions. Piston pumps are more sensitive to inlet pressure than gear pumps. Many need a flooded inlet (tank above the pump) or a short, large suction line with no strainer restriction, and some open-circuit pumps need 0.8 to 1 bar absolute at all times. Cavitation erodes the valve plate kidneys and barrel ports quickly.
Running in the heat. In the UAE the oil’s viscosity in summer can fall towards the minimum the pump maker allows, typically around 10 cSt. Below that, the oil film under the slippers and on the valve plate gets too thin. Check the oil grade against the real operating temperature (lesson 5), not the temperature of a European data sheet.
Key numbers
| What | Typical value |
|---|---|
| Continuous working pressure | 280 to 350 bar |
| Maximum swashplate angle | 15 to 21° |
| Overall efficiency at rated point | about 88 to 92% |
| Healthy case drain flow | 1 to 3% of maximum flow |
| Case drain flow that means “worn” | above about 5% |
| Typical maximum case pressure | about 2 bar (check the data sheet) |
Work out your own figures with the pump flow, power and torque calculator.
