Fixed vs variable & pump selection
A pump can deliver a fixed slug of oil every turn, or change that slug while it runs. The choice decides how much energy the machine wastes — and which pump you should buy.
Source: Rabie, Fluid Power Engineering, Ch. 4.
Before you start
What you need first
- The three pump types and what each is good at (Topic 21).
- Displacement \(V_g\) sets the flow per turn, \(Q = V_g\,n\,\eta_v\) (Topic 19).
What you'll be able to do
- Tell fixed from variable displacement, and read the symbol.
- Explain how a pressure-compensated pump saves energy.
- Choose a pump from pressure, flow, varying load and budget.
Start here · the choice
Fixed vs variable displacement
A fixed-displacement pump always delivers the same \(V_g\) per turn; its flow only changes if the drive speed changes. Simple and cheap — but it always pumps full flow, even when the machine needs less, and the excess is dumped over the relief valve as wasted heat.
A variable-displacement pump can change its \(V_g\) while running — for example by tilting the swash plate of a piston pump, or shifting the cam ring of a vane pump. So it can deliver just the flow the machine needs at that moment.
Why a variable pump saves energy
A pressure-compensated variable pump watches its own outlet pressure. It delivers full flow while the machine is moving, but once the pressure reaches its set value (the load is held, or stalled) it cuts its own displacement to almost zero.
Delivering almost no flow at that pressure means almost no power (\(N = pQ\), and \(Q\to 0\)) — so it stops wasting energy as heat. A fixed pump, by contrast, keeps pumping full flow over the relief valve, turning all that power into heat.
Fixed
- Cheap, simple, robust
- Wastes power at part-load (heat)
Variable
- Saves energy — delivers only what is needed
- Costlier, more complex
Choosing a pump
Weigh four things — pressure, flow/noise, whether the load varies, and budget:
| Type | Pressure | Efficiency | Noise | Cost |
|---|---|---|---|---|
| Gear | Low–medium | Fair | Higher | Lowest |
| Vane | Medium | Good | Low | Medium |
| Piston | Highest | Best | Medium | Highest |
| Type | Typical max working pressure |
|---|---|
| Gear pump | up to ≈ 250 bar |
| Vane pump | up to ≈ 175 bar |
| Piston pump | ≈ 350–400 bar (and higher) |
Rough guide — exact limits depend on the maker and the size.
Worked example — which pump?
A machine needs 350 bar, high efficiency, and its load changes a lot through the cycle. Which pump?
✏️ Try it yourself — no numbers needed
- A cheap, robust pump for a simple 100-bar machine in a dusty workshop?
- A quiet pump for a medium-pressure machine used indoors?
- Why pick a variable pump when the load changes a lot?
Recap — the whole topic on one screen
| Idea | What you own now |
|---|---|
| Fixed | Constant \(V_g\); flow only via speed; cheap but wastes part-load power |
| Variable | Changes \(V_g\) while running (tilt the swash plate); delivers only what is needed |
| The symbol | A slanted arrow through the circle = variable |
| Pressure compensation | Flow cut to ~0 at the set pressure → big energy saving |
| Selection | Pressure (→piston), noise (→vane), varying load (→variable), budget (→gear) |