The oil's jobs & fluid types
The oil is not just "the stuff that carries pressure." It does four jobs at once — and which fluid you choose can decide whether a machine is safe, reliable, or a fire risk.
Source: Rabie, Fluid Power Engineering, Ch. 2.
Before you start
What you need first
- What fluid power is — the pump raises oil pressure to do work (Topic 1).
What you'll be able to do
- List the oil's four jobs.
- Tell apart the main fluid families and what additives do.
- Choose a fluid from fire risk, temperature and environment.
Start here · what the oil does
The oil does four jobs at once
Pick the wrong oil and any of these four jobs can fail — and a failure in one usually wrecks the others.
| Job | What it means |
|---|---|
| Transmit power | Carry pressure and flow from pump to actuator (the main job). |
| Lubricate | Form a film between moving parts (pump gears, valve spools) so they slide, not scrape. |
| Cool | Carry heat away from where it is made back to the tank/cooler. |
| Clean | Carry dirt and wear particles to the filter. |
Families of hydraulic fluid
Most machines run on mineral oil. Where fire is a danger, a fire-resistant fluid is used instead — usually trading some lubrication or cost for safety.
| Fluid | What it is | Good / weak |
|---|---|---|
| Mineral oil | Refined petroleum oil + additives. | Cheap, great lubricity · flammable |
| HFA (oil-in-water) | ~95% water emulsion. | Very fire-safe, cheap · weak lubrication |
| HFC (water-glycol) | Water + glycol + thickener. | Fire-resistant, OK lubrication · watch temperature limit |
| HFD (synthetic) | Phosphate-ester synthetic. | Fire-resistant, good lubricity · costly, seal-sensitive |
| Bio-oil (HEES) | Synthetic/vegetable ester. | Biodegradable (near water) · costlier |
Additives — small doses, big effect
A few percent of additives tune the base oil to the job:
| Additive | What it does |
|---|---|
| Anti-wear (AW) | Strengthens the lubricating film under high load. |
| VI improver | Keeps viscosity steadier as temperature changes (see Topic 9). |
| Anti-foam | Helps trapped air bubbles break and rise out (see Topic 12). |
| Anti-oxidant | Slows the oil "ageing" (oxidising) when hot. |
| Anti-corrosion | Protects metal surfaces from rust, especially with any water. |
Choosing the fluid
Match the fluid to the working conditions, in roughly this order:
- Fire risk? Near furnaces, foundries, mines → fire-resistant (HFC/HFD).
- Temperature range? Sets the viscosity grade and how high a VI you need (Topic 9).
- Environment? Near water/soil → biodegradable (HEES).
- Seal & metal compatibility, then cost.
✏️ Try it yourself
A die-casting shop runs hydraulic clamps right beside furnaces full of molten metal. A colleague suggests "just use cheap mineral oil." What is the danger, and what family would you choose instead — and what do you give up?
Recap — the whole topic on one screen
| Idea | What you own now |
|---|---|
| Four jobs | Transmit power · lubricate · cool · clean (+ seal) |
| Families | Mineral (default) · fire-resistant HFA/HFC/HFD · bio HEES |
| Additives | AW, VI improver, anti-foam, anti-oxidant, anti-corrosion |
| Choosing | Fire → temperature → environment → compatibility → cost |