Fluids, Filtration and Reservoirs
Oil viscosity and cleanliness, filters, reservoir size, heat generation and cooling.
What the oil does
Hydraulic oil carries the power, but it also lubricates, seals the tiny clearances in the pumps and valves, carries heat away and protects against rust. It has to do all that at the temperature and cleanliness the machine runs at. Most systems use mineral oil with additives (anti-wear, anti-oxidant, anti-foam, rust inhibitors), classified as HLP (ISO 6743-4, with the grade HM in ISO 11158).
Viscosity
Viscosity is the oil’s resistance to flow and the single most important property. It is measured as kinematic viscosity in centistokes (mm²/s) and graded by ISO VG: the number is the viscosity at 40 °C (VG 32, 46 and 68 are the common ones). It falls steeply as the oil warms (see the figure) and the viscosity index (VI) measures how steady it is: a VI of 100 is a standard mineral oil, 150 or more a multigrade (HVLP) oil.
Most pumps want 16 to 36 cSt at running temperature, with limits of about 10 to 1 000 cSt (a minimum for lubrication and a maximum at cold start). VG 46 oil is 134 cSt at 20 °C, 46 at 40 °C, 21 at 60 °C and 11 at 80 °C: the viscosity drops to 45 percent between 40 and 60 °C, so an oil that runs too hot becomes too thin, wears faster and leaks more through the clearances, and one that is too cold starves the pump.
Cleanliness and filtration
Contamination causes most hydraulic failures. Particles bigger than the clearances (a few micrometers in a servo valve, 10 to 25 µm in a gear pump) wear the surfaces and jam the valves. Cleanliness is coded by ISO 4406 as three numbers for the counts of particles larger than 4, 6 and 14 µm per milliliter: the code 19/17/14 means 2 500 to 5 000, 640 to 1 300 and 80 to 160 particles. Each step up in the number doubles the count.
| Component | Target code (ISO 4406) | Typical filter rating (β ≥ 200) |
|---|---|---|
| Servo valves and high-pressure variable piston pumps | 16/14/11 to 17/15/12 | 3 to 5 µm |
| Proportional valves, piston pumps | 18/16/13 | 5 to 10 µm |
| Vane pumps and general industrial valves | 19/17/14 | 10 µm |
| Gear pumps, low pressure systems | 20/18/15 | 10 to 25 µm |
A filter is rated by its beta ratio βx = (particles of size x upstream) ÷ (particles of size x downstream). Efficiency = 1 − 1 ÷ β: a β10 of 200 removes 99.5 percent of 10 µm particles in a single pass, and a β of 75 only 98.7 percent. Place a pressure filter before sensitive valves, a return-line filter on every system, a suction strainer (125 µm, never a fine filter on the pump inlet), a breather with a filter on the tank, and an off-line (kidney loop) filter on critical or dirty systems. Replace on the pressure-drop indicator: a bypass valve opens when a filter is clogged and passes the dirt downstream.
The reservoir
The tank is more than storage: it lets air bubbles rise out, dirt settle, water separate and heat escape. Rule of thumb: 3 times the pump flow per minute (more for mobile machines that run hot, less where space is short). For 30.2 L/min that is 90 L, so a 100 L reservoir. Fill it to about 80 percent of its volume at the lowest level; use baffles between suction and return, with the return line below the oil surface and away from the pump inlet, a sloping bottom with a drain, a sight glass, a magnetic plug and a breather that keeps moisture out.
Heat generation and cooling
All the power lost in a hydraulic system turns into heat in the oil: pump inefficiency, valve pressure drops, throttling, relief valve flow and pipe friction. In steady state the heat has to go somewhere: through the tank walls and pipes by convection (Q = U A ΔT, see Heat Transfer) and, if that is not enough, through a cooler. The oil temperature should stay below about 55 to 65 °C: every 10 K above 60 °C roughly halves the oil’s life.
Worked example: the press heat balance
Worked example: does the press need a cooler?
- The motor takes 9.2 kW. If the circuit overall is 65 percent efficient (pump, valves, lines) the loss is 35 percent: 3.22 kW of heat into the oil.
- The 100 L tank (about 1.5 m² of surface, U ≈ 12 W/m²·K to the air) in a 25 °C shop rejects 12 × 1.5 × (55 − 25) = 540 W at an oil temperature of 55 °C.
- The remaining 2.68 kW must go through a cooler. Water-cooled with a 5 K rise it needs 461 L/h of water; an air-cooled cooler needs a fan.
- Without it, the oil would warm at 1.2 K per minute (87 kg of oil, cp = 1 900 J/kg·K, when the tank loses no heat), so it would pass 55 °C in about 26 minutes of continuous running (the tank’s own cooling slows it down).
- The real fix is to cut the loss: the press only works part of the cycle, so unloading the pump when idle removes most of the heat (see Designing a Fluid Power System).
Other fluids
| Fluid | Use | Notes |
|---|---|---|
| Mineral oil (HM, HV) | the standard | cheapest; flammable (flash point about 200 °C) |
| Water-glycol (HFC) | fire-resistant: foundries, die casting | lower pressure and temperature limits (to 60 °C); shorter pump life |
| Phosphate ester (HFD) | fire-resistant: steel mills, power plants | attacks some seals and paint; expensive |
| Biodegradable (HEES, HETG) | forestry, marine, near water | synthetic ester or vegetable oil; seal compatibility and oxidation need care |
| Water (HFA) | food, mining | very low viscosity: pumps and valves designed for it |
Entrained air is a hidden fluid property: only 1 percent of air bubbles in the oil cuts the effective bulk modulus from 1.4 to 0.86 GPa at 160 bar, making the system spongy and noisy and starting cavitation.
Related
- Previous: Circuits and Accumulators. Next: Pneumatic Systems.
- Fluid Properties and Statics for viscosity and density.
- Heat Transfer for the cooling calculation.
- Hydraulics and Pneumatics: the whole guide.
FAQ
What does ISO VG 46 mean?
A mineral oil with a kinematic viscosity of 46 cSt at 40 °C (between 41.4 and 50.6). It is the most common industrial hydraulic grade.
Can I mix oils?
Avoid it. Different additive packages can precipitate. Flush the system when changing type.
How often should I change the oil?
On a condition: test for particle count, water, acid number and viscosity. Typical lives are 2 000 to 8 000 hours, or longer with good filtration and a cool tank.
Why is new oil not clean?
It is often filled at ISO 21/19/16 or worse. Filter it through the system or a kidney-loop filter when filling.
References
- ISO 6743-4 and ISO 11158 (hydraulic fluid classes), ISO 3448 (viscosity grades), ISO 4406 (cleanliness code), ISO 16889 (filter beta-ratio test).
- Esposito, A., Fluid Power with Applications, Pearson; Totten, G. E., Handbook of Hydraulic Fluid Technology, CRC Press.
- Parker Hannifin, Fluid Conditioning Guide; Eaton, Fluid Power Contamination Control.
Last reviewed: October 2026. Viscosities and targets are typical; use the manufacturers’ data.