Hydraulic fluids
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Frequently asked questions
How do HM, HV and HLP differ?
Under ISO 6743-4, HM is mineral oil with anti-wear additives, used in most industrial hydraulics. HV adds viscosity index improvers for outdoor equipment or wide temperature swings. HLP is the DIN 51524 part 2 name and is equivalent to HM.
Should I choose ISO VG 32, 46 or 68?
Follow the pump maker's recommendation and the running oil temperature. In hot plants such as typical Vietnamese conditions, VG 46 is the usual grade for presses and injection moulding machines; VG 32 for precision or cooler systems; VG 68 for hot-running systems or worn pumps. Oil that is too thin at operating temperature wears the pump quickly.
When must hydraulic oil be changed?
Change on condition rather than fixed hours: check cleanliness (ISO 4406 code), water content, acid number and viscosity. Milky oil means water; dark oil with a burnt smell means oxidation from overheating. Change the filters at the same time.
Can I top up with another brand's oil?
Topping up with the same type and VG grade is possible when needed, but additives from different brands may react and form deposits, especially between zinc and zinc-free oils. Never top up a mineral oil system with ester or glycol based fluid.
Does new hydraulic oil need filtering before filling?
It should be filtered. New oil from a drum is often less clean than piston pumps or proportional valves require. Filling through a filter cart with an element around 3 to 10 µm helps reach a suitable ISO 4406 code and extends pump and valve life.
Why does hydraulic oil foam and how is it fixed?
Foam usually comes from air drawn into the suction line, a tank level that is too low, or a return line discharging above the oil surface. Mixing with a different oil or water contamination can also defeat the antifoam additive. Check suction line joints, keep the oil at the correct level and make sure the return pipe ends below the oil surface.
What hydraulic oil temperature is too high?
Most systems run well at about 40 to 60 °C in the tank. Above roughly 70 °C, mineral oil oxidises markedly faster, viscosity falls and seals harden early. When the oil runs hot regularly, check the cooler, a relief valve that is bypassing continuously, and pump wear.
When are fire-resistant hydraulic fluids used?
Fire-resistant fluids under ISO 6743-4, such as HFC (water glycol) or HFDU (synthetic ester), are used near ignition sources, molten metal, die casting machines or mines. They require checking seal materials, tank paint and filters, since not all are compatible. Converting from mineral oil needs thorough draining and flushing according to the fluid maker's procedure.
