Short answer
DIN 51524 is the German standard for mineral-oil hydraulic fluids, published in three parts: Part 1 covers HL oils, Part 2 covers HLP oils and Part 3 covers HVLP oils. HLP oils add anti-wear performance to HL oils, and HVLP oils add a higher viscosity index for wide temperature ranges. European-built machines frequently specify oils by these codes, such as HLP 46 or HVLP 32.
The three parts
Part 1 (HL) describes oils with oxidation and corrosion inhibitors for systems with moderate pressures and modest thermal demands. Part 2 (HLP) is aimed at hydrostatic systems with higher thermal loads, possible water contamination, or pumps and motors that need anti-wear additives under mixed-friction conditions. Part 3 (HVLP) covers the same kind of service as HLP but with improved viscosity-temperature behavior, for systems that see wide ambient or operating temperature swings.
All three are minimum-requirement standards. Each lists required tests and limits by viscosity grade, and the grade number after the letters is the ISO VG grade, so HLP 46 is an HLP oil of ISO VG 46.
What the tests look at
The requirements cover the properties a hydraulic system depends on: viscosity at defined temperatures, pour point, flash point, oxidation stability, rust and copper corrosion protection, demulsibility, air release, foaming, seal compatibility and filterability. For HLP and HVLP oils the list also includes wear protection, checked with a mechanical gear-rig test and a vane pump wear test.
The vane pump test, once known as DIN 51389 and now published as ISO 20763, runs the oil in a vane pump for an extended period and measures wear on the vanes and ring afterward. It is one of the tests that separates an anti-wear oil from a plain inhibited oil.
HVLP and viscosity index
HVLP oils must meet a minimum viscosity index, which in practice means they thin less as they heat and thicken less in the cold. Because many high-VI oils use polymer additives, shear stability also matters: the oil should keep enough viscosity after the polymers are sheared in pumps and valves. Product data for an HVLP oil usually reports a viscosity after a shear test as well as fresh-oil viscosity.
Relationship to ISO and AW oils
DIN 51524 HLP and HVLP are close counterparts of the ISO 6743-4 HM and HV categories, and many anti-wear hydraulic oils sold in North America list both. They are not identical documents, though, and editions have been revised over time, so when a manual cites a specific DIN 51524 part, look for that part in the current product data.
Frequently asked questions
What does HLP stand for?
In German designations, H stands for hydraulic oil, L for oxidation and corrosion inhibitors, and P for additives that reduce wear under load. HVLP adds V for improved viscosity-temperature behavior.
Is HLP 46 the same as AW 46?
They are similar types: an anti-wear hydraulic oil of ISO VG 46. Only DIN 51524-2 defines HLP, though, so check that the product data lists that standard if your manual requires it.
What is HLPD?
HLPD is a German designation for an HLP-type oil with detergent and dispersant additives. It is not one of the three DIN 51524 parts, so follow the machine maker’s guidance if it is specified.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
