Hydraulic Oils

Hydraulic Oils

The primary purpose of a hydraulic oil is to transmit force applied at one point in a system to another. As well as this, it must also protect seals, lubricate, transfer heat and protect against wear.

Qualities of a good Hydraulic oil are -

Anti Wear Protection - to prevent Rust & Corrosion of the hydraulic system. Typically, these products contain anti wear, anti rust/ corrosion and anti oxidation inhibitors. These may be ashless (non-metallic) or use a zinc di-thiophosphate type system. Some older, high zinc additives can be corrosive to silver, so many manufacturers may specify a zinc free oil.

Demulsibility - to separate water from the oil, which can lead to pump wear and poor filtration

Anti-Foaming - to protect from cavitation, as air in the fluid can generate heat and damage pumps

Film Strength / Viscosity to maintain pressure and lubricate components within the typical operating temperature. The viscosity of the oil is important to ensure efficient power transfer. Too heavy, and high-pressure drops may occur, the system becomes sluggish and power usage increases. If too light, then wear can be a problem, efficiency decreases and leaks may occur.
Hydraulic oils may also be of a high viscosity index. These oils provide better flow when cold and also don’t thin out as much when at operating temperatures.

Mineral Based Hydraulic Oils

Hydraulic Oils are generally mineral oil-based, with suitable additives. The requirements for hydraulic oil are set out in ISO 6743-4 with the designations HL, HM, HV. Germany designations - HL, HLP, HVLP are standard, in accordance with DIN 51524.

 TYPE

 DESCRIPTION

 H & HH

 Mineral oil with no active ingredients – (Obsolete)

 HL

 Contains active ingredients to increase the corrosion protection and resistance to aging.

 HM

 Contains active ingredients to increase the corrosion protection and resistance to aging and to reduce wear due to scoring in the mixed friction area.

 HLP

 Further active ingredients in addition to HL oil to reduce wear and increase resistance in the mixed friction area – widest application in practice.

 HV & HVLP

 Similar to HLP with increased resistance to aging, with improved high temperature-viscosity.

 HLPD

 Like HLP, but with additives to improve particle transport (detergent effect) and dispersion capacity (water carrying capacity) and active ingredients to increase the corrosion protection (German designation, not standardised)

 

Flame resistant fluids

HFAE: Oil in water emulsions

  • Water content is above 80% and is mixed with a mineral oil or soluble polyglycol-based concentrate
  • With a mineral-oil based concentrate, there is the risk of  separation and microbe growth
  • Flame resistant, can be used at temperatures between +5°C and +55°C

HFAS: Synthetic concentrates dissolved in water

  • No risk of separation, since this is a true solution, which means the hydraulic components are considerably more susceptible to corrosion

HFB: Water in oil emulsions

  • The water content is above 40% and is mixed with a mineral oil. This emulsion is rarely used.
  • Flame resistant, can be used at temperatures between +5°C and +60°C.
  • In Germany, HFB fluids are not permitted due to the lack of fire protection properties

HFC: Water glycols

  • the water content is more than 35% in a polymer solution,
  • flame resistant, can be used at temperatures between -20°C and +60°C.
  • Can be used at pressures of 250 bar.

HFD: Synthetic liquids

  • HFD-S: anhydrous chlorinated hydrocarbons
  • HFD-T: mixture of HFD-R and HFD-S
  • HFD-U: anhydrous other composition (consisting of fatty acid esters)
  • Synthetic liquids have a higher density than mineral oil or water (not HFD-U), they can cause problems with the suction performance of pumps and affect a lot of gasket materials.
  • Flame resistant, can be used at temperatures between -20°C and +150°C.

Biodegradable

Biodegradable hydraulic liquids are manufactured using plant oils (e.g. rapeseed oil) and used in biologically critical environments (construction machinery in water protection areas, food industry, snow moving equipment in mountains, etc.). The fluids are class 1 harmful substances.

Labelling : HE = Hydraulic Environmental

Classification:

  • HETG (triglyceride base = plant oils),
  • HEES (synthetic ester base),
  • HEPG (polyglycol base)
  • HEPR (other base liquids, primarily Poly-alpha-olefins).

Water

Water can be used as hydraulic liquid but offers no resistance to corrosion. It is not normally used in powered hydraulics but is mixed with oil to form an emulsion, where it can separate from the oil. The first technical use of hydraulics employed water as the fluid. Water has a practically constant low viscosity.

Classification:

  • Tap water (filtered)
  • Technical water (water-oil emulsion)
  • Sea and salt water (filtered, not suitably due to aggressiveness)