Hydraulic Oils Selection and Specifications

Hydraulic Oils

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This discussion focuses on mineral-based hydraulic fluids.

Hydraulic Fluids are one of the most common, and important lubricants for industry.  The fluids are compressed in hydraulic systems, and essential in order to generate, transfer and control force.  The power generated may then be utilized to perform heavy mechanical actions, such as pushing, pulling, bending, and lifting.

Fluid is compressed (BLUE) to generate, transfer and control force

Applications may be found in heavy equipment (backhoes, loaders, dump-trucks, etc), metal working machines (CNC lathes & mills, die cast machines, plastic injection machines, etc), lifting devices (jacks, car lifts, etc), and in all other industries.

A hydraulic fluid is essential in a hydraulic system as it also performs the following functions:

  1. Lubricate (reduce friction and wear, and prolong the useful life) of the hydraulic pumps and cylinders,
  2. Ensure smooth, non-jerky motion of the cylinders (pistons),
  3. Prevent rust and corrosion,
  4. Absorb the heat generated in the system, and,
  5. Ensure that the system is clean by transporting any contaminants to the filter/s.

hydraulic system

Typical hydraulic system

In selecting the right hydraulic oil for one’s application, care must be given in evaluating different parameters or specifications:

VISCOSITY – the resistance to flow, measured in cst at 40oC, or as ISO VG.   The correct viscosity ensures the separation, via an oil film, of all sliding and rotating frictional surfaces, prolong component life.  For most hydraulic applications, ISO VG 32, 46, 68 and 100 are used.   Moreover, equipment manufacturers (OEM) typically have a recommended viscosity recommendation, which in all cases should be followed.   Using a fluid of too low viscosity can cause abnormal friction and wear, leakages, and the loss of precision control.

Too high viscosities, on the other hand, lead to sluggish mechanical action, increased operating temperatures and power consumption, as well as leaving some lubricating areas unfilled.

VISCOSITY INDEX – for applications requiring mineral-based hydraulic fluids, the standard is normally a VI of > 90.

ANTI-WEAR PROTECTION – while the right viscosity provides an oil film for friction reduction purposes, there are many instances wherein the oil film is not sufficient to provide protection.  This occurs since viscosity may be reduced at high temperatures, and may also fluctuate to unacceptable levels depending on oil-life and condition.  The presence of both liquid and solid contaminants can also sacrifice the friction reduction properties of viscosity.

To aid lubrication, Anti-Wear (AW) additives are normally formulated into the hydraulic fluid.  AW additives, at elevated temperatures, activate to form a strong, adherent layer on the metal surfaces where viscosity protection is not present.

Action of anti-wear additives

Please ensure that they hydraulic oil you use contains AW additives.

DEMUSLIBILITY – a good hydraulic fluid should separate water readily, allowing for its removal via gravity drain.  Water or moisture is always detrimental, as it does not provide rust and corrosion protection, lubrication, and may cause reactions that oxidize and degrade the lubricant prematurely.

AIR-RELEASE AND ANTI-FOAM – foam may be generated due to the agitating action of the hydraulic pumps, and should be avoided.   Anti-foaming additives are a normal part of the hydraulic fluid formulations.

OXIDATION AND THERMAL RESISTANCE – as a lubricant is exposed to constant high-temperatures, moisture, and oxygen, the lubricant degrades (or oxidizes).  While this is a normal occurrence, oil-life should at least be at 12 months, or more, in order to maximize operations and up-time.

FILTERABILITY OF ADDITIVES – modern systems utilize very fine filters, with mesh sizes as low as 25 microns.  Therefore, the additives’ molecular size should be in small enough to pass through filters at all times.

MATERIAL COMPATIBILITY – as a hydraulic system employs a myriad of materials other than metals, hydraulic oils should not cause any negative reactions of seals (natural or synthetic rubbers), o-rings, gaskets, paints, and plastics.

An easy and safe practice in hydraulic lubricant selection is to patronize globally-known brands to ensure quality and performance.  However, ‘lesser-brands’ may also be considered upon careful inspection of the manufacturer’s technical data sheet (TDS), and a review of the specifications as stated above.

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