Which Hydraulic Oil Should I Use?

Hydraulic Fluid

The most commonly used hydraulic fluids are mineral oil-based ISO 32 and ISO 46 fluids. The numbers in their names represent fluid viscosity in centistokes at 100 °F. Additive packages in hydraulic fluids provide the properties necessary to turn base oils into fluids suited for hydraulic applications, setting them apart from lubricating oils. Whereas hydraulic fluids use ISO designations, lube oils use SAE designations (SAE 10, SAE 20, SAE 10W30).

Temperature & Viscosity

An important issue to always keep in mind when selecting hydraulic oils for your applications is that viscosity varies with temperature. Therefore, viscosity needs to be high enough to provide lubrication to your system, yet low enough as to not cause excessive pressure losses or problems drawing sufficient fluids through the pump (cavitation).

So, how do you determine the correct temperature to operate your hydraulic system? If you can place your hand on the reservoir and feel heat, yet comfortably keep you hand there, your hydraulic system is operating at a good temperature. This trick works for almost any hydraulic system.

Applications & Industries

Hydraulic fluids have also been developed for special circumstances that require special attention:

  • Food Grade
  • Fire Resistant: developed for applications in which hot surfaces or open flames are present.
  • For low-temperature applications, automatic transmission fluid (ATS) is often used, as it has a very low pour point.

Food grade oils were developed for applications in which there is a possibility that fluid can contaminate consumable goods. The fluid typically used in these instances is vegetable oil. For applications in which hot surfaces or open flames are present, fire resistant oils should be used. Fire resistant fluids range from polyol esters (biodegradable, miscible with oils) to phosphate esters (very high flash point, very caustic, very expensive) to water glycol (a mixture that must be monitored for proper content). In situations where low temperature applications are present, the most commonly used oil is automatic transmission fluid (ATS), as it has a very low pour point.

The fluids that you should use largely depend on the environment of your application.

For more information on hydraulic fluids or for help selecting the proper fluid for your environment, please comment below or use our contact form to request additional information.

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Frank Stilwagner
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