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Difference Between AW & HD Hydraulic Fluid

Kimberly Hawthorne

Hydraulic fluids give working mechanisms energy, and should provide high demulsibility for the separation of water, air and other contaminants. Anti-wear (AW) and high-density (HD) are two types of hydraulic fluids that are commonly used in industry.

Hydraulic machines operate with specially designed hydraulic fluids.


Viscosity is the most important part of any hydraulic fluid. The viscosity grade determines the operating temperature range of the hydraulic system. Using the wrong hydraulic fluid in a hydraulic system will shorten the life of moving components.


AW hydraulic fluids contain anti-wear additives. Hydraulic systems that experience a wide operating temperature range, such as pumps, will work best with AW hydraulic fluids. Some AW fluids also contain viscosity index improvers. Hydraulic fluids containing improvers should not be used unless the equipment specifically requires it, as they can have a negative effect on air separation properties and demulsification, or seperation, of the fluid.


HD hydraulic fluids are designed for systems bearing heavy loads. This type of hydraulic lubricant is thick and commonly used to lubricate gears and pistons. Lubricants like this are oil-based and help to prevent rust and wear due to metal-on-metal contact by providing a barrier between moving parts.