Properties of mil-h-5606 hydraulic fluid

Written by puriry makandi
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Properties of mil-h-5606 hydraulic fluid
Hydraulic fluids help to lubricate various engine parts. (hydraulic machine image by Heng kong Chen from

Hydraulic fluids effect the transfer of energy throughout the hydraulic system. These liquids, which are commonly used in applications such as brakes and power steering, also provide lubrication to avoid friction, wear, rust, oxidation and corrosion. Mil-H-5606 is a now-superseded product standard for U.S. military aircraft hydraulic fluid.


Petroleum-based Mil-H-5606 hydraulic fluid also contains other additives to protect the fluid from reacting with oxygen. Petroleum-based hydraulic fluids find use in submarines, aircraft automatic pilots, in control mechanisms and in shock absorbers, among other hydraulic systems. Among the main disadvantages of hydrocarbon-based hydraulic fluids are their toxicity and low biodegradability. This makes them susceptible to cause environmental degradation in situations where leaks occur.


Petroleum-based hydraulic fluids can be quite flammable, and care should be taken to ensure they are kept away from open fires or high temperatures. Mil-H-5606 is a highly flammable hydraulic fluid, a factor which limits its application in other areas. The flammability of Mil-H-5606 has resulted in its substitution with less flammable hydraulic fluids in the aircraft industry.

Low Flash Point

A flash point refers to the levels of temperature at which the hydraulic fluid surface emits enough vapour in order for it to ignite. Some flammable liquids are known to have a low flash point of below 100F. The flash point of a hydraulic fluid corresponds with the vapour pressure. Mil-H-5606 has a very low flash point of about 203F and this makes the fluid very flammable. However, flash points are useful indicators of fluid contamination because of their sensitive low boiling point.

Temperature Range

Mil-H-5606 hydraulic fluid is a mineral oil combined with other petroleum products in order to enhance its viscosity index, which is a significant factor in the operation of hydraulic systems. Viscosity index measures how much a liquid's viscosity changes over its temperature range. Fluids with a high index vary little in viscosity over a wide temperature range. Mil-H-5606 has an operating temperature range of between 65 below zero to 275F.

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