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Hydraulic Fluid 101

April 26, 2023

What is Hydraulic Fluid?

Often taken for granted, hydraulic fluid is the literal lifeblood of the hydraulic systems found in heavy equipment manufactured by Bobcat, Caterpillar, Kubota, and other brands. Are you raising the boom on your mini-excavator or dumping its bucket? This only happens thanks to the hydraulic fluid and the hydraulic components found in this machine. Also called hydraulic oil, this vital fluid transfers power within the hydraulic equipment and is carefully engineered to perform in specific ways. Understanding how hydraulic fluid works gives insight into how your equipment functions.

What is Hydraulic Fluid Made Of?

Hydraulic fluid is usually made from mineral oil, called base stock or base oil, and a small amount, about 1%, of additives. Specialty purpose hydraulic fluids can use other base stocks such as silicone-based oils for enhanced lubrication or water-glycol for moisture control. Traditional hydraulic fluids contain hazardous substances that can cause problems if leaked into the environment. Hydraulic fluid producers are engineering more environmentally sensitive base oils as an alternative but with a higher price. Depending on suitability and cost considerations, today’s heavy equipment manufacturer can use biodegradable hydraulic fluids based on vegetable oils like Canola.

Additives Help with Hydraulic Fluid Functionality

While hydraulic fluid contains a minimal amount of additives, these chemical compounds are vital in helping hydraulic fluid perform the function it was designed for. Depending on the equipment and the setting it’s used in, additives can reduce corrosion and oxidation, aid in lubrication, foster anti-wear, and maintain fluid viscosity.

Critical Characteristics of Hydraulic Fluid: Viscosity and Incompressibility

Viscosity is the measure of a fluid’s resistance to motion or deformation. In simpler terms, viscosity is the thickness of the hydraulic fluid. It has to be thin enough to have a lubricating effect on the most remote internal components while maintaining consistent thickness at a lower temperature range. Automatic transmission fluid is a form of hydraulic fluid. When these transmissions fail and slip gears, it’s often because the transmission fluid has suffered a breakdown in viscosity.Hydraulic fluid works simply because it is incompressible—it cannot be compressed. This is how power is efficiently transmitted within the equipment. “Air gaps” are formed if the hydraulic fluid can be compressed. This can occur if the wrong fluid is used, the fluid has contaminants, or the fluid is affected by an extreme operating temperature. These gaps cause equipment to work harder, resulting in low operating performance and possible malfunction.

Cavitation in Hydraulic Fluid

While viscosity and incompressibility affect hydraulic fluid performance, cavitation can also impact the function of hydraulic fluid. This occurs when a quick pressure change causes the formation of small vapor bubbles. These “cavities” generate high temperatures with the potential to damage equipment and reduce the effectiveness of the fluid. Cavitation can occur, for example, at the discharge side of a hydraulic pump.

Keeping It Safe with Hydraulic Fluid

Hydraulic fluid needs to be treated with caution. Exposure can occur by
  • Touching contaminated water or soil
  • Touching or ingesting the fluid
  • Breathing the air near a hydraulic machine
In the United States, according to the Centers for Disease Control (CDC), little is known about the effects of breathing in air with high levels of hydraulic fluid. Drinking large amounts of the liquid, says the CDC, can cause pneumonia, intestinal bleeding, and death. People who regularly handle these fluids have reported weakness and skin irritation of the hands.There have also been reports of weakness and skin irritation of the hands for people who handle these fluids regularly.Hydraulic fluid is dangerous in mist or vapor form or when heated to its flashpoint. The U.S. Occupational Safety and Health Administration (OSHA) reports that typical hydraulic fluid has a flashpoint of 300-600°F. The fluid and contaminated cloth material should be stored in sealed metal containers and disposed of properly.Also, OSHA and many state agencies mandate that Material Safety Data Sheets (MSDS) for most chemicals, including hydraulic fluid, should be made available to all workers who could come in contact with these substances. MSDS information is supplied by the manufacturers or distributors of these products.


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