List common hydraulic fluids and key selection criteria for each.

List common hydraulic fluids and key selection criteria for each.

• A. Mineral oil, white oils, silicone oil, and kerosene; select based on color.
• B. Mineral oil, phosphate ester (FR), polyalkylene glycol esters (PAG), and water-glycol; select based on viscosity, fire resistance, compatibility with seals, temperature range, environmental impact.
• C. Only water-based fluids are used.
• D. Air is a recommended hydraulic fluid.

Answer: (B)

In hydraulic systems, the fluid choice drives lubrication, heat transfer, seal life, safety, and environmental impact, so you select fluids based on their properties and how they match the application. Common hydraulic fluids include mineral oil–based fluids, phosphate ester (FR) fluids, polyalkylene glycol esters (PAGs), and water-glycol blends. Mineral oil–based fluids are the general-purpose workhorse: they offer good lubricity and a wide range of viscosities at a relatively low cost, but they can be flammable and less favorable for fire-safety or environmental concerns. Phosphate ester fluids are designed for fire resistance and high-temperature stability, making them attractive in high-risk or high-heat environments, though they can be harsher on some elastomer seals and are pricier. PAGs are synthetic fluids with excellent lubricity and temperature performance, but they require compatible seals and system materials because they can swell or degrade certain elastomers. Water-glycol blends provide inherent fire resistance and good cooling, but they typically have lower lubricating properties and may require corrosion inhibitors and materials chosen to resist water-based fluids. The best answer highlights the types of common fluids and the key criteria used to pick among them: viscosity (and how it changes with temperature), fire resistance, compatibility with seals and materials, operating temperature range, and environmental impact. Colors or other ad-hoc criteria aren’t reliable guides, and air isn’t used as a hydraulic fluid in fluid-power systems.

In hydraulic systems, the fluid choice drives lubrication, heat transfer, seal life, safety, and environmental impact, so you select fluids based on their properties and how they match the application. Common hydraulic fluids include mineral oil–based fluids, phosphate ester (FR) fluids, polyalkylene glycol esters (PAGs), and water-glycol blends. Mineral oil–based fluids are the general-purpose workhorse: they offer good lubricity and a wide range of viscosities at a relatively low cost, but they can be flammable and less favorable for fire-safety or environmental concerns. Phosphate ester fluids are designed for fire resistance and high-temperature stability, making them attractive in high-risk or high-heat environments, though they can be harsher on some elastomer seals and are pricier. PAGs are synthetic fluids with excellent lubricity and temperature performance, but they require compatible seals and system materials because they can swell or degrade certain elastomers. Water-glycol blends provide inherent fire resistance and good cooling, but they typically have lower lubricating properties and may require corrosion inhibitors and materials chosen to resist water-based fluids.

The best answer highlights the types of common fluids and the key criteria used to pick among them: viscosity (and how it changes with temperature), fire resistance, compatibility with seals and materials, operating temperature range, and environmental impact. Colors or other ad-hoc criteria aren’t reliable guides, and air isn’t used as a hydraulic fluid in fluid-power systems.