Hydraulic Fluid

Pascal’s Law and Your Machines: The Science Behind Hydraulic Fluids


If you ever wondered how a power steering system works, or how brake fluid can keep brakes functional, it all starts with a theory from 15th Century France.

French physicist Blaise Pascal contributed to the study of fluids, eventually recognizing how pressure points are transmitted throughout fluid matter. Pascal’s discovery, later termed “Pascal’s law,” influenced the study and application of hydraulics.

Liquid matter, existing with qualities of both solid and gas, provides unique capabilities for hydraulic use. Pascal’s law states that pressure applied to one point in a fluid is distributed, undiminished, throughout the rest of the fluid, hitting any surface at a right angle.

Essentially, you can’t compress liquids. And that incompressibility fuels many mechanical applications. Let’s look at how hydraulic fluid works and how it impacts performance in your systems.

Under Pressure: How Hydraulic Fluid Works

Pascal provided a theoretical basis for hydraulics, but the practice has existed for millennia. Ancient Egyptians and Greeks used water as a hydraulic fluid for irrigation and flood control. However, water has relatively low temperature control–heat it too much, and it evaporates. Cool it too much, and it freezes.

In the early 20th Century, scientists replaced water with mineral oil as a hydraulic base stock. Mineral oil holds a higher boiling point and contains inherent lubrication properties, which helps in higher-temperature applications. Most hydraulic fluid now stems from mineral oil bases, though some higher-performance fluids use chemically produced synthetic base oils.

Mineral-oil-based hydraulic fluid negates the deficiencies of water, as it can sustain itself in higher and cooler temperatures. But that’s not the only benefit. Specialized hydraulic fluid operates with these key properties:

  • It’s incompressible. Pascal’s law applies here. Like all other fluids, hydraulic fluid resists compression. Pressure applied in one area of the fluid is distributed evenly throughout, feeding the applicable system with the same amount of fluid power.
  • It’s viscous. Hydraulic systems often need specific thickness levels from their fluids. Too thin, and the fluid won’t transfer pressure. Too thick, and it might stall the system or increase friction. If a hydraulic system leaks, overheats, or breaks down, it’s likely because of poor or contaminated fluid.
  • It’s consistently viscous. The viscosity index (VI) measures changes in a fluid’s viscosity over temperature. Poor or contaminated hydraulic fluid may fluctuate viscosity under high or low temperatures. Ideally, we aim for a consistent viscosity level in high-pressure systems.
  • It’s lubricated. Friction can paralyze hydraulic systems, especially when delivered through poorly lubricated fluids. Premium hydraulic fluid reduces friction between moving parts, limiting wear and tear on the system.
  • It releases trapped air quickly. Even in high-performing hydraulic systems, air bubbles inevitably appear. This is called cavitation, and it can wreck a hydraulic system. Too many air pockets will limit pressure transfer, so they need to be released. Premium hydraulic fluid can quickly release these air pockets and maintain optimum fluid pressure.
  • It resists corrosion. If hydraulic fluid combines with water, it can corrode metal components. Rust eventually forms from the corroded elements. Premium hydraulic fluids balance the risks of emulsification with achieving peak performance.

How Hydraulic Fluid Impacts Your Machines

Hydraulic fluids carry specific properties for high-powered, high-pressured systems. But whether you’re operating an excavator, installing an automatic transmission system, or designing an aircraft hanger door, the performance impacts are the same: better power transmission, better temperature management, reduced wear and tear, and improved air resistance.

Premium hydraulic fluids perform well in all of these areas:

  • Premium hydraulic fluids improve power transmission. We’re back to Pascal’s law again. Because hydraulic fluids are incompressible and consistently viscous, they should efficiently distribute pressure and power. Poor or contaminated hydraulic fluids may lack compressibility, which could cause servo failure, efficiency loss, and cavitation.
  • Premium hydraulic fluids prevent overheating. This is called thermal stability, or the fluid’s ability to withstand high temperatures. Poor or contaminated fluids break down at high temperatures, eventually turning into sludge and clogging hydraulic systems.
  • Premium hydraulic fluids help reduce wear and tear. In many hydraulic systems, water and moisture can severely damage metal components. Poor or contaminated fluids raise the risk of emulsification and rust.
  • Premium hydraulic fluids limit pressure deficiencies. Too many air bubbles can reduce pressure transmission. Poor or contaminated fluids can’t disperse these bubbles, which limits machine performance.

Centuries after Pascal discovered how fluids operate under pressure, we now apply hydraulics for countless machinery uses. But the best machinery can still fail with the wrong type of fluid. That's why selecting the right one is crucial for maximizing performance and lifespan. Connect with a Keller-Heartt Associate if you have questions about selecting hydraulic fluids per your application. It is our priority to keep your machines running as smoothly and efficiently as possible. 


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