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Hydraulic Fluid: Definition, Types, Principles, and Applications

What is hydraulic fluid?

Hydraulic fluid is a specialized liquid used in hydraulic systems to transmit power, lubricate moving parts, and dissipate heat. It operates by following Pascal’s Law, where pressure applied to the fluid is distributed equally, allowing controlled movement and force generation in machinery. Hydraulic fluids are commonly composed of mineral oils, synthetic oils, or water-based fluids, depending on system needs. They are essential in various industries, such as construction, manufacturing, and aerospace, powering equipment like excavators, presses, and aircraft systems. Below are some hydraulic fluids that can used in aviation:

Aeroshell Fluid 31 (MIL-PRF-83282D)

Aeroshell Fluid 31

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AeroShell Fluid 41 (MIL-PRF-5606J)

Aeroshell Fluid 41

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Why use hydraulic fluid?

Hydraulic fluid is used to efficiently transmit power, lubricate components, and maintain temperature regulation in hydraulic systems. By transmitting force through pressurized fluid, hydraulic fluid enables smooth and controlled movement in machinery. It also reduces wear and tear on system components, enhances efficiency, and prevents overheating. This makes it essential in industries like construction, manufacturing, and aerospace, where heavy machinery and complex systems rely on precise, high-force operations.

Difference between hydraulic oil and hydraulic fluid

The terms hydraulic oil and hydraulic fluid are often used interchangeably, but there is a subtle difference:

  • Hydraulic fluid is a broad term that refers to any liquid used in hydraulic systems to transmit power, lubricate, and control temperature. It can include water-based fluids, mineral oils, or synthetic fluids.

  • Hydraulic oil specifically refers to a type of hydraulic fluid that is oil-based, typically made from mineral oil or synthetic oils. It is the most common type of hydraulic fluid, favored for its lubrication and anti-wear properties.

In essence, all hydraulic oils are hydraulic fluids, but not all hydraulic fluids are hydraulic oils.

Types of Hydraulic Fluids

1. Mineral-Based Hydraulic Oil

This is the most common type, made from refined petroleum oils. It offers good lubricating properties, anti-wear characteristics, and temperature stability. It’s widely used in industrial machinery and construction equipment.

2. Synthetic-Based Hydraulic Oil

Made from synthetic compounds, these fluids provide better performance in extreme temperatures and pressures. They offer superior lubrication, longer service life, and greater stability compared to mineral oils, making them ideal for high-performance applications.

3. Fire-Resistant Hydraulic Oil

These fluids are designed to resist ignition in high-heat environments, such as steel mills or mining operations. They often contain water or phosphate esters to reduce the risk of fire while maintaining hydraulic performance.

4. Rapidly Biodegradable Hydraulic Oil

These fluids are formulated to break down more quickly in the environment, minimizing ecological impact in case of leaks. They are commonly used in industries where spills may occur in sensitive environments, like forestry or agriculture.

5. Food-Grade Hydraulic Oils

Designed for use in food processing and packaging equipment, these hydraulic fluids comply with strict safety regulations and are non-toxic. They must meet FDA and NSF requirements, ensuring they are safe in food-related environments.

6. Universal Mobile Hydraulic Oils (STOU & UTTO)

These oils are versatile fluids used in agricultural and construction machinery, often labeled as Super Tractor Oil Universal (STOU) or Universal Tractor Transmission Oil (UTTO). They provide lubrication for engines, transmissions, and hydraulic systems in one fluid.

7.Aircraft Hydraulic Oils

These oils are specially formulated for use in aircraft systems, where reliability and performance are critical. They must function well under extreme pressures and temperatures, ensuring smooth operation of flight control systems, landing gear, and more.

 

What is Hydraulic Fluid in Aviation?

Hydraulics in aviation refer to the use of pressurized fluids to power various systems within an aircraft. These systems control critical components such as flight control surfaces (e.g., ailerons, rudders), landing gear, brakes, and flaps. Hydraulic systems provide the necessary force for these operations, enabling precise and reliable control of the aircraft.

Why do planes need hydraulic fluid?

Planes need hydraulic fluid to efficiently transmit power across the aircraft’s hydraulic systems. The fluid enables the movement of control surfaces, deployment of landing gear, and operation of other essential systems. Hydraulic fluid is favored for its ability to transmit large amounts of power with minimal weight, a crucial factor in aircraft design.

Types of hydraulic fluid used in aviation

There are primarily three types of hydraulic fluid used in aviation:

  1. Mineral-Based Fluids: These are typically used in older or general aviation aircraft. They offer good lubrication and corrosion protection.
  2. Phosphate Ester-Based Fluids: Commonly used in commercial and military aviation, these fluids are fire-resistant and function well in high-pressure environments.
  3. Synthetic Fluids: These fluids provide superior performance in extreme temperatures and are used in specialized systems, such as those in high-performance aircraft.

What is AeroShell Fluid 41 used for?

AeroShell Fluid 41 is a mineral hydraulic oil manufactured to a very high level of cleanliness, and possesses improved fluid properties. AeroShell Fluid 41 contains additives which provide good low temperature fluidity, anti-wear, oxidation-corrosion inhibition and shear stability. In addition metal de-activators and foam inhibitors are included in this high viscosity index fluid to enhance performance in hydraulic applications. AeroShell Fluid 41 is capable of wide temperature range operation.

Principles and Functions of Hydraulic Fluid

What is the principle of hydraulic fluid in aviation?

The principle of hydraulic fluid in aviation is based on Pascal’s Law, which states that pressure applied to a confined fluid is transmitted equally in all directions. In aviation, hydraulic fluid is pressurized in a closed system to transfer energy to different components such as flight controls, landing gear, and brakes. This enables precise and reliable operation of aircraft systems, with minimal weight and space requirements.

Functions of hydraulic fluid/oil

Hydraulic fluids in aviation serve several essential functions:

  1. Power Transmission: Hydraulic fluid transmits force to operate flight controls, landing gear, brakes, and other critical systems.
  2. Lubrication: It reduces friction and wear between moving parts within hydraulic systems, extending the life of components.
  3. Heat Dissipation: Hydraulic fluid absorbs heat generated during system operation and helps dissipate it to prevent overheating.
  4. Corrosion Protection: It prevents rust and corrosion by forming a protective layer on metal components within the hydraulic system.
  5. Sealing: Hydraulic fluid helps to seal gaps between components, maintaining system integrity and preventing leaks.

Additives in hydraulic fluids

Hydraulic fluids often contain various additives to enhance performance and longevity. Common additives include:

  1. Anti-Wear Agents: These additives reduce friction and wear between metal components, prolonging the lifespan of the hydraulic system.
  2. Corrosion Inhibitors: These prevent the formation of rust and protect the system from corrosion caused by moisture or contaminants.
  3. Anti-Foaming Agents: These additives prevent air bubbles from forming in the fluid, which could reduce system efficiency.
  4. Viscosity Modifiers: These help maintain consistent fluid viscosity across different temperatures, ensuring smooth operation in varying conditions.
  5. Oxidation Inhibitors: These additives prevent the breakdown of fluid caused by exposure to heat and air, improving fluid stability over time.

Hydraulic Systems & Pressure Considerations

What are the 2 basic types of hydraulic systems?

The two basic types of hydraulic systems are open-loop and closed-loop systems. Open-loop systems rely on the continuous flow of fluid from a reservoir and back, while closed-loop systems recirculate fluid, maintaining a constant flow within the system.

What is the psi for hydraulics?

The psi (pounds per square inch) for hydraulics typically ranges from 1,500 psi to 5,000 psi, depending on the system’s application and design. For heavy-duty systems, pressures can reach up to 10,000 psi.

What is psi in hydraulic?

  1. PSI (pounds per square inch) is a unit of pressure used to measure the force exerted by hydraulic fluid within a system. It indicates the intensity of pressure required to transmit power in hydraulic systems, influencing performance and efficiency.

     

Properties of Hydraulic Oil

Hydraulic oil viscosity

Viscosity is the measure of a hydraulic oil’s resistance to flow. It is a critical property that determines how well the oil flows under different temperatures. Hydraulic oils are categorized based on their viscosity grades, ensuring efficient pump operation and fluid movement. High viscosity oils offer better sealing and lubrication, while lower viscosity oils provide better flow at lower temperatures.

Hydraulic oil density

Density refers to the mass of hydraulic oil per unit volume. It impacts the oil’s ability to store energy, as well as its flow rate. Hydraulic oils typically have a density around 0.85 to 0.90 g/cm³ at 15°C. Density variations affect pump efficiency and system pressure, especially at high temperatures.

Most important properties of hydraulic oil

  1. Viscosity: Essential for smooth operation of pumps, valves, and actuators, and for maintaining optimal flow under varying temperatures.
  2. Lubrication ability: Protects system components from wear and reduces friction.
  3. Oxidation resistance: Prevents the oil from breaking down, ensuring longer fluid life and consistent performance.
  4. Thermal stability: Ensures that the oil maintains consistent performance across a wide temperature range.
  5. Water content: Water in hydraulic oil can cause corrosion, reduce lubricating properties, and affect system performance.
  6. Foam resistance: Reduces the formation of foam, which can compromise hydraulic system performance.

Benefits & Applications of Hydraulic Fluid

Uses and benefits of hydraulic oil

Hydraulic oil is essential for transmitting power, lubricating, and cooling within hydraulic systems. The key uses and benefits include

  1. Power transmission: Hydraulic oil efficiently transmits power to various components such as pumps, actuators, and motors in machines. This enables smooth operation of construction equipment, manufacturing machinery, and automotive systems.
  2. Lubrication: Hydraulic oil acts as a lubricant, reducing friction between moving parts. This minimizes wear and tear on system components, extending their lifespan.
  3. Heat dissipation: Hydraulic oil absorbs heat generated during system operation, preventing overheating and maintaining optimal performance.
  4. Corrosion prevention: The oil creates a protective film on system components, safeguarding them from rust and corrosion, especially in moist or humid environments.
  5. Contaminant removal: Hydraulic oil helps carry away contaminants such as dirt and metal particles, ensuring a cleaner system and preventing damage to components.
  6. System efficiency: The proper viscosity and stability of hydraulic oil ensure that power is transmitted efficiently, minimizing energy loss and maximizing system performance.
  7. Sealing properties: Hydraulic oil forms effective seals around pistons and other moving components, preventing leaks and enhancing system pressure.
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