Short Answer:

Fluids used in hydraulics must have special properties that help transmit power efficiently and safely. The main properties include high lubricating ability, proper viscosity, low compressibility, and chemical stability. These properties ensure smooth operation of hydraulic systems, reduce wear and tear, and prevent damage due to friction or heat.

Hydraulic fluids should also resist oxidation, corrosion, and foaming to ensure long service life and reliability. They must be non-toxic, non-corrosive, and compatible with the materials of the hydraulic components. The correct fluid properties allow the system to work efficiently under varying pressure and temperature conditions.

Detailed Explanation:

Properties of Fluids Used in Hydraulics

Hydraulic systems work on the principle of transmitting power through pressurized fluid. This fluid acts as a medium to transfer energy from one point to another. The performance and life of any hydraulic system largely depend on the properties of the fluid used. A good hydraulic fluid not only transmits power but also lubricates moving parts, removes heat, and protects the system from corrosion and wear.

Below are the important properties that a hydraulic fluid must have for proper operation and efficiency.

1. Viscosity:
   Viscosity is the measure of a fluid’s resistance to flow. It is one of the most important properties of hydraulic fluids. The viscosity should neither be too high nor too low.

• If viscosity is too high, the fluid will resist flow, causing more friction and energy loss.
• If viscosity is too low, leakage may occur, and lubrication between moving parts will be poor.
  The correct viscosity ensures smooth movement, less friction, and proper sealing within the hydraulic system.

2. Lubricating Ability:
   Hydraulic fluids also act as lubricants to reduce friction between moving parts such as pistons, pumps, and valves. Good lubrication prevents wear, overheating, and damage of components. The fluid should form a thin film between parts to reduce metal-to-metal contact and enhance efficiency. Proper lubrication also increases the lifespan of the hydraulic components.
3. Low Compressibility:
   For efficient energy transmission, the fluid should be nearly incompressible. This means that when pressure is applied, the volume of the fluid should not change significantly. A fluid with low compressibility transmits force quickly and accurately without delay, making the system more responsive. Incompressible fluids are essential for achieving precise control in hydraulic systems.
4. Chemical Stability:
   The hydraulic fluid should remain chemically stable under varying operating conditions. It should not react with air, moisture, or metal parts. Chemical instability can cause sludge formation, oxidation, or acid generation, leading to system failure. A chemically stable fluid maintains its properties even under high temperature and pressure for long periods.
5. Corrosion and Rust Resistance:
   Hydraulic fluids should protect metal parts from corrosion and rust. The presence of moisture or oxygen can cause rust formation inside the system, reducing its efficiency. Therefore, additives are mixed with fluids to prevent oxidation and corrosion, ensuring longer life of the components.
6. Oxidation Resistance:
   When hydraulic fluids are exposed to oxygen at high temperatures, they can oxidize and form gum or sludge. This can block valves and filters, reducing system performance. A good hydraulic fluid should resist oxidation and maintain cleanliness and flow even after long use.
7. Thermal Stability:
   Hydraulic systems often operate under high temperatures. The fluid should not degrade or lose its properties when exposed to heat. Good thermal stability means that the fluid maintains its viscosity, lubrication, and chemical composition even when the temperature fluctuates. This prevents overheating and ensures steady performance.
8. Anti-Foaming Property:
   When air mixes with hydraulic fluid, foam can form. Foaming causes erratic motion and reduces system efficiency. It can also lead to air pockets, resulting in cavitation and damage to pumps. Fluids are often treated with anti-foaming additives to prevent the formation of bubbles and maintain smooth operation.
9. Compatibility with Materials:
   The fluid should be compatible with rubber seals, hoses, and metal components used in the hydraulic system. Incompatible fluids can cause swelling, hardening, or cracking of rubber parts, leading to leakage and failure. Therefore, fluids are specially formulated to match the materials used in specific systems.
10. Non-Corrosive and Non-Toxic:
    Hydraulic fluids should be safe to handle and should not corrode system components. Non-toxic and non-corrosive fluids reduce environmental hazards and ensure the safety of operators. In sensitive industries like aerospace or food processing, special non-toxic fluids are used for safety.
11. Fire Resistance:
    In industries where there is a risk of fire, such as steel plants or aircraft systems, fire-resistant hydraulic fluids are used. These fluids do not catch fire easily and are made to operate safely even at high temperatures. Common examples include water-glycol mixtures and phosphate ester fluids.
12. Density:
    Density affects how much pressure the fluid can generate and how much energy is needed to move it. A proper balance of density ensures efficient power transmission and smooth operation without overloading the system components.

Conclusion:

The performance, safety, and reliability of hydraulic systems depend largely on the properties of the fluid used. A good hydraulic fluid must have the right viscosity, high lubricating ability, low compressibility, and good chemical stability. It should resist oxidation, corrosion, foaming, and thermal breakdown. By maintaining these properties, hydraulic fluids ensure smooth, efficient, and long-lasting operation of the system. Selecting the correct hydraulic fluid is therefore one of the most critical steps in designing and maintaining any hydraulic system.