Lubrication oils - Hydraulic oils

Hydraulic oils

Purpose of Hydraulic Fluids

a. Power transmission. The primary purpose of any hydraulic fluid is to transmit power mechanically throughout a hydraulic power system. To ensure stable operation ,the fluid must flow easily and must be incompressible.

b. Lubrication. Hydraulic fluids must provide the lubricating characteristics and qualities necessary

to protect all hydraulic system components against friction and wear, rust, oxidation, corrosion, and

demulsibility. These protective qualities are provided through the use of additives.

c. Sealing. Many hydraulic system components, such as control valves, operate with close tolerances where seals are not provided. In these applications hydraulic fluids must provide the seal between the low pressure and high-pressure side of valve ports. The amount of leakage will depend on the the tolerances between adjacent surfaces and the fluid viscosity.

d. Cooling. The circulating hydraulic fluid must be capable of removing heat generated throughout the system.

                Physical Characteristics

The physical characteristics of hydraulic fluids are similar to those of other  lubricating oils.

a. Viscosity. Viscosity is the most important characteristic of a hydraulic fluid and has a significant impact on the operation of a hydraulic system.

A higher viscosity results in higher friction, pressure drop, power consumption, higher heat generation .Which results in sluggish operation of valves and servos

Lower viscosity results in increased internal leakage under higher operating temperatures. The oil film may be insufficient to prevent excessive wear and a  possible seizure of moving parts, reduced  pump efficiency and sluggish operation.

b. Compressibility. Compressibility is a measure of the amount of volume reduction due to pressure.

Compressibility is sometimes expressed by the “bulk modulus,” which is the reciprocal of compressibility.

Petroleum fluids are relatively incompressible,. Compressibility increases with pressure and temperature and has significant effects on high-pressure fluid systems.

Problems associated include failure of servos in maintaining static rigidity and experience adverse effects in system amplification or gain, resulting in an overall  loss in efficiency.

Cavitation effects  as a result of compressibility can cause, metal fracture, corrosive fatigue, and stress corrosion on a long run

c. Stability. The stability of a hydraulic fluid is the most important property affecting service life.

The properties of a hydraulic fluid can be expected to change with time. Factors that influence the changes include: mechanical stress and cavitation, which can break down the viscosity improvers and cause reduced viscosity.

 Oxidation and hydrolysis cause chemical changes, formation of volatile components, insoluble materials, and corrosive products. The types of additives used in a fluid must be selected carefully to reduce the potential damage due to chemical breakdown at high temperatures.

                Quality Requirements for operation

The quality of a hydraulic fluid is an indication of the length of time that the fluid’s essential properties will continue to perform as expected, i.e., the fluid’s resistance to change with time.

 The selection and compatibility of additives is very important to minimize adverse chemical reactions that may destroy essential properties.

a. Oxidation stability. Oxidation, or the chemical union of oil and oxygen, is one of the primary causes for decreasing the stability of hydraulic fluids. Once the reactions begin, a catalytic effect takes place. The chemical reactions result in formation of acids that can increase the fluid viscosity and can cause corrosion. Polymerization and condensation produce insoluble gum, sludge, and varnish that cause sluggish operation, increase wear, reduce clearances, and plug lines and valves. Main accelerators for loss of oxidation stability are as follows:

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          Temperature. The rate of chemical reactions, including oxidation, approximately doubles for every 10 deg C  increase in temperature. The reaction may start at a local area where the temperature is high. However, once started, the oxidation reaction has a catalytic effect that causes the rate of oxidation to increase.

            Pressure. As the pressure increases, the fluid viscosity also increases, causing an increase in friction and heat generation. As the operating temperature increases, the rate of oxidation increases.

Pressure also  increases, the amount of entrained air and hence associated oxygen to accelerate reaction..

          Contaminants. Contaminants that accelerate the rate of oxidation may be dirt, moisture, joint compounds, insoluble oxidation products, or paints. A 1 percent sludge concentration in a hydraulic fluid is sufficient to cause the fluid to oxidize in half the time than it would take if no sludge were present. Therefore the contaminated fluid’s useful life is reduced by 50 percent.

          Water and metal. Certain metals, such as copper, are known to be catalysts for oxidation reactions, especially in the presence of water. Due to the production of acids during the initial stages of oxidation, the viscosity and neutralization numbers increase. The neutralization number for a fluid provides a measure of the amount of acid contained in a fluid.

 The most commonly accepted oxidation test for hydraulic fluids is the ASTM Method D 943 Oxidation Test. This test measures the neutralization number of oil as it is heated in the presence of pure oxygen, a metal catalyst, and water. Once started the test continues until the neutralization number reaches a value of 2.0. One series of tests provides an indication of how the neutralization number is affected by contaminants.

With no water or metal contaminants, the neutralization number reached 0.17 in 3500 hours.

When the test was repeated with copper contaminant, the neutralization number reached a value of 0.89 after 3000 hours.

The test was subsequently repeated with copper and water contamination and the neutralization number reached 11.2 in approximately 150 hours.

      Agitation. To reduce the potential for oxidation, oxidation inhibitors are added to the base hydraulic fluid. Two types of inhibitors are generally used: chain breakers and metal deactivators. Chain breaker inhibitors interrupt the oxidation reaction immediately after the reaction is initiated. Metal deactivators reduce the effects of metal catalysts.

b. Rust and corrosion prevention. Rust is a chemical reaction between water and ferrous metals.

Corrosion is a chemical reaction between chemicals (usually acids) and metals. Water condensed from

entrained air in a hydraulic system causes rust if the metal surfaces are not properly protected. In some

cases water reacts with chemicals in a hydraulic fluid to produce acids that cause corrosion. The acids

attack and remove particles from metal surfaces allowing the affected surfaces to leak, and in some cases to

seize. To prevent rust, hydraulic fluids use rust inhibitors that deposit a protective film on metal surfaces.The film is virtually impervious to water and completely prevents rust once the film is established throughout the hydraulic system.

Rust inhibitors are tested according to the ASTM D 665 Rusting Test.

This test subjects a steel rod to a mixture of oil and salt water that has been heated to 60 degC  , If the rod shows no sign of rust after 24 hours the fluid is considered satisfactory with respect to rust inhibiting properties. In addition to rust inhibitors, additives must be used to prevent corrosion. These

additives must exhibit excellent hydrolytic stability in the presence of water to prevent fluid breakdown and the acid formation that causes corrosion.

c. Air entrainment and foaming. Air enters a hydraulic system through the reservoir or through air leaks within the hydraulic system. Air entering through the reservoir contributes to surface foaming on the  oil. Good reservoir design and use of foam inhibitors usually eliminate surface foaming.

(1) Air entrainment is a dispersion of very small air bubbles in a hydraulic fluid. Oil under low pressure absorbs approximately 10 percent air by volume. Under high pressure, the percentage is even greater. When the fluid is depressurized, the air produces foam as it is released from solution. Foam and

Higher air entrainment in a hydraulic fluid cause erratic operation of servos and contribute to pump cavitation.

 Oil oxidation is another problem caused by air entrainment. As a fluid is pressurized, the entrained air is compressed and increases in temperature. This increased air temperature can be high enough to scorch the surrounding oil and cause oxidation. The amount of foaming in a fluid depends upon the viscosity of the fluid, the source of the crude oil, the refinement process, and usage. Foam depressants are commonly added to hydraulic fluid to expedite foam breakup and release of dissolved air.

However, it is important to note that foam depressants do not prevent foaming or inhibit air from dissolving in the fluid. In fact, some antifoam ants, when used in high concentrations to break up foam, actually retard the release of dissolved air from the fluid.

d. Demulsibility or water separation. Water that enters a hydraulic system can emulsify and promote the collection of dust, grit, and dirt, and this can adversely affect the operation of valves, servos,

and pumps, increase wear and corrosion, promote fluid oxidation, deplete additives, and plug filters.

Highly refined mineral oils permit water to separate or demulsify readily. However, some additives such as antirust treatments actually promote emulsion formation to prevent separated water from settling and

breaking through the antirust film.

e. Antiwear properties.

(1) Conventional hydraulic fluids are satisfactory for low-pressure and low-speed applications.

However, hydraulic fluids for high-pressure (over 6900 kPa) and high-speed (over 1200 rpm) applications that use vane or gear pumps must contain antiwear additives. These applications do not permit the formation of full fluid film lubrication to protect contacting surfaces--a condition known as boundary lubrication. Boundary lubrication occurs when the fluid viscosity is insufficient to prevent surface contact. Antiwear additives provide a protective film at the contact surfaces to minimize wear. Use of a hydraulic fluid without the proper antiwear additives will cause premature wear of the pumps and cause inadequate system pressure. Eventually the pumps and components will be damaged.

Antiwear properties of a hydraulic fluid are performed in accordance withASTM D 2882.

This test procedure is generally conducted with a variety of high-speed, high-pressure pump models manufactured by Vickers or Denison. Throughout the tests, the pumps are operated for a specified period. At the end of each period the pumps are disassembled and specified components are weighed. The weight of each component is compared to its initial weight; the difference reflects the amount of wear experienced by the pumps for the operating period. The components are also inspected for visual signs of wear and stress.