HYDRAULIC FLUIDS:Density and Compressibility

Density and Compressibility

A fluid with a specific gravity of less than 1.0 is desired when weight is critical, although with proper system design, a fluid with a specific gravity greater than l can be tolerated. Where avoidance of detection by military units is desired, a fluid that sinks rather than rises to the surface of the water is desirable. Fluids having a specific gravity greater than 1.0 are desired, as leaking fluid will sink, allowing the vessel with the leak to remain undetected.

Under extreme pressure a fluid may be compressed up to 7 percent of its original vol­ ume. Highly compressible fluids produce sluggish system operation. This does not present a serious problem in small, low-speed operations, but it must be considered in the operating instructions.

Foaming Tendencies

Foam is an emulsion of gas bubbles in the fluid. In a hydraulic system, foam results from compressed gases in the hydraulic fluid. A fluid under high pressure can contain a large volume of air bubbles. When this fluid is depressurized, as when it reaches the reservoir, the gas bubbles in the fluid expand and produce foam. Any amount of foam­ ing may cause pump cavitation and produce poor system response and sponge con­ trol. Therefore, defoaming agents are often added to fluids to prevent foaming. Minimizing air in fluid systems is discussed later in this chapter.


Cleanliness in hydraulic systems has received considerable attention recently. Some hydraulic systems, such as aerospace hydraulic systems, are extremely sensitive to contamination. Fluid cleanliness is of primary importance because contaminants can cause component malfunction, prevent proper valve seating, cause wear in compo­ nents, and may increase the response time of servo valves. Fluid contaminants are dis­ cussed later in this chapter.

The inside of a hydraulic system can only be kept as clean as the fluid added to it. Ini­ tial fluid cleanliness can be achieved by observing stringent cleanliness requirements (discussed later in this chapter) or by filtering all fluid added to the system.


There have been many liquids tested for use in hydraulic systems. Currently, liquids being used include mineral oil, water, phosphate ester, water-based ethylene glycol compounds, and silicone fluids. The three most common types of hydraulic liquids are petroleum-based, synthetic fire-resistant, and water-based fire-resistant.

Petroleum-Based Fluids

The most common hydraulic fluids used in shipboard systems are the petroleum­ based oils. These fluids contain additives to protect the fluid from oxidation (antioxi­ dant), to protect system metals from corrosion (anti-corrosion), to reduce tendency of the fluid to foam (foam suppressant), and to improve viscosity.

Petroleum-based fluids are used in surface ships’ electro-hydraulic steering and deck machinery systems, submarines’ hydraulic systems, and aircraft automatic pilots, shock absorbers, brakes, control mechanisms, and other hydraulic systems using seal materials compatible with petroleum-based fluids.

Synthetic Fire-Resistant Fluids

Petroleum-based oils contain most of the desired properties of a hydraulic liquid. However, they are flammable under normal conditions and can become explosive when subjected to high pressures and a source of flame or high temperatures. Nonflammable synthetic liquids have been developed for use in hydraulic systems where fire hazards exist.

Phosphate Ester Fluid

As a maintenance person, operator, supervisor, or manager, you must understand the hazards associated with hydraulic fluids to which you may be exposed. This type of fluid contains a controlled amount of neurotoxic material. Because of the neurotoxic effects that can result from ingestion, skin absorption, or inhalation of these fluids, be sure to use the following precautions:

1. Avoid contact with the fluids by wearing protective clothing.

2. Use chemical goggles or face shields to protect your eyes.

3. If you are expected to work in an atmosphere containing a fine mist or spray, wear a continuous-flow airline respirator.

4. Thoroughly clean skin areas contaminated by this fluid with soap and


5. If you get any fluid in your eyes, flush them with running water for at least 15 minutes and seek medical attention.

If you come in contact with this type of fluid, report the contact when you seek medical aid and whenever you have a routine medical examination.

Silicone Synthetic Fire-Resistant Fluids

Silicone synthetic fire-resistant fluids are frequently used for hydraulic systems that require fire resistance, but that have only marginal requirements for other chemical or physical properties common to hydraulic fluids. Silicone fluids do not provide the corrosion protection and lubrication of phosphate ester fluids, but they also lack those fluids’ detrimental characteristics, and they are excellent for fire protection.

Water-Based Fire-Resistant Fluids

The most widely used water-based hydraulic fluids may be classified as water-glycol mixtures and water-synthetic base mixtures. The water-glycol mixture contains addi­ tives to protect it from oxidation, corrosion, and biological growth and to enhance its load-carrying capacity.

Fire resistance of the water-mixture fluids depends on the vaporization and smother­ing effect of steam generated from the water. The water in water-based fluids is con­ stantly being driven off while the system is operating. Therefore, frequent checks to maintain the correct ratio of water are important.


Hydraulic fluid contamination may be described as any foreign material or substance whose presence in the fluid is capable of adversely affecting system performance or reliability. It may assume many different forms, including liquids, gases, and solid matter of various composition, sizes, and shapes. Solid matter is the type most often found in hydraulic systems and is generally referred to as particulate contamination. Contamination is always present to some degree, even in new, unused fluid, but must be kept below a level that will adversely affect system operation. Hydraulic contami­ nation control consists of requirements, techniques, and practices necessary to mini­ mize and control fluid contamination.

Classification of Contaminants

There are many types of contaminants, which are harmful to hydraulic systems and liquids. These contaminants may be divided into two different classes-particulate and fluid.

Particulate Contamination

This class of contaminants includes organic, metallic solid, and inorganic solid con­ taminants. These contaminants are discussed in the following paragraphs.

Organic. Wear, oxidation, or polymerization produces organic solids or semisolids found in hydraulic systems. Minute particles of 0-rings, seals, gaskets, and hoses are present, due to wear or chemical reactions. Synthetic products, such as neoprene, sili­ cones, and hypalon, though resistant to chemical reaction with hydraulic fluids, pro­ duce small wear particles. Oxidation of hydraulic fluids increases with pressure and temperature, although antioxidants are blended into hydraulic fluids to minimize such oxidation. The ability of a hydraulic fluid to resist oxidation or polymerization in ser­ vice is defined as its oxidation stability. Oxidation products appear as organic acids, asphaltics, gums, and varnishes. These products combine with particles in the hydrau­ lic fluid to form sludge. Some oxidation products are oil soluble and cause the hydraulic fluid to increase in viscosity; other oxidation products are not oil soluble and form sediment.

Metallic solids. Metallic contaminants are almost always present in a hydraulic sys­ tem and will range in size from microscopic particles to particles readily visible to the naked eye. These particles are the result of wearing and scoring of bare metal parts and plating materials, such as silver and chromium. Although practically all metals commonly used for parts fabrication and plating may be found in hydraulic fluids, the major metallic materials found are ferrous, aluminum, and chromium particles. Because of their continuous high-speed internal movement, hydraulic pumps usually contribute most of the metallic particulate contamination present in hydraulic sys­ tems. Metal particles are also produced by other hydraulic system components, such as valves and actuators, due to body wear and the chipping and wearing away of small pieces of metal plating materials.

Inorganic solids. This contaminant group includes dust, paint particles, dirt, and sili­ cates. Glass particles from glass bead peening and blasting may also be found as con­ taminants. Glass particles are very undesirable contaminants because of their abrasive effect on synthetic rubber seals and the very fine surfaces of critical moving parts. Atmospheric dust, dirt, paint particles, and other materials are often drawn into hydraulic systems from external sources. For example, the wet piston shaft of a hydraulic actuator may draw some of these foreign materials into the cylinder past the wiper and dynamic seals, and the contaminant materials are then dispersed in the hydraulic fluid. Contaminants may also enter the hydraulic fluid during maintenance when tubing, hoses, fittings, and components are disconnected or replaced. It is there­ fore important that all exposed fluid ports be sealed with approved protective closures to minimize such contamination.

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