950G Series II Wheel Loader Caterpillar




Introduction to Hydraulic Fluids and Fluid Conditioners

The selection and care of the hydraulic fluid has an important effect on the life of the system. Just like the hardware components of a hydraulic system, the hydraulic fluid must be selected on the basis of the hydraulic systems characteristics and properties in order to accomplish the designed task.

In hydraulic systems, a filter is used to keep the fluid clean, in order to prevent system damages. In hydraulic systems, a cooler is used to keep the fluid cool, in order to prevent system damages.

Functions of Hydraulic fluids

Fluids are virtually incompressible. Therefore, fluids can transmit power instantaneously in a hydraulic system. For example, petroleum oil compresses approximately 1% for every 13800 kPa (2001 psi). Therefore, petroleum oil can maintain a constant volume under high pressure. Petroleum oil is the primary fluid that is used in developing most hydraulic oils.

The following items are the primary functions of hydraulic fluids:

  • Power transmission

  • Lubrication

  • Sealing

  • Cooling

Power Transmission

Because hydraulic fluids are virtually incompressible, once the hydraulic system is filled with fluid it can instantly transmit power from one area to another. This does not mean that all hydraulic fluids are equal and will be able to transmit power with the same efficiency. Choosing the correct hydraulic fluid depends on the application and the operating conditions.


Hydraulic fluid must lubricate the moving parts of the hydraulic system. The rotating components or sliding components must be able to function without touching other surfaces. The hydraulic fluid must maintain a thin film between the two surfaces in order to prevent friction, heat, and wear.


Many hydraulic components are designed to use hydraulic fluid instead of mechanical seals within the component. The viscosity of the fluid helps to determine the ability to function as a seal.


When a hydraulic system develops heat, mechanical energy is transfered to hydraulic energy. Then the hydraulic system transfers hydraulic energy back to mechanical energy. As the fluid moves throughout the system, heat flows from the warmer components to the cooler fluid. The fluid gives up the heat to the reservoir or to the coolers that are designed to maintain fluid temperatures within design limits.

The following items are other properties that are expected of the hydraulic fluids:

  • Prevention of rust and corrosion on metal parts

  • The resistance to foaming and oxidation

  • The ability to separate air

  • The ability to separate water

  • The ability to separate other contaminates from the fluid

  • The ability to maintain stability over a wide range of temperatures

Illustration 1g01061729

Controlling Hydraulic Oil Temperature

As hydraulic components work, heat builds up in the oil. Many lower pressure hydraulic systems are able to dissipate the heat through lines, through cylinders, through tanks, and other component surfaces in order to control the oil temperature. Most high pressure hydraulic systems require an oil cooler, in addition to the other components, in order to control the oil temperature.

There are two types of hydraulic oil coolers that are used on mobile equipment:

  1. Air to oil (shown on the left) where the oil passes through tubes covered with fins. A fan or a machine speed will blow air over the tubes and the fins in order to cool the oil.

  1. Water to oil (shown on the right) is where the oil passes through a bundle of tubes. Engine cooling water will pass around the tubes in order to cool the oil.

Hydraulic oil temperature must normally be maintained below 100 °C (212 °F) in order to prevent damage to components. Oil above this temperature causes seals to deteriorate. The oil also becomes thin enough to allow metal to metal contact between moving components in the system.


Viscosity is the measurement of a fluid's resistance to flow at a specific temperature. A fluid which flows easily has a low viscosity. A fluid which does not flow easily has a high viscosity.

A fluid's viscosity is affected by temperature. When a fluid becomes warmer, the fluid's viscosity becomes lower. Likewise, when a fluid cools, the viscosity increases. Vegetable oil is a very good example of how viscosity changes with a change in temperature. When vegetable oil is very cold, vegetable oil thickens and is very slow to pour. As vegetable oil is heated, vegetable oil becomes thinner and pour's more readily.

Saybolt Viscosimeter

Illustration 2g01061723

The most common tool of measuring viscosity is the Saybolt Viscosimeter as shown in illustration 2. The Saybolt Viscosimeter was invented by and named after George Saybolt.

The Saybolt Viscosimeter unit of measurement is the Saybolt Universal Second (SUS). In the original viscosimeter a container of fluid was heated to a specific temperature. When the temperature was reached, a stopcock (orifice) was opened and the fluid flowed out of the container and into a 60 mL. flask. A stopwatch was used to measure the time it took to fill the flask. The viscosity was recorded as the number of seconds the flask took to fill at a given temperature. If a fluid, when heated to a temperature of 24 °C (75 °F), took 115 seconds to fill the flask, the viscosity is 115 SUS @ 24 °C (75 °F). If the same fluid was heated to 38 °C (100 °F) and took 90 seconds to fill the flask, it's viscosity would be 90 SUS @ 38 °C (100 °F)100°F.

Viscosity Index

Viscosity Index (VI) is a measure of a fluid's change in thickness with respect to changes in temperature. If a fluid's consistency remains the same when the temperatures vary, the fluid has a high VI. If a fluid becomes thick at low temperatures and very thin at high temperatures,the fluid has a low VI. In most hydraulic systems, fluids with a high VI are acceptable over fluids with a low VI.

Petroleum Oil

All petroleum oils will become thin when the temperature rises and the oil will thicken as the temperature decreases. If the viscosity is too low, there may be excessive leakage past seals and from joints. If the viscosity is too high, sluggish operation may be the result and extra power is needed to push the oil through the system. Viscosity of petroleum oil is expressed by the Society of Automotive Engineers (SAE) numbers: 5W,10W,20W, 30W,40W,etc. The lower the number, the better the oil will flow at a low temperature. The higher the number, the more viscous the oil and the more suited to high temperatures.

Synthetic Oils

Synthetic oils are formed by processes which chemically react with materials of a specific composition in order to produce a compound with planned and predictable properties. Synthetic oils are specifically blended for extreme service at a high temperature and a low temperature.

Fire Resistant Fluids

There are three basic types of fire resistant fluids: water/glycol, water-oil emulsions and synthetics.

Water/glycol fluids contains 35% to 50% water (water inhibits burning), glycol (synthetic chemical similar to some anti-freeze), and a water thickener. Additives are added in order to improve lubrication. Additives are added in order to prevent rust, corrosion, and foaming. Water/glycol fluids are heavier than oil and may cause pump cavitation at high speeds. These fluids may react with certain metals and seals that cannot be used with some types of paints.

Water-oil emulsion are the least expensive of the fire resistant fluids. A similar amount (40%) of water is used, as in water/glycol fluids, to inhibit burning. Water-oil can be used in typical hydraulic oil systems. Additives may be added in order to prevent rust and foaming.

There are certain conditions that require synthetic fluids to be used in order to meet specific requirements. The fire resistive synthetic fluids are less flammable than oil and are more suitable for use in areas of high pressure and high temperature.

When fire resistant fluids react to polyurethane seals you may be required to use special seals with these fluids.

Oil Life

The hydraulic oil never wears out. The use of filters that are used to remove solid particles and some chemicals add to the useful life of the oil. However, eventually the oil will become so contaminated, that it will have to be replaced. In construction machines, the oil is replaced at regular time intervals.

The contaminates in the oil may also be used as indicators of high wear and prospective problem areas. One program that will use oil contaminates as a source of information is the Caterpillar Schedule Oil Sampling Program(SOS).

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