Hydraulic oils and transmission fluids transmit power in hydraulic equipment and are used in power transmission applications. They are incompressible fluids used as the power transmitting media in hydraulic systems. Hydraulic power systems commonly involve a pump (as a power source), a series of tubes or elastomeric hoses for transmitting pressurized fluid, and some type of control (typically a series of valves, actuators, or cylinders). As the power transmission media, hydraulic fluids are indispensable in these systems.

Hydraulic oils and fluids are used to provide large amounts of power using relatively small tubes and hoses. Typical hydraulic applications include excavator booms, dippers, and buckets; hydraulic brakes; power steering systems; mechanical transmission systems; lifts; and general industrial machinery.

A basic hydraulic system. This system uses a pump (C) to move fluid from the reservoir (A) in order to provide linear motion via a cylinder (H).

Fluid Characteristics

Hydraulic fluids are typically classified by their function within a system; a single fluid can fulfil more than one of these functions. Five broad function types include:

• Transference of hydraulic energy
• Lubrication of system pumps, valves, and cylinders
• Avoiding corrosion
• Removing impurities and abrasive elements
• Dissipating heat

Regardless of primary function, all hydraulic fluids share certain properties which render them suitable for use in hydraulic systems.

• Lubricity: The fluid must lubricate each contacted surface or component, especially those which are dynamic or have tight tolerances. Viscosity is a related characteristic which is discussed in detail below.
• Chemically and physically stable: The fluid must retain its characteristics throughout large pressure fluctuations as well as during long-term storage. Low volatility and non-toxic makeup are also desirable properties.
• Temperature: The oil or fluid must dissipate heat buildup caused by pressure drops, friction, and leakages. In outdoor systems, oils must remain stable in low-temperature environments.
• Low foaming properties: The fluid must be able to release gases without foaming. Foaming causes loss of fluid and increases system temperature.
• Fire and flash resistance: In critical applications or explosive environments, fluids must have relatively high flash points. Oils intended for these applications should include a non-petroleum makeup or contain large amounts of water.
• Other physical characteristics: Low coefficient of expansion and low specific gravity to reduce system volume and weight, respectively.

Viscosity

While density, specific gravity, and other physical properties are important in hydraulic oil composition and use, viscosity is key to design and system compatibility. Simply defined, viscosity is a fluid's resistance to flow. Fluids with high viscosity, such as molasses, flow slowly, while low-viscosity materials such as gases flow freely. Water is often regarded as a medium-viscosity fluid.

Analysis of an oil's viscosity is key to predicting its behavior when moving through a hydraulic system. Oils which are too thin (low viscosity) will not seal properly, leading to pressure loss, leakage, and accelerated component wear. Fluids which are too thick (high viscosity) reduce the system's efficiency due to excessive strain on pumps and valves.

Viscosity can be measured as absolute or kinematic. Absolute (or dynamic) viscosity is the force required to move one horizontal plane with respect to another at unit velocity, while both planes are maintained a unit distance apart by the fluid. Absolute viscosity is defined by Newton's Law of Friction, described by the following differential equation:

where:

τ = shearing stress (force divided by area; or F/A)

μ = dynamic viscosity

du/dy = local shear velocity

Once absolute viscosity is defined, this value can be used to find the related value of kinematic viscosity, or the ratio of absolute viscosity to fluid density.

where:

ν = kinematic viscosity

μ = absolute viscosity

ρ = fluid density

Kinematic viscosity is typically expressed in m²/s, a measure often referred to as stokes (St). Because this unit is too large to practically express typical oil viscosity, centistokes (cSt) are usually employed.

Kinematic viscosity of oil decreases as temperature increases. This attribute must be considered when selecting a fluid for a specific system environment.

Viscosity Classifications

ISO 3448, a common international standard for viscosity measurement, defines a number of classes for viscous industrial fluids. The standard specifies viscosity measurement at 40° C (a typical reference temperature in industrial machinery).

Several viscosity classes are listed below. ISO 3448 defines an acceptable midpoint and allows for variation of ±10%; this rule was used to calculate the minimum and maximum values for each class. Values are provided in centistokes.

Oils manufactured for more extreme, outdoor conditions typically have higher VG class numbers. For example, VG 10 fluids are most appropriate for indoor control systems, while VG 32 is suitable for temperate outdoor use and is common in automotive applications. VG 150 and above is reserved for extremely high temperatures.

ASTM D2422 provides a similar classification system.

Hydraulic Fluid Composition

The table below lists most of the common, standardized hydraulic oils. Nearly 85% of manufactured oils are based on mineral oil. The remaining 15% of fluids are based on environmentally acceptable bio-bases or fire-resistant (sometimes aqueous) solutions.

Transmission and Final Drive Fluids

Transmission and final drive fluids are related to but distinct from hydraulic fluids. They may technically be used in dedicated hydraulic systems but are typically designed for compatibility with seals and other components within transmissions.

Transmission Fluids

Transmission fluids are used to operate and lubricate transmission components. Transmissions may be designed as manual or automatic, and the fluids involved are drastically different for each system type.

Most automatic transmission systems use automatic transmission fluid (ATF) as a hydraulic fluid, as well as a gear lubricant. Automatic transmission fluids may be pressurized by a pump and regulated by valves in a similar manner to other hydraulic systems. ATFs must be precisely designed to accommodate temperature extremes, exacting lubrication qualities, and compatibility with all transmission components. For the latter reason, major manufacturers design specific fluids to be used with their vehicle transmissions. The stability, precision, and long lifetime of ATFs is achieved by using multiple additives—including dispersants, antioxidants, and friction modifiers—in their design.

Manual transmissions use proprietary gear lubricants, which can range from straight mineral oil to synthetic fluids with multiple additives. Gear oils are typically less viscous than ATFs and operate at lower temperatures. The American Petroleum Institute (API) classifies gear oils using six service designations which are shown at right.

Final Drive Fluids

Final drive fluids are used to lubricate the last set of transmission components, which includes the final drive and differential. Many final drive fluids are specialty, silicone-based gear oils which feature superior heat dissipation to accommodate high temperatures.

References

Engineering ABC - Viscosity Classifications and Calculators

LubeWhiz - Hydraulic Fluids

Image Credit:

Dow Corning - Molykote | MechGuru | Torco | Hydraulic Valve