Temperature Stability of Lubricants and Hydraulic Fluids

E. C. Fitch, Tribolics, Inc.
Tags: contamination control, hydraulics, industrial lubricants

Fluid temperature stability is essential to the success of mechanical systems. All hydraulic and lubricating fluids have practical limits on the acceptable operating temperature range - both high and low levels. The machine loses stability and experiences conditional failure whenever the system’s fluid temperature violates these limits. If left unabated, the conditional failure ultimately results in both material and performance degradation of machine components.

Temperature extremes have a pronounced effect on component materials as well as machine performance. When temperature is too low, fluid viscosity is high. At low temperatures, the fluid often reaches the point where it actually congeals and will no longer flow (pour point). High temperature also accelerates wear, destroys hydrodynamic lubrication regimes, increases the oxidation rate, fosters additive depletion and affects other critical aspects of the machine.

Fluid temperature instability is the result of various machine operating factors such as component integrity (design, selection, manufacture, application and maintenance), duty cycle severity (load application, magnitude and duration), environmental hostility and heat absorption/desorption. Operation and maintenance personnel should thoroughly investigate an occurrence of temperature instability to understand the effects on machine operation in order to optimize its performance and prolong equipment service life.

Low-Temperature Effects
Low temperature can damage the temperature stability of a hydraulic fluid or lubricant just as much as high temperature. Very low fluid temperatures usually result from exposure of some system part to the external environment, particularly when operation takes place in arctic or high-altitude conditions. Such low temperatures can cause petroleum-based fluids to increase in viscosity and eventually reach the critical point where the fluid actually congeals and will no longer pour or flow. Such fluid immobility can starve a pump, cause damaging vaporous cavitation and produce high fluid and mechanical friction, not to mention lubricant starvation of bearing surfaces. Certainly, the usefulness of a fluid as a lubrication medium at low temperature hinges upon its viscosity and pour-point characteristics.

For hydraulic circulating systems, high oil viscosity causes a drastic drop in the oil’s static pressure as suction draws the oil into the pump’s inlet. This pressure reduction results in the creation of vaporous bubbles and causes air normally dissolved in the oil to be desorbed and become entrained as air bubbles. When the pump compresses this bubbly oil, the bubbles violently implode on the high-pressure side, creating loud noises, strong vibrations and wear of internal pump parts. Under these high-viscosity conditions, other system problems arise, such as filters that go into bypass, and on occasion, even collapse.

High-Temperature Effects
As industry continues to design systems of higher power density, fluid temperatures well above the current norms will become increasingly common. Such high-temperature conditions can disrupt the stability of conventional working fluids, compromise system performance and significantly reduce the life of operating components. In many systems exposed to hostile environments and severe duty cycles, the need for supplemental heat transfer capability and/or synthetic fluids will become apparent.

Fluid exposed to high temperature can experience permanent deterioration. For example, a substantial reduction in fluid viscosity normally accompanies asperity contacts (mechanical rubbing) and an increase in temperature. In addition, irreversible viscosity change can also occur when a fluid having poor shear stability encounters high temperature. Whether through rapid oil oxidation promoted by high temperature with its accompanying sludge formation production, or simply accelerated component wear, the influence of high temperature on oil properties is serious and generally deserves prompt consideration and attention.

The reduction in fluid viscosity is one of the most obvious effects of high-temperature operation. Viscosity decreases rapidly with increasing temperature because the mobility of the fluid molecules becomes hyperactive as gas is desorbed and lighter fractions of the fluid vaporize. Engineers commonly express the change in fluid viscosity with variations in temperature on an ASTM Standard Viscosity Temperature Chart. This particular chart is popular because the associated relationship tends to plot as a straight line. Deviations from a straight line most notably occur at both ends of the curve - at low temperatures where certain constituents of the fluid begin to revert to a solid phase, and at high temperatures where lighter fractions of the fluid vaporize. In general, measured values of viscosity are higher at lower temperatures and lower at higher temperatures. Consequently, engineers should extrapolate on ASTM charts with caution, keeping in mind the log2 nature of the viscosity axis.

Some fluids are very viscosity-sensitive with respect to temperature. To improve this situation, engineers commonly add polymers called Viscosity Index (Vl) improvers. These improvers consist of long molecular chains which increase the Vl of the blended oil over that of the base stock - that is, they flatten the viscosity-temperature curve.

Because the effectiveness of a Vl-improved oil depends upon the chain length of the molecules, any breakdown, scission or shearing of these critical molecular bonds destroys an otherwise favorable viscosity characteristic of a VI-improved fluid. The high shear rates and turbulent flow conditions normally existing in fluid systems can cause a continual but often tolerable reduction in fluid viscosity. However, when such conditions exist in tandem with high temperature, fluid degradation greatly accelerates and any artificial improvement in viscosity sensitivity to temperature of the oil is permanently sacrificed. The shear stability of an oil is the property which reflects the susceptibility of a given blend to viscosity degradation.

Figure 1. Effect of Temperature and
Operating Time on Shear Stability of Fluid

Figure 1 illustrates this fundamental characteristic of a typical system fluid at two different temperatures.

Hydrocarbon fluids have an affinity for gases and tend to dissolve air and other gaseous substances. The amount of gas an oil will absorb or dissolve is proportional to the partial pressure of the gas in contact with the fluid. Note that gas solubility increases significantly with temperature for all petroleum products. The increased level of oxygen resulting from greater air content seriously affects the oxidation rate of the fluid and lowers its expected service life.

Surface tension is a critical property of a lubricating fluid; it helps establish the airtightness, leakage rate, capillary flow and boundary lubrication conditions of a system. Surface tension decreases with increased pressure. High temperature also significantly reduces surface tension.

Fluid temperature grossly affects chemical stability and particularly the oxidation rate of the basic elements of the oil. The primary accelerator of all oxidation reactions is temperature. Like any other reaction, the oxidation rate of hydrocarbons will approximately double for every 18 degrees Fahrenheit increase in temperature. Below 140°F, the reaction is comparatively slow, but engineers estimate that the life of an oil is reduced 50 percent for every 18 degrees Fahrenheit temperature rise above 140°F, according to the Arrhenius equation for chemical reaction rates. Hence, for high-temperature applications, the oxidation stability of an oil can have great significance and users should assess it carefully.

The thermal stability of a fluid is its ability to resist decomposition due to temperature alone. It establishes the ultimate high-temperature limit for a tribological system fluid that will ensure continual unimpaired service. The most significant change in fluid properties caused by thermal decomposition of organic molecules is an increase in vapor pressure caused by the shearing of molecules into smaller, more volatile fragments.

Modern formulations of lubricating fluids contain vital additive packages to help the fluid satisfy essential operating functions. Unfortunately, high temperature operation can deplete all such additives, but especially rust inhibitors, foam depressants, antioxidants and antiwear ingredients.

Another factor that deserves consideration in high-temperature operation is the resistance of the component materials to oxidation. Under normal conditions, a metal’s resistance to oxidation is a function of the thickness of the built-up protective oxide film produced. However, because the oxidation rate is accelerated at high temperatures and any film built-up in fluid components is exposed to cyclic stresses, the protective coat continually ruptures and flakes off. Thermal cycling also intensifies the situation by causing severe compressive stresses due to differences in the coefficients of thermal expansion of the film and the underlying material.

High fluid temperature can cause a chain reaction leading to total system destruction. High-temperature operation has a pronounced effect on the wear of all bearing type surfaces in a system. Engineers can evaluate this effect for a particular fluid by using the Gamma Wear Test System.

Figure 2. Antiwear Characteristics of New and Used Hydraulic Fluids vs. Time and Temperature

For example, Figure 2 shows the antiwear characteristics of new petroleum-based oil at three different temperatures (150°F, 200°F and 250°F). Notice the impact of increasing temperature on gamma wear. After the system uses the same fluid for a significant length of time, the wear curves at the same three temperatures become seriously elevated (see Figure 2).

Close comparison of Figure 2 reveals depletion of the antiwear additive in the used oil, significantly reducing its effectiveness. Also, the fluid viscosity may have been sheared down to the point where the lubrication film thickness has become totally inadequate to prevent asperity contact wear. Notice that when engineers add a special antiwear additive (identified as ER) to both new and used fluids and perform the same wear test at the most severe operating temperature of 250°F, the wear rate becomes trivial.

Heat Generation and Removal
Heat cannot be created, only derived from some other form of energy. Fluid systems generally produce heat by converting mechanical energy or fluid pressure energy. Friction is the conversion process in a fluid type system. Because molecular friction generates heat in a sheared fluid, the higher the viscosity, the more heat this friction produces.

Many points in the system can add heat, particularly points with high frictional resistance. Good examples include such sources as bearings, fluid being pushed through orifices and various restrictions, and frictional drag on the fluid as it courses through restricted passages. The larger the pressure drop, the greater the amount of heat generated. Pressure-activated piston seals create high contact pressures to minimize internal leakage. The result is that friction is high, thus creating a massive heat generator that elevates fluid temperature. Low-viscosity fluid can also contribute to heat generation because it inherently fails to maintain a crucial lubrication film between moving surfaces. This failure to separate the running surfaces results not only in wear (abrasion and adhesion of the two surfaces) but also in excessive leakage. Both factors reduce the efficiency of the system and the lost energy is converted to heat.

Engineers often overlook compression heating of aerated fluid as a heat source. Because temperatures as high as 2000°F will occur when the pump compresses air bubbles passing through it, compression heating can have a major impact on the fluid temperature of a mechanical system. The solution in this case is to reduce the amount of air entrained in the fluid.

Intense heat sources can be devastating to hydraulic systems required to operate in their immediate vicinity. A fluid system located near an external heat source or in a place where it cannot receive good ventilation must rely on some artificial means of dissipating system heat. Such a situation is not only a heat source problem but also a heat dissipation problem.

No matter how careful designers of fluid systems are, excessive heat generation sometimes occurs. If a machine like a hydraulic system has an overall efficiency of 80 percent, rough approximations would indicate that the amount of generated heat for an average fluid system is equal to 20 percent of the connected shaft power. This heat must be dissipated to the surroundings in some way, otherwise the fluid temperature will keep rising until the system either stabilizes (where the heat dissipated to the environment balances the heat generated by the system) at some undesired elevated temperature or destroys itself.

The first avenue of escape from generated heat is by natural dissipation. With natural cooling, heat in the system fluid dissipates into the surrounding air, primarily by conduction and convection. All metal surfaces in contact with the fluid serve as heat-transfer surfaces. If engineers design sufficient heat transfer surface area into the machine and expose the external surface to ambient air sufficiently cooler than the required system temperature, then much or all of the heat the system generates dissipates by natural cooling.

Systems utilize heat exchangers or oil coolers to relieve the system fluid of excess heat and lower its operating temperature. Basically, the amount of heat a system must remove and transfer to a cooling medium is equal to the difference between the power input to the hydraulic pump and the power output of all the system actuators. This implies that the ambient temperature is not adding more heat than is dissipating by natural cooling and that environmental conditions are not adding or subtracting heat from the fluid, which is seldom the case.

Figure 3. Vicious Cycle of Thermal Failure

Under extreme cold and hot environmental conditions, heat exchangers may be necessary primarily to counteract environmental conditions rather than to satisfy operating conditions needed to maintain the oil temperature within operable limits. For example, when systems operate in northern climates, users frequently add heat to the fluid to decrease its viscosity. In hot climates or in systems operating near furnaces, users must subtract heat from the fluid to increase fluid viscosity and reduce the temperature.

Oil-to-water heat exchangers require a source of cold water and a means of disposing of the water after the system fluid warms it. This type of exchanger routes the less viscous fluid (water) through a bundle of tubes and the thick fluid (hydraulic oil) through the shell or housing. This type of heat exchanger requires some means to regulate the water flow, perhaps a valve controlled by a sensing element in the reservoir. The controller keeps the fluid temperature nearly constant, thus reducing the cyclic variation in performance and water consumption.

On mobile equipment, hydraulics or in other applications where water is not readily available, the use of oil-to-air heat exchangers with a suitable radiator and fan may prove a good choice. As a coolant, air offers several advantages over water. Piping and sewer charges are saved and air is unaffected by freezing weather. It can also be located on the machine without regard to water supply or sewer connections and is suitable for mobile equipment.

Air-cooled heat exchangers require that oil temperatures be at least 10 to 15 degrees Fahrenheit above the cooling air temperature. They are most effective when the operating fluid temperature is in the vicinity of 200°F. Oil-to-air heat exchangers are least effective when they are needed most, at high ambient temperatures where their efficiency is lowest.

Mechanical refrigeration systems have found broad application where the amount of space available and the heat generation are not compatible with either water-cooled or air-cooled heat exchangers. In addition, for those applications that require limited cooling for short periods, refrigeration type heat exchangers have proven particularly useful. The refrigeration system employs a standard liquid chiller that combines the compressor, evaporator, condenser and circulating pump into a completely self-contained unit that is compact and portable.

Serious damage to a fluid system can occur if the system does not achieve fluid temperature stability within an appropriate range and maintain it throughout the operating period. The importance of conducting a heat balance on troubled systems is great. Strange conditions such as high ambient temperatures, high altitude, low suction line pressure conditions, localized external heating, etc., can fool experts. Machines routinely assigned to various climatic and geographic conditions should have a dedicated heat balance program that users can consult in order to anticipate conditional failure operations.

When oil gets hot and breaks down, it looks dark and smells burnt. Thermally degraded oil placed between the thumb and forefinger feels definitely thinner and much less slippery than new oil. The dark color is varnish - that is, oxidized particles. Even when a filter has removed the burned particles, the oil will still smell slightly burnt and feel thinner and less slippery. Oil analysis provides advance warning of and quantitative data on the extent of, and mechanism resulting in the damage to the fluid.

The temperature of the reservoir oil is not a true representation of the actual oil temperature. In reality, the fluid temperature on the discharge side of the pump is a much better guide. Even then, some regions are generally hotter due to local oxidation, dieseling, compression heating and/or areas having high operating friction forces. The most important action required when overheating occurs, localized or generalized, is discovering the cause. This requires that someone trained to recognize aberrations in system operation analyzes the system. After heat dissipation practices have been applied, users may finally have only one simple solution - to go to a higher temperature system. Such an option is feasible today but often proves expensive, because such a system requires heat-resistant materials, elastomers, fluids and components.