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There are many criteria to consider when selecting a high temperature grease for hot, grease-lubricated equipment.

The selection must include consideration of oil type and viscosity, oil viscosity index, thickener type, stability of the composition formed by the oil and the thickener), additive composition and properties, ambient temperature, operating temperature, atmospheric contamination, loading, speed, relubrication intervals, etc.

With the variety of details to resolve, the selection of greases that must accommodate extreme temperature conditions poses some of the more challenging lubrication engineering decisions.

Given the variety of options, the potential for incompatibility problems and high prices for a given high-temperature product, the lubrication engineer must be selective and discriminating when sourcing products to meet high-temperature requirements.

High-Temperature
‘High’ is relative when characterizing temperature conditions. Bearings running in a steel mill roll-out table application may be exposed to process temperatures of several hundreds of degrees, and may experience sustained temperatures of 250oF to 300oF (120oC to ±150oC).

Automotive assemblers hang painted metal parts on long conveyors and weave them through large drying ovens to dry painted metal surfaces. Operating temperatures for these gas-fired ovens are maintained around 400oF (205oC).

In these two cases, the selection criteria differ appreciably. In addition to heat resistance, the grease to be used in a hot steel mill application may require exceptional load-carrying capability, oxidation stability, mechanical stability, water wash resistance and good pumpability, and at a price suitable for large-volume consumption. With all of the important factors to consider, it is useful to have a grease selection strategy.

Selection Strategies
A reasonable starting point for selecting a high temperature grease is to consider the nature of the temperatures and the causes of product degradation. Greases could be divided by temperatures along the lines in Table 1.

There is general correlation between a grease’s useful temperature range and the expected price per pound. For instance, a fluorinated hydrocarbon-based (type of synthetic oil) grease may work effectively as high as 570oF (300oC) in space applications but may also cost hundreds of dollars per pound.

The grease’s long-term behavior is influenced by the causes of degradation, three of which are particularly important: mechanical (shear and stress) stability, oxidative stability and thermal stability. Oxidative and thermal stresses are interrelated. High-temperature applications will generally degrade the grease through thermal stress, in conjunction with oxidative failure occurring if the product is in contact with air. This is similar to what is to be expected with most industrial oil-lubricated applications.

Large production facilities have a variety of grease-lubricated equipment, ranging from steady-state applications to applications that vary significantly in speed and load, and operate in aggressive (wet or dusty) environments.

If machine designers address equipment lubrication needs based strictly on a dynamic loading requirement, they might have to specify a wide variety of greases to meet the many existing needs. In this approach, the added system complexity would likely increase the cost and the risk of failure due to misapplication and cross-contamination.

To maximize grease lubrication effectiveness, minimize cost and minimize risk of application-induced failure, lubricant manufacturers have made an effort to formulate greases that cover a variety of applications. These greases range from slow to high speeds, and from low to high loads, in an effort to provide a single product to meet a multitude of requirements. The result is general purpose grease.

What is General Purpose Grease?
A general purpose (GP) grease is designed to meet a broad range of requirements. It is grease manufactured to medium consistency with a medium viscosity base oil and medium wear, washout and oxidation resistance properties. Essentially, it is a product designed to fit the largest possible cross-section of grease lubricated components in an operation.

If the demand on the lubricated components in a plant could be rated on a curve according to speed, load and environment/application severity, the resulting curve would likely resemble a Pareto chart. The typical application in most plants does not severely challenge a GP grease. Some applications would be considered tough, and may or may not be suitable for a GP grease. A few extreme applications will require a grease with one or more special qualities.

The proportion of typical, tough and extreme applications might vary considerably. This distribution has no correlation to the criticality of the mission of the grease-lubricated machines. Many of the tough and most of the extreme applications will require a thorough technical review to determine what special lubricant properties might be required. Lubricant criteria are fairly narrow at the extremes of load and speed, and therefore may require products that do not suit the vast majority of lubricated components.

However, it is advisable to cover as many of the lubricated components as possible with as few products as possible. With this in mind, begin by examining the lubricated components for an average requirement and work selectively toward extremes in load and speed.

Equipment Properties to Consider
Given the wide range of characteristics that may exist in the greases at any given plant, it is best to first characterize the equipment and plant conditions, then select a general purpose grease to meet the conditions.

Equipment Condition Considerations
Keeping the objective in mind, a general purpose (GP) grease is used as a multi-application grease in a production process to reduce complexity and the potential for component failure due to misapplication. Consider the following operational characteristics when selecting the GP grease.

Size and Type
Ball screws, cables, linear bearings, plain bearings, rolling element bearings, slide-ways and seals are just a few of the many different component types that are grease lubricated. If sliding friction is the dominant contact type, then there may be a greater reliance on heavy viscosity oils, polymers and solid additives to support the load and provide lubricating film protection.

If rolling friction is the dominant contact type, then greases with lighter viscosity base oils and minimal use of polymers, solids and antiwear (AW) and extreme pressure (EP) additives can be effectively used.

Load
As the load increases, the grease’s base oil viscosity must also increase to support the load. If the majority of the components in the mill/plant environment are heavily loaded, it may make sense to use high-viscosity base oils for a general purpose product. This might be the case in a cement, steel or paper mill environment. It is not uncommon to find GP greases made from 460 cSt (40°C) and heavier oils in these types of environments.

Speed
As the speed increases or the load diminishes, the required base oil viscosity also diminishes. In operations with predominantly moderate to high-speed and lightly to moderately loaded applications, the grease’s oil viscosity would fall to an ISO 46 to 150 range. It is unusual to find highly loaded applications that also operate at high speeds that are lubricated with grease. This type of application would likely warrant special consideration and therefore fall outside this discussion.

Atmosphere
The three atmospheric factors that must be accounted for are temperature, moisture and airborne solid contaminants (particles). Although the influence of atmospheric factors can be significant, these factors are considered after the viscosity selection is complete.

Lubrication Intervals
The method of application combined with the application cycle dictates the rate of application. The rate of relubrication is the amount of lubricant fed into the component in a given time.

Greased components require a constant supply of lubricant at the load zone to sustain the hydrodynamic film much the same as oil lubricated components. The reserve grease contained in the cavity in the housing serves as an oil reservoir that components draw from for lubrication.

When grease is resupplied to the housing, the oil reservoir is replenished. The longer the duration between cycles, the greater the likelihood that the reservoir will deplete and the component will run to a semi-dry (mixed film) condition.

The oil in the load zone is squeezed and pushed away over time. If the relubrication volume is insufficient, or the cycle is sporadic (greater risk with manual lubrication), the likelihood that the oil film will dissipate leading to mixed film conditions increases. When these conditions are prevalent, the grease selection must be one that resists the squeezing action and tendency to dissipate. Greases formulated with heavier viscosity base oils and chemical and mechanical film forming additives can be helpful in these circumstances.
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