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Large production facilities have a variety of grease-lubricated equipment, ranging in application from steady-states and aggressive environments to varying speeds and loads.
If machine designers address equipment lubrication needs based solely on dynamic loading requirements, they will have to specify a variety of greases that are needed to meet the many existing needs. With this approach, the added system complexity will increase the overall cost and the risk of failure due to grease misapplication and cross-contamination.
To maximize grease lubrication effectiveness, minimize costs, and reduce the risk of application-induced failure, lubricant manufacturers have begun formulating greases that cover a wide array of applications, including varying speeds and load sizes. These efforts were made to provide a single product that could meet a multitude of requirements. The result was the creation of general-purpose greases.
A general-purpose (GP) grease is designed to meet a broad range of requirements. It is manufactured to a 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 lubricated components could be rated on a curve according to speed, load, and environment/application severity, the resulting curve would likely resemble a Pareto chart. Typical plant applications do not severely challenge GP greases, but there are certain applications that could be considered rough conditions. These conditions may or may not be suitable for a GP grease, and a few of these extreme applications may require a grease with one or more specialty 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 extreme applications will require a thorough technical review to determine what special lubricant properties might be required. Lubricant criteria are 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 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.
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 grease to meet the conditions.
Keeping the objective in mind, a general-purpose 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.
Ball screws, cables, linear bearings, plain bearings, rolling bearing elements, 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.
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 cement, steel, or paper mill environments. It is not uncommon to find GP greases made from 460 cSt (40°C) and heavier oils in these types of environments.
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 and are lubricated with grease. This type of application warrants special consideration and therefore falls outside this discussion.
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.
The atmospheric conditions have a greater influence on the heavily loaded components than the high-speed components. Heat causes oxidation and reduces the base oil’s viscosity. As the base oil’s viscosity decreases, its load-bearing capability is diminished. Heavily loaded components operating at elevated temperatures are susceptible to inadequate lubricant film and aggressive wear.
Water can have similar effects on the oil film because water has a very low load-bearing capability. If water is allowed into the bearing in a free or emulsified form, the film may be compromised, leading to corrosive, adhesive, abrasive, and hydrogen-induced wear.
A continuous grease purge is sometimes used to prevent water and solid contaminants from entering grease-lubricated components. Because most greased components are not constantly purged, there is potential for moisture and particles to enter the component cavities through seals and fittings. Silica-laden atmospheric dust and dirt particles that enter into the load zone can scratch and abrade surfaces, generating more abrasive particles (wear debris).
The potential for particles to damage load zone surfaces increases in high-temperature and/or high-moisture applications. The combination of reduced base oil viscosity, free water, and particulate at the load zone can be particularly detrimental.
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 within 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.
After gaining a thorough understanding of the conditions that the majority of the equipment operates within, consider the lubricant properties that are useful in meeting the equipment requirements.
There are a few lubricant properties that influence lubricant selection, whether GP grease, special purpose grease, oil bath, or circulation lubricated applications. Oil viscosity, oxidation resistance, wear resistance, and corrosion resistance certainly qualify. Beyond this, the thickener type influences the dropping point and water wash resistance, both of which play a role in the selection of a GP product. Each of these warrants further discussion.
Given the relationship between speed, load, and viscosity, and given that the viscosity of the oil in the grease is the primary load-bearing mechanism, the first selection criterion is the grease’s base oil viscosity.
Fluid friction produces heat which locally reduces the base oil’s viscosity, causing high-viscosity oils to reduce under load. Because of this, it’s advised to initially defer to the heaviest base oil viscosities the machine can tolerate and move toward lighter viscosities as high-speed conditions are considered. As noted earlier, general purpose grease base oil viscosities in the ISO 460-plus range may be seen in operations running heavily loaded machinery.
If extreme temperatures, both high and low, are frequently encountered, consider the oil type and the influence of temperature change on the various types available. Synthetics offer a broader range of operating temperatures than petroleum oils, but even within the family of synthetics, there are variations. Figure 4 shows common synthetic oils by effective working temperature ranges.
Figure 4. Common Synthetic Oils by Effective Working Temperature Ranges
Paraffinic mineral-based oils often have a viscosity index (VI) around 95. With few exceptions, synthetic base oils offer greater flexibility, with VI numbers ranging from 120 to 175. The higher the VI, the broader the temperature range in which the product can effectively operate.
If the application involves high temperatures and poor machine access, then it is preferable to select a product with a base oil and thickener that can withstand the temperatures and resist breakdown over prolonged relubrication intervals. Oxidation resistance is largely a subset of the base oil selection – the base oil type will have a significant influence on oxidation resistance. Thickeners and additive system selection also play an important role.
Wear performance is also related to base oil selection because the viscosity of the base oil at operating temperature is the primary determinant of the lubricating condition (hydrodynamic, elastohydrodynamic, mixed film, or boundary) that the component will experience.
Where there is a strong likelihood of shock loading and extreme loading, chemical AW and EP additives can be useful in supplementing base oil properties to provide additional protection from wear and seizure. Inorganic additives such as moly, graphite, and PTFE are often added to provide mechanical load support in addition to the chemical additives.
Applications where they employ water-based coolants or process chemicals have multiple problems to address. Water resistance is characterized by the grease’s ability to withstand one of four water-related problems, including:
1. Washout Resistance
The ability of the lubricant to stay in the bearing while operating partially or fully submerged (ASTM D1264).
2. Water Absorption
The ability of the grease to deal with the presence of water by either absorbing or resisting the washing and diluting action of the water. The grease may either absorb a large percentage of ingressed water and then de-gel (lose consistency but retain the water), absorb a lesser amount of water and retain consistency, or resist absorption of water altogether (which leaves water in a separate phase in the component or system).
3. Corrosion Resistance
The ability of the grease to prevent corrosion of surfaces when water is present (ASTM D1743).
4. Spray Resistance
The ability of the grease to resist displacement from a direct impingement of water on the greased surface (ASTM D4049).
The thickener, in large part, determines the grease’s response to moisture. If the thickener is particularly good at absorbing moisture, such as with sodium soap greases, it may form an emulsion that traps and draws water away from metal surfaces. The downside to greases that emulsify is that they can be easily washed out.
Coupled with water washout resistance, if the typical operating condition is constantly exposed to water, process chemicals, or high humidity conditions, then an additional measure of corrosive resistance may be warranted. Some thickeners, such as aluminum, have inherently good corrosion resistance.
If the thickener resists water, such as lithium or calcium soaps, then rust and corrosion inhibitors will be added to protect surfaces from water. If the grease is expected to remain in the bearing and prevent corrosion under wet conditions for a prolonged period (between lubrication cycles), then an added anticorrosive capability is highly desirable.
There are several thickeners available for consideration. Generally, the criteria for selection of the GP grease thickener type will be compatibility with other greases being used and the dropping point (which represents the grease’s temperature maximum range) that the thickener imparts to the grease.
Lubricant thickeners are generally compatible with other similarly named thickeners, with Polyureas being the exception. For example, lithium thickeners are compatible with other lithium-thickened greases. This table shows typical compatibility characteristics.
A second rule of thumb is that the complex thickeners can be used in higher temperature ranges, frequently to 400°F (204°C), with frequent relubrication. If the equipment operates at sustained temperatures above 400°F (204°C) then it probably would not qualify for a general-purpose lubricant.
The issue of the thickener type and dropping point is central to this decision because technicians are inclined to use greases when oil won’t stay in place. The thickener must remain stable over a sufficiently broad temperature range so that an incidental spike in temperature will not cause the grease to thin and bleed from the application.
Finding an acceptable general-purpose grease requires consideration of many aspects, including lubricant factors such as oil viscosity, oxidation resistance, corrosion resistance, thickener type, and water wash resistance properties.
There are key operational factors to consider as well, including speed, load, temperature, contaminant type, concentration, and relubrication cycles. In food handling and processing plants, toxicity and edibility create a separate set of questions that should also be addressed.
Selecting a grease to meet broad operational requirements requires an understanding of how the equipment runs and how the grease will respond to those operational factors. A well-reasoned and planned consolidation to a general-purpose grease can bring dividends through reduced risk of contamination, reduced complexity, and improved overall lubrication conditions.