According to John Sander, vice president of technology at Lubrication Engineers, this test has been the standard for many years. However, in a presentation at the OilDoc Conference in Rosenheim, Germany, in January, Sander contended, The simplicity of the test has contributed to complacency within the industry in considering the true value of the test. It is well known that greases with the same NLGI grade often have noticeably different physical properties.

He asked, Why, then, is the grease consistency number important? For a grease to be used, it must be pumped into an application, then flow into the areas needing to be lubricated. It can be proven that various greases with the same NLGI consistency do not have the same pumpability or flow properties, said Sander. Therefore, what is the real value of the consistency number? He then outlined alternative tests that he and co-author Wade Flemming, the companys laboratory manager, considered that could potentially provide a better way to characterize greases.

To Flow or not to Flow

Grease is commonly used in applications where oil would run out, such as a bearing with no seals, he explained. Therefore, it would seem the goal is for grease not to flow. However, for grease to lubricate moving parts, it must flow, but under the proper circumstances.

As a result, grease is often described by consistency, which is a measure of its hardness or softness, rather than by its viscosity. And grease products are typically sold according to their NLGI consistency number.

As such, grease consistency is a physical parameter used by lubricant manufacturers, equipment manufacturers and users to specify, recommend and purchase a grease for their applications. Therefore, the importance of consistency to the end user is very high.

But, Sander contended, It is obvious to anybody who has actually looked at, touched or pumped grease that two products of the same NLGI consistency are not necessarily the same. Various researchers have shown that some greases pump harder, are stickier or look harder or runnier than other greases with the same penetration number.

While penetration has been the standard for measuring consistency, newer methods have been developed that could provide a more complete picture of grease consistency. It is not my goal to suggest an immediate replacement for the penetration test, said Sander, but rather to provoke thought and spur discussion to eventually create a new standard for evaluating grease consistency.

Selecting Consistency

Sander presented a table relating grease consistency to specific end uses. It is apparent from this table many aspects must be considered, including pumpability, bearing speed, operating temperature, grease bleed and environmental conditions, said Sander. The grease penetration test is insufficient to provide detailed information about any of these.

Two types of flow must be considered when selecting a grease: flow from the grease gun and into the bearing, and flow within the lubrication contact zone. In turn, flow is controlled by two other properties; namely, cohesion and adhesion, Sander explained.

NLGI defines cohesion as the molecular attraction of the molecules within a grease, and adhesion as the forces that cause two substances, such as grease and metal, to stick together. Both can affect how difficult or easy it is to make a grease begin to flow.

The first challenge is to get the grease moving, known as overcoming the yield stress, the minimum amount of stress needed to make a plastic-like material flow. In general, flow is affected by the dynamic viscosity of the lubricant, which is the viscosity under conditions of shear, such as moving equipment, Sander said. Unfortunately, grease viscosity is difficult to measure because most greases do not flow easily due to their cohesive and adhesive properties.

Another important property is pumpability. Simply put, the grease will not work if it cannot be transported to the lubricating point. Today, centralized lubricant systems are frequently used to improve efficiency, convenience and safety, Sander noted. These are great tools, but one drawback is the distance grease must be pumped to reach the contact area. Various factors affect pumpability, including temperature, distance to be pumped, pump input pressure and the inner diameter of the line.

Operating temperature also has a significant effect on grease flow. The standard penetration test is performed at 25 degrees C, but this is not a typical operating temperature. In addition, studies have shown that grease flow changes with temperature in a nonlinear way, but the NLGI rating system is linear.

Bleed is the next factor to consider, and Sander explained there are two uses of the term. For most users, bleed is when a grease becomes thin enough (or the dynamic viscosity is low enough) for it to run out of the bearings. For our purposes, that type of bleed would actually be considered flow.

The technical definition of grease bleed, he said, is when the base fluid and additives release from the grease. Sometimes, this is observed in the container as a puddle of oil. This condition is unfavorable because it can create housekeeping problems and can result in the grease appearing to be thicker than it should if the oil is not reincorporated.

Environmental conditions affect grease consistency both in storage and in service. Besides temperature, contamination is an important environmental factor. Contamination can come in various forms, he said. One is contamination from another grease. This is such an important consideration that it has been given a name, compatibility.

Most often, two greases get mixed when an operator pumps the wrong grease into the application. Other possible sources of grease mixing are poor manufacturing practices or changing a product formula without ensuring compatibility, he noted.

Environmental contaminants include dirt, water, chemicals and wear metals. Depending upon concentration, any of these can affect grease consistency, some dramatically. Therefore, consideration should be given to selecting grease ingredients such that consistency can be optimized to manage contamination.

Alternative Tests

A number of tests are available that might provide a better picture of grease performance than the penetration test. In fact, Sander contended that more than one test may be needed to properly measure consistency. In support of this notion, he noted that the performance of fluid lubricants is often described using several tests and suggested that a similar approach could be taken with greases.

One of the more important considerations when selecting grease is how well it flows at low temperatures, he said. This is especially important for centralized lubricant systems.

To address this situation, ASTM D1092 was developed to determine the apparent viscosity of grease at varying shear rates. The results provide a better indication of pressure drops in grease distribution systems under steady-state flow at constant temperatures. The test determines apparent viscosity at 16 different shear rates, data that can then be used to predict the flow characteristics of the grease through various pipes, lines and dispensing equipment.

The main drawback to ASTM D1092 is that it requires several pounds of grease. It also can be very time consuming, especially if apparent viscosity determinations are required at several different temperatures.

Like ASTM D1092, the U.S. Steel Grease Mobility test was developed to measure grease flow resistance at various temperatures and pressures. It helps predict the pumpability characteristics of grease at low temperatures. By varying temperature and pressure, grease can be evaluated for its performance in known supply systems. Unfortunately, the method is not standardized, so its repeatability and reproducibility are unknown.

The Lincoln Ventmeter test was developed as a rapid alternative to ASTM D1092. Results can be used to approximate the maximum or minimum size of supply line needed for a particular grease at a given temperature. Pumpability can be predicted by comparing the Ventmeter viscosity reading with supply line charts supplied with the instrument. One of the known drawbacks of the test is that its repeatability and reproducibility are in question.

Grease must be able to be pumped to some applications at subambient temperatures, then be able to flow and provide adequate lubrication at the contact. ASTM D1478 evaluates the extent to which a grease retards the rotation of a slow-speed ball bearing by measuring the starting and running torques of a bearing packed with the test grease at temperatures below minus 20 degrees C.

The method is helpful for applications requiring greases with low yield stress and that can maintain suitable consistency at low temperatures and speeds. However, it requires a substantial amount of time and large samples to determine data at several temperatures.

A few standard methods are available for testing oil separation from grease. ASTM D1742 and D6184 determine the amount of oil bleed from a grease under static conditions. These methods help evaluate the amount of oil bleed that could be detrimental to grease consistency at elevated temperatures.

However, ASTM D6184 includes the disclaimer, Test results obtained with this procedure are not intended to predict oil separation tendencies of grease under dynamic service conditions. Therefore, Sander related that the dynamic test procedure ASTM D4425 may provide a better indication of grease bleed in applications producing high centrifugal forces such as flexible shaft couplings, universal joints and rolling element thrust bearings.

Rheological testing to determine the deformation and flow of matter has been used in many other industries for years. Controlled stress/strain rheometers can evaluate the consistency, flow properties, pumpability, yield stress, thixotropic performance, temperature limits and even tackiness of grease at various shear rates and temperatures. The small amount of grease required for this evaluation is also ideal for evaluating used grease samples.

Sander conceded that using a controlled stress/strain rheometer can be more complicated than using other established test methods. However, he noted that the flexibility in test parameters and the data generated make rheology testing an ideal way to evaluate consistency, flow and pumpability. As a result, research in using these rheometers to evaluate greases continues to grow.

Another important attribute to consider when selecting a grease is tackiness. Tackiness is defined as the ability of the grease to form threads when pulled to enable easy transfer of grease to the contacting areas, he explained. Tackiness can be a desired attribute for some applications, while it may be detrimental to others.

The amount of tack can be controlled to a large extent during formulation. Currently, there are no established methodologies that efficiently and accurately evaluate the adhesion and tackiness of grease, Sander explained. However, current research is making strides in developing effective methods and equipment.

Proposed Specification

Sander proposed a specification, comprised of a variety of tests, to better characterize grease consistency. This list is not intended to be a final product, but rather a launch pad for future thought, he said. The penetration test is missing, he noted, because it did not fit into any of the suggested categories. Therefore, it was not felt that it provided any useful information about grease consistency.

Like the SAE viscosity classifications, a consistency specification could provide a much more useful description of a greases physical attributes than a single 60-stroke worked penetration, Sander explained. This is not to suggest that all of the tests listed should be used, but they could be beneficial. In particular, the rheometer test shows promise because it can be used to evaluate several different properties.

Sander suggested that a new consistency specification might act as a catalyst for change. At the December 2016 meeting of the ASTM D02.0G.07 Grease Working Group on Research, data was shared from various labs that had analyzed grease samples using the rheometer. A debate ensued as to whether the report should also include correlation data between the rheometer and the penetration test. Unfortunately, no consensus was reached. Yet, this could be a good indication that the revolution has started, and innovations will be made, leading to the retirement of the grease penetration test, Sander concluded.