**1. Introduction**

The consistency of a lubricating grease is often considered to be its most important rheological property. It generally dictates the suitability of a grease for a particular application. Consistency broadly refers to the firmness of a grease, indicating a grease's ability to remain in place (resist leakage) within bearings and to form stable channels of lubricant [1]. These channels are important to the functionality of a bearing because they serve as reservoirs from which moving parts draw lubricating fluid over the operational lifetime of a machine [2]. Hence, consistency serves as an important metric for selecting a grease, and the capability to properly quantify this property is of grea<sup>t</sup> importance.

The main test to measure grease consistency is the cone penetration test given by ASTM D217 [3]. In this test, a cone is dropped into a grease sample for 5 s and the depth to which the cone penetrates is used as a measure of consistency. This test requires a large sample of grease—approximately 450 grams according to ASTM D217—so the alternative scaled-down tests given by ASTM D1403 [4] are able to provide penetration results with smaller samples. The penetration depth obtained from any of these tests is usually used to assign a grade to a grease for succinctly characterizing its nature to a broad market of consumers.

Using specialized equipment such as a rheometer is becoming increasingly popular for assessing the rheological properties of grease. This apparatus requires a very small sample— less than two grams—and provides quantitative results that can assess grease consistency. It is perhaps even more precise than the cone test with the advantage of requiring a simpler operational procedure while allowing for temperature control. These tests are often used to evaluate a "critical" stress but have also been used as a way of conducting a penetration test [5,6]. The methodology of conducting a penetration test is quite simple, whereas the calculation of critical stresses demands an understanding of viscoelasticity.

**Citation:** Gurt, A.; Khonsari, M.M.Testing Grease Consistency. *Lubricants* **2021**, *9*, 14. https://doi.org/10.3390/ lubricants9020014

Received: 5 January 2021 Accepted: 30 January 2021 Published: 2 February 2021

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A viscoelastic material is one that has properties of both a liquid and a solid. Grease, for instance, can be considered a viscoelastic solid material since it behaves as a solid unless sheared. Once sheared past a critical point, it begins to flow. The quantification of this state is of grea<sup>t</sup> interest for characterizing a substance, but the exact definition of this state has been a subject of debate for years [7]. In addition to having different definitions, there are various methods for evaluating this state, including the use of steady flow curves, creep measurements, stress ramps, stress sweeps, and oscillatory tests [8,9]. Nevertheless, in the context of lubricating grease, two distinct points found through oscillatory tests—the yield point and flow point—have come to be popular choices for characterizing a grease.

Oscillatory rheometry involves the analysis of viscoelastic materials by monitoring their stress response to oscillating strain (or vice-versa). The solid-like behavior is characterized by the storage modulus, which describes the ability of a material to store energy through elastic deformation and is in phase with the oscillating input. Alternatively, the liquid-like behavior is characterized by the loss modulus, which describes the ability of a material to lose energy through viscous dissipation and is out of phase with the input. These two parameters both change as the state of stress within material changes, so imposing an increasing stress or strain amplitude (often called an amplitude sweep) in an oscillatory test is often an ideal way of examining viscoelastic behavior. Results of an amplitude sweep can then be used to calculate the yield point and/or flow point.

The yield point, characterized by the yield stress, is considered by many to be the end of the linear viscoelastic range at which the structure begins to be considered damaged. One choice is to define the linear viscoelastic range as the region where the storage modulus is independent of strain. Another choice is to use the end of stress-strain linearity [10] as the yield stress. Due to the recent popularity of the second choice in analyzing grease [9–14], it will be used as the definition of yield stress throughout this paper.

The flow point, characterized by the crossover stress, is the point at which the storage modulus and loss modulus cross over each other. Upon commencing an amplitude sweep, the storage modulus will quickly reach a plateau value, but eventually start decreasing. The flow point will be considered the point where the storage modulus decreases enough to cross over the loss modulus, temporarily signifying a material with properties more closely resembling a liquid than a solid. Both this point and the yield point are indications of how "solid" a given material is and are considered properties akin to consistency.

Over time, overall grease consistency can permanently change due to the shear irreversibly breaking the structure, due to oil bleed, leakage, and evaporation, due to contamination, and due to chemical reactions [15]. This change in consistency has been used to track the degree of degradation a grease has undergone [5,16,17]. A sampling of in-service grease in conjunction with a degradation model can be used to estimate the remaining useful life. However, the cone penetration tests given by ASTM D217 and ASTM D1403 require a larger sample of grease than is used for many applications. This means the consistency of an in-service grease cannot be measured with these methods. Therefore, it is advantageous to have alternative consistency tests that allow the use of a small sample of grease. One such test intended for field use is provided in a test kit given by the bearing manufacturer SKF, but the resolution of the results is limited to the grade. Therefore, rheometer tests allowing more precise results and temperature control using a very small sample have a significant value. A closer examination of these tests is the main focus of this paper.

This paper examines common consistency tests in Section 2, takes a closer look at a proposed method for assessing consistency through a rheometer penetration test in Section 3, compares results of these tests to each other in Section 4, presents a discussion of results in Section 5, and gives concluding remarks in Section 6.

#### **2. Examination of Consistency Tests**

The most common tests of grease consistency will be overviewed and a discussion of each is provided.

#### *2.1. Cone Penetration Test*

The accepted method for testing grease consistency is the cone penetration test given by ASTM D217 [3]. In this test, a cone is dropped into a cup of grease and the depth of penetration is used as a measurement of consistency. This value of penetration is used to assign a grade to a grease from 000 to 6. The National Lubricating Grease Institute (NLGI) defines the ranges of penetration corresponding to the grade as given in Table 1. The values of penetration are given in tenths of a millimeter, often called decimillimeters and abbreviated as dmm.


**Table 1.** National Lubricating Grease Institute (NLGI) grades and applications [18].

The grades in this scale are defined so that they span 30 dmm and have gaps of 15 dmm between grades. This leads to some ambiguity in labeling a grease, where many choose to assign half grades. For instance, some would choose to assign a grade of 2.5 to a grease with a worked penetration of 260 dmm.

As the cone penetrates deeper into the grease sample, grease begins to lift out of the cup as is demonstrated in Figure 1. As penetration increases, this geometry-dependent behavior begins to influence penetration readings. ASTM D217 indicates that the penetration for soft greases is a function of cup diameter if its penetration is above 265 dmm. However, the grease eventually reaches a point where the complicated nature of the squeezing of grease between the cone and the lip of the cup becomes even more of a determinant of the final penetration value. To account for this, the standard mandates that the cone be perfectly centered when the penetration reading is above 400 dmm.

Not only does the cone penetration test require a substantial quantity of grease, it also demands a skilled operator in order to obtain consistent results. A significant source of error in this test is the presence of pockets of air within the grease sample. This is particularly relevant for measuring the unworked penetration of a grease sample, as loading the sample into the grease cup can easily form large pockets of air. Therefore, having an unskilled operator can cause inconsistent measurements of consistency using the cone penetration method.

**Figure 1.** Grease cup (**a**) before and (**b**) after cone penetration.

## *2.2. Rheometer Oscillatory Tests*

A rheometer—such as the one pictured in Figure 2—has the capacity to perform numerous useful tests to analyze viscoelastic materials. Through oscillatory strain sweep measurements, properties of viscoelastic materials can be found without potential errors from sample fracturing [19] and with minimal sensitivity to gap height and plate roughness [9,10,20]. Two parameters will be investigated here: the yield point—corresponding with the yield stress—and the flow point—corresponding with the crossover stress. The definition set forth by Cyriac et al. [10] will be used for yield stress, where the end of stressstrain linearity measured from an oscillatory amplitude sweep is considered the yield point. For the crossover stress, the first point at which the storage modulus and loss modulus reach the same value will be considered the definition.

**Figure 2.** Anton Paar MCR 301 rheometer used in experiments

When performing these tests, there are many choices for geometry, but parallelplate geometry is perhaps the most common. In addition, there are many choices of

plate diameters, but 25 mm and 50 mm are among the most common. For rheological measurements, a smaller diameter will measure higher stresses than a larger diameter if all other variables are held constant. Therefore, the results obtained from different plate diameters cannot be reliably compared. Nevertheless, a smaller diameter is generally advantageous because it is easier to load the sample, and only a small quantity of the sample is needed despite having slightly reduced testing precision.

Another important consideration for rheometer tests is the sample loading procedure. In fact, different methods of loading a sample can cause errors of up to 30% in some cases [21]. The typical procedure is to apply a sample to the lower and/or upper plate and then lower the top plate to a gap slightly larger than the measurement gap. At this point, a tool is used to clear (trim) all the sample that is not directly between the plates. The top plate is then lowered to the measurement gap and the measurement is performed. A standard procedure is to trim the sample at a gap 5% greater than the measurement gap, but trimming the sample at 2.5% above the measurement gap is also reasonable.

Another consideration is the state of stress within the sample as it is being measured. Upon lowering the top plate, there can be a large resistive force acting on the top plate due to the compression of the sample. This effect is especially pronounced when using a plate with a large diameter. This is generally dealt with in one of three ways: waiting for the sample to relax (relaxation), shearing the sample a small amount (pre-shear), or doing nothing and immediately commencing measurement. For many measurements, this choice is inconsequential. However, in some cases, this can have a significant effect, and this is explored in Sections 3 and 4.

Overall, performing these measurements requires an operator to learn how to use a rheometer, but the actual treatment of a sample is quite straightforward and far less prone to operator error than the cone penetration test.

#### *2.3. Alternative Penetration Tests*

Though the cone penetration tests given by ASTM D217 and ASTM D1403 are the officially recognized penetration tests, there are other penetration tests that can possibly be used to assess grease consistency. Two of the most prominent include the consistency test found within the SKF grease test kit [22] and the rheometer penetration test.

The consistency test provided in the SKF grease test kit is intended for the in-service sampling of grease and needs only a very small sample. This consistency test is an example of a constant-volume squeeze flow [23]. It is done by first applying a cylindrical-shaped sample of grease to a glass plate using a jig. Then, another glass plate is carefully placed above this one and a weight is put atop it. The weight is allowed to cause the grease to spread between the plates for 15 s and the final diameter of the sample is compared to rings on a sheet of paper to determine consistency. This only has the resolution to determine the NLGI grade and relies on the final sample shape being close to circular. In practice, a used grease sample may be quite nonhomogeneous and contaminated, leading to a non-circular spread, which will add difficulty to determining the result.

The rheometer penetration test has been used in the past [5,6] but will be examined more in-depth in Section 3.

#### **3. Details of Rheometer Penetration Test**

The geometry given by the rheometer penetration test is an example of "imperfect squeeze flow" [23,24], with the geometry having neither a constant area nor constant volume. In this configuration, there is a complicated variable pressure imposed by the fluid squeezed as the gap closes [23]. This leads to some difficulty in deriving analytical equations describing the relationships among parameters, such as displacement, force, area, and velocity. Therefore, these parameters were investigated empirically.

Many variables were identified for the rheometer penetration test, including sample preparation, gap height, penetration time, and normal force imposed. Similar to the oscillatory tests done in a rheometer, the test geometry and sample preparation are key variables that must be arbitrarily chosen. For the same reasons as in the oscillatory tests, it appears that the 25 mm diameter plate configuration is a good choice, and this was used in the previous works mentioned.

All rheometer experiments were conducted using an Anton Paar MCR 301 rheometer at room temperature. Each sample was measured three times and the average value is presented with error bars corresponding to one standard deviation. In most cases, the standard deviation is quite low and error bars are not visible. An overview of the greases used in the experiments is provided in Table 2.

**Table 2.** Greases used.


\* The measured consistency of this grease was approximately grade 1.

In choosing the gap height, the main constraint is that the ratio of the plate's radius to gap height should be greater than or equal to 10 to avoid edge effects [25]. The selection of the plate diameter is another important parameter, as results of similar experimentation [24] indicate that there is a complicated dependency of rheological measurements on plate geometry. For these measurements, a 25 mm diameter top plate was selected. This is a common size and can allow for using a small sample of grease while allowing a reasonable gap. In order to satisfy the radius-gap constraint given above for a 25 mm diameter, the gap must be below 1.25 mm. Hence, a 1 mm gap was chosen for the experiments reported in this paper.

The next thing to be examined is sample preparation. Measurements of the same sample were taken using a standard 5% trim with no pre-shear, a 100% trim without preshear, a 5% trim with pre-shear, and a 5% trim with a 20-min relaxation period. Using a 5% trim without pre-shear or a relaxation period means that the top plate is initially lowered to a height 5% above the measurement gap where the sample is trimmed before lowering the top plate to the measurement gap and immediately commencing the measurement. Using a 100% trim means the top plate is initially lowered to twice the measurement gap where the sample is trimmed before lowering the plate and immediately commencing measurement. The 5% trim with pre-shear means the sample was trimmed at 5% above the measurement gap but after the plate was lowered to the measurement gap, a shear rate of 5 s<sup>−</sup><sup>1</sup> was induced for 5 s before commencing measurement. Finally, the 5% trim with relaxation period means that after trimming the sample at 5% above the measurement gap, the top plate is lowered to the measurement gap and a pause of 20 min is taken before commencing measurement.

Results of the sample preparation investigation are displayed in Figure 3 and show that using pre-shear does not appear to influence the result when compared to the 5% trim. Allowing the sample to relax for 20 min before subjecting it to penetration appeared to slightly decrease the penetration, but also led to a higher standard deviation in this case. Adding 20 min to a 20 s test is also impractical. Nevertheless, the method with the lowest standard deviation of results is the case of the overfilled gap, where the sample was trimmed at 2 mm and immediately subjected to penetration. This method yielded a consistently lower penetration when compared to the 5% trim, which is an expected result when considering the nature of squeezing flow. The repeatability of this method led to its implementation in subsequent measurements.
