The Effects of Thickeners on the Low Temperature Properties of Open Gear Greases, Raymond Drost, Donald

The Effects of Thickeners on the Low Temperature Properties of Open Gear Greases

Raymond Drost, Donald Howard, Huafeng (Bill) Shen, PhD

Bel-Ray Company, LLC

Abstract

Open gears operate under severe conditions, often at their design limits. Severe conditions include extreme pressures, varying speed and contamination. Possibly the most important aspect of open gear lubricant selection is low temperature pumpability and the ability of the lubricant to be applied through a centralized lubrication system at the lowest anticipated ambient temperature. Considering the cost of downtime of an electric rope shovel or walking dragline in a mining operation, choosing the correct open gear lubricant for the application is critical. When it comes to selection, open gear greases using various thickeners are available and their low temperature performance needs to be examined. While keeping the additive system constant, lab samples of an open gear grease were blended with four thickener types; Aluminum Complex, Lithium Complex, Calcium Sulfonate Complex and Organo-clay. The open gear lubricants’ low temperature properties were evaluated using standard tests to determine the effects of the thickener type on their low temperature properties.

Keywords

Open gear lubricant; Thickener; Ventmeter; Grease Low Temperature Properties.

Introduction

The earliest open gears have been used since the beginning of rotational machinery and were made using wood with cylindrical pegs and lubricated with animal fat. Engineers developed metal gears during the British industrial revolution in the mid-18th century. It was not until a century later that open gears and the way they were lubricated were modernized. Large heavy-duty open gears are a very common method of transmitting power. In addition, they are the most economical type of gear drive where high load-carrying capacity is required under severe shock load conditions for extended periods of time.

Typically, there are two varieties of gear systems; Type 1 and Type 2. For the extent of this paper, only Type 1 gear systems will be discussed. Type 1 open gear systems consist of a motor, typically an actuator and a rack system on machinery such as the hoist and drag drives of mining shovels and draglines. The main difference between Type 1 and Type 2 gear systems is the operation. During operation, Type 1 systems change speeds and are bi-directional, whereas Type 2 systems operate at constant speeds and in the same direction.

Due to the nature of Type 1 systems, different physical and performance properties need to be examined in order to properly lubricate these systems with maximum efficiency for reduced cost of operation and down time of the equipment. Variable speed and directional changes under heavy loads could pose the toughest conditions the lubricant can handle. For this reason, EP properties to protect against friction and wear, tackiness for adhesion to the gears, corrosion resistance and the ease of pumpability are all important factors in choosing the correct open gear lubricant for the application.

Several industry standards have been established for performance properties as well as another important factor; base-fluid viscosity. As aforementioned, open gear lubricants encounter some of the harshest conditions a lubricant can face. Therefore, a proper film thickness of the lubricant is critical in ensuring optimal performance

of the machinery. In addition, due to the continuous speed and directional changes of Type 1 gear systems, open gear lubricants face boundary, mixed and hydrodynamic lubrication. Heavy base-fluids are crucial in keeping the proper film thickness of the lubricant. However, this also creates the challenge of pumpability.

With regards to lubrication, it is not only the additive system or film thickness that can affect the performance of the product. Thickener type can also effect physical and performance properties of open gear lubricants. As it is widely known in the industry, there are various types of thickeners available all with their advantages and disadvantages for each type of application. This paper will focus on and offer an explanation on the effects of thickener type on the performance properties of open gear greases; specifically on the low temperature properties.

Before discussing the preparation of the samples used for testing, it is important to note the test methods used in evaluating the open gear greases:

Grease Sample Preparation

For the scope of this paper, four thickener types were examined: Aluminum Complex, Lithium Complex, Calcium Sulfonate Complex and Organo-Clay. All samples were prepared using the same proprietary base oil and additive system consisting of EP additives, corrosion inhibitors, antioxidants, mineral oils and high viscosity synthetic oils. To make the comparison, the samples were each blended by mixing the components mentioned above in a Kitchen Aid mixer with each respective thickener to make a total of four blends. In order to achieve the same consistency, a different amount of each thickener was needed. The thickener content needed to achieve an unworked penetration of an NLGI Grade 0 grease is shown below in Table 2. To evaluate the performance of each thickener, preliminary testing was conducted on the samples prepared to ensure consistent results for each sample. A dropping point and unworked penetration were tested. As a note, for the extent of this research, only the unworked penetration was tested for consistency according to ASTM D217.

There were multiple tests conducted in order to determine the low temperature properties of each of the formulas. The test conditions are inclusive of 2 temperature parameters, 0°C and 25°C. To obtain a base line for comparison, all of the tests considered necessary in measuring low temperature properties were performed at 25°C first.

The samples were then evaluated at 0°C on each test performed above to determine the values.

*Cone & Plate test was run using CPE-52 Spindle @ 2.5 rpm **Brookfield viscosity was run using a T-Bar Spindle (T-C) and Helipath stand @ 30 rpm ***Brookfield viscosity was run using a T-Bar Spindle (T-F) and Helipath stand @ 30 rpm

In order to correctly evaluate the low temperature properties of each thickener, it is necessary to determine which test results directly correlate to the performance in the application of the open gear lubricants. Two tests that that are relevant but do not necessarily correlate to the application were run for data; Brookfield viscosity and the Cone and Plate test. Both of these tests are rotational viscometers that measure the viscosity by the torque required to rotate an object in a fluid at a constant speed. A contributing factor in the difference in rheological properties of each thickener can be due to the size of thickener fibers and its gel structure. Although the exact size of the thickener fibers were not determined, the structure can affect the low temperature properties of open gear greases.

As previously mentioned, the “ease of pumpability” of the lubricant is very important. The first test that was conducted which corresponds to this is the Lincoln ventmeter test. The Lincoln ventmeter test effectively measures the temperature flow limits of a grease. It determines the greases yield stress and shear thinning index to approximate the apparent viscosity at a given temperature. In a paper by Conley and Shah1, the significance of this test is described: “By measuring the flow properties of grease, a lubricant designer or an application engineer or technician can select the pump and line size to ensure good performance of the centralized grease lubrication system, or select an appropriate lubricant for an existing system. The Ventmeter provides this information.”

After evaluating the results for the Lincoln Ventmeter test at 0°C, the Aluminum Complex formula performed best. The table below shows the percent increase in venting pressure of each sample when compared to the Aluminum Complex formula:

The second test that is important in determining low temperature performance in open gear greases is the Grease Mobility test. According to the U.S. Steel method using S.O.D. cylinder and capillary tube2 “Grease Mobility is a measure of resistance to flow under prescribed conditions”. Open gear greases can face sub-zero temperatures for extended periods of time. This makes grease mobility very important, especially in centralized lubricating systems. With regards to this test, a larger grease flow rate (g/min) indicates better mobility.

Conclusion

After evaluating the low temperature performance of open gear lubricants using various thickeners, it was determined that using Aluminum Complex as the thickener is the preferred choice. This conclusion is supported by the Lincoln Ventmeter and Grease Mobility test data. The data also shows that, although the performance is not quite as good as Aluminum Complex, Calcium Sulfonate Complex performed significantly better than Lithium Complex and Organo-Clay thickeners. This data also shows how the thickener type has a significant impact on the low temperature properties of open gear greases. Since choosing the correct open gear grease is critical in optimizing the performance of the applied machinery, thickener type needs to be carefully considered.

References

1. Conley, P., Shah, R. “Ventmeter Aids Selection of Greases for Centralized Lubrication Systems”.

Machinery Lubrication. (January 2004) 2. The Lubrication Engineers Manual. 2nd ed. Pittsburgh, PA. Association of Iron and Steel Engineers, 1996. (p.154)

Acknowledgments

The authors would like the thank Calumet Specialty Products Partners, L.P. for the support on conducting research on the effects of thickeners on low temperature properties of open gear greases. The authors would also like to thank our colleagues in the R&D lab for evaluating physical properties of open gear greases and sample preparation for this study.