10 Analytical Instrumentation Table 3. NLGI grades with their corresponding penetration [21].


NLGI Grade 000 00 0 1 2 3 4 5 6


Worked Penetration Range, 25°C 445 – 475 400 – 430 355 – 385 310 – 340 265 – 295 220 – 250 175 – 205 130 – 160 85 – 115


performance limits are more complex than a single test, involving multiple assessments and factors, such as grease life and friction, to accurately determine them.


The LTPL refers to the grease’s ability to fl ow and deliver oil to the lubricated surfaces during startup and cold operation. Factors infl uencing low temperature performance include oil viscosity, thickener type, and the presence of pumpability improvers. Below this limit, the grease may become too stiff, hindering oil bleeding, which can lead to increased friction, wear, and potential startup issues.


The HTPL defi nes the maximum temperature at which the grease maintains its lubricating properties and structural integrity. Various test methods assess high temperature performance, such as the high-temperature grease life test (ASTM D3336) in Figure 3 and the dropping point test (ASTM D2265) in Figure 4 [29].


ASTM D3336 measures the duration a grease can lubricate a bearing under high temperatures without failure. On the other hand, ASTM D2265 determines the temperature at which the grease changes from a semi-solid to a liquid state. These tests measure factors like wear rate, torque increase, and changes in consistency at elevated temperatures. While the dropping point provides a general indication of high- temperature resistance, it is not the sole indicator. A safety margin is always applied to account for real-world conditions and ensure reliable performance.


Within the established LTPL and HTPL, grease formulations can be optimized for specifi c needs, such as extending bearing life, reduced friction, or noise control, depending on the application and service conditions. Thus, it is essential to select a grease with an appropriate temperature range that aligns with the expected conditions of the application. Greases with broader operating ranges offer greater versatility and can accommodate potential fl uctuations in operating temperatures. However, it is equally important to remember that a broader range of greases might involve trade-offs in terms of other performance characteristics compared to a more specialized option.


3.3 Stability


Stability is a performance parameter that encompasses several aspects, including structural, chemical, oxidation, and thermal stability [21]. These factors determine the ability of a grease to maintain its desired properties and performance under various operating conditions and environmental exposures.


3.3.1 Structural integrity


Structural integrity, also known as mechanical stability or shear stability, refers to the ability of grease to maintain its consistency and structure under mechanical stress or shearing forces. This parameter is particularly important for applications involving repeated or continuous motion, such as bearings, gears, or other dynamic components. The structural integrity of grease is typically assessed using the cone penetration test (ASTM D217), which measures the change in consistency after the grease is subjected to mechanical working or shearing for a specifi c period [30, 31]. ASTM D217 simulates the shearing forces experienced by the grease during operation but does not directly measure performance. This test helps determine how well the grease can withstand dynamic conditions, such as those in rolling element bearings or gear systems.


Greases with high structural integrity or mechanical stability exhibit minimal changes in consistency after being worked or sheared, indicating their ability to maintain their structure under dynamic conditions. Structural integrity is infl uenced by factors such as the type and concentration of thickener used in the grease formulation, as well as the interactions between the thickener and the base oil [32]. Greases with higher thickener concentrations exhibit better mechanical stability, as the thickener network provides resistance to shearing forces through stronger chemical bonding. Conversely, greases with poor structural integrity may experience thinning or softening under shear, leading to potential leakage, decreased lubrication performance, and increased wear on components.


Therefore, selecting greases with adequate structural integrity for the specifi c application is crucial to ensure reliable performance and extended service life of the lubricated components. This selection can help reduce maintenance costs and downtime, ultimately contributing to more effi cient and cost-effective operations.


3.3.2 Water stability


Figure 3. High temperature grease life tester (230 V, 50 Hz): Conforms to ASTM D3336.


Water stability, or water resistance, is an important aspect of chemical stability, particularly for lubricants used in environments prone to water exposure, such as marine and outdoor applications where moisture or high humidity is common. Water resistance addresses the ability of a grease to stay put and maintain its lubricating properties when faced with water.


To evaluate this characteristic, the water washout tester depicted in Figure 5, which adheres to ASTM D1264 specifi cations, is employed. This test evaluates this by rotating a lubricated ball bearing at 600 rpm while spraying it with water at controlled fl ow rates and temperatures of 100°F and 175°F (38°C and 79°C) [29]. The test setup includes a reservoir with a heater and thermoregulator for precise temperature control, and a circulation system with a pump and fl owmeter directing water through a 1 mm nozzle aimed at the bearing. After testing, the bearing and shields are weighed to determine the amount of grease loss, thereby assessing the grease’s resistance to water washout. Although this test does not measure direct lubricating performance, it is essential for determining a grease’s ability to maintain its position and function in wet conditions.


Figure 4. High temperature dropping point apparatus: Conforms to ASTM D2265 and ASTM D4950.


Figure 5. Water washout tester: Conforms to ASTM D1264, D4950 and related specifi cations.


Advancements in grease formulations have led to the development of sustainable grease products that incorporate water-resistant additives. These additives are often non-toxic and biodegradable, enhancing the environmental compatibility of the grease [33]. They function by forming a protective barrier around the grease, repelling water, and minimizing the risk of emulsifi cation or breakdown of the grease structure. This protective barrier not only prevents washout but also ensures long-lasting protection against water ingress.


In addition to water resistance, these additives may also provide improved corrosion protection, oxidation stability, and overall grease life. However, it is important to note that while water resistance is crucial, it should be balanced with other performance requirements, such as compatibility with seals and materials, and overall environmental impact and biodegradability, to ensure a comprehensive and sustainable grease solution [6].


3.3.3 Oxidation stability


Oxidation stability measures the resistance of grease to chemical reactions with oxygen, which can lead to the formation of harmful deposits like gum, sludge, and lacquer [34]. These deposits can cause sluggish operation, increased wear, and reduced clearances, shortening the grease’s service life. Long-term exposure to high temperatures accelerates the oxidation process.


To assess the oxidation stability of greases, methods such as the rotating bomb oxidation test (ASTM D942) using the apparatus shown in Figure 6 and the pressurized differential scanning calorimetry (PDSC) are commonly used [29]. The ASTM D942 test evaluates the grease’s ability to resist oxidation by measuring the pressure drop in a sealed vessel containing the grease and oxygen at elevated temperatures. In contrast, PDSC assesses the oxidative stability by measuring the heat fl ow associated with oxidation reactions under controlled temperature and pressure conditions. These tests provide valuable insights into how greases will perform over time when exposed to oxidative environments.


Figure 6. Oxidation stability test apparatus for lubricating greases: Conforms to ASTM D942, IP 142, DIN 51808, FTM 791-3453.


Greases with better oxidation stability exhibit minimal changes in consistency, color, and other properties over time when exposed to oxidative conditions. Improving oxidation stability can be achieved using antioxidant additives, which inhibit or slow down the oxidation reactions [35]. Common antioxidants include amine-based and phenolic compounds,


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