11 Analytical Instrumentation
which can interrupt the oxidation process by neutralizing free radicals and decomposing peroxides, respectively [36].
Additionally, selecting base oils and thickeners with inherent resistance to oxidation can also contribute to enhanced oxidation stability. For instance, synthetic base oils such as PAOs and esters are known for their excellent oxidation resistance compared to mineral oils [11]. PAOs, derived from polymerizing alpha-olefi n monomers, provide high oxidative stability due to their uniform molecular structure and lack of impurities. Esters, on the other hand, offer superior thermal and oxidative stability due to their chemical composition and strong ester bonds.
Similarly, thickeners also play a role in determining the oxidation stability of greases. Lithium complex thickeners, for example, provide excellent oxidative stability and are widely used in high-performance greases [22]. Calcium sulfonate thickeners are another option, known for their inherent resistance to oxidation and ability to maintain consistency under severe conditions.
3.3.4 Thermal stability
Thermal stability refers to the grease’s ability to resist changes in its physical and chemical properties when exposed to high temperatures. It is a measure of the heat resistance of the grease, defi ning the upper temperature limit at which it can retain its structure and lubrication performance. The Dropping Point Test (ASTM D2265) is commonly used to assess thermal stability [29]. This test determines the temperature at which the grease becomes fl uid enough to drip through an orifi ce under standardized conditions. However, it is important to note that the dropping point does not directly indicate the grease’s performance at elevated temperatures.
High-temperature life tests, such as ASTM D3527 (Figure 7) and ASTM D3336 (Figure 8), are more accurate for evaluating thermal stability in practical applications [29]. ASTM D3527 measures the durability of grease in wheel bearings by assessing its ability to maintain performance over extended periods at elevated temperatures, simulating real-world conditions in automotive applications. Similarly, ASTM D3336, already discussed, evaluates how long a grease can effectively lubricate a bearing at high temperatures without signifi cant degradation. These tests offer a better indication of a grease’s thermal stability and suitability for high- temperature applications.
3.3.5 Thermo-oxidative stability
Thermo-oxidative stability refers to the ability of a grease to resist chemical breakdown and maintain its properties under prolonged exposure to elevated temperatures and oxygen [21]. Greases with high thermo-oxidative stability ensure consistent lubrication, prevent the formation of harmful deposits, and extend the intervals between re-lubrication.
The degradation of lubricating grease due to heat and oxygen can lead to the formation of sludge, varnish, and acidic compounds, which can impair the lubricating fi lm, cause corrosion, and increase wear on metal surfaces. To combat these issues, high-quality greases are formulated with antioxidants and thermal stabilizers. Antioxidants inhibit the oxidation process by neutralizing free radicals and decomposing peroxides, thereby preventing the oxidative degradation of the grease. Thermal stabilizers enhance the grease’s ability to withstand high temperatures without breaking down. Among these additives, alkylated naphthalene is particularly effective due to their exceptional thermal stability and antioxidant properties, making them ideal for high-temperature applications [38].
3.4 Lubrication
Effective lubrication is crucial for minimizing friction and wear in bearings and other moving components. The lubrication effi ciency of a grease depends on two key factors: the properties of the base oil and the grease’s ability to release enough oil (bleeding rate) to maintain a protective lubricating fi lm [6]. Importantly, the optimal balance between these factors will vary depending on the specifi c application.
For optimal lubrication performance in rolling element bearings, formation of an EHL lubricating fi lm is essential. This fi lm separates the moving surfaces, preventing direct metal-to-metal contact and reducing friction and wear [21]. The thickness of the EHL fi lm depends on various factors, including the operating conditions (speed, temperature, and load), as well as the presence of contaminants. Base oil properties, such as viscosity and viscosity index, play a crucial role in determining the grease’s ability to form and maintain an adequate EHL fi lm [40]. Additionally, the bleeding rate of the grease is vital for continuously replenishing the lubricating fi lm as it is depleted or displaced during operation.
For conventional greases, a minimum bleeding rate is necessary to ensure suffi cient oil supply to the lubricated surfaces. This bleeding rate dictates the maximum viscosity of the base oil that can be effectively utilized in the grease formulation. Higher viscosity base oils may provide better load-carrying capacity but may also require trade-offs with formulation adjustments to ensure adequate oil release.
3.5 Friction management
Figure 7: High temperature wheel bearing grease tester: Conforms to ASTM D3527, D4290 and D4950.
Effective friction management in machinery extends beyond environmental benefi ts to deliver substantial cost savings and enhance overall equipment effectiveness (OEE). Studies indicate that minimizing friction can signifi cantly reduce energy consumption, with potential savings ranging from 5% to 24% [41]. This translates to tangible fi nancial advantages, considering friction was estimated to cost the global economy €2.54 trillion ($2.37 trillion USD) in 2017, as shown in Figure 9 [42]. By implementing friction management strategies, businesses and industries can achieve greater operational effi ciency for reduced energy expenditures and lower overall operating costs, all while contributing to a smaller CO2 footprint. Additionally, proper friction management can minimize wear and tear on components, extending their lifespan and reducing maintenance requirements. This contributes to improved uptime and machine availability, ultimately boosting OEE.
Grease formulations play a key role in optimizing friction management. These lubricants are specifi cally designed with additives that minimize friction between moving components [43]. This aligns with implementing friction management as a driving principle for modern grease formulations. Effective friction management requires consistent friction behavior during operation, avoiding unpredictable friction behavior like stick-slip, which can be detrimental in different industrial contexts.
3.6 Noise reduction
Noise reduction is an essential aspect of grease performance, particularly in precision applications such as electric motors and household appliances, where noise levels can signifi cantly impact user experience and product quality [43]. Noise generation in bearings is infl uenced by the quality and composition of the lubricating grease.
Low-noise greases are characterized by their high purity and the use of fi nely dispersed thickeners. The structure of the grease, known as the grease matrix, is formed by the base oil, thickener, and any additives and plays a role in noise reduction. A well-formulated grease matrix can minimize noise by preventing hard particles from forming or existing within the grease. Noise in bearings can be generated by particles traveling through the rolling element-ring contacts, which can originate from contaminants or the thickener itself. Solid additive particles, such as graphite and calcium carbonate, can increase noise levels by introducing additional abrasive elements into the bearing system [6]. In contrast, fi nely dispersed thickeners help suppress noise by eliminating interactions between hard particles.
To further decrease noise levels, greases with higher base oil viscosity, typically up to 100 cSt at 40°C can be utilized. These greases help reduce noise by providing better dampening of high-frequency excitations and vibrations [44]. The increased viscosity contributed to a smoother lubrication fi lm, minimizing the transmission of vibrations and noise. In addition to base oil viscosity, advanced thickeners and additives can be incorporated into grease formulations to enhance their noise-reducing capabilities. These specialized components work by dampening vibrations and absorbing high-frequency excitations, resulting in quieter operation and improved product quality.
Noise reduction is important in both industrial and consumer applications, as excessive noise can lead to worker discomfort, product dissatisfaction, and even regulatory compliance issues [45]. Sustainable grease formulations now incorporate additives and design considerations that prioritize noise reduction by damping vibrations and providing a smooth, consistent lubrication fi lm. These low-noise greases are particularly valuable in applications such as electric motors, automotive components, and household appliances, where noise reduction contributes to overall product quality, user comfort, and environmental impact.
3.7 Corrosion resistance
Corrosion resistance is another key performance parameter for lubricating greases, especially in environments where metal surfaces are exposed to moisture, salts, and other corrosive agents [21]. Effective corrosion resistance extends the life of machinery, reduces maintenance costs, and ensures operational reliability. Lubricating greases that excel in corrosion protection typically incorporate advanced additive technologies that form a protective barrier on metal surfaces. This barrier prevents the ingress of water and corrosive substances, thus safeguarding the metal components from oxidative damage and corrosion.
Figure 8: ASTM D3336 test: High temperature bearing endurance test machine [36].
Achieving high thermal stability in grease formulations typically involves the use of high-performance base oils and thickeners that are resistant to thermal degradation. Bio- based and synthetic esters are increasingly used due to their excellent thermal properties, ensuring that the grease remains effective at both high and low temperatures, thereby reducing the need for frequent re-lubrication and minimizing waste [37].
One of the primary mechanisms through which greases provide corrosion resistance is by including inhibitors that neutralize acids formed during operation. These inhibitors are carefully selected to ensure compatibility with other grease components while providing robust protection against rust and corrosion. Additionally, some greases utilize fi lm-forming agents that create a continuous, hydrophobic layer over metal surfaces. This layer repels water and other corrosive agents, further enhancing the corrosion resistance of the lubricated components.
3.8 Fretting corrosion and false brinelling
Figure 9. Global friction and wear effects on energy, costs, and CO2 emissions [42].
Fretting corrosion is crucial in applications involving oscillatory motion that causes wear in contact areas. Fretting occurs between the outer ring and housing or inner ring and shaft, while false brinelling occurs in rolling element-ring
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