9 Analytical Instrumentation


fi lm may disappear entirely, leading to direct metal-to-metal contact between moving surfaces. This direct contact can cause signifi cant wear, increased friction, and potentially catastrophic seizure or failure of the machinery components. Reaching the starvation phase represents the failure point of the grease’s protective capabilities and must be avoided. Visual examples of each phase are illustrated in Figure 1.


Proper grease formulation aims to minimize the occurrence and prolong the onset of the starvation phase, thereby preventing premature machinery failure. This is achieved by optimizing the balance and compatibility of the base oils, thickeners, and additives [28]. Selecting high-quality base oils that provide the necessary viscosity, thermal stability, and oil- bleed characteristics is crucial. Moreover, choosing thickeners that can withstand mechanical stress, extreme temperatures, and environmental conditions helps maintain the grease structure and consistency. Incorporating performance- enhancing additives, such as antioxidants and anti-wear agents, further extends the grease’s service life and protective capabilities.


Figure 1. The phases in the grease lubrication process and the lubrication mechanisms [6].


to stabilize emulsions and reduce surface tension effectively. This dual affi nity makes metallic soaps excellent thickeners, as they can interact with both the base oil and the polar additives present in the grease formulation.


Table 2 presents a comprehensive list of organic soap greases alongside their respective physical and tribological properties, providing a useful reference for selecting the appropriate thickener based on the specifi c application requirements.


2.1.3 Additives


Although comprising only a small percentage (typically 0-10%) of the grease composition, additives are vital components that enhance the performance and functionality of the grease formulation. These additives impart additional properties, such as improved oxidation stability, corrosion resistance, and extreme pressure (EP) tolerance, tailored to meet the specifi c needs of different applications and operating conditions [23].


Oxidation inhibitors, or antioxidants, help to prevent the base oil from degrading due to exposure to oxygen and high temperatures, prolonging the grease’s service life and preventing premature breakdown [21]. Anti-wear additives, such as zinc dialkyldithiophosphates (ZDDPs) and other organic compounds, form protective fi lms on metal surfaces, reducing friction and wear, particularly in boundary lubrication regimes [21].


Rust inhibitors and corrosion inhibitors protect metal components from corrosive attack by forming barrier fi lms or neutralizing corrosive agents [6, 24]. EP additives, like sulfur-based compounds or chlorinated paraffi ns, improve the grease’s ability to withstand high loads and prevent seizure or welding of metal surfaces under extreme pressure conditions. Other additives, such as viscosity index improvers, pour point depressants, and friction modifi ers, can be incorporated to optimize the grease’s performance in specifi c applications or operating environments [25]. Viscosity index improvers help maintain a consistent viscosity across a wide temperature range, ensuring the grease remains effective under varying conditions. Furthermore, pour point depressants reduce the temperature at which the oil forms a thick wax layer, which


is particularly benefi cial during the winter months. Friction modifi ers decrease the energy required for surfaces to move past one another, enhancing effi ciency. Collectively, all these additives contribute to improved grease performance across various applications.


By carefully selecting and combining the appropriate additives, grease manufacturers can tailor the formulation to meet the specifi c performance requirements of various applications, ensuring optimal lubrication, protection against wear and corrosion, and extended service life for machinery and equipment under diverse operating conditions.


2.2 Grease lubrication


Grease lubrication operates through three primary phases or mechanisms: churning, bleeding, and starvation [6]. In the initial churning phase, the freshly applied grease undergoes macroscopic fl ow, providing a fully fl ooded lubrication regime. During this phase, the grease is distributed evenly across all contact surfaces of the machinery components, ensuring they are adequately coated and lubricated. This even distribution is essential during initial start-up, as it helps to prevent immediate wear and friction between moving parts.


As the machinery continues to operate, the grease transitions into the bleeding phase. In this phase, the grease releases, or bleeds, a thin fi lm of base oil by phase separation from the thickener matrix [26]. This oil release is governed by a balance of supply from the grease reservoir and loss mechanisms such as evaporation, centrifugal forces, or absorption into porous materials. The released oil creates a starved elastohydrodynamic lubrication (EHL) regime, ensuring that a thin lubricating fi lm is consistently present at the contact surfaces. This fi lm reduces friction and wear between moving components. The bleeding phase allows the grease to perform its protective lubrication functions over an extended period, even under varying operational conditions.


However, in extreme conditions or near the end of the grease service life, when the grease can no longer maintain an adequate supply of oil to sustain the EHL regime, it enters the starvation phase [27]. During this phase, the lubricating


Table 2. Common organic soap greases with their respective physical and tribological properties as well as common applications [22]. Thickener


Key characteristics Lithium soaps Calcium soaps Sodium soaps Aluminum soaps


Good lubricity, shear stability, thermal resistance, low oil separation, high dropping points (~180°C)


Improved water resistance over the lithium greases, good shear stability


High dropping points (~175°C), good shear stability and lubricity


Excellent oxidation resistance, good water resistance


Applications


Bearings in automotive and industrial applications


Used in applications up to 110°C, bearings of water pump, wheel bearing and agricultural vehicles


Bearings in aerospace, wheel bearings, universal joints, and axle journal boxes


Vibrating screens, elevator drive motors and governors where reverse motion occurs and large electric motors with bearings operating at high linear speeds


Figure 2. Penetrometer with a penetration cone: Conforms to ASTM and related specifi cations for penetrometers.


3 Key Performance Parameters of Lubricating Grease 3.1 Consistency


Consistency is a critical parameter that determines the ability of grease to resist deformation under applied force, indicating its thickness or stiffness. It is a measure of the grease’s semi-solid nature and its ability to stay in place and maintain continuous contact with the lubricated surfaces.


The consistency of grease is typically measured using the cone penetration test, as outlined in ASTM D217 or ASTM D1403. In this test, a standardized cone, as depicted in Figure 2, is allowed to sink into the grease sample at a controlled temperature (usually 25°C) for a specifi ed time (typically 5 seconds) [29]. The depth of penetration, measured in tenths of a millimeter, is used to classify the grease into various consistency grades, established by the NLGI, ranging from 000 (semi-fl uid) to 6 (block-like solid), shown in Table 3 [21]. Examples of such greases include Tufoil, which is classifi ed as a 000-grade grease suitable for low-viscosity applications. On the other end of the spectrum, 6-grade greases are used in applications where maintaining containment is imperative. These examples illustrate the range of applications and performance characteristics associated with different grease consistencies.


Higher penetration numbers indicate softer consistencies, while lower numbers denote harder or stiffer greases. The appropriate consistency grade is selected based on the specifi c application requirements, such as the type of bearing, operating temperature range, and the degree of mechanical stress involved. For example, greases with higher consistencies are typically preferred for applications involving high loads or high temperatures, as they resist being squeezed out or thinning under these conditions.


Maintaining the desired consistency is crucial for ensuring that the grease can continue to provide effective lubrication over an extended period. This is especially important in applications where the grease is subject to mechanical working, as it must retain its consistency to function properly.


3.2 Operating temperature range


The operating temperature range is a critical parameter that defi nes the temperature window within which a grease can perform effectively. This range is typically defi ned by two key limits: the Low Temperature Performance Limit (LTPL) and the High Temperature Performance Limit (HTPL) [21]. These


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