32 Analytical Instrumentation


RECENT ADVANCES IN PREDICTIVE MAINTENANCE TECHNIQUES FOR LUBRICANT CONDITION MONITORING


1. Introduction


Lubrication is a critical component in maintaining the effi ciency and reliability of mechanical systems. The primary function of lubricants is to reduce friction and wear between moving parts, which signifi cantly extends the operational lifespan of equipment. However, over time, lubricant conditions degrade due to contamination, oxidation, and thermal stresses, which can lead to machinery failure if left unchecked. Lubricant condition monitoring (LCM) is thus employed to maintain the health of lubricants to prevent machine malfunctions. Traditional maintenance strategies, such as time-based or reactive approaches, often fail to account for real-time variations in lubricant conditions, leading to unexpected downtime and costly repairs. Predictive maintenance techniques, particularly those involving lubricant condition monitoring, off er a solution by utilizing advanced modeling and sensor data to predict and prevent failures before they occur. This paper details the potential incorporation of diff erent techniques, such as sensors, vibration analytics, and soft computing models, to make LCM more mainstream and easily incorporated into already established systems.


2. Global Performance Passport


LCM refers to proactive maintenance strategies that can assess the current lubricant health or predict the lubricant’s future performance. The monitoring of lubricant properties over time allows for preventative maintenance and helps preserve the lifespan of machinery by preventing equipment failure. Real-time tracking of these properties can reduce maintenance costs by eliminating over-maintenance and the uncertainty that problems with the machinery are a result of lubricant degradation. Thus, it would be benefi cial for all industries to adopt tested LCM techniques and share data collected from different applications.


A practical and perhaps traditional approach to standardizing lubricant maintenance and health across industries involves compiling key characteristics and performance metrics into a “global performance passport”4


. Just as Safety Data Sheets (SDS) provide workers with essential information about the proper handling and use of potentially hazardous chemicals5 , the global


performance passport would serve a similar purpose for lubricants. It would contain identifying characteristics of the lubricant, including the lubricant’s name, physical properties and chemical composition. This would allow employees to easily identify the specifi c type of lubricant in use, especially when working with various machines that require different lubricants. While current lubricant manuals containing this information are typically obtained from the manufacturer, a global performance passport would ideally standardize this information across all industries and lubricant types in a consistent format, making it easier to use, share and apply universally.


Beyond identifi cation, the global performance passport would also include the lubricant’s ideal performance test results and operating conditions as well as maintenance history4


. Performance


metrics such as viscosity, acidity, and water content would set a standard for the lubricants’ usable range, while operating conditions will defi ne the specifi c application and environment the lubricant is suitable for. For instance, Figure 1 compares the tribological properties of fresh and worn hydraulic power transmission H46 oil4


Figure 1: Tribological feature of global performance passport for hydraulic power transmission H46 oil4


healthy lubricant use. The system could also be abused through data falsifi cation from companies or individuals attempting to avoid necessary maintenance or repairs. The unifi cation of such information can best be put into place by established standardization organizations. For instance, the American Society for Testing and Materials (ASTM) International is a global organization that develops and delivers voluntary consensus standards with over 12,000 standards in use around the world6


. ASTM International already has standards on lubricant properties and testing, but it


does not cover everything a global performance passport would provide. If large organizations develop and publish LCM standards, industries that adhere to their policies will follow suit, contributing to the global standardization of LCM.


3. Sensors


Sensors play a critical role in LCM, as they can directly measure key indicators of lubricant health in real-time. This eliminates the need for frequent disassembly of machinery to check for lubricant degradation. Excessive maintenance could cause the loss of lubricant, introduce contaminants to otherwise healthy lubricants, increase downtime, or damage machine components due to constant disassembly and reassembly. Often, lubricants are replaced before the end of their life cycle, but the integration of sensors into LCM would optimize both lubricant and machinery lifespan, reducing waste from unnecessary lubricant changes7


.


In railway applications, lubrication is essential for maintaining the smooth operation of moving parts and extending the longevity of equipment. Axle box bearings, in particular, have a high load rating as they support the axles and wheels of high-speed railway vehicles8


were lubricated with mineral oil but were later replaced with lithium grease due to heavy oil loss leading to reduced lubricant effectiveness and environmental contamination9


. The higher viscosity allows for a reduction in initial lubricant . Initially, these bearings . As shown in Table


1, a commonly used grease in axle box bearings called Arapen RB 320 is made from mineral oil thickened with a lithium calcium soap, resulting in a kinematic viscosity about 5 times greater than that of pure mineral oil at 40°C10,11


amount and loss during operation. LCM for lubricating grease will primarily focus on its water content since water can alter its properties (e.g., reducing viscosity) and cause corrosion in the metal bearings.


Researchers at the Austrian Center of Competence for Tribology tested commercially available humidity sensors in axle box bearings to evaluate their effectiveness and robustness12


. These


sensors successfully detected increases in humidity–indicating water intake–and withstood challenging railway environmental factors, like thermal cycling, relative humidity, and mechanical load. Figure 2 shows temperature and relative humidity measurement results from random vibration tests and shock tests by comparing the sensor’s measurements to reference values12


. . The noticeable dip seen in the worn oil’s performance


would set a clear threshold for its useability, enabling workers to easily identify when the oil is nearing the end of its effective life. The inclusion of an up-to-date maintenance log would allow for easy tracking of changes over time, so that employees can visually see a decreasing trend in lubricant health and schedule an inspection and replacement in advance.


However, measurements of performance may be too generalized to apply to all types of lubricants, and storing information in a centralized database may leave it vulnerable to data breaches. Different industries and countries may also fi nd it hard to agree on the same set of standards for


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The sensor in the random vibration test showed relatively similar outputs with the reference values, while the shock test indicated no change in the sensor’s capabilities when subjected


Table 1: Viscosity comparison of grease and oil lubricants10,11


to abrupt acceleration. This suggests that the humidity sensor can perform accurately under stressful conditions, potentially making them a non-invasive tool for LCM in railway applications.


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