Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1399684
www.machinerylubrication.com | July - August 2021 | 43 ML ML and contaminants. It aids in trending the depletion of additives, oxidation and changes in oil viscosity as the oil ages. Oil manufacturers can be consulted to clarify if the elements present are part of a lubricating oil formulation or if they are a result of contamination or debris. 2. Sampling location Critical to the analysis of oil condition is the consistency in both time and location that the data is collected (Figure 1). Due to economic and resource variables, it is also important to focus on assets that are considered critical to the user's operation. Location of oil sampling, e.g., the gearbox's sump, will yield statistically significant gauge that is repeatable and reproducible. Deviating from consistently sampling oil from the same point and oper- ating conditions may result in discrepant data, and/or the data will be more difficult to trend. 3. Oil sample intervals It is imperative that the oil is sampled at regularly scheduled intervals. is will allow consistent trending of the used oil sample results among multiple similar gearboxes, leading to more effective corrective actions. Oil change intervals are usually established by OEMs. It may be possible to extend both used oil samples and oil change intervals depending on the application. However, care should be taken when doing so since the equipment may be damaged and/or machinery warrantees may be voided. Main- tenance professionals should be familiar with both gearbox condition and application before making this decision. 4. Representative used oil sample It is best to take a used oil sample while the unit is running during normal operating condi- tions after the unit has reached thermal stability where the oil has turbulent flow (Figure 2). Strict adherence to all safety guidelines should be followed when taking oil samples from any equipment in operation. Should it not be safe to do so, the oil sample should be taken imme- diately after the gearbox is shut down. Taking a used oil sample once the unit has cooled down or has been shut down for an extended period of time will not provide a representative sample, as contaminants will collect at the bottom of the sump. Also, when taking samples on sections where oil has a laminar (smooth) flow, the contaminants or wear particles shown in the oil analysis result may not be representative. Recommended oil tests for gearboxes and interpretation e following tests are useful to determine the condition of a gearbox, as well as under- standing how the oil is affected by both the operation of the unit and application, including environmental conditions. Viscosity Description: Viscosity tests "indicate fluid's resistance to flow at a given temperature," usually measured and reported at 104°F (40°C). e units of measurement are reported in centistokes (cSt). Interpretation: e viscosity of the oil is prone to change due to wear and chemical reac- tions. e results of other tests can be helpful to determine what happened with the oil during operation. As an example, an increase in oxidation trending levels could serve as a symptom that explains how the oil viscosity increased in the system. e possible causes of changes in oil viscosity are the following: • Increased Viscosity: Oxidation, thermal failure, water contamination and wrong oil. • Decreased Viscosity: VI improver (addi- tive) shear-down, base oil shear-down, base oil cracking and wrong oil. e Viscosity Analysis can help to deter- mine if both the baseline and used oil are within specifications. For example, assuming that we have a gearbox that uses a lubricating oil with a viscosity of 220 cSt, the expectation is that the viscosity test result for that partic- ular oil is within a 5% range of 220 cSt. As a reference, the most common designations according to ISO Standard 3448 are listed in Table 1. ey are a good reference to compare the oil viscosity with the product we should be using for a particular gearbox, even though this reference applies to new oil only. Figure 1: Oil sampling on in-line gearbox. Figure 2: Sampling by the reduction com- ponent's oil level ensures a turbulent flow for a representative oil sample. Table 1: Viscosity Grades based on ISO 3448. ISO VG MIDPOINT VISCOSITY cSt @ 40°C KINEMATIC VISCOSITY LIMIT cSt @ 40°C Lower Upper 100 100 90 110 150 150 135 165 220 220 198 242 320 320 288 352 460 460 414 506