Gear pairings often run under very high loads. That can result in different kinds of failure modes limiting their lifetime. Many of the known gear failure modes are tribologically influenced. Especially for gear pairs running with lower circumferential speeds or with different surface hardness, (continuous or slow speed) wear is often the lifetime limiting factor. Slow speed wear appears continuously over a longer period of runtime. In many cases, such applications are lubricated with greases. Since the standardized calculation methods (e.g. ISO 6336) do not cover any determination of wear, one common way to predict the wear lifetime is the calculation method according to Plewe. In the associated Plewe diagram the worn off amount of material is correlated to the minimal lubricant film thickness in the tooth contact. The wear intensity decreases for higher film thicknesses. However, this method has certain limits for greases, because the film thickness of a grease, its bleed oil and the base oil is not necessarily the same. Additionally, the consistency and the flow properties have to be considered, because they influence the lubrication supply mechanism (circulating or channeling). Under certain circumstances channeling could be predominant. Although in theory a grease should build a thicker lubricating film than its base oil, experimental investigations have shown higher wear rates in comparison to oil lubrication.
Gear flank changes caused by wear do not only affect the dynamic behavior of gear systems, but they can also compromise the load-carrying capacity of gear teeth up to critical failure. To help avoid unintended consequences like downtime or safety risks, a condition monitoring system needs to be able to estimate the current wear during operation based on available sensor measurements. While many condition monitoring approaches in research rely on vibrational analysis with manual feature engineering, gearboxes running at slow speed do not reveal much excitation information for this purpose. We therefore introduce an approach for slow-speed gear wear monitoring that is based on the dynamic gear transmission error and that contains an automated feature selection process. For this purpose, we extract a large set of features from the preprocessed transmission error samples. Applying combined filter and embedded feature selection methods enables us to automatically identify and remove features with low relevance. The selection process consists of filtering features with no statistical dependence on the target wear value, removing redundant features with a correlation analysis and a recursive feature elimination process with cross-validation based on a random forest regressor. The remaining relevant set of features is the basis for model training and subsequent wear estimation. For this, the present research employed two independent ensemble models, random forest regression and gradient boosted regression trees. To train and test the proposed approach, we conducted slow-speed gear experiments with developing gear wear on a single-stage spur gear test rig setup. The results of both models show good gear wear estimation performance compared to the actual wear mass loss, even for small quantities. Hence, the proposed transmission error-based approach with automated feature selection is able to quantify the degree of slow-speed wear and offers a possible way for condition monitoring and fault diagnosis.
Greases have a variety of advantages when special operating conditions apply. Mainly related to large, slow-running gear drives such as used in heavy industry applications, grease lubrication can also be the preferred solution for small, fast running gear drives. Conse-quently, the calculation of the wear service life within the gear design process is essential. Due to their flowing properties, there is a danger of losing the lubrication supply depending on the operating conditions, boundary conditions and grease properties. While a circulating lubricant ensures a continuous lubricant supply to the gear mesh, channeling includes the risk of starved lubrication and consequentially, even a discontinuation of the lubricating film that can lead to heavy damages of the gearbox. The experimental investigations on a modified FZG back-to-back test rig show a strong effect of operating conditions and grease properties on the lubricant supply: A higher amount of grease in the gearbox and a higher lubricant temperature support circulating, whereas, a higher consistency of the grease supports channeling. Based on the results, a first calculation approach is developed that approximates the lubricant supply of grease lubricated gears for the gear design process.
In large gear drives, through-hardened ring gears or internal gears are often in contact with case-hardened pinions. This offers economic advantages, but involves an increased risk of wear during its operation. Various test methods exist to evaluate the expected wear of the used gear-lubricant-system, but typically with case-hardened gears only. A direct transfer of the test results to the application in field is not known so far. In this study, influences from different surface hardness and surface roughness values as well as lubricating conditions were systematically investigated on a FZG back-to-back test rig. The results prove a higher risk for strong wear for hard-soft gear pairings. Based on these results, a calculation method for the wear lifetime of gears was extended to allow an estimation of the wear lifetime of gear pairings with different surface hardness based on standard wear tests.
Greases have a variety of advantages when special operating conditions apply for gear lubrication, e. g. improved properties against continuous (or slow speed) wear. Often, gears with different surface hardness are combined, in order to reduce costs during manufacturing and heat treatment. However, this has detrimental effect on the wear lifetime. Consequently, the calculation of the wear service life within the gear design process is essential. One common way of wear lifetime prediction is the calculation method acc. to Plewe that is typically combined with a standardized wear test with case carburized gears. It allows to calculate the wear carrying capacity for different gear-lubricant-systems, but a conversion between gear stages of different gear materials is not possible so far. Based on theoretical studies, influences from different surface hardness and surface roughness values as well as lubricating conditions were investigated on FZG back-to-back test rig. The results prove that hard-soft gear pairings have a higher risk for strong wear compared to case hardened gear stages, but basically follow the same trends and mechanisms. Furthermore, surface roughness of the harder gear has an effect on the wear life time of the softer gear. Based on these results the existing calculation method for the wear lifetime of gears was extended.
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