Karsten Stahl

• Prof. Dr.-Ing.
• Full Professor @ Gear Research Center (FZG) at Technical University of Munich

In gearboxes, the transmission of power and motion is accompanied by power losses. Load-independent power losses are particularly relevant at higher circumferential speeds. In the literature, load-independent power losses are classified according to their phenomenological characteristics. The identified types of interaction include churning, windag...

According to the current state of the art, wear and efficiency of worm gears can be calculated using ISO/TS 14521 [1] which is based on experimental investigations of the past decades. Concerning wear, the material properties of the typically softer worm wheel are major influence factors on the wear behaviour. Herein, the microstructure of the whee...

Case hardening is the dominant heat treatment process for modern gear applications due to decades of continuous development. It is used to produce a hard and wear-resistant surface layer while maintaining a ductile core structure. In conventionally case-hardened components, the surface layer consists mainly of martensite and a remaining proportion...

The present study delves into the connection between the driving comfort and safety of electric vehicles and the e-drive transmission fluid (ETF), examining how water influx arising from environmental factors and iron particles resulting from wear of various transmission components can lead to performance degradation in the lubricant. The performan...

In the design process of gear systems, various computational tools are used to optimize the operational behavior. Due to time constraints, the initial stages of development typically assume ideal, deviation-free gear geometries. However, deviations resulting from manufacturing significantly affect performance in later applications. This publication...

In the design process of gear systems, various computational tools are used to optimize the operational behavior. Due to time constraints, the initial stages of development typically assume ideal, deviation-free gear geometries. However, deviations resulting from manufacturing significantly affect performance in later applications. This publication...

Condition monitoring systems are widely used in gearboxes. Gears are one of the most crucial components for power transmission. Hence, the optimal sensor positions for condition monitoring of gears should be investigated to maximize reliability and to minimize costs. This work aims to analyze measured signals from rotating sensors at gears and comp...

Gear and gearbox design require reliable strength numbers to calculate the tooth root load carrying capacity. These values can be determined directly by experiments on the one hand, which is often associated with considerable effort and costs, or, on the other hand, with the aid of established calculation standards, such as ISO 6336‑3. This is base...

During the last two decades, electricity generation by wind power increased rapidly. This is mainly due to efforts to reach CO2 neutrality and the low levelized cost of wind energy. Also, the power generated by a single wind turbine with a gearbox steadily grew during the last years, reaching up to 15 MW in prototypes by 2023. High power density du...

Water-soluble polyalkylene glycols have become a growing subject of research to achieve liquid superlubricity in elastohydrodynamically lubricated contacts. While the influence of various factors, including water content and viscosity, has been extensively studied, the underlying mechanisms responsible for liquid superlubricity under elastohydrodyn...

The primary design criteria for gearboxes are (i) power density and load capacity, (ii) noise, vibration, and harshness, and (iii) efficiency and heat balance. To meet these design criteria, it is essential to use accurate calculation models throughout the design process. The total power loss can be divided into portions of gears, bearings, sealing...

Wet brakes and clutches are crucial drivetrain components widely used in many sectors. However, their drag losses can account for a considerable share of the overall drivetrain losses. Therefore, investigation of the drag loss behavior of wet brakes and clutches has been the subject of research since the 1970s. This paper aims to provide a comprehe...

Vorwort Sehr geehrte Damen und Herren, die rasante Transformation im Maschinen- und Fahrzeugbau – getrieben durch Elektrifizierung, Digitalisierung und Nachhaltigkeitsziele – stellt etablierte Systeme wie Kupplung und Bremse vor neue Aufgaben. Und gleichzeitig eröffnet sie faszinierende Möglichkeiten zur Weiterentwicklung dieser für mechatronische...

Vorwort Sehr geehrte Damen und Herren, die rasante Transformation im Maschinen- und Fahrzeugbau – getrieben durch Elektrifizierung, Digitalisierung und Nachhaltigkeitsziele – stellt etablierte Systeme wie Kupplung und Bremse vor neue Aufgaben. Und gleichzeitig eröffnet sie faszinierende Möglichkeiten zur Weiterentwicklung dieser für mechatronische...

Advancements in manufacturing technology provide users with new opportunities for the design of gears. Special gearings with customized non-involute tooth profile geometries offer a promising alternative to conventional involute gears, addressing limitations, such as undercutting and small radii of curvature near the tooth root. However, the absenc...

Condition monitoring is crucial for the predictive maintenance of gear transmissions to avoid unexpected failures with downtimes, costly repairs, or dangerous incidents. While vibration-based condition monitoring is widespread in industry and research, using angle measurements as a primary source still offers much potential. This review provides in...

Zusammenfassung Seit der Covid-19-Pandemie wird den Lehrenden immer wieder empfohlen, das Beste aus den zwei Welten „Online-“ und „Präsenz“-Lehre in die eigene Lehre zu integrieren. Aber woran erkennen Lehrende, an welcher Stelle sich eher analoge, digitale, asynchrone oder synchrone Lehrelemente anbieten? Ziel des Projekts „NewNormal – Kompetenz-...

For designing gears reliable strength values are essential. The determination of strength values for the tooth root load-carrying capacity is usually based on experimental fatigue tests in a pulsator due to various advantages compared to back-to-back tests. In contrast to steel gears, it is currently unclear how polymer gears perform in the pulsato...

In the Priority Program 2305 of the German Research Foundation, so-called Sensor-integrating Machine Elements (SiME) are to be developed. These are essentially highly standardized components with integrated microelectronics. The present article presents the development of a model-based construction kit to support the design of integrated sensor sys...

Thermoplastics show great potential due to their lightweight design, low-noise operation, and cost-effective manufacturing. Oil lubrication allows for their usage in high-power-transmission applications, such as gears. The current design guidelines for thermoplastic gears lack reliable estimates for the coefficient of friction of oil-lubricated rol...

A fundamental understanding of wet clutches’ drag loss behavior is essential for designing efficient clutch systems. It has been widely recognized that the separation behavior immediately after disengaging the clutch and the resulting clearance distribution influence the drag loss behavior. However, these influencing factors have yet to be systemat...

The efficiency of bevel and hypoid gears is, alongside load capacity, one of their most important design criteria. To consider the efficiency of bevel and hypoid gears during the development and design process, validated calculation methods based on experimental investigations are necessary. However, the isolated experimental investigation of the l...

High-speed electric drive units promise improved power density and, theoretically, driving range of battery electric vehicles. An essential step of the development process is extensive testing of the drive unit on a test rig. In particular, at a high rotational speed level, experimental testing can be challenging. This paper describes a test rig fo...

For a drive unit for axes of robots and robot-like systems (RLS) usually a motor-gearbox arrangement is chosen due to its high-power density. The combination of a high-ratio gearbox and a high-speed electric motor ensures a very compact and efficient design of the drive train. The transmission properties primarily determine the properties of the ax...

The reduction of frictional power losses in power transmitting gears takes a crucial role in the design of energy- and resource-efficient drivetrains. Water-containing lubricants like glycerol and polyalkylene glycols have shown great potential in achieving friction within the superlubricity regime with coefficients of friction lower than 0.01 unde...

Purpose Industrial drivetrains use wet disk clutches for safe and reliable shifting. Advances over the past decades regarding the formulation of lubricants and the composition of friction materials have led to reliable clutch systems. In this context, the friction behavior is crucial for the correct operation of the clutch. Nevertheless, the fricti...

The drive system of robots and robot-like systems (RLS) is often designed with a combination of an e-motor and a gearbox with a high transmission ratio to optimize performance. The various types of possible robot gearboxes can be selected based on their characteristics, which strongly influence the performance of the entire robotic system. Planetar...

Resource and energy efficiency are of high importance in gearbox applications. To reduce friction and wear, an external lubricant supply like dip or injection lubrication is used to lubricate tribosystems in machine elements. This leads to the need for large lubricant volumes and elaborate sealing requirements. One potential method of minimizing th...

An important characteristic of gear drives is the noise behavior, which can be influenced by the design of the gears. The dynamics and NVH behavior of gearboxes depend to a large extent on the flank geometry of the meshing teeth. The most common profile shape of the meshing teeth is the involute, especially for cylindrical gears. Involute gearings...

Gearboxes of modern drivetrains, e.g., in wind turbines, can be exposed temporarily to low ambient temperatures during their manufacturing or during operation. During manufacturing or assembly, for example, very low temperatures sometimes occur during joining processes due to the cryogenic treatment of the components. In these cases, the temperatur...

For the reliable operation of a gearbox, consistently sufficient lubrication of tooth contacts and bearings is necessary. Thus, gearbox oil flow plays an important role. Additionally, air flow may affect the oil flow in the gearbox. The state of the art indicates that Computational Fluid Dynamics (CFD) enables targeted support of the gearbox develo...

This contribution demonstrates the potential of machine learning (ML) algorithms in predicting elastohydrodynamic lubrication (EHL) film thickness in elliptical contact with varying direction of lubricant entrainment, ranging from wide to slender elliptical configurations. The input parameters pertain to worm gear contacts, which are characterized...

This research paper presents a comprehensive methodology for analyzing wet clutches, focusing on their intricate thermomechanical behavior. The study combines advanced encoding techniques, such as Principal Component Analysis (PCA), with metamodeling, to efficiently predict pressure and temperature distributions on friction surfaces. By parametrica...

The safe and efficient torque transmission of wet disk clutch systems requires high coefficients of friction. To achieve good controllability and high comfort, a positive slope of the coefficient of friction over sliding velocity is ensured by a reasonable formulation of the lubricant and choice of the friction pairing. This results in low transmit...

Monitoring systems for rolling contacts in machinery are widespread. There are multiple solutions in various applications like Health Usage Monitoring Systems (HUMS) for Helicopters and smart bearings from different manufacturers. The number of solutions and possibilities is rising. This paper outlines how the vibrational behavior depends on the me...

Nitriding is a thermochemical surface hardening process, which can be used to improve the performance properties of gears. In particular, the tooth flank and tooth root load-carrying capacity can be significantly increased by nitriding. Previous work has shown that tribological dominated failure modes and limits, such as the micro-pitting and wear...

Enabling gears to withstand loss of lubrication in gearboxes without secondary oil supply systems can reduce weight and space demand and thus fuel consumption. This study investigates the potential of surface and material technologies on the loss of lubrication performance of gears. Thereby, superfinished, coated, and nitrided gears are compared to...

Residual stresses have a significant influence on the load carrying capacity of gears. High near-surface compressive residual stresses can increase pitting strength and tooth root bending strength up to 50%. Tooth flank fracture (TFF) is a gear fatigue failure mode, which is in contrast to pitting or tooth root breakage not initiated at the surface...

The design of gear boxes is a complex challenge characterized by conflicting requirements and seemingly circular dependencies. Existing tools support engineers but focus on a single predefined design, often leading to costly iterative processes and non-optimal solutions. Solution Space Engineering (SSE) alleviates this by generating multiple design...

Calculating the service life of gears under variable loads requires a description of the load-carrying capacity. The current standard for this is the use of the S/N curve. International standards such as ISO 6336 stipulate the use of this approach for the calculation of the service of gears under variable loads. In this paper, five new approaches a...

Fundamental knowledge of wet clutches’ drag loss behavior is essential for designing low-loss clutch systems. In contrast to the widely investigated injection lubrication, more comprehensive knowledge is needed on the drag loss behavior of dip-lubricated wet clutches. In the development phase, data-driven models allow drag loss predictions with low...

The drag loss behavior of a disengaged wet clutch is influenced, among other things, by the movement of the plates. Therefore, knowledge about the plate movement is essential for investigating and optimizing the drag loss behavior. This paper presents a methodology for image-based measurement of plate movement in disengaged wet clutches. A drag tor...

Increasing the speed of the electric motor can significantly improve the power density of the powertrain of EVs (Electric Vehicles), resulting in a smaller size and thus weight and cost advantages. Achieving acceptable NVH behavior becomes more difficult at higher speeds, partly because the excitation frequencies of the gears cover a wider frequenc...

Geared turbofans need to withstand loss of lubrication due to inevitable off-design conditions. Additionally, loss of lubrication specifications that have to be fulfilled are getting stricter. For a loss of lubrication event, today's geared turbofans are equipped with secondary oil supply systems. Their omission would reduce space and weight, and c...

In the design of rear-axle locking differentials, the desired high locking effect is often achieved using wet multi-plate clutches. This study conducts an in-depth investigation into the spontaneous damage behavior of these clutches through a series of methodical experimental tests. It focuses on three different clutch variants, each equipped with...

This contribution summarizes the current state of research regarding so‐called sensor‐integrating machine elements as an enabler of digitalization in mechanical engineering and——if available—their application in industry. The focus is on the methodical aspects of the development of these machine elements in general as well as specific sensor‐integr...

Properties and environmental dependencies of mechatronic components like gear-drives and motors impact the overall performance of automated Production Systems. These properties, often in the form of characteristic curves, are only accessible from suppliers’ documents, like operating instructions or online catalogs. The proposed SysML’ profile integ...

Today, mineral or synthetic oils that are made out of fossil raw materials, are the most common lubricants in gear drive applications. Most of them are non-biodegradable and may pose a risk to the environment. An important step to minimize the risk and the ecological footprint is the use of biodegradable and eco-friendly lubricants. Former research...

Due to increasing performance requirements and sometimes tribologically challenging operating conditions in gears (such as those found in wind turbines), alternative heat treatment processes to quenching and tempering or case hardening are becoming more and more important. Nitriding can significantly increase the load-carrying capacity of gears com...

Planetary gear sets have a more complex structure than layshaft gearboxes which leads to challenges in design, analysis and operation. In this paper, design critical phenomena in planetary gear sets are addressed and latest test bench findings are presented. These focus on the impact of carrier shaft and planet pin misalignments on the load distrib...

Today, mineral or synthetic oils that are made out of fossil raw materials, are the most common lubricants in gear drive applications. Most of them are non-biodegradable and may pose a risk to the environment. An important step to minimize the risk and the ecological footprint is the use of biodegradable and eco-friendly lubricants. Former research...

This work focuses on the effect of time-dependent errors in the behavior of planetary transmissions. More precisely, this work analyses the response of low-speed planetary transmissions to the presence of time-varying manufacturing errors such as run-out and index errors, which are some of the most common errors inherent to the gear manufacturing p...

The driving comfort and safety of the automotive powertrain are significantly related to the performance, lifetime, and functionality of the lubricant. The presented study focuses on investigating the performance loss of the lubricant due to water contamination resulting from environmental influences and iron particles originating from the wear of...

Face gear drives are special angular gears in which an involute pinion and face gear wheel mesh. They enable high transmission ratios in small construction spaces. The face gear is typically manufactured by gear hobbing or gear shaping using an involute pinion-type cutter. Face gear drives with identical pinion and cutter parameters form a conjugat...

In gearboxes, rolling bearing power losses often make up a large amount of the total gearbox power losses. Therefore, an exact calculation of rolling bearing power losses is very important for resource and energy efficient gearbox design. The state of the art offers numerous friction calculation methods with different levels of detail to calculate...

Fundamental knowledge of the oil flow in a disengaged wet clutch is essential for optimizing the cooling performance and the drag losses. However, no fundamental information on the oil flow and drag torque generation is available for dip-lubricated wet clutches. Therefore, the oil flow and drag torque generation in the sub-millimeter gap of a dip-l...

Important goals in gear design are high load carrying capacity, good noise, vibration, and harshness (NVH) performance and high efficiency. Regarding the load carrying capacity, the international series of standards ISO 6336 [13–15] is the state of the art for its calculation. To ensure reliable calculation of the load carrying capacity of gears an...

Pitting is, besides scuffing, tooth flank fracture and the phenomenon micropitting, one of the primary failure modes of the flank load-carrying capacity of bevel and hypoid gears. The international standard ISO 10300-2:2014 is widely used for the estimation of the flank load-carrying capacity of bevel and hypoid gears based on operational data and...

Tooth flank fracture (TFF) is a gear fatigue failure mode with a crack initiation in larger material depths. It has recently been observed more frequently, especially with larger gear dimensions, such as can be found e.g., in wind turbines and heavy industry applications. In the past, experimental investigations were performed by single load testin...

Purpose Numerous metals can be processed using the additive manufacturing process laser-based powder bed fusion of metals (PBF-LB/M, ISO/ASTM 52900). The main advantages of additive manufacturing technologies are the high degree of design freedom and the cost-effective implementation of lightweight structures. This could be profitable for gears wit...

Fiber-reinforced materials feature transversely isotropic elasticity. Although its influence on pressures, shapes, and sizes has been studied extensively for dry contacts, the transferability to lubricated contacts is fragmented. This numerical study investigates how the content and orientation of short fibers in fiber-reinforced polymers (FRP) aff...

Planetary gear sets combine the characteristics of high-power density and compact design which are beneficial for a wide range of high-performance applications. Embedded in the efforts for resource efficiency and cost reduction, new challenges arise regarding a lightweight design. An accurate prediction of the load sharing on the individual planeta...

This work shows the possibility of utilizing compressive residual stresses that are induced by the fineblanking process to increase the load-carrying capacity regarding tooth root breakage up to the level of case-carburized gears. Pinions and wheels made of S355MC (1.0976) and S500MC (1.0984) are fineblanked using parameters determined in previous...

Gear condition monitoring can prevent unexpected downtimes or sudden failure of machinery. Since gear damage usually results from tooth contact, data for reliable fault detection should be acquired as close as possible to this engagement to reduce other components' disturbances (such as vibrations). One typical gear damage mechanism is pitting. Alt...

The electrification of vehicle powertrains and the expected engineering labor shortage are ongoing key challenges in the gear transmission development. Because traditional methods reach limits, the solution is further automating the design process while enabling flexible and optimal design solutions even with rapidly changing constraints and requir...

Geared turbofans need to withstand loss of lubrication due to inevitable off-design conditions. Additionally, loss of lubrication specifications that have to be fulfilled are getting stricter. For a loss of lubrication event, today's geared turbofans are equipped with secondary oil supply systems. Their omission would reduce space and weight, and c...

Involute gears have been developed for a long time and already used in many applications. In addition, special tooth profile, such as cycloid, are used in special gearing. To facilitate rapid gear design and load analysis, loaded tooth contact analysis (LTCA) approaches have become essential and efficient tools. In this paper, the results of involu...

Worm gears are usually used in industrial applications with the material pairing of hard worm and softer worm wheel, where the failure mode wear often limits the service life. Up to now, extensive, costly and time-consuming component wear tests need to be carried out for each tribological pairing in order to calculate the amount of wear on worm whe...

Plastic gears are produced cost-effectively in high volume but mainly for applications under low load. Current trends are leading to the design and application of plastic gears for more demanding operating conditions. This requires finding the optimum macrogeometry for the application, which is determined in practice using iterative design processe...

The scuffing load carrying capacity is a decisive design criterion in the development of gears. In addition to the gear geometry, the entire tribological system of the gearbox must also be taken into account. Besides the loads and speeds occuring during the operation of the gears, the lubricant used and its additives have a significant influence on...