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S. Kunert:
"Modellierung hydrodynamischer Zahnradschmierung unter Berücksichtigung von Wärmeübergang und Kavitation";
Supervisor: B. Scheichl; Institut für Strömungsmechanik und Wärmeübertragung, 2022; final examination: 2022-11-08.

In the current state of affairs, technical machines are very important with regard to the
efficiency of energy and resources. Reduction of raw materials and global warming force
the emission of greenhouse gases to be reduced in every aspect. CO2 emission is the ability
of CO2 to be released into the atmosphere. The increase in greenhouse gases and aerosol
concentrations led to a change in the anthropogenic climate on earth. The greenhouse effect
is caused by thermal radiation on the earth´s ozone layer, which absorbs the gases carbon
dioxide, methane and ozone in particular. Since barely heat is given off, the middle surface and oceans heats up automatically. The warming has the effect of an irreversible increase in the mean surface temperature, sea level and precipitation as well as acidification of the oceans and a melting of the remaining ice deposits. The intergovernmental committee on climate change, also ICPP (Intergouvernemental Panel on Climate Change) publishes a comprehensive report every few years, which explicitly describes climate change and presents the prospects for improvement. Industrialization is primarily responsible for the sharp rise in carbon dioxide levels in the atmosphere and the associated global warming. In 1750 the amount of carbon dioxide in the atmosphere was still approx. 315 ppm (parts per million); in 2005 this rose to 401 ppm, currently it is approx. 407 ppm, and the trend is rising. The permissible range for CO2 concentration is approx. 500 ppm. If this value is exceeded, this can have severe consequences for the climate and life on our planet. The earth stays in contact with the solar and cosmic ray system and works like a sphere which takes in more heat than its releases.
A huge amount stays in the dome, so the resulting heat is released to land and oceans. A
functioning ecological maritime system should ideally be in equilibrium. In order to keep the
earth habitable, methods must be developed to maintain the CO2 concentration and, at best,
to reduce it [1]. Around 95% of the intercontinental and 62% of the intra-European exchange
of goods take place via sea transport. Almost 542 million tons of goods were handled in world
sea trade in 2021. Due to the advancing international division of labor, the improvement in
productivity and the increasing global demand for energy, a further expansion in the area of
shipping is to be expected in the coming years. The mineral oil, which is used in shipping as
a lubricant in ship´s gearbox, accounts for around 10% of global marine oil pollution, along
with leaks from oil tankers and drilling rigs [2].
Intensive research is already being carried out on the replacement of oil lubrication by water lubrication on machine elements, which is the focus of this work. Based on the physical
properties of oil, its load-bearing capacity is already known. It is to be investigated whether the geometry of the gears also allows lubrication with water and even salt water. Therefore, the efficiency of conventional transmissions and other machine elements in the transport industry must be improved and adapted. This applies not only to gears but also to increasing the efficiency of accompanging technical systems such as engines and plain bearings.
This work concentrates on the hydrodynamic lubrication in a gear pair of a marine gear. If
the conventional lubricant oil is to be replaced by water in the gear unit, it is particularly important that the performance and load-bearing capacity of the loads to be transferred are guaranteed during operation. The size of the lubrication gap in the gear pairing provides information as to whether the gear drive is stable with hydrodynamic lubrication and can
be used in industry. The marine and yacht transmission company Reintrieb has started a
collaboration with the research company AC2T research GmbH in the field of tribology to
develop marine transmissions that can be operated entirely by water lubrication.
This work concentrates on hydrodynamic water lubrication and an evaluation of the loadbearing
capacity of the lubrication gap in the gear pairing of the transmission. The geometric
models will be explained in section 3 for the lubrication gap are first set up and solved
simultaneously in order to determine the number of degrees of freedom. Then the geometric
kinematics and Navier-Stokes equations are coupled to determine the speed and pressure
profile and thus the size of the minimum gap between the moving gear flanks. In addition,
the heating and the associated temperature gradients between the lubricant and the gearwheel
material are determined. The properties of the viscosity of water and conventional gear oils
are compared with each other and individually evaluated as a lubricant for the gap. Finally,
the possibility of cavitation is examined. A pressure distribution of the gear with a hypothesis of a cavitation model with JFO (Jacobson, Floberg, Olsson)-condition is set up in order to
determine the resulting pressure distribution in the event of cavitation. Various approaches
can be used to improve the tribology to lubricants and gears. This includes the optimization
of the component surface texture and the further development of materials, lubricants and,
if necessary, their additives. As the lubrication gap increases, the likelihood of mixed friction due to the interaction of roughness peaks as well as hydrodynamic friction due to shear of the lubricant decreases. In the case of higher stressed EHD (elastohydrodynamic) contacts, intensive research has been carried out on this topic to this day. The aim of this work is to examine and improve the load-bearing capacity and service life of hydrodynamic lubrication of tribological contacts in machine elements (here gear pairing). The main point is on higher loaded rolling / sliding contacts, for which the elastohydrodynamic lubrication theory (see section 5) applies. For this purpose, an overview of the fundamentals of gear geometry and kinematics is given in section 3 and of fluid mechanics in section 4 created. So far, slip-prone elastohydrodynamic contacts have been little investigated. With EHD contacts, the elastic deformation of the contact partners creates the size of a realistic, load-bearing lubrication gap. In addition a pressure profile of the gear pair will be callculated and created with a hyphothesis of cavitaion (will be explained in section 6). A résumé of the results and future outlook to this work will be summerized in section 7. This work is intended to provide a better understanding of the evaluation of various lubricants and their effect on the geometric lubrication gap and gear kinematics [3].

Hydrodynamische Zahnradschmierung; Wärmeübergang; Kavitation