Gear | EPFL Graph Search

A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic principle behind the operation of gears is analogous to the basic principle of levers. A gear may also be known informally as a cog. Geared devices can change the speed, torque, and direction of a power source. Gears of different sizes produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine. The rotational speeds, and the torques, of two meshing gears differ in proportion to their diameters. The teeth on the two meshing gears all have the same shape. Two or more meshing gears, working in a sequence, are called a gear train or a transmission. The gears in a transmission are analogous to the wheels in a crossed, belt pulley system. An advantage of gears is that the teeth of a gea

Étude des engrenages de microtechnique

Quentin Theurillat-Bonjour

The research presented in this thesis addresses the design of gear drives for micro engineering applications, and was performed in collaboration with a watch manufacturer. Gears have long been found in many fields of mechanical engineering. Their use in watches satisfy specific constraints: small dimensions, little or no lubrication and unusual performance specifications. A literature review allows us to establish a catalogue of existing gear profiles, and a synthesis of profile analysis and generation methods. On the basis of this assessment, we define knowledge gaps which we propose to bridge in the present work by reaching the following objectives: 1. Characterize the behavior of existing gear profiles used in the watchmaking industry (nominal profiles, but also profiles affected by shape errors and axes misalignment errors); determine their kinematic and static performances, and their transmission efficiency. Use these results to compare various profiles on an objective and rigorous basis. 2. Formulate the equations necessary to calculate the tooth profiles using conjugate conditions differing from the tradition-al kinematic conditions ( i.e. imposing the transmission ratio) 3. Work out a design methodology for gear profiles relying on the results obtained while working on the first two objectives To reach those objectives, we develop a computer code allowing us to evaluate the performances of the profiles. The diversity of existing profiles led us to formulate a generic geometrical representation applicable to all profiles, while allowing us to use the same analysis code. The developed tool is then validated by simulating profiles, the behavior of which is known analytically. To calculate conjugate tooth profiles respecting a static criterion, we use the classical theory again, adding a new equation to it, which we derive from a balance of power. We present this equation for the three dimensional case, and the planar case. The resolution method for this equation, valid for static conjugated cams (i.e. a unique pair of teeth), must satisfy a supplementary condition that ensures that the imposed torque ratio leads to an average kinematic ratio compatible with the desired number of teeth. A numerical method was developed for the planar gears case that establishes the range of torque ratios satisfying the imposed average transmission ratio. The lower and upper bounds of the range of solutions are defined in terms of geometric and efficiency considerations, respectively. The proposed approach is illustrated and validated by some profiles computations. In parallel to those theoretical developments, we designed and built a test bench to characterize experimentally the performances of gear profiles, (altered or not by various shape or misalignment errors) under operating conditions similar to those prevailing in a watch. The last part of the thesis provides a critical discussion of these developments places them in a broader context and comments how they could be generalized in future research works.

EPFL2016

The dynamical and chemical evolution of dwarf spheroidal galaxies with GEAR

Pascale Jablonka, Yves Revaz

We present the fully parallel chemo-dynamical Tree/SPH code GEAR, which allows us to perform high resolution simulations with detailed chemical diagnostics. Starting from the public version of Gadget-2, we included the complex treatment of the baryon physics: gas cooling, star formation law, chemical evolution, and supernova feedback. We qualified the performances of GEAR in the case of dwarf spheroidal galaxies (dSphs) galaxies. Our code GEAR conserves the total energy budget of the systems to better than 5% over 14 Gyr and provides an excellent convergence of the results with numerical resolution. We showed that models of dSphs in a static Euclidean space, where the expansion of the universe is neglected are valid. In addition, we tackled some existing open questions in the field, such as the stellar mass fraction of dSphs and its link to the predicted dark matter halo mass function, the effect of supernova feedback, the spatial distribution of the stellar populations, and the origin of the diversity in star formation histories and chemical abundance patterns. Strong supernova-driven winds seem incompatible with the observed metallicities and luminosities. Despite newly formed stars being preferentially found in the galaxy central parts, turbulent motions in the gas can quickly erase any metallicity gradient. The diversity in properties of dSph are related to a range of total masses, as well as a range of dispersion in the central densities, which is also seen in the halos emerging from a Lambda CDM cosmogony.

2012

Étude des engrenages de microtechnique

Quentin Theurillat-Bonjour

EPFL2016