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ELEMENTS DE MACHINES & TRIBOLOGIE 

RESEARCH

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RESEARCH RW N° 2174. (EMT - PROF. J. BOZET) Physical and chemical properties of environment friendly lubricants by C. CLOESEN and A. KABUYA  
This research is devoted to the assessment of tribological, oxidative and environmental properties of refined vegetable oil based lubricants.The products currently tested are the following : soyabean, rapeseed and sunflower oils, a mineral lubricant for comparison (pure paraffinic oil of the same ISO grade), and a rapeseed methylic ester. 
In terms of viscosimetric and tribological properties, the pure vegetable tested oils proved better than the mineral reference. They actually showed a higher viscosity-temperature index (over 170), their oiliness, due to their polar nature, allows a lower friction coefficient in boundary lubrication conditions (studied with a Stribeck machine) and equal values in hydrodynamic lubrication. Their extreme pressure performances (whether expressed in pressure wear index, mean Hertz load or wear scar diameter), measured on a 4-balls machine, also outperformed non added mineral equivalents. 
The oxidative resistance was assessed using a Triboxy machine, specially built to oxidize great amounts of oil, by means of high shear stress, high temperature (194°F) and air micro-bubbles injection over a period of 100 hours. The reference mineral oil did not show any sign of degradation; on the other hand, vegetable oils were very sensitive to oxidation: for soyabean, sunflower and rapeseed oils, the dynamic viscosities measured at 68°F were respectively multiplied by 8, 16 and 65. Other oxidation indicators were used, such as the acidity value, which reached 13 mg KOH / g oil (for rapeseed), and the peroxide number, whose curves reached their highest points at 300 meq / kg oil. 
One of the most interesting properties of vegetable oils is their presumed high biodegradability rate. In order to check this assertion, different experiments were carried out: classical CEC-L-33-T-82 tests (extended to 40 days) and assessment of biodegradability in soil, using the same analysis process as for the CEC test. Both of these experiments were conducted with fresh oils, but also with oxidized (thus polymerised) ones. CEC tests clearly showed that vegetable oils are fully or almost fully debased after 21 days, but also that the mineral oil is characterized by a biodegradability over 70 % after 40 days. The difference of biodegradation betweeen mineral and vegetable oils is very pronounced when measured in soils: fresh and polymerized vegetable oils reached biodegradability values over 87 %, whereas mineral oil barely reached a 6 % degradation value after 40 days. 
In conclusion, we can say that, considering their good tribological and environmental properties, vegetable oils should be suited for renewable environmentally acceptable lubricant use, if they are used at low temperature or in « single use » applications. These oils are too oxidation sensitive for high temperature use, but further existing solutions, whether derived from plant genetics or from additives providers, will be explored.

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RESEARCH RW N° 1845 - 3023. (EMT - PROF. J. BOZET) Self lubricating function of porous bearings, a simple surface treatment to improve the tribological behaviour by M. NELIS 

The purpose of this research is to characterize the tribological behaviour of porous bronze bearings containing oil, working with no additional feeding. 
Porous bearings come from powder metallurgy and are common in many industrial applications. Their use is nevertheless limited by the relatively bad tribological behaviour observed during the lapping process. 
When using the test rig presented in this web site we measured friction coefficients of about 0.1. This means that many applications will not be able to afford this kind of bearing, even though their use should be very advantageous in reducing the cost of the final product and, therefore, making it more attractive on the market. 
We then studied the influence of the main bearing parameters on the tribological behaviour. These are respectively :  We found that the friction coefficient was lower when using a fluid lubricant after a severe lapping process. We then tried to combine these two observations by using a simple surface treatment in order to close the superficial porosity and to use a fluid oil directly. This led to the obtaining of a friction coefficient of about 0.005, which permits the use of porous bearings in more technological applications. 

This success was reached experimentally by studying a huge number of results. Optimum parameters were found and will soon be applied to real mechanical applications. This research was also conducted on the theoretical level by making a mathematical model in order to physically explain the results measured and also to study more efficiently the influence of some parameters. 

Finally, the influence of the chemistry of the lubricant was introduced. We conducted some tests on special fluids such as perfluoroalkylether oils.

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RESEARCH RW N° 1151. (EMT - PROF. J. BOZET) Numerical Solutions of Non Steady-State Conditions of Lubrication In Internal Combustion Engine Bearings by M. NELIS  

The design of internal combustion engine bearings is a complex problem because it requires, not only the knowledge of the bearing loads, but also the determination of the non steady-state conditions of lubrication. In practical applications, the bearing loads vary both in magnitude and direction during an engine cycle. Fortunately, all parameters are cyclic and the corresponding shaft motion describes a closed trajectory which can be represented by the shaft centre locus. 

This trajectory may be obtained using theories incorporating different levels of simplifications. ISB, the simplest theory, gives a fast approximate solution. FLB theory, which involves a longer computation time, gives a realistic trajectory. The description of the shaft center locus is then used to predict the value and location of the minimum oil film thickness. 

This problem is important in the industrial environment because the malfunction of engine bearings may cause severe mechanical failures, generally in the crankshaft, which will then need costly repairs. 

We studied this problem by making a user-friendly computer program in order to evaluate rapidly and accurately the shaft centre locus. We present here one example of calculated results. 

The same kind of problem has also been solved in the case of the running-in process of hydrostatic ball mill bearings. The research was the RW n° 2580 by F. THOUMSIN. 

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RESEARCH RW N° 3235. (EMT - PROF. J. BOZET) Calculation of non contacting face seals used in space and aeronautic environment by O. BREGAND and A. KABUYA  

Seals are essential components within gas turbines or pumps. They do not directly perform any of the fundamental thermodynamic tasks but rather allow other components to carry out these processes in an effective manner. In general they control the flow of fluids. In particular, in lubrication systems, their task is to separate oil from air reducing firstly the possibility of an oil fire then the undesired presence of oil in the cabin conditioning system. In cryogenic turbopumps found in spatial applications they prevent LOx from migrating further and causing a fire hazard. 

The main objective of this research is to produce a numerical model for a type of seal : the mechanical non-contacting seal. Performances of the seal such as leakage flow, temperature rise across the face and overall friction coefficient will be calculated for several interface shapes and for different sealing fluid. Effects like cavitation and phase change will be taken into account in the solution of the Reynolds equation. It will also be possible to make allowance for tribological effects such as wear and contact pressure for the determination of the pressure field. 

Our aim is to further incorporate into the main program the results of thermo-elastic FE calculations. These have the purpose of modelling the deformation of the solid components under pressure and under the heat generation caused by friction at the interface.Finally we hope that our program will also be able to function in non steady-state conditions, making it complete.

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RESEARCH RW N° 2326. (EMT - PROF. J. BOZET) Comparative analysis of the tribological behaviour of industrial coatings by JP. HUQUE  

At present, the informations concerning friction and wear of the coatings and surface treatments are qualitative rather than quantitative. 

We can observe that : 

The current study, then, attempted to define the extreme functioning conditions of coatings, the purpose being to reduce friction and wear by an appropriate design and so increase the life span of mechanical parts. 

The next plan involved : 

This research was performed in conjunction with two industrial partners : Advanced Coating and Nitruration moderne 

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RESEARCH RW N° 1791. (EMT - PROF. J. BOZET) Characterization of couples of materials used in Space Applications by J.-L. BOZET and by M. QUILLIEN  

This research was performed with the intensive collaboration of TECHSPACE AERO. 

The next plan involved : 

  • The main thrust of this research focusses on the fact that under such special conditions, we often observe chemical and physical transformations of the surfaces which then modify the tribological behaviour.
  • We present here a surface which has been physically (roughness) and chemically (transfer) modified during a tribological test performed in cryogenic conditions.
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    EXPERT ACTIVITIES. (EMT - PROF. J. BOZET)   

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    Some references 

    CMI, CBR, DECLOEDT, DIAMANT BOART, DUPONT DE NEMOURS, MAGOTTEAUX, SEP, TECHSPACE AERO, ... 

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