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Using an unidirectional oscillator including a mass-spring system, the authors evidenced the important variations of the static friction coefficient both for very low contact pressure (0.002 MPa) and for high contact pressure (386 MPa).The experiments were realised by the CETR UMT-2 Tribometer with a variation of the sliding speed between 0.02 mm/s to 8 mm/s. The oscillator mass have 0.242 grams and the stiffness of the elastic spring had 77 N/m. The contact surface of the oscillator mass have a roughness Ra = 0.1 μm and the opposite plane surfaces have roughness of Ra = 0.1 μm and Ra = 2.5 μm. Two types of experiments were realized: the sliding between two plane surfaces with a nominal pressure of 0.002 MPa and the sliding between a plane surface and three steel balls of 6 mm with a maximum Hertz contact pressure of 386 MPa. For very low pressure the obtained static friction coefficient varied between 0.08 to 0.3 and the dynamic friction coefficient varied between 0.08 to 0.16. For high pressure the obtained friction coefficient varied between 0.2 to 0.4 and the dynamic friction coefficient varied between 0.15 to 0.2. To simulate the sliding speed the model of Zuleeg [5] for dynamic friction coefficient has been adapted to the experimental parameters and the dynamic equation of mass displacement as function on time has been numerical integrated.

The dry sliding friction between glass surface and soft materials like paper, towel and cloth for wiping glasses was explored experimentally. The UMT- 2 Tribometer has been fitted with a unidirectional oscillator including a mass-spring system. With a linear speed ranging from 0.05 to 8mm/s, a mass weighing 242 grams was sliding across planar surfaces covered with soft materials. The values of static and dynamic friction coefficients have significant variation as a function of linear speed and soft materials. The transition between the stick-slip process and continuum sliding has also been demonstrated experimentally. A theoretical model for variation of the dynamic friction coefficient as function of linear speed has been included in the dynamic equations of mass displacement. The dynamic equations have been solved by Runge Kuta method and compared with experimental results.

Estimation of the power loss in miniature ball bearing grease lubricated is a complex problem. Usually the applied loads (radial and axial) have small values and the methodologies recommended by the bearing companies are cannot applied for these conditions. For a ball bearing, some friction processes have differential contribution to the total friction torque and power loss. For very low loads, the lubricant is the most important source for friction torque. In the present paper the authors determined experimentally the friction torque both in a standard 7000C angular contact ball bearing (ACBB) and a modified 7000C ACBB containing only 3 balls without cage, operating with very low axial load and lubricated with lithium soap grease. The experimental values of the friction torque have been correlated with the theoretical Houpert’s IVR model developed for hydrodynamic rolling resistances in ball race contacts considering the viscosity of the base oil of grease.

In this paper the authors experimentally investigated the influence of the linear speed and the stiffness of a single degree-of-freedom mass-spring system on the evolution of the stick-slip process. Using the UMT-2 Tribometer a steel mass was sliding on glass surface with a linear speed varied between 0.02 mm/s and 1 mm/s and spring stiffness varied between 40 N/m and 442 N/m. The experimental results evidenced that increasing both the linear speed and the spring stiffness can reduce the stick-slip process. Thus, the authors proposed an original friction model to simulate the stick-slip process by considering the dependence of the dynamic coefficient of friction with the sliding speed in the “slip” phase. The theoretical model is based on dynamic equilibrium of all the forces acting on the mass in transient phases. The proposed theoretical model was validated with experiments by simulating the stick-slip process for given linear speed and spring stiffness.

The presence of the lubricant in a ball-race contact has two principal effects on friction: reduces the sliding friction, as a result of developing a film thickness between the contact surfaces and increases the rolling friction, as result of developing a hydrodynamic rolling friction. The influence of the hydrodynamic rolling friction on the torque of a modified 7205B angular contact ball bearing has been theoretically and experimentally studied. The experiments demonstrated that, for very low contact loads between balls and races, the effect of the hydrodynamic rolling force is dominant in the total friction torque. The experiments have been validated by using Biboulet’s transition model for hydrodynamic rolling forces developed in ball-race contacts.

Cellulose and some of its derivatives form liquid crystalline solutions in a variety of solvents. The present work investigates cellulose acetate phthalate/hydroxypropyl cellulose blends in N,N-dimethylacetamide, in function of concentration and composition, by rheology, considering dynamic viscometry and oscillatory shear tests. The effects of composition, concentration, shear rate, and oscillatory deformation on the rheological functions determines the orientation or mobility of the chain segments in the shear field. Atomic force microscopy and polarized light microscopy studies on corresponding films show that specific interactions, such as the hydrogen bonds between cellulose acetate phthalate and the liquid crystalline component—hydroxypropyl cellulose, stabilizes the morphology, leading to the emergence of different formations typical for lyotropic liquid crystal phases. These studies contribute to a better knowledge of the specific interactions that generate and modify the liquid crystalline properties of cellulose derivatives, required by the applications in electronic domains.

A comparative study of the influence of acetone, isopropyl alcohol and mixtures of ethanol-acetone and ethanol-isopropyl alcohol on the cellulose structure during alkalization is presented. The kinetic behavior and distribution of substituents in the carboxymethylation stage are explained tentatively. It is shown that the decrystallization is more pronounced and the proportion of cellulose I is higher for samples alkalized in ethanol-isopropanol and ethanol-acetone mixtures containing up to about 50% ethanol, as compared with those produced in isopropyl alcohol or acetone. The distribution of substituents in the C 6 position, as evaluated by 1 H NMR spectroscopy, is correlatedwith the change of the cellulose structure revealed by X-ray analysis data.

This paper reports a novel method for the synthesis of wholly aromatic polyazomethines by oxidative polymerization of monomers having a preformed azomethine linkage. Two monomers having a Schiff base type structure: N -pyrrolylmethylen-1-aminonaphthalene ( I ) and N -pyrrolylmethylen-2-aminonaphthalene ( II ) were synthesized by coupling of pyrrole 2-carboxaldehyde with 1- and 2-aminonaphthalene, respectively. Their cation-radical polymerization was performed by chemical oxidation with ammonium persulfate and iron trichloride. Aromatic polyazomethines were obtained with good thermal properties. The structures of the polymers were studied by IR spectroscopy and were correlated with Huckel molecular orbital calculations for evaluating the monomers reactivity and the most probable linkage sites of the monomer units in the polymer chain.

In agreement with isothermal EHD theory, increases in speed and lubricant viscosity determine the increase in film thickness. Beyond certain limits, both speed and viscosity lead to a decrease in film thickness, as a result of thermal effects and starvation. To estimate starvation limits and film thickness in high-speed, mist-lubricated ball bearings, a new analytical methodology has been developed. This methodology is based both on the oil flow rate in ball-race contacts and Hamrock and Dowson's starvation theory. This new methodology has been used to estimate minimum film thickness in a 7206C angular contact ball bearing, axially-loaded, and rotated between 0 and 35,000 r/min−1. Good correlation between computed and experimental values of the minimum film thickness have been obtained.

In a 7206 C high-speed, mist-lubricated ball-bearing, both the film thickness and lubricant parameter λ in ball—race tribo-systems have been analytically investigated in terms of starvation and thermal effects, and good correlations with experimental results were obtained for 5,000 to 35,000 rpm rotational speeds. Including a lubricant parameter in ball—race tribosystems as a safety criterion for ball-bearings, friction losses, temperature, and limit speeds have been correlated with this criterion for a 7206 C ball-bearing. Friction losses in ball-outer race tribosystems have been computed for both EHD and mixed lubrication regimes. The influence of the bricant parameter on friction losses has been shown both theoretically and experimentally. A new speed limit concept including the lubricant parameter λ, has been developed by the authors.

Die bei der Einwirkung elektrischer HF-Entladungen auf Cellulose-, Polyester- und Polypropylenfasern ablaufenden Prozesse, wie Abbau- und Vernetzungsreaktionen, Masseverlust, anderung der relativen Molekulmasse sowie der thermischen und mechanischen Eigenschaften, werden in Abhangigkeit von den Behandlungsbedingungen untersucht. Die in den Fasern entstehenden Makroradikale bilden bei Anwesenheit von Monomeren die Grundlage zur Synthese von Pfropfcopolymeren, wobei die HF-behandelten Fasern als aktivierte Tragerpolymere dienen. The processes caused by electric high frequency discharges on cellulose, polyester and polypropylene fibres, as degradation, crosslinking, weight loss, change of molecular weight, and the changes in thermal and mechanical properties have been investigated in dependence on the experimental conditions. The macroradicals formed in the fibre provide, in the presence of monomers, the basis of the synthesis of graft polymers, since the fibres treated by high frequency discharge include activated backbone chains.