Erik Kuhn

Effect of rheological behaviour of lithium greases on the friction process

Purpose – The aim of this work is to investigate the relationship among rheology, tribology and traditional standardized technological parameters of lithium lubricating greases.Design/methodology/approach – Lubricating greases having the same composition but differing in processing protocols have been manufactured and characterized in order to isolate the rheological behaviour from the formulation.Findings – Some successful empirical correlations between rheological (viscous and viscoelastic) and technological standardized parameters, with the friction factor obtained from a ball‐disc tribometer, have been established in order to elucidate the role of the rheological behaviour of lubricating greases on the friction process. In addition to this, an energetic evaluation of the structural degradation of greases during the friction process has been carried out by performing stress‐growth experiments. Thus, the storage energy density, which is related to the grease capacity to accumulate energy in the elastic ...

Description of the energy level of tribologically stressed greases

Abstract Lubricating greases play a special role in the tribological system. The peculiarities of greases are discussed from the energy point of view. An apparent theological energy density is in a first step modelled for the description of the tribologically behaviour. The influence of the material properties is estimated via a newly introduced quantity, named limiting energy density. So it is possible to describe grease behaviour as a function of material and process parameters.

Experimental grease investigations from an energy point of view

This paper is concerned with the study of grease reaction by tribological stress. To observe the change of the viscoelastic properties by shearing the grease, rheometer tests were done. In addition to the flow behaviour, we investigated the friction process by using some tribometer. The tribological characteristics of the lubricant are determined with the help of energetic parameters. The interpretation of the results shows a correlation between viscoelastic properties and friction behaviour and an interesting dependence of storage modulus and shear stress. The results are considered to be useful information on the tribology of greases.

Experimental procedure for the evaluation of the friction energy of lubricating greases

Abstract The paper is concerned with the study of a tribological system characterised by fluid friction. The main parameter of the system is the density of the friction energy, a physical quantity which is put in a direct relation with the viscous dissipation function of the lubricant. The lubricant under study is grease, a viscoelastic material which exhibits a yield stress at low strain rates. For this type of material, the shear stress function has a relevant plateau behaviour previous to the beginning of the flow. The rheological characteristics of the lubricant are determined in simple shear motions, both in creeping and stressing modes. The specific energy is graphically computed for these corresponding non-steady shear motions. Below the yield shear stress, an important time variation of the specific energy has been observed. This result is considered to be useful information on the non-steady tribological regime, implicit in the relative intensity of the associated wear process.

Investigations into the degradation of the structure of lubricating greases

The influence of the time dependence of grease behavior was studied. In using rheometer experiments an interpretation of the obtained flow curves is given. A parameter for the description of the structural degradation, the limiting energy density, is quantified and compared for different temperatures. Also, the deformation properties of some greases were investigated from the viewpoint of time dependence. Finally, for the state of maximum degradation, the friction coefficient is estimated.

Tribological characterization of green lubricating greases formulated with castor oil and different biogenic thickener agents: a comparative experimental study

Purpose – The aim of this paper is to characterize some selected formulations based on castor oil and a variety of biogenic thickeners from a tribological point of view and compare them with some traditional lithium greases.Design/methodology/approach – The evolution of the friction coefficient in several tribological tests performed using several ball‐on‐disc configurations and coupling materials was monitored for the different oleogels proposed as biodegradable lubricating greases. Both a rotational ball‐on‐disc classical tribometer designed at MuT laboratory and a nanotribometer were used in rotational and oscillatory modes.Findings – Generally, the use of castor oil‐based formulations potentially applicable as biodegradable lubricating greases provides similar or lower values of the friction coefficient than traditional lithium greases, depending on the nature of thickener agent employed and tribological contact. In all cases, biodegradable formulations provide significantly lower values of the fricti...

Friction and Wear of a Grease Lubricated Contact — An Energetic Approach

Lubricating greases are colloid disperse systems with visco-elastical properties. These special lubricants have a wide range of application. More than 90% of the ball bearings are grease lubricated but also gears and journal bearings are application examples for greases. The composition of greases consists of a base oil and a thickener (of course some additives can be found in commercial lubricants). The thickener forms a network which leads to a complex rheological and tribological behaviour. Tribological contacts lubricated by greases are often working under mixed friction conditions. The situation inside the grease film determines the content of fluid friction. The situation inside the tribological gap influences the content of solid friction. A special phenomenon is the structural degradation due to the effects of friction. It leads to a dependence on time of the grease behaviour. Aims of this chapter are the description of the liquid friction process inside the grease film and of the procedure of structural degradation. Both phenomena have a strong influence on the friction and wear behaviour of the whole tribo-system.

An algorithm to estimate the friction energy of a grease lubricated contact

This paper presents a tribological analysis of a grease lubricated contact with a consideration of the grease structure (density distribution).

Analysis of a grease-lubricated contact from an energy point of view

A number of grease lubricated contacts are working under mixed friction conditions. In this paper, we try to model the grease-lubricated contact considering the density distribution within the grease film. A proposal is presented to quantify the friction energy for the fluid friction part by using experimental work. The friction leads to a process of structural degradation (rheological wear) of the lubricating grease. This process is analysed and some new parameters are defined. A proposal for quantifying the irreversible change of the grease properties is made. All this work is done from an energetical point of view and is supported by experimental work.

Transient Shear Flow of Model Lithium Lubricating Greases

This work deals with the analysis of the transient shear flow behaviour of lithium lubricating greases differing in soap concentration and base oil viscosity. The shear‐induced evolution of lithium grease microstructure has been studied by means of stress‐growth experiments. With this aim, different lubricating grease formulations were manufactured by modifying lithium 12‐hydroxystearate concentration and base oil viscosity. Different rheological parameters, related to both the elastic response and the structural breakdown of greases, have been analysed. In this sense, it has been found that the elastic properties of lithium lubricating greases were highly influenced by soap concentration and oil viscosity. Moreover, an asymptotic tendency has been found for the stress overshoot by increasing shear rate. The asymptotic values of this parameter have been correlated to the friction coefficient obtained in a ball‐disc tribometer.