P. De Baets

The tribological behaviour of engineering plastics during sliding friction investigated with small-scale specimens

Abstract For economical, ecological and even technical reasons for some years there has been a tendency to introduce self-lubricating materials for bearing applications. In this way external lubricants such as oil or grease can be excluded, the design can be simplified and maintenance cost can be reduced. Among the self-lubricating materials the so-called engineering plastics have increasing importance. Unfortunately, data on their friction and wear characteristics are very disparate and often there is a lack of general understanding of the physical phenomena involved. In the present paper some basic types of engineering plastics (PA, oil-filled PA PTFE-filled PETP, POM-H) are experimentally investigated by means of small-scale reciprocating tribotesting. The tribological behaviour is explained in correlation with the chemical and mechanical properties of the materials. The basic failure processes are described for mild wear conditions as well as for overload conditions.

The friction and wear behaviour of polyamide 6 sliding against steel at low velocity under very high contact pressures

Abstract The friction and wear behaviour of polyamide 6 sliding against steel under huge contact pressures and at very low sliding velocities was examined using large scale test specimens. The applied contact pressures and PV factors exceed the maximum allowable values cited in the literature. It is found that polyamide 6 resists huge contact pressures and allows relative sliding against steel without damaging the steel surface. The wear (material loss) is moderate, even at the huge contact pressures for which plastic flow (creep) is considerable. Polyamide 6 sliding against steel is found to be sensitive to stick-slip motion, which complicates accurate positioning, and the resulting vibrations can also lead to damage of the bearing material or even of the construction in which it is used.

The tribological behaviour of paper friction plates for wet clutch application investigated on SAE#II and pin-on-disk test rigs

Abstract The friction behaviour of wet clutches for automatic transmission applications strongly influences the dynamic behaviour of the entire machine or vehicle including the transmission. The wear, but also the friction curve, determines the lifetime of the clutch. The role of wear is obvious. The friction coefficient of the material couple friction plate/separator plate decreases with number of engagement cycles. As a result, the possible torque of the transmission decreases with time. Under a certain threshold, the clutch has to be revised. But because manufacturers tend to oversize their clutches the decrease in friction coefficient does not yield a limitation to the lifetime of the clutch. In this paper, both SAE#II and pin-on-disk tests are used to investigate the wear and friction characteristics of the friction material. Although the pin-on-disk tests fail in correctly reproducing the wear rate of the SAE#II tests, they can be used for a qualitative analysis of the influence of material parameters and operating conditions on both friction coefficients and wear rate.

Finite element approach for modelling fatigue damage in fibre-reinforced composite materials

Today, a lot of research is dedicated to the fatigue behaviour of fibre-reinforced composite materials, due to their increasing use in all sorts of applications. These materials have a quite good rating as regards to life time in fatigue, but the same does not apply to the number of cycles to initial damage nor to the evolution of damage. Composite materials are inhomogeneous and anisotropic, and their behaviour is more complicated than that of homogeneous and isotropic materials such as metals. A new finite element approach is proposed in order to deal with two conflicting demands: (i) due to the gradual stiffness degradation of a fibre-reinforced composite material under fatigue, stresses are continuously redistributed across the structure and as a consequence the simulation should follow the complete path of successive damage states; (ii) the finite element simulation should be fast and computationally efficient to meet the economic needs. The authors have adopted a cycle jump approach which allows to calculate a set of fatigue loading cycles at deliberately chosen intervals and to account for the effect of the fatigue loading cycles in between in an accurate manner. The finite element simulations are compared against the results of fatigue experiments on plain woven glass/epoxy specimens with a [#45°]8 stacking sequence.

The relation between friction force and relative speed during the slip-phase of a stick-slip cycle

Abstract In order to better understand the stick-slip phenomenon, several attempts have been performed in the past to measure the friction force-relative speed relation during the slip portion of a stick-slip cycle. In literature, two different shapes for this relation are reported: clockwise loops and counter-clockwise loops. In this paper, it is shown that both shapes can be obtained by changing the tangential stiffness of the test rig. Likewise, it is shown that the counter-clockwise friction-speed loop does not really occur, but that it results from neglecting the velocity variation (by vibration) of the most rigidly connected sliding specimen. Finally, a sufficiently large difference between the stiffness of both tangential sliding specimen-frame connections is recommended for accurate friction measurements during stick-slip.

The friction force during stick-slip with velocity reversal

Abstract Stick-slip is the well-known phenomenom of intermittent motion caused by a changing friction force in combination with mechanical system elasticity. Usually, no velocity reversal occurs during the slip-phases of a stick-slip cycle. However, velocity reversal is observed during stick-slip motion of a laboratory test rig with oil lubricated centre-pivoted sliding shoes and a very flexible tangential spring connection. The general condition for velocity reversal during the slip-phase of a stick-slip cycle is derived and the friction force during stick-slip with velocity reversal is measured and discussed with reference to the Stribeck-curve, which represents the friction force in steady-state conditions. In order to better understand the friction force during velocity reversal, free vibration tests with friction are also performed and discussed in this paper.

Comparison of the wear behaviour of six bearing materials for a heavily loaded sliding system in seawater

Abstract A problem of severe wear occurred at the guidance system of a door of a sea-lock. The seawater immersed guidance was a linear system with aluminium bronze bearing material on stainless steel rail material. A satisfactory laboratory simulation of the wear behaviour of the guidance system was achieved by testing with large test specimens. Six bearing materials were tested. Nodular and lamellar cast iron suffered severe scuffing and cannot be used. Sintered graphite bronze shows a wear coefficient about six times smaller than the original aluminium bronze. Two plastics were tested: PETP and PA. PA has the best friction and wear characteristics and is suggested as bearing material for the sliding system.

A new approach of stick-slip based on quasi-harmonic tangential oscillations.

Abstract It is generally accepted that stick-slip behaviour is caused by a negative slope of the friction force-relative sliding velocity relation. In this paper, it is shown that the F - v rel gradient derived from steady-state friction fundamentally differs from the instantaneous F - v rel gradient. which is determined by the instantaneous change of the friction force at a sudden infinitesimal change of the relative sliding velocity. This instantaneous F - v rel gradient, not the stationary F - v rel gradient, is the real initiator of stick-slip. The occurence of quasi-harmonic tangenitial oscillations during friction induced by a negative instantaneous F - v rel gradient is explained. Moreover, it is shown that a correlation exists between the amplitude of the quasi-harmonic oscillations occurring during friction and the instantaneous F - v rel gradient. In literature, stick-slip is always explained with an initial stick-phase. In this paper, it is explained how stick-slip can arise from quasi-harmonic tangential oscilations during pure slip (without stick). It is of great interest to explain the occurrence of stick-slip after a decrease of the impressed velocity, starting from above the critical (stick-slip free) velocity. For this new approach of stick-slip, the same qualitative influences of the system parameters (velocity, damping, stiffness) on intemittent motion are found as for the classic models with an initial stick-phase.

Mathematical approach of the influencing factors on stick-slip induced by decelerative motion

Abstract Experiments performed in our laboratory raised the suspicion that decelerative motion can induce stick-slip whereas on stick-slip appears during steady sliding. In this paper, a mathematical model is presented which proves that stick-slip can be induced by deceleration. The parameter combinations for which stick-slip induced by deceleration can be expected are also marked and the influence of each parameter is discussed. Dimensional analysis was used in order to reduce the number of influencing parameters.

Postmortem raman spectroscopy explaining friction and wear behavior of sintered polyimide at high temperature

With their thermal stability and high strength, polyimides are an aromatic type of polymers that are used in sliding equipment functioning under high loads and elevated temperatures. However, their tribological behaviors under high temperature and atmospheric conditions are not fully understood. It has been reported that a transition from high to lower friction occurs “somewhere” between 100 and 200 °C; however, correlation with changes in the polyimide molecular structure remains difficult to illustrate, and it is not certain whether this transition is correlated to lower wear. In the present work, sliding experiments under controlled bulk temperatures between 100 and 260 °C are performed. A transition in both friction and wear at 180 °C is observed that is further examined with microscopic analysis of the transfer film on the steel counterface and Raman spectroscopy of the worn polymer surfaces. Close examination of the spectra reveals transitions in the relative intensities of certain absorption bands, which suggests different orientation effects of the molecular conformation at the polymer sliding surface at 180 °C.