Experimental study of air squeeze effect on high-frequency friction contact

A special tribometer was developed which was used to test sliding friction force between PTFE-based composites and bronze with normal (out-of-plane) or transverse (in-plane) high-frequency vibrations under three different environmental pressures. The influences of environmental pressure, vibration amplitude and sliding velocity on sliding friction coefficient were studied. The results show that the effect of environmental pressure on reduction of sliding friction is outstanding. With the increase of vacuum, the reduction of sliding friction by high-frequency vibrations decreased, especially the reduction of sliding friction by normal vibration. The sliding friction coefficient with high-frequency vibrations slowed down as the vibration amplitude increased. With increase of sliding velocity, the time-averaged friction coefficient with transverse vibration increased.     

Reduction of the sliding friction of metals by the application of longitudinal or transverse ultrasonic vibration

The influence on sliding friction of ultrasonic vibration both parallel and perpendicular to the sliding direction has been studied for samples of aluminium alloy, copper, brass and stainless steel sliding against tool steel. Experiments were performed at a mean sliding speed of 50 mm s⁻¹, and at mean contact pressures up to 0.7 MPa, with vibration amplitudes up to 10 μm at 20 kHz. Significant reduction in sliding friction was observed (up to >80%) and good agreement was found between the measured values and the predictions of two simple models for the effects of longitudinal and transverse vibrations. Longitudinal vibration produces greater reduction in friction than transverse vibration at the same amplitude and frequency. At high vibration amplitudes, the reduction in friction was less than that predicted by the models, because significant metallic transfer occurred from the softer metals to the tool steel counter surface.     

Tribological property investigation on a novel pneumatic actuator with integrated piezo actuators

Pneumatic piston–cylinder actuators are commonly used in industry for a variety of automation and robotics applications. In order to suppress leakage, these actuators comprise seal rings which unfortunately introduce friction and affect the positioning accuracy and output force. This article investigates vibrations of the seal generated by integrated piezo actuators to reduce friction force. For this, two piezoelectric stacks are integrated in the cylinder and used to excite vibration modes. This concept was studied in a compact cylinder pneumatic actuator with a bore diameter of 5 mm and a stroke of 10 mm. Dry friction measurement shows a 52% reduction from the original friction force at a driving frequency of 18.29 kHz and vibration amplitude of 0.05 μm. In the wet friction experiments, the friction force can be reduced by 54% from the original wet friction with vibrations at amplitude of 0.04 μm.     

The frictional behavior of mild steel under horizontal vibration

The present paper investigates experimentally the effect of external horizontal vibration on friction property of mild steel. To do so, a pin-on-disc apparatus having facility of vibrating the test samples in horizontal direction was designed and fabricated. Horizontal vibration is created along the sliding direction of vibration and perpendicular to the direction of vibration. The experimental setup has the facility to vary the amplitudes and frequencies of vibration while velocity of vibration is kept constant. During the experiment, the frequency and amplitude of vibration were varied from 0 to 500 Hz and 0 to 200 μm, respectively. Results show that the friction coefficient increases with the increase in amplitude and frequency of vibration for mild steel. The horizontal vibration can be of two types: one along the direction of sliding (longitudinal direction) and the other along the perpendicular direction of sliding (transverse direction). For both the cases, test sample slides horizontally. It is found that the magnitude of friction coefficient for longitudinal vibration is less than that for under transverse vibration. These results are analyzed by dimensional analysis to correlate the friction coefficient with sliding velocity, frequency and amplitude of vibration. The experimental results are also compared with those available in literature and simple physical explanations are provided.     

滑动摩擦振动噪声的非线性显式动力学分析

利用有限元软件ABAQUS/Explicit(显式动态求解器)对球-平面接触条件下的滑动摩擦振动噪声进行了数值模拟分析。对比试验结果,探讨了摩擦噪声的发生机制,并分析了摩擦噪声发生时接触界面的运动特性。结果表明,摩擦噪声主要是由摩擦系统的自激振动引起的,法向振动与切向振动的耦合是系统产生自激振动和摩擦噪声的一个重要因素。当摩擦系统发生自激振动时,从面节点与主面的接触并不是连续不变的,两者在相对运动的过程中具有黏着-滑动-分离-黏着的特性。     

干气密封滑动密封端面高频微幅摩擦振动研究

为揭示干气密封端面间摩擦振动规律,通过对密封端面在干摩擦状态下的微观形貌与受力进行分析,基于分形理论构建包含分形参数的密封端面法向位移激励,以及干气密封在干摩擦状态下的两自由度摩擦振动系统模型,并对摩擦振动系统模型的影响因素进行数值分析.研究结果表明:法向振动位移和速度随着分形维数与转速的增大先增大后减小;密封环面分形...     

Influence of Ultrasonic Oscillation on Static and Sliding Friction

Vibrations of varying frequency and amplitude are used in many technological areas to control and reduce friction. In this study we report the results of systematic high-precision measurements of the static and sliding friction under the influence of ultrasonic oscillations. We investigate the effect of ultrasonic oscillations for in-plane and out-of-plane oscillations in the completely relevant interval of oscillation amplitudes and sliding velocities and for various material pairings. The experimental results are interpreted on the basis of both macroscopic and microscopic models. There are two main effects which are of interest for tribological applications. Firstly, the frictional force typically decreases with increasing oscillation amplitude, with an oscillation amplitude of about 0.1???m typically being sufficient for a significant decrease of frictional force. Secondly, the decrease of force is larger at smaller sliding velocities; therefore, at sufficiently large oscillation amplitudes, the frictional force always increases with sliding velocity. This effect can be used to suppress frictionally induced vibrations.     

Wear of water lubricated silicon nitride in ball on disc test

Abstract

The ball on disc test configuration is preferred to the flat ended pin on disc because the ball is self-aligned and measurement of wear on the ball is of higher accuracy, compared to the pin. Silicon nitride, sliding on itself in water, was tested with the ball on disc tribometer. Misalignment of the test ball from its proper position behind the disc axis of rotation leads to friction measurement errors, which were analysed. The disc wears non-uniformly, the wear track depth and width vary in longitudinal direction by a factor of 2–3. The uneven wear of the disc is explained by the combined effects of sliding surface anisotropy and disc material non-homogeneity on the one hand and by the friction force and the normal load periodic variation on the other hand. During the running-in process at particular sliding velocity amplitude modulated friction force was observed and an explanation by the mechanical vibration 'beating' phenomenon was suggested. Predictive model of the running-in process is presented, which describes the evolution of the ball wear scar area, the contact pressure and the wear rate. The model predictions are consistent with the experimental data.     

湿式离合器考虑纸基衬面孔隙弹性特性摩擦振动理论

本文提出考虑纸基摩擦材料界面间变形和其中造成润滑油流动的摩擦振动新理论,推荐了一摩擦振动模型,采用引入的粘性液体浸透多孔体,联合假定在切向和法向两个方向的振动导出一稳态准则,和实验相比较,确认所推荐的准则可预测该稳态极限比常规的更精确。它不考虑摩擦材料的变形,只与摩擦系数与滑动速度的斜率有关。基于本准则,讨论了材料特性摩擦振动对稳态的影响。     

Friction-induced vibration — with and without external disturbance

The dynamics of friction-induced vibration resulting from a velocity-dependent friction characteristic is studied theoretically and experimentally. The frictional system is further studied in the presence of an external dynamic force. The amplitude of quasi-harmonic vibration is shown to increase with sliding velocity until oscillation ceases at some upper velocity boundary depending on the friction characteristic. The vibration, which can exist at high sliding velocities, may influence the operation of automatic transmissions, brakes and clutches. Introduction of strong damping to the system can reduce or eliminate the induced-vibration. Similar results could be achieved by modifying the friction-velocity characteristic of the fluid and materials at the interface. An external excitation in the transverse direction, with properly chosen frequency and force ratios, can also reduce the amplitude of the friction-induced vibration. The theoretical and experimental results are in good agreement.     

湿式离合器考虑纸基衬面孔隙弹性特性摩擦振动理论(二)

本文提出考虑纸基摩擦材料界面间变形和其中造成润滑油流动的摩擦振动新理论,推荐了一摩擦振动模型,采用引入的粘性液体浸透多孔体,联合假定在切向和法向两个方向的振动导出一稳态准则,和实验相比较,确认所推荐的准则可预测该稳态极限比常规的更精确。它不考虑摩擦材料的变形,只与摩擦系数与滑动速度的斜率有关。基于本准则,讨论了材料特性摩擦振动对稳态的影响。     

游梁式抽油机井皮带滑动效率的仿真研究

针对游梁式抽油机皮带传动装置负载扭矩波动大的特点,研究了游梁式抽油机皮带的纵向振动特性、皮带与带轮之间的相对滑动速度以及皮带传动的瞬时滑动效率。将皮带简化为纵向振动的弹性体,建立了皮带在交变摩擦力激励下的纵向振动力学模型与波动方程形式的数学模型;应用振型叠加法建立了皮带纵向振动的数值仿真模型;建立了皮带相对于带轮的瞬时滑动速度、瞬时滑动效率的仿真模型;仿真分析了抽油机曲柄扭矩波动幅值、皮带预紧力对皮带滑动效率的影响。仿真结果表明：抽油机曲柄扭矩波动会降低滑动效率;当抽油机曲柄扭矩存在负扭矩时,皮带滑动效率明显降低;在预紧力能保证皮带传动正常工作的情况下,再增加皮带预紧力会增大皮带的传动效率。
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Experimental investigation into squeal under reciprocating sliding

Experiments on squeal under reciprocating sliding were performed by means of a ball against a block. Vibration accelerations, sound pressure level of squeal and tangential force were measured simultaneously. Under certain test conditions, the reciprocating sliding can create a whole process from squeal generation to disappearance. Based on power spectral density (PSD) and short-time Fourier transform (STFT) analyses on the vibration accelerations in that process, it was found that the dominant frequencies of the friction-induced vibrations associated with squeal is not varied. Examination of the friction–velocity slope shows that there is no invariable correlation between the negative friction–velocity slope and occurrence of squeal. Squeal can occur in regions with both negative and positive friction–velocity slopes.     

Investigation on the Molecular Shear-Induced Organization in a Molecularly Thin Film of N-hexadecane

The frictional properties of a thin hexadecane film confined between two atomically smooth surfaces of mica were studied using the surface forces apparatus equipped with a 3D actuator–sensor attachment specially designed to investigate static and dynamic forces in three orthogonal directions simultaneously. The use of this attachment allows the relative alignment of the reciprocal sliding motion to be changed by an angle of 90° while maintaining the film under the same confinement conditions. The effects of the commensurability of the confining mica surfaces as well as the relative sliding direction on the frictional behavior of the hexadecane film were determined for different temperatures (18–29 °C) and sliding velocities (4 nm/s to 4 μm/s). The confined hexadecane film exhibited smooth sliding friction whose amplitude increased with the commensuration of the surfaces. A progressive evolution in the kinetic friction force toward a steady-state value was observed over reciprocal sliding motion for given experimental conditions of applied load, sliding velocity and environmental temperature. This friction evolution shows to be dependent on the sliding history of the film and could result from a partial molecular ordering, occurring during shear.     

单辊驱动轧机水平自激振动定性分析

应用非线性力学模型分析了轧辊水平振动。在分析轧辊轧件接触界面之间相对运动及其相对变形与接触面上摩擦力的关系基础上,建立了轧辊水平振动的非线性动力学方程。分析了粘滑共存摩擦转变为全滑动摩擦对轧辊水平振动的影响。用相平面方法分析了轧辊水平运动规律,并确定了粘滑—滑动运动的极限环。定性地分析了轧辊水平运动的动力学特性,着重分析单辊驱动轧机水平自激振动产生的条件及其机理,得出影响轧辊水平自激振动的因素。提出轧辊水平自激振动模型。     

Influence of sliding velocity on friction force in rubber seal rings under reciprocal motion

The influence of sliding velocity on the value of maximum and steady friction force in rubber seal rings under reciprocal motion is studied. It is established that the nature of the change in a static friction force and a steady-state friction force as a function of the sliding velocity is the same all other conditions being equal. The greater the sliding velocity, the faster the decrease and stabilization of a friction force after endurance of rubber seal rings in fixed contact with a cylinder. The maximum of the friction force dependence on the velocity in an investigated pair decreases with increasing temperature of a sealed medium.     

Friction-induced vibrations of reciprocating contacts

The tangential and normal oscillations for three reciprocating and sliding pairs of different materials are experimentally obtained at different normal loads and reciprocating frequencies. The magnitude and phase shift of the transfer function have been measured to correlate the normal and tangential induced oscillations. The results show a remarkable effect of the properties of sliding pairs on both the normal and tangential vibration amplitude during the prescribed motion. The maximum normal amplitude of oscillation is found to vary linearly with the maximum contact pressure regardless of the material type of the sliding surfaces. It is also found that at high normal loads, the tangential amplitude is proportional to the sonic speed in the sliding pair material. Analytical justification has been introduced throughout the work to substantiate and explain the experimental findings.     

Dynamic Friction Model of Lubricated Surfaces for Precise Motion Control

A model is developed to describe dynamic friction effects in lubricated surfaces. The model covers the hydrodynamic, mixed and boundary lubrication regions. The dynamic friction model can predict the friction force for time-varying velocity, and is useful in precise motion control. The model presented is for a short journal bearing, but can be extended to other geometries of sliding surfaces, such as point and line contacts or rolling element bearings. The friction is related to a time variable fluid film thickness, resulting from journal vibrations relative to the sleeve. The proposed model agrees qualitatively with experimental results for lubricated line contact. Both show similar hysteresis-type friction curves under oscillating velocity.     

The effect of amplitude of vibration on the coefficient of friction for different materials

This work examines how friction coefficients are affected by amplitude of normal vibration at different frequencies. Variation of friction coefficient with the amplitude of normal vibration is investigated experimentally when mild steel pin slides on different types of material such as glass fiber reinforced plastic, cloth reinforced ebonite, polytetrafluoroethylene (PTFE), rubber and mild steel. For this, a pin-on-disc apparatus having facility of vibrating the test samples at different amplitudes and frequencies of vibration was designed and fabricated. During the experiments, the effects of sliding velocity, roughness, normal load and duration of rubbing were also investigated. Studies have shown that the friction coefficient decreases with the increase of amplitude of vibration within the observed range. The observed ranges of amplitude of vibration were 10–200 μm. In this study, it is also observed that the rate of reduction of friction coefficient has a particular relationship on the amplitude and frequency of vibration. The experimental results are compared with those available in the literature and simple physical explanations are provided.