Use of ‘chalk’ in rock climbing: sine qua non or myth?

Magnesium carbonate, or ‘chalk’, is used by rock climbers to dry their hands to increase the coefficient of friction, thereby improving the grip of the holds. To date, no scientific research supports this practice; indeed, some evidence suggests that magnesium carbonate could decrease the coefficient of friction. Fifteen participants were asked to apply a force with the tip of their fingers to hold a flattened rock (normal force), while a tangential force pulled the rock away. The coefficient of friction – that is, the ratio between the tangential force (pulling the rock) and the normal force (applied by the participants) – was calculated. Coating (chalk vs no chalk), dampness (water vs no water) and rock (sandstone, granite and slate) were manipulated. The results showed that chalk decreased the coefficient of friction. Sandstone was found to be less slippery than granite and slate. Finally, water had no significant effect on the coefficient of friction. The counter-intuitive effect of chalk appears to be caused by two independent factors. First, magnesium carbonate dries the skin, decreasing its compliance and hence reducing the coefficient of friction. Secondly, magnesium carbonate creates a slippery granular layer. We conclude that, to improve the coefficient of friction in rock climbing, an effort should be made to remove all particles of chalk; alternative methods for drying the fingers are preferable.     

柴油机凸轮－挺柱磨损分析与摩擦副匹配试验研究

凸轮与挺柱是发动机中最容易产生早期磨损的一对摩擦副,设计不当产生磨损会严重影响发动机的可靠性。通过改进凸轮型线,增大最小曲率半径以及减小弹簧刚度来降低接触应力;同时对凸轮—挺柱二者的形状结构、空间位置、材料、硬度进行了匹配优化分析。对改进后的结果设计了合理的试验方案,进行了详细的试验验证。     

Understanding the effect of finger–ball friction on the handling performance of rugby balls

Handling errors are often seen in professional rugby games and even more so in amateur rugby. This paper analyses the problem of ball mishandling using high-speed video footage of passes and a bespoke finger friction rig. The high-speed video analysis showed that when the ball is caught, often there is a fluctuating movement of the fingers over the surface of the ball. It also showed that the fingers move over the surface of the ball when the ball is thrown, confirming that the dynamic friction is a good measure of how easily a ball can be handled. Rugby ball surface samples were used, on a finger friction rig, to assess the coefficient of friction between the finger and the balls. The currently manufactured balls displaying the highest coefficients of friction in clean, dry conditions were the design with square, ‘sharp’ pimples and also the design with a mixture of small and large pimples. The most consistent ball across wet and dry conditions was the ball with round, large, densely populated pimples. It was also shown that when water is added to the surface of the ball or finger, there was little variation in performance between the ball varieties.     

发电机碳刷发热原因及对策

碳刷主要是连接发电机转动部分和静止部分的,并将整流柜输出的电流供给发电机转子,但是在运行中由于通风不良、机械摩擦、接触不好等因素的影响,会引起发热,危及发电机的安全运行。文章通过分析碳刷发热的原因,提出解决措施。     

提升科技期刊国际影响力的策略与实践——以Friction为例

【目的】 以Friction的创刊实践为例,探讨“中国科技期刊国际影响力计划”项目对我国科技期刊的促进作用,与我国英文科技期刊工作者分享办刊经验。【方法】 以Friction为例,结合摩擦学学科特点,探索创办国际一流英文科技期刊的经验。【结果】 在中国科学技术协会D类和B类项目资助下,Friction发展迅速,国际影响力明显提升,2016年6月正式被SCI收录,首个影响因子位于Q2区,并连续2次获得“中国最具国际影响力学术期刊”荣誉称号。【结论】 Friction通过坚持自身办刊特色,建立高水平编委和审稿专家队伍,组织高水平稿件,提高稿件时效性,加强期刊的宣传与推广,为我国英文科技期刊不断提升国际影响力提供借鉴。     

Optimization of handheld gauge sizes for rocker measurements of skate blades and bobsleigh runners

This work uses numerical methods to investigate the feasibility of modifying an instrument used in speed skating to analyze blades from four different ice sports. The instrument, a handheld rocker gauge, is adapted to create a device that can effectively profile other types of skate blades and bobsleigh runners. Since there are significant differences between short and long-track blades one could expect a difference in the gauges used to study these blades. Despite this expectation, the same gauge is used in both disciplines. The usefulness of these gauges has been proven in speed skating so it is expected that they should also be useful to study hockey blades and bobsleigh runners. To optimize the gauge size for different blade types we digitize the profile of a blade, which we use to simulate gauge data. Then we use that gauge data to reconstruct the profile and compare it to the original digital profile. The result is compared for various gauge sizes and the gauge size is optimized for each of the four disciplines. The only commercially available device seems optimal for bobsleigh and long track speed skating. Smaller gauges are recommended for analyzing short track speed skates and hockey skates.

Construction of a Precursor Model for the Concept of Rolling Friction in the Thought of Preschool Age Children: A Socio-cognitive Teaching Intervention

The aim of this study was to explore the extent to which the characteristics of two teaching interventions can bring about cognitive progress in preschoolers with regard to the factors rolling friction depends on, when it is applied to an object that is freely rolling on a horizontal surface. The study was conducted in three phases: pre-test, teaching intervention, and post-test. Two teaching strategies were compared: one inspired by Piaget’s theory (Piagetian approach) and one inspired by post-Piagetian and Vygotkian assumptions (socio-cognitive approach). A statistically significant difference was found between the pre-test and post-test, providing evidence that the socio-cognitive approach allows for the creation of a more appropriate teaching framework compared to the Piagetian one.     

高精度伺服系统Stick-Slip摩擦的机理建模与非线性动力学分析

低速摩擦问题是高精度伺服系统设计所面临的难点之一,直接影响系统的稳态精度和稳定性,不仅会使系统产生稳态误差,而且还会导致Stick-Slip运动,甚至会使系统进入混沌状态.为了能提高系统性能,很多学者对含摩擦环节的伺服系统进行了大量研究,得出了很多有益的结论.本文主要综述近年来在摩擦特性和机理研究、摩擦建模以及摩擦非线性动力学三个方面的研究现状,并对今后的研究方向进行了分析.     

基于试验的制冷机节流管沿程阻力系数的确定

鉴于紊流运动的复杂性,其沿程阻力系数确定起来较为困难.本文针对无回热且进口处制冷剂为过冷液体的节流管介绍确定其沿程阻力系数的两种方法.     

Al2O3颗粒对丁腈橡胶/316L不锈钢配副摩擦磨损行为的影响

目的：研究弹性体/金属配副在硬质颗粒环境下的摩擦磨损行为,分析有无颗粒及颗粒尺寸大小对摩擦学特性的影响,为橡塑密封设计提供参考。创新点：基于橡胶O型圈常见失效机制,模拟橡胶密封圈在颗粒介入时的摩擦磨损行为,探讨硬质颗粒及其颗粒尺寸对橡胶/金属摩擦配副的影响。方法：1.采用球/平面接触方式,开展丁腈橡胶/金属（316L）配副在Al2O3颗粒环境下的摩擦磨损行为,通过考察摩擦系数时变曲线、摩擦副磨损形貌及其损伤机制等特性,揭示 Al2O3颗粒对丁腈橡胶/316L不锈钢配副摩擦磨损行为的影响。结论：1. Al2O3颗粒进入橡胶/金属摩擦配副明显降低摩擦系数、硬质颗粒的犁削作用,加剧金属偶件的磨损;2.大尺寸的Al2O3颗粒能嵌入橡胶基体并加速橡胶的磨损,对金属有微切屑作用;然而随着颗粒尺寸的减小,颗粒反而减缓橡胶的磨损;3.在有无颗粒和不同颗粒尺寸的情况下,橡胶和金属均表现出不同的损伤机制。