Understanding elastohydrodynamic (EHL) film formation and the pressure-viscosity response of lubricants is necessary for designing rolling/sliding tribological contacts. This article investigates the EHL behaviour of four formulated water-based lubricants (glycerol-water, glycol-water, and ionic liquid-water) and one reference oil under moderately high pressures, typical in gears and bearings applications. A ball-on-disc tribometer with optical interferometry was employed to measure the film thickness of the water-based lubricants. The results highlight the sensitivity of film formation to entrainment speed, slide-to-roll ratio (SRR), temperature, and lubricant composition. Water loss due to evaporation significantly impacts film formation at high temperatures. Additionally, an unusual increase in film thickness was observed for the glycol-water solution, likely due to complex tribological conditions. The limitations of the classical Hamrock-Dowson film thickness equation for water-based lubricants are also discussed. Furthermore, pressure-viscosity coefficients of the water-based lubricants were estimated using both optical interferometry and high-pressure viscometer methods. The effect of water content on the pressure-viscosity coefficient was also examined, revealing that higher water content leads to reduced pressure and temperature dependence of viscosity.

Grinding is regularly conducted on railway tracks to prevent crack propagation and surface deterioration. However, grinding can introduce roughness on rail surfaces, potentially leading to stress and strain concentration that increase the likelihood of crack initiation. This paper proposes the utilization of surface roughness obtained by replicating the ground rail surface to assess its impact on train wheel-rail interactions. A novel approach which integrates the replicated roughness into an elastoplastic contact model, allows for a detailed assessment of its effects on contact pressure, and residual strain and stress distributions. The findings highlight the importance of considering surface roughness in predictive maintenance planning for railway infrastructure, as it can significantly influence the structural integrity and long-term performance of the track system.

This study evaluates the negative effects of grip wax application on the dynamic ski–snow coefficient of friction and subsequent performance during the double poling cycle in cross-country skiing. Utilising a linear ski tribometer, friction tests were performed on classic cross-country skiing skis prepared with no, thin, and thick grip wax under controlled laboratory conditions. The dynamic coefficient of friction was estimated under various load conditions, reflecting dynamic skiing motions. Results indicated a clear increase in coefficient of friction with the addition of grip wax, with significant differences observed between thin and thick applications. Specifically, compared to skis with no wax, the coefficient of friction for skis with thin and thick wax layers experienced a negative increased by 1.8% and 3.2% during double poling, and by 1.7% and 2.6% whilst gliding, respectively. These friction increases were associated with higher power requirements during skiing or a consequent time loss. This underscores the need for meticulous application of grip wax application, tailored to the snow conditions, ski camber profile and racecourse demands, to minimise impact on gliding performance whilst maintaining sufficient static coefficient of friction for effective use of the diagonal stride technique. Furthermore, the skier should utilise a skiing technique to minimise the risk of encountering load conditions that increase the coefficient of friction. Overall, this research provides quantitative insights into the trade-offs between grip enhancement and friction-related performance losses in cross-country skiing.

In cross-country skiing, athletes expend large amounts of energy to overcome friction as their skis interact with snow. Even minor reductions in the friction can significantly influence race outcomes. Over the years, researchers have found many ways of quantifying ski–snow friction, but there are only a few methods that consider the glide of real-sized skis under natural conditions during both accelerating and decelerating movements. This study introduces a novel experimental setup, consisting of a sled equipped with authentic cross-country skis and a base station that uses satellite receivers to communicate via radio, constituting a real-time kinematic positioning system with centimetre accuracy. While the sled was running on a classic ski track with natural height variations, altitude and velocity data were recorded for quantification of the coefficient of friction (COF), both for accelerating and decelerating motion, employing a model based on Newton’s second law. The results show that the COF during acceleration was more than 20% higher than during deceleration, demonstrating dynamic changes in the frictional behaviour between these phases. This finding is crucial for the execution of all types of cross-country skiing techniques, where the athlete either accelerates or decelerates while moving forward. The ability of the current experimental set-up to distinguish between the COF during acceleration and deceleration has considerable implications for further developments.

In this study, non-equilibrium molecular dynamics (NEMD) simulations with a reactive force field were used to investigate the tribochemical properties of glycerol, with and without water, confined between two ferrous surfaces. The results demonstrated that glycerol significantly reduced friction on α-Fe slabs more effectively than on functionalized amorphous magnetite. A numerical method was introduced to identify the interface region and evaluate the dissociated surface atoms. It was found that the dissociation rate of glycerol molecules increased with applied normal pressure and shear stress. Additionally, the production rate of water molecules from glycerol dissociation was consistently positive for all solutions above 80 % wt. The assumption of linear velocity distribution across the film thickness was validated for all systems studied.

Archard’s wear law encounters challenges in accurately predicting wear damage and volumes, particularly in complex situations like asperity–asperity collisions. A modified model is proposed and validated, showcasing its ability to predict wear in adhesive contacts with better accuracy than the original Archard’s wear law. The model introduces an improved wear coefficient linked to deformation energy, creating a spatially varying relationship between wear volume and load and imparting a non-linear characteristic to the problem. The improved wear model is coupled with the Boundary Element Method (BEM), assuming that the interacting surfaces are semi-infinite and flat. The deformation energy is calculated from the normal contact pressure and displacements, which are the common outputs of BEM. By relying solely on these outputs, the model can efficiently predict the correct shape and volume of the adhesive wear particle, without resorting to large and often slow models. An important observation is that the wear coefficient is expected to increase based on the accumulated deformation energy along the direction of frictional force. This approach enhances the model’s capability to capture complex wear mechanisms, providing a more accurate representation of real-world scenarios.

A better understanding about the rolling contact fatigue and micropitting performance of machine component surfaces lubricated with environmentally friendly lubricants is critical to designing and further formulating new lubricants intended to be used in rolling–sliding contacts such as those found in gear and bearing applications. In this work, the frictional behaviour and rolling contact fatigue (RCF) performance of DLC, Cr/a-WC:H/a-C:H and a-C:Cr coatings under glycerol-based lubrication in rolling sliding contact conditions have been investigated. Traction maps, Stribeck curves, and fatigue plots have been generated by using a micropitting test rig (MPR). The initiation and progression of micropitting was monitored by means of white light optical interferometry and scanning electron microscopy (SEM). Results indicated that glycerol-based lubricants exhibited a significant friction reduction as the hydrodynamic effect is enhanced at higher rolling-speeds. Under boundary lubrication the friction coefficient was significantly higher compared to the values obtained with a commercial mineral-based transmission oil. Compared to uncoated steel surfaces, DLC coatings effectively reduced the volume loss and micropitting progression. Irrespective of the coating thickness, DLC showed an excellent tribological behaviour when the base lubricant favours the onset of mild-wear, over micropitting. When the lubricant formulation favoured the onset of micropitting, the coatings tended to prematurely fail due to debonding from the substrate, and local micro-spallation. The experiments demonstrated that friction reduction does not necessarily correspond with a reduction of micropitting.

Of the medals awarded during the Winter Olympics Games, most are awarded for sports involving cross-country (XC) skiing. The Double Poling (DP) technique, which is one of the sub-techniques used most frequently in XC skiing, has not yet been studied using simulations of the ski–snow contact mechanics. This work introduces a novel method for analysing how changes in the distribution of pressure on the sole of the foot (Plantar Pressure Distribution or PPD) during the DP motion affect the contact between the ski and the snow. The PPD recorded as the athlete performed DP, along with an Artificial Neural Network trained to predict the geometry of the ski (ski-camber profile), were used as input data for a solver based on the boundary element method, which models the interaction between the ski and the snow. This solver provides insights into how the area of contact and the distribution of pressure on the ski-snow interface change over time. The results reveal that variations in PPD, the type of ski, and the stiffness of the snow all have a significant impact on the contact between the ski and the snow. This information can be used to improve the Double Poling technique and make better choices of skis for specific snow conditions, ultimately leading to improved performance.

Ski-orienteering, which combines cross-country skiing with orienteering, dates back to the late 1800s, with the first World Championships in 1975. While researchers have explored the physiological and biomechanical determinants of success in cross-country skiing and orienteering separately in detail, scientific knowledge concerning ski-orienteering remains limited. Based on the information that is presently available together with interviews with elite ski-orienteers, we explore here for the first time the historical development, physiological, biomechanical, and psychological demands, certain training strategies, and future prospects and challenges associated with this sport, including its potential to become an Olympic event. A demanding endurance sport (with racing times of 12–120 min), ski-orienteering requires both considerable aerobic and anaerobic capacity, as well as well-trained upper and lower body muscles. In addition, ski-orienteering demands advanced skiing technique on various types of terrain, with frequent changes between sub-techniques, on both wide and narrow tracks and with numerous turns on downhill terrain. Moreover, success in this sport requires accurate and rapid orienteering—the ability to navigate a complex network of ski tracks with numerous intersections/crossings in a manner designed to pass the multiple control points in the order indicated on the map as rapidly as possible, i.e., advanced spatial cognition and highly developed navigational skills. Thus, ski-orienteering requires training designed to improve both relevant physiological characteristics and orienteering skills, which should become the focus of future interdisciplinary research on this complex sport.