This work introduces two innovative rolling pair concepts to minimize slippage and reduce mass in cam-roller systems of large-scale hydraulic drivetrains: The variable contact length and the Shifting Contact Geometry concepts. Both aim to improve traction in the low contact force phase in cyclically loaded rolling contacts. The shifting contact geometry concept was validated using three custom rolling contacts: a line contact, a double elliptical contact, and a combination of both (i.e., shifting contact geometry). The tests were conducted under synchronized cyclic loading to mimic the conditions in a hydraulic drivetrain. Furthermore, a model from previous work was implemented to make predictions and compare them against the experimental results. During preliminary tests, the double elliptical contact displayed superior tractive behavior than the line contact under the same load thanks to higher contact pressures. Under synchronized cyclic loading, the line contact displayed high sensitivity to applied resisting torques at low contact forces, leading to high slide-to-roll ratios and traction force peaks. In contrast, the rolling pair with shifting contact geometry exhibited minimum slippage even under high resisting torques, resulting in substantially lower (and in most cases negligible) slide-to-roll ratio and traction force peaks. The simulations also captured this behavior, proving the validity of the model for predicting and comparing the rolling-sliding dynamics of these two different rolling pairs. This study demonstrates that rolling pairs with shifting contact geometry can not only improve the tribological performance of cam-roller contacts in large-scale hydraulic drivetrains but also yield a more favorable dynamic behavior.

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The rolling-sliding dynamics of large-scale cam-roller contacts are strongly influenced by the inertia of the roller, particularly when slippage occurs. Slippage can potentially impact the reliability of these rolling interfaces. This study introduces an approach to replicate the rolling-sliding dynamics of cam-roller contacts in a large-scale hydraulic drivetrain, on a small scale. For that, we have upgraded our two-roller tribometer to enable cyclic loading, allow the application of resisting torques, and generate inertia torques. These are three essential elements required to mimic the dynamics observed at large scales. A method has been proposed for scaling the roller inertia accordingly. Furthermore, we have implemented a modeling framework from previous work to make predictions under various dynamic conditions. The results show that our small-scale approach can replicate five key characteristics anticipated at a large scale, including those linked to slippage. Small increments in the resisting torque significantly increased the slide-to-roll ratio (SRR) and peak traction force, among others. The simulations also predicted these effects, capturing trends and producing reasonable predictions of the magnitude and relevant features of key parameters. The use of cyclic loading, extra inertia, and adjustable resisting torques, effectively generated repeatable and controllable dynamic rolling-sliding conditions. Our work is significant for the design and development of novel large-scale hydraulic drivetrains. Our findings highlight the importance of reducing slippage at low contact forces to prevent the brusque change in the rolling conditions during the high contact force phase. By doing so, surface damage and detrimental dynamic effects can be prevented.@en

This study introduces a method based on fine torque control to evaluate traction in rolling—liding line contacts under small slide-to-roll ratios (SRRs). To accomplish this, we engineered an innovative testing machine—a two-roller tribometer capable of precisely applying resisting torques to one of the rollers. Two types of tests were designed and conducted to validate our method and showcase the capabilities of the novel test setup. The first type, named the “Traction Decay Test”, proved to be effective in evaluating changes in the SRR over time. The second, named the “Torque-Mode Traction Test”, demonstrated its effectiveness in achieving ultra-low SRRs, in the order of 0.01%. As a result, traction curves with high resolution in the low SRR domain were constructed. This advancement provides the means for gaining a deeper understanding of traction coefficients, wear behavior, and tribological performance at ultra-low SRRs across diverse applications.@en

The frictional performances of surgical sutures have been found to play a vital role in their functionality. The purpose of this paper is to understand the frictional performance of multifilament surgical sutures interacting with skin substitute, by means of a penetration friction apparatus (PFA). The influence of the size of the surgical suture was investigated. The relationship between the friction force and normal force was considered, in order to evaluate the friction performance of a surgical suture penetrating a skin substitute. The friction force was measured by PFA. The normal force applied to the surgical suture was estimated based on a Hertzian contact model, a finite element model (FEM), and a uniaxial deformation model (UDM). The results indicated that the penetration friction force increased as the size of the multifilament surgical suture increased. In addition, when the normal force was predicted by UDM, it was found that the ratio between the friction force and normal force decreased as the normal force increased. A comparison of the results suggested that the UDM was appropriate in predicting the frictional behavior of surgical suturing.

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