Assessment of Abrasion-Induced Visual Defects in Twin Screw Wet Granulation Using Wall Friction Measurements

Starting point of the presented study were abrasion effects occurring during a twin screw wet granulation (TSG) process of a new chemical entity (NCE) formulation, resulting in gray spots on the final tablets. Several actions and systematic changes of equipment and process parameter settings of TSG process were conducted which reduced the visual defect rate of the tablets, i.e., gray spots on the surface, below the specification limit. To understand the rationale and mechanism behind these improvements, correlations of defect rates and wall friction measurements using a Schulze ring shear tester were evaluated. To check the suitability of the method, a broad range of wall materials as well as powder formulations at various moisture levels were investigated with regard to their wall friction angle. As differences in wall friction angle could be detected, further experiments were conducted using wall material samples made out of different screw materials for TSG. Evaluation of these screw wall material samples gave first hints, which screw materials should be preferred in regard of friction for TSG process. In the finally presented case study, wall friction measurements were performed using the above mentioned NCE formulation with known abrasion issues at TSG processing. The results confirmed that changes which led to a reduced visual defect rate of tablets correlated with a decreased wall friction angle. The results suggest wall friction measurements as a potent tool for equipment selection and establishment of a suitable process window prior to conducting TSG experiments. Graphical abstract

Petrus van Musschenbroek (1692–1761), man of tribology

Petrus van Musschenbroek was a famous scientist and inventor, natural philosopher, experimental physicist, engineer, instrument builder, experimenter, in the continental Newtonian tradition of Boerhaave and ‘s Gravesande. And: a tribologist. He is one who deserves more fame than he has received so far, and which is well documented in this publication. Van Musschenbroek coined the name tribometer for his device to measure friction in a journal bearing, and some authors rightly refer to this. However, what seems to have remained unnoticed until now is that he also published quantitative results of his friction measurements and tried to arrive at general laws of friction based on them. He reported in detail on friction experiments on sliding, dry as well as lubricated, sliders and journal bearings, a novum in his time, as early as in 1734. When the data from Van Musschenbroek's tables are mapped into graphs, a method which was not in use at that time, two Stribeck curves for journal bearings emerge. Van Musschenbroek's work deserves much more acclaim in the tribology community than it has now.

On the interest of a semi-empirical model for the tooth friction coefficient in gear transmissions

Accurate modelling of friction coefficient is of primary importance in efficiency, vibration and failure analyses of enclosed gear drives. After showing the influence of surface/lubricant interactions on friction, the authors used a semi-empirical model which can take all these aspects into account. Lubricant is modelled as an Eyring–Reynolds fluid and rough surfaces are described with two parameters via a stochastic approach. A specific two-disc machine is used to perform series of friction measurements on smooth and rough discs. Smooth discs allow to operate under full film lubrication and to measure a reference shear stress of lubricant, whereas rough discs reproduce gear tooth roughness and generate a representative value of friction on asperities. The purpose of this present paper is to describe calculations of this physical-based friction coefficient model and to present the experimental process. On the basis of new results, the impact of a surface finishing process is assessed as well as the consequence of calculating friction coefficient based on oil injection instead of local bulk temperature.

Drop-On-Demand Lubrication of Gears: A Feasibility Study

Different lubrication methods such as oil dip or injection lubrication are used in gearboxes to lubricate tribological contacts and to dissipate frictional heat. To improve resource and energy efficiency, novel needs-based lubrication methods like the drop-on-demand lubrication are being developed. It includes an ink-jet nozzle driven by a piezo element to generate picoliter droplets injected to tribological contacts. This study evaluates the feasibility of drop-on-demand lubrication of gears. Friction measurements in rolling-sliding contacts indicate the formation of typical elastohydrodynamic contacts. Power loss measurements of gears show a similar behavior compared to continuous minimum quantity lubrication. Hence, the study confirms that the operation of gears with drop-on-demand lubrication is possible. It introduces the possibility of dynamic and flexible oil supply on a contact needs-based level.

Design of bidirectional frictional behaviour for tactile contact using ellipsoidal asperity micro-textures

Tactile perception and friction can be modified by producing a deterministic surface topography. Change of surface feature arrangement and texture symmetry can produce an anisotropic frictional behaviour. It is generally achieved through skin hysteresis by promoting its deformation. This work investigates whether a bidirectional friction can be created with microscale ellipsoidal asperity textures, thus relying on the adhesive component of friction. For this purpose, four textured samples with various asperity dimensions were moulded with a silicone rubber having an elastic modulus comparable to that of the skin. Coefficient of friction measurements were conducted in-vivo in two sliding directions with a range of normal loads up to 4 N. Finite element method (FEM) was used to study elastic deformation effects, explain the observed friction difference, and predict surface material influence. Measurements performed perpendicular to the asperity major radii showed consistently higher friction coefficients than that during parallel sliding. For the larger asperity dimensions, a change of the sliding direction increased friction up to a factor of 2. The numerical analysis showed that this effect is mostly related to elastic asperity deflection. Bidirectional friction differences can be further controlled by asperity dimensions, spacing, and material properties.

Acoustic signature identification of damage and wear mechanisms in a steel/glass sliding contact

The tribological behavior of a steel/glass ball-on-flat contact was studied by synchronizing the friction measurements with an acoustic emission device and a vision system. The results highlight two distinct friction regimes identified with low and high friction values. Their transition is characterized by a modification of acoustic emission signals. In addition, two main damage and wear mechanisms are identified: the creation and propagation of Hertzian cracks visible on the glass surface and the constitution of an interfacial layer of debris. The different accommodation mechanisms, activated successively or simultaneously, are identified for acoustic emission frequencies between 300 and 700 kHz. Eventually, this approach allows a real-time wear mechanisms identification and gives better insights about acoustic emission signals in relation to tribological systems.

Effects of Fe Addition on the Phase Transformation Internal Friction in Ni-Al-Based Alloys

The influences of chemical compositions on the internal friction of phase transformation were investigated for the quenched Ni-Al-based alloys. The internal friction measurements were completed using a low-frequency mechanical spectrum apparatus through forced vibration method. It was shown that the chemical compositions have the great influences on the internal friction of phase transition for the oil-cooled Ni-Al-based alloys. The peak-temperature of internal friction is lowered, and the peak-height is increased and the peak-width is narrowed when Fe content is elevated. The reducing of the peak-temperature is attributed to the increase of stability of γ phase due to the addition of Fe. The changes of the parameters of the peak are also related to the Aleq (equivalent Al) contents.

Wax-oil lubricants to reduce the shear between skin and PPE

Prolonged use of tight-fitting PPE, e.g., by COVID-19 healthcare workers leads to skin injuries. An important contributor is the shear exerted on the skin due to static friction at the skin-PPE interface. This study aims to develop an optimised wax-oil lubricant that reduces the friction, or shear, in the skin-PPE contact for up to four hours. Lubricants with different wax-oil combinations were prepared using beeswax, paraffin wax, olive oil, and mineral oil. In-vivo friction measurements involving seven participants were conducted by sliding a polydimethylsiloxane ball against the volar forearms to simulate the skin-PPE interface. The maximum static coefficient of friction was measured immediately and four hours after lubricant application. It was found that the coefficient of friction of wax-oil lubricants is mainly governed by the ratio of wax to oil and the thermal stability and morphology of the wax. To maintain long-term lubricity, it is crucial to consider the absorption of oil into the PPE material. The best performing lubricant is a mixture of 20 wt% beeswax, 40 wt% olive oil, and 40 wt% mineral oil, which compared to unlubricated skin, provides 87% (P = 0.0006) and 59% (P = 0.0015) reduction in instantaneous and 4-h coefficient of friction, respectively.

A Digital Twin for Friction Prediction in Dynamic Rubber Applications with Surface Textures

Surface texturing is an effective method to reduce friction without the need to change materials. In this study, surface textures were transferred to rubber samples in the form of dimples, using a novel laser surface texturing (LST)—based texturing during moulding (TDM) production process, developed within the European Project MouldTex. The rubber samples were used to experimentally determine texture-induced friction variations, although, due to the complexity of manufacturing, only a limited amount was available. The tribological friction measurements were hence combined with an artificial intelligence (AI) technique, i.e., Reduced Order Modelling (ROM). ROM allows obtaining a virtual representation of reality through a set of numerical strategies for problem simplification. The ROM model was created to predict the friction outcome under different operating conditions and to find optimised dimple parameters, i.e., depth, diameter and distance, for friction reduction. Moreover, the ROM model was used to evaluate the impact on friction when manufacturing deviations on dimple dimensions were observed. These results enable industrial producers to improve the quality of their products by finding optimised textures and controlling nominal surface texture tolerances prior to the rubber components production.

Full Scale Evaluation of Surface Treatments for Airfield Concrete Pavement Repair

Surface deterioration of concrete pavements requires maintenance. Highway and airfield pavements exhibit many of the same maintenance issues, but airfields have several additional unique issues and requirements. Among these are petroleum contamination on aircraft parking areas and a high potential for failed concrete or maintenance materials to damage aircraft. To address these issues, commercially available surface-applied treatment products were assessed for use on concrete pavements with particular focus on the special requirements of airfields. Fourteen products encompassing numerous chemistries were evaluated in a full-scale field experiment. The specific objectives of this study were to investigate materials for field application issues, adhesion to concrete (for both clean and oil contaminated concrete), the ability to seal cracks, behavior under aircraft traffic loads including surface friction, and durability over time with exposure to environmental conditions. Test strips of each material were applied to deteriorated concrete slabs. Half of the concrete was intentionally contaminated with oil while the other half was left clean. Simulated aircraft traffic was applied and periodic visual observations and surface friction measurements were made. Two years after material application, a final visual assessment was made. Many of the products performed well on clean concrete; however, oil contaminated concrete detrimentally affected many of them. Of the fourteen products evaluated, two of the epoxy based materials clearly emerged as the best performing.