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In a combustion engine, the valve system controls the flow of gases in to and out of the combustion chamber. The contacting surfaces experience a harsh tribological situation with high temperatures, high speed impacts, corrosive environment and high closing forces causing micro sliding in the interface. The components have to endure in the range of hundreds of millions to a billion operational cycles, resulting in extreme demands on low wear rate. Such low wear rates can be accomplished by the protective action from tribofilms forming from oil residues, avoiding a pure metal-to-metal contact. Such tribofilms are found on well-functioning engine valves from a variety of engines, but some stationary gas engines experience problems with wear occurring seemingly randomly at normal running conditions. For some reason, the tribofilm has not protected the surfaces sufficiently, causing wear. One way to combat the random behaviour could be to promote robust function of the protective tribofilms by texturing the valve sealing surfaces to improve the capture and storage of oil residues. By stabilising the supply in this way, the damage from periods with low access to tribofilm forming material could be reduced. The present work demonstrates that turning of the valve seat inserts, creating valleys perpendicular to the sliding motion, can be developed into a useful solution. The amount and localisation of tribofilms became more predictable and stable than without the texture, leading to reduced component wear. The valleys should not be too wide, since this increased the amount of exposed metal if the tribofilm flaked off. When having the same width, the deeper valleys showed less flaking off of the tribofilm.

A theoretical investigation of the hydrodynamic lubrication of the top compression piston ring in a large two-stroke marine diesel engine is presented. The groove mounted piston ring is driven by the reciprocal motion of the piston. The ring shape follows a circular geometry and the effect of changes in radii is analysed. A numerical model based on the finite difference method in 1D has been developed for solving Reynolds equation in combination with the load equilibrium equation together with flow continuity between the piston ring surface and liner for analysis of the lubricant transport. The cyclic variation throughout one stroke is presented for the minimum film thicknesses at different interesting locations of the piston ring surface together with the friction and the pressure distribution history. The aforementioned parameters have been investigated numerically. The numerical results are presented and discussed.

The wear resistance of carbide-free bainitic microstructures have recently shown to be excellent in sliding, sliding-rolling, and erosive-abrasive wear. Boron steels are often an economically favorable alternative for similar applications. In this study, the erosive-abrasive wear performance of the carbide-free bainitic and boron steels with different heat treatments was studied in mining-related conditions. The aim was to compare these steels and to study the microstructural features affecting wear rates. The mining-related condition was simulated with an application oriented wear test method utilizing dry abrasive bed of 8–10 mm granite particles. Different wear mechanisms were found; in boron steels, micro-cutting and micro-ploughing were dominating mechanisms, while in the carbide-free bainitic steels, also impact craters with thin platelets were observed. Moreover, the carbide-free bainitic steels had better wear performance, which can be explained by the different microstructure. The carbide-free bainitic steels had fine ferritic-austenitic microstructure, whereas in boron steels microstructure was martensitic. The level of retained austenite was quite high in the carbide-free bainitic steels and that was one of the factors improving the wear performance of these steels. The hardness gradients with orientation of the deformation zone on the wear surfaces were one of the main affecting factors as well. Smoother work hardened hardness profiles were considered beneficial in these erosive-abrasive wear conditions.

Highly additized low viscous lubricants, new coatings, and surface treatments have been employed by original equipment manufacturers in several tribosystems to reduce emission and fuel consumption. In this sense, this work investigates the tribological response of four different advanced fully formulated gear oils and three different materials (coatings and topography) in terms of friction and wear using a ball-on-disc test rig under pure unidirectional sliding condition and boundary lubrication. The tested lubricants had different base oils: mineral, semi-synthetic, and synthetic with different additive packages. The ball's material was AISI 52100 bearing steel and the bulk material of the tested specimens (discs) were SAE 4320 steel with surface as follows: (i) ground; (ii) subjected to ceramic shot peening (CSP) and, (iii) coated with WC/C. Optical and scanning electron microscopy and 3D profilometry were used to evaluate the wear track and tribofilm formation. It was found that the frictional dependence on the surface topography and lubricant type is not significant, whilst the wear mechanisms were highly dependent on material and surface conditioning. The harder and rougher the contact body, the higher the wear produced in the counter body. At the harsher conditions base oil type control wear more effectively than the additive package.

In this study, the friction and wear properties of six different new and used wind turbine gear oils (ISO VG 320), with different base oil formulations and additives packages, were investigated. For that purpose, a four-ball tribometer and an Optimol SRV were used. Moreover, the lubricants extreme pressure properties were also evaluated, using the same four-ball tribometer. The study also includes a characterization of the lubricants. The main objective was to compare the new and used gear oils in order to identify performance differences and predict oil change intervals. The results indicate that a use of 3 to 4 years is within the lifetime of the lubricant.

Shaft seals with a seal lip are used in various applications to separate gases and fluids, but their sealing function and durability are affected by the extent of lubrication at the dynamic seal faces between a rotating shaft and the seal lip. A new sealing system consisting of two hydrated seal rings and non-Newtonian lubricants was designed by a biomimetic approach to minimise frictional loss and provide high waterproof function when separating air and water. This system demonstrates significant superiority in low-speed rotation, conditions under which the lubrication mechanism of an oil seal or mechanical seal does not generally work well. Fibre reinforcement of the seal ring material was performed to prevent the deformation of the soft seal rings because of the frictional force in the tangential direction of the shaft and water pressure. This reinforcement enabled the new sealing system to obtain both high waterproof performance and low frictional characteristics in the speed range of 20–100 r/min.

In recent years, 3D scanning and printing of plastics has rapidly matured while printing of metallic parts is only gaining popularity due to required refinements of technology combined with cost- and resources effectiveness for the main components of printers and consumables. The 3D printing allows producing complicated shapes that can be hardly produced by conventional mechanical tools and can provide the functionalization of surfaces. In this work, several different stainless steel (AISI 316 L) surface patterns (flat, gecko’s fibrils, dimples, pyramids, mushrooms, mesh, brush, inclined brush) intended for controlling the coefficient of friction were printed with the help of a 3D metal printer by selective laser melting technique. Unidirectional sliding tests were performed with pin-on-disc configuration. Sliding velocity of 5 × 10−3m/s and continuously increasing load ranged from 5 to 103 N has been applied in the course of “scanning” mode and accompanied by simultaneous recording of the coefficient of friction. A stainless steel (AISI 316) disc counterbody was used in this series of the tests. It was found that the 3D printed structures allow to control the value and stability of the coefficient of friction in a wide range of loads. Microstructural analysis of the worn samples was performed to support the conclusions regarding wear mechanism.

A ferrofluid pocket bearings is a type of hydrostatic bearing that uses a ferrofluid seal to encapsulate a pocket of air to carry a load. Their properties, combining a high stiffness with low (viscous) friction and absence of stick-slip, make them interesting for applications that require fast and high precision positioning. Knowledge on the exact performance of these types of bearings is up to now not available. This article presents a method to model the load carrying capacity and normal stiffness characteristics of this type of bearings. Required for this is the geometry of the bearing, the shape of the magnetic field and the magnetization strength of the fluid. This method is experimentally validated and is shown to be correct for describing the load and stiffness characteristics of any fixed shape of ferrofluid pocket bearing.

Non-lubricated clutches are still subject of numerous investigations. Measuring the temperature distribution in the friction contact would deliver a more detailed view of the live friction process. Main restriction remaining to establish this information is the low spatial measurement density of conventional thermal measurement facilities. On this account, fibre optic sensing technology has been used for the first time to measure the temperature distribution with high spatial resolution in a pressure plate of a clutch system. One sensing fibre offers not less than 700 measurement points in the pressure plate. An academic example was carried out to validate the new measuring method. On one hand, the focus of this investigation was the comparison of the measured temperature between fibre and thermocouple. On the other hand, the manner of inserting the fibre, especially the influences on the part by inserting holes, was investigated. The target is minimally changing the friction contact by the measurement instrumentation. By means of this measurement method, it is possible to analyse the thermo-mechanical behaviour of a friction system with a high level of details. This method enables the control of temperature distribution in the counter plate. Currently, an optimization method is in development, which should make it possible to rate design changes, for example of the pressure plate, linked to the temperature distribution.