The study analysed the tribological performance of five different polymer composites: polyetheretherketone reinforced with 30% carbon fibres—PEEK CF30, polyetheretherketone reinforced with 10% carbon fibres, 10% graphite and 10% polytetrafluoroethylene—PEEK MOD, polytetrafluoroethylene reinforced with 25% carbon fibres—PTFE CF25, polyoxymethylene with 30% carbon fibres—POM CF30 and ultra-high molecular weight polyethylene—UHMW PE. The polymers were tested under the sliding regime of a reciprocating stainless-steel ball on a polymer disc, with test parameters expected for hydraulic valves. Two environmentally safe lubricants were used: glycerol and water. The selected polymer materials and their tribological properties were compared based on the coefficient of friction and the specific wear rate. The worn surfaces were examined using scanning electron microscopy, and the transfer film was analysed using the energy dispersive spectroscopy technique. When tested in glycerol, a comparable and low coefficient of friction was measured for all polymers (~0.02). At the same time, a significantly lower coefficient was measured for all polymers in glycerol compared to water-lubricated conditions (~0.06–0.22). The polymers differed in the measured specific wear rate, which increases significantly in water for all polymers. A lower specific wear rate was measured for three polymers with higher microhardness: PEEK CF30, PEEK MOD and POM CF30. In water, PEEK CF30 showed superior tribological properties under harsh conditions but was well followed by POM CF30, which showed the most intense transfer film.

In this study, the frictional behaviours of three different guide elements—guide rings, labyrinth seals, and hydrostatic bearings—in hydraulic cylinders is investigated experimentally. A modular, double-acting hydraulic cylinder was designed to compare these three different design elements under different pressures (0 bar, 120 bar, and 240 bar), velocities, and radial loads. The results show that the guide rings exhibit the highest friction, especially at high pressures. Labyrinth seals exhibit significantly lower friction and extend the service life of the components. Hydrostatic bearings allow low friction but require precise control of the fluid, which limits their use. The results provide practical guidelines for selecting guide elements and optimising the friction performance, durability, and efficiency of hydraulic systems. We found that the best solution from the points of view of design, friction, and economics is to use labyrinth seals as guiding elements for the fast reciprocal moving rods of hydraulic cylinders.

Fatigue failure remains a key design concern in many mechanical components, particularly in applications where cyclic loading is unavoidable. In experimental practice, uniaxial fatigue testing is still the most widely used approach, mainly because it is simple, cost-effective, and supported by long-established standards. However, many components in service are subjected to combined axial, bending, and torsional loads, for which single-axis tests provide only limited insight. This discrepancy between laboratory testing and real operating conditions has driven the development of multiaxial fatigue testing systems. Existing review studies on multiaxial fatigue largely focus on failure criteria and life prediction models, while the experimental machines required to generate multiaxial loading are seldom examined in a systematic and comparative manner. In this paper, uniaxial and multiaxial fatigue testing machines reported in the literature are reviewed from an experimental perspective, with attention given to mechanical layout, actuation principles, load control, and synchronization. The discussion highlights the trade-offs between loading realism, system complexity, and cost, and illustrates industrial relevance through examples from aerospace and automotive engineering. Recent developments in control, automation, and data analysis are briefly discussed in relation to future experimental fatigue testing strategies.