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The transmissions of mining conveyors are exposed to very harsh conditions. These are primarily related to the contamination of the gear oil with hard particles coming from coal and lignite, which can cause intensive abrasive wear, scuffing, and even pitting, limiting the life of gears. One of the ways to prevent this problem is the deposition of a wear-resistant coating onto gear teeth. However, a proper choice of gear oil is an important issue. The abrasion, scuffing, and pitting tests were performed using simple, model specimens. A pin and vee block tester was employed for research on abrasion and scuffing. To test pitting, a modified four-ball pitting tester was used, where the top ball was replaced with a cone. The test pins, vee blocks, and cones were made of 18CrNiMo7-6 case-hardened steel. A new W-DLC/CrN coating was tested. It was deposited on the vee blocks and cones. For lubrication, three commercial industrial gear oils were used: A mineral oil, and two synthetic ones with polyalphaolefin (PAO) or polyalkylene glycol (PAG) bases. The results show that, to minimize the tendency forabrasion, scuffing, and pitting, the (W-DLC/CrN)-8CrNiMo7-6 tribosystems should be lubricated by the PAO gear oil. MoS 2 /Ti, C/Cr [4], TiN, and CrN [15]. Concerning DLC coatings, they are either doped coatings: W-DLC [2,15-17], Cr-DLC [18], and Si-DLC [19]; or non-doped coatings: a-C:H [ [18][19][20] or ta-C [19]. The review of the literature allows one to state that it is the DLC coating that is currently used in the majority of tribological research works.Apart from testing thin coatings, it is also important to select a proper oil to lubricate the coated parts [15]. For decades, a lot of research works have been devoted to investigating the interaction between the lubricating additives in the oil with the steel surface [21][22][23][24][25][26][27][28][29][30][31][32], and the mechanisms of the interaction between steel surfaces and lubricants are well recognized.Concerning the interaction between the oil and the thin coating, the publications are less frequent [15,18,[33][34][35][36][37][38] and have been mostly issued in the last 20 years. Unlike the oil-steel interactions, when testing coatings, one can find different statements and observations in the literature. Some authors point out an effect of the coating's elemental composition, occurrence of the transfer of material between the samples, forming of protective films on the surface, or even chemical reactions of the coating with the lubricating additives in the oil.In a review paper, Kalin et al.[33] compared oil-coating interactions when lubricating with oils with a mineral, synthetic ester, and polyalphaolefin (PAO) base using various tribosystems. They stated that non-doped DLC coatings can react with different types of additives (e.g., a friction modifier (FM), antiwear (AW), and extreme-pressure (EP) additives), and that the hydrogen content in DLC coatings plays a crucial role in the tribological performance under lubricated conditions.Michalczewski et al.[15...

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A new test rig for tribological tests was developed and manufactured. It consists of a mobile device for measurement of the start-up friction torque of transmissions, in particular planetary gearboxes, and the friction torque in dynamically steady conditions, as well as a climatic chamber to stabilize the temperature of the tested gearbox in its extreme range: from -50 to +50°C. In the series of devices for tribological tests, developed and manufactured at the Institute, the new test rig is marked with the symbol T-34. The verification results correspond with the churning losses related to the viscosity characteristics of the lubricating oils. As the temperature increases, both the start-up friction torque and the friction torque under dynamically steady conditions decrease.

In spite of long-term development of the technology of bevel gear production, the automotive industry reports various operational demands such as the need of the size and mass reduction of gears without lowering their durability and reduction of friction leading to a decrease in the energy losses and a decrease of the tendency to scuffing. What is more expected, EU regulations may impose the use of new generation gear oils (ecological-friendly) providing the proper operational properties of the transmission. In view of these demands, a new, bevel gear test rig and scuffing test method have been developed at ITeE-PIB. The idea resulted from a necessity to improve reliability of tests-popular gear tests are run mostly on spur gears having the tooth geometry significantly different than bevel gears. The test rig, test method, and results of verification testing are presented. The effect of various gear oils and the deposition of a low-friction coating on the resistance to scuffing were investigated. It is shown that the new test rig fulfils the research requirements and that the new test method has a good resolution.