Comparative analysis of tribological characteristics of stern-tube bearing materials working by the scheme of a direct and an opposite friction pair

A comparative analysis of the tribological characteristics of the coupling “propeller shaft lining material – nylon” was carried out when working according to the schemes of the forward and reverse friction pairs under conditions of boundary lubrication when lubricated with fresh water. The tests were carried out on the MI-1 friction machine according to the “rotating roller – fixed ring” scheme. When testing the direct friction pair, the rollers were made of nylon, and the rings were made of stainless austenitic steel and tin bronze BrO5Ts5S5; when testing the reverse pair, on the contrary, the rollers were made of metal material, and the rings were made of nylon. Water lubrication was carried out by partially immersing the roller in a tub of water in combination with jet water supply to the friction zone. The water was supplied in a closed cycle: water flowed from the tub into a 10-liter container and was pumped by a submersible pump into the friction zone. Part of the water was constantly taken from the tank and passed through a cooling device, which made it possible to maintain water temperatures in a constant range of 20 ± 3 °C. Regardless of the coupling scheme, the roughness of the metal counterbody practically does not change, and the values of the height parameter of the nylon roughness change very much and approach the values of the metal roughness parameter. The transition from the reverse pair scheme to the direct friction pair scheme leads, all other things being equal, to an increase in pressure in the contact zone by almost three times, which worsens the conditions for the existence of a film of water on the friction surface. The latter circumstance, combined with the peculiarities of working out the elements of the friction pair “propeller shaft lining alloy – nylon”, leads to the fact that the proportion of the deformation component of friction increases in the direct friction pair. It is concluded that in direct pairs, compared with reverse pairs, the wear of the nylon element and the coefficient of friction increase many times.