the pressure was gradually increased the rotating discs against which the first test pieces of the shoe of the present invention were pressed were made of a high silica aluminum alloy including 20 weight percent silica and the rotating disc against which the test pieces of the conventional type shoe were pressed were made of a spherulitic graphite cast iron the selected test conditions common to both test pieces were as follows 2 the pressure the pressure was increased from 40 kg cm 2 by increments of 20 kg cm 2 each to about 180 kg cm 2 each pressure was applied for thirty minutes 3 the lubricating oil used a low viscosity oil ssu saybolt universal seconds 70 4 the lubricating method the lubricating oil is supplied at the rate of 0 8 cc minute by using a felt cloth 5 the surface roughness of the test pieces the maximum roughness was 0 4 through 0 6 6 the surface roughness of the discs the maximum roughness was 0 4 through 0 6 the measurement results are shown in fig5 and 6 respectively as is indicated in fig5 the frictional coefficient of the first test pieces was lower than that of the test pieces of the conventional type shoe over the entire pressure range and was kept almost constant without being affected by an increase in the pressure further as.