Functional properties

Wear on seal and cylinder bore

During hydraulic operation, conventional soft seals of polymeric material will initially wear in the cylinder bore during the first 1000 strokes. The wear will take place both on the surface profile peaks and valleys but mostly on the peaks. As a rule, the profile depth will already after a few thousands of strokes be reduced to about half the original value. The wear of the sealing surface and the cylinder bore will then proceed, but at a reduced rate. During this relative short wearing in period, many small particles will be generated. These will be a threat to the cleanliness of the whole system, if the filtration is not functioning. The wear on the cylinder bore can increase as time goes on and become a crucial problem if hard particles are trapped in the surface of the soft polymeric seal or the guide ring. These trapped particles can wear grooves in the cylinder bore. The wear of the soft polymeric seal can increase with time and be critical if the cylinder bore surface becomes damaged. For instance if it becomes scored for any reason or if it spoilt as a consequence of existing surface imperfection, such as a small crater caused by slag inclusions in the material close to the cylinder surface. The soft polymeric seals are totally dependent on very mooth surfaces without scratches and craters. On the whole there are only roller burnished and honed surfaces with Ra-values of 0,2 or better that will give acceptable results for demanding applications. One of Metseals best qualities is that it is practically unaffected by the surface finish of the cylinder bore surface and the possible occurrence of scratches and craters. This results in a superior length of life for the seal and cylinder surface and a possibility to lower the cost for manufacturing the surface. In addition to roller burnished or honed surfaces the Metseal seal can be used with the inferior surface smoothness as found for instance on drawn or fine turned surfaces The Metseals piston seal can be used with these comparatively rough surfaces, without generating large amount of particles. It will still provide a longer and more reliable length of life than the best combinations of smooth surfaces and piston seals in polymeric materials. As the Metseal seal works, it will improve the surface of the cylinder bore so that the piston guiding ring will have better conditions, a longer length of life and a lower generation of abrasive particles. Metseals piston seals for hydraulic applications are made in hardened high-alloyed steel and both the sealing surface and surfaces of the flank faces have very low friction with a surface finish under Ra 0,2 μm.

When the Metseals piston seal makes the first strokes under high pressure, it will press down on the high peaks of the softer steel surface in the cylinder bore and plastically reduce their height. As the seal is considerably harder than the cylinder bore, the seal's surface will not be damaged. The bore surface will already after a few hundred strokes start to acquire a surface profile with pronounced plateau's that all have the same height. After an additional comparatively large number of strokes, these plateaux will assume a very smooth surface and be like a mirror. As a rule the wear on the cylinder bore is so small that it is not measurable. In fact, it is a gradual development of the surface and an expansion of the area of the plateaux. When the seals sealing surface is sliding against the cylinder bore the contact will only be against the smooth surfaces of the plateaux. The presence of scratches and craters will only influence the maximum contact pressure marginally. Scratches, craters and miscellaneous cavities are therefore not a problem as regards the length of life, nor, as a consequence of the surface phenomenon with half solid boundary layers, do they influence the static and dynamic leakage. Soft polymeric seals on the other hand, can elastically slide down in craters and scratches where the soft surface will be rapidly sliced or worn away and a breakdown of the seal is a consequent. Fig. 1 show respectively how soft seals and the hard Metseal seal wear or deform the surface profile in the cylinder bore.

Wear in the cylinder bore