Surface roughness - Surface machining methods
The Metseal piston seals are very tolerant of poor bore surfaces. Score marks, scratches, surface inclusions and other material defects can be tolerated. The Metseal seal can cope with very high Ra-values. Earlier we stated a maximum of Ra=4 μm. However, it seems meaningless to set up such a limitation, as the methods that can be considered, will give a surface finish better than Ra=4μ and without difficulty and additional costs. Another factor in deciding if a certain surface finish can be accepted is determined from whether the surface can be worn in and used for a long time without harmful generation of wear particles. Consideration must also be given to the surface functioning without harmful wear on the pistons guiding device. As the Metseal piston seals will deform and polish a rough surface so the wear on the piston guiding equipment will be reduced, a number of rational and cost saving surfaces can be used as a result. Smooth rollers burnished or honed surfaces are of course acceptable even if they are better than required. It is also clear that these surfaces can be permitted to have minor local surface defects as scratches and craters. Turned and smooth drilled surfaces can also be accepted. With today's specially adapted and commonly available hard metal cutting tools for fine surfaces, surface finishes around or better than Ra=1 μm are easily produced. Small scratches and craters can of course be accepted even here. Drawn surfaces with oxide coating from heat treatment operations can also be accepted. Obviously, there is a limit to how heavy these deposits can be. Many times, it looks worse than it really is. The Metseal steel ring will relatively quickly deform and compress the material parts that stick up. The surface in the bottom of scratches will not hurt the rings sealing surface in the same way that it would seals of polymeric material. For all above mentioned surfaces and other possible surfaces of the same type, it is a rule that a comparatively rough surface pattern between the plateaux not is an obvious disadvantage. Full knowledge of this subject is still lacking but it seems that an entirely smooth surface should not necessarily be better than a surface with dales and plateaux. To specify a surface demand for Metseal is not very easy. Leading manufacturers of seals of soft polymeric material, give the following approximate recommendations:
Rmax = 0,63 to 2,5 μm
Rz = 0,40 to 2,5 μm
Ra = 0,05 to 0,4 μm
MR = 50 to 95 %
One can see that permitted Ra-values are slightly higher, when the demand for MR is in the range 80 to 95 %. The demand is, despite smaller differences, in reality similar for soft seals. Especially the highest Rz-value 2,5 seems to be a common value. Only surfaces without scratches and craters can consequently be accepted.
For Metseal can following recommendations be given:
Rmax= Rz < 10 μm
Ra < 2 μm
MR > 30 %
The recommendation is set to simultaneously give low generation of wearing particles and a low negligible wear on cylinder surface, piston steering ring and seal.
The demand for MR is especially difficult to specify with the conventional methods to describe the quality of a surface. For the Metseal piston seals, when they are in contact with soft ductile steel surfaces, it is a better to specify that the surface shall have such a surface profile that after wearing in it will have plateaux surfaces that are at least 30 to 40 % of the total surface.