The sealing groove for Metseals piston seals is machined into a solid piston. The seal is located in the sealing groove in a similar way to how a tyre is mounted on a rim. To facilitate the passing of the seal over the piston’s outer diameter, the piston is shaped in a special way. Before the sealing groove, there is first a cylindrical surface that is 1 mm wide, followed by a radius of 2 mm which changes over to a conical section with an inclination of 45°. The first step in the assembly operation is to lubricate the O-ring with oil or grease and mount it in the seal groove. The O-ring, according to American standard, has been selected to provide a small pre stress against the bottom of the groove. The second step is to tilt the seal ring towards the piston groove and move the seal in to the groove against the O-ring on one side of the piston. The seal is now also in contact with other parts of the pistons circumference. First against the cylindrical surface with 1 mm width, then against the radius 2 mm and then, for most of the circumference, against the conical section with the inclination of 45°. See Figure 4. The third step is to press against the seal's plain flank with a tool starting alternately on both sides of that part of the seal that already is seated in the groove against the O-ring. Then alternately further and further round so that more and more of the seal is tensioned against the conical surface and compresses the O-ring. The seal gradually slides over the radius and the inclined plane and down into the groove. When the seal is pressed sideways and stretched circumferentially, it is also bent sidewise a little. Before the last piece of the seal slides down in the groove, a small piece of the O-ring may stand proud. Before the seal is allowed to slide completely home, the O-ring must be pressed down into the groove so that it will not become jammed between the side planes of the groove and the seal. See Figure 4, which shows how with a simple hand tool of for instance plastic or aluminium, one can mount the seal. Please note that solid hard tools like screwdrivers etc. which can hurt the surface on the seal's side plane should not be used. After some training, assembly by hand can be carried out without problems and be appropriate for individual assembling operations. For production line assembly however, it is better to use an assembly tool, which can be easily adjusted for varying diameters. See Figure; Page 29, shows functional principle of such a device. The tool has a bearing that rolls against the side plane of the seal.
The piston is placed in the centre of a fixture. In the same centre a revolving spindle is mounted with an arm, which can move axially along the piston, this arm carries the roller bearing. The radius between the centre of the fixture and the roller bearing can be easily adjusted to suit the diameter of the piston and the seal. One starts by locating a part of the seal in the groove on one side and then rolling the arm under axial pressure, back and forward from side to side until the whole seal slides over the edge and down in the groove. Outside the piston, a tube is axially movable and can be slid down in the final moment of mounting and so avoids the O-ring jamming between the piston groove and the seal. To simplify assembly, the width of the groove can be made equal to or greater than the diagonal of the cross section. In this way, the seal will not be jammed in the groove. In practice it has been shown however, that it is quite in order to mounting the seals in a groove that is only 0,03 mm wider than the maximum width of the seal. The width of the groove for seal diameters 40 to 95 mm, can then be made 3,03 +0,05/0 mm. For diameters 100 mm and bigger the width can be made 4,42 +0,05/0 mm. Both widths have functioned well. In the first hand, we recommend the narrower width. If the seal is to be disassembled, the steel ring is pushed into the groove in one direction so that a gap between the seal and the piston is established on the opposite side. By help of a tool, for instance, a small screwdriver, the O ring can be lifted out and cut away, piece by piece. When the O-ring is removed, it is easy to take the Metseal seal away.