Contact pressure

The contact pressure between the seals contact surface and the cylinder bore is build up partly as a result of the sealing rings pre-stress and partly from the pressure drop over the seal. Thanks to the Metseal rings very stiff cross section, one can achieve a contact pressure that is suitably spread over contact surface and with a low maximum value. By varying and thinning of the seal's cross section, one can influence that portion of the contact pressure that comes from the pre-stress By minimizing that part of the seal's width, which is influenced by the pressure drop over the seal, one can counterbalance a big percentage of the power, which provides the contact pressure. The total force and mean value for the contact pressure will now be lowered. Above all, the maximum contact pressure, which is close to the low-pressure side, can be reduced, benefiting the life span and the wear decisively. This is achieved, thanks to the torque, that twists the seal towards the gap between the piston and the cylinder will redistribute the total contact pressure, but not change its mean value. Therefore the contact pressure nearest to the low-pressure side will be reduced and moved over to the contact surface on the high pressure side. Metseals seals can in this way, by determination of sizes and shapes in the cross section, be given both a low mean value and a low maximum value for the contact pressure. As an example of how the contact pressure is affected, picture 2 shows the contact pressure for the double acting piston seal, type D, when the seal is only working as single acting with pressure loss in only one direction. Picture 3 shows the same seal, type D, after it has been worn in to an arrow like form, when the seal works double acting, i.e. with variable direction of pressure loss. Over the sealing surface, the pressure drop is linear. The pressure drop, i.e. the difference in pressure between the high-pressure side and the low-pressure side, is consequently influencing the contact pressure only by about 50 % or by the mean value of the pressure loss over the sealing surface. In soft seals with polymeric material, the contact pressure in the important sealing zone, will be approximately the same as the pressure loss over the seal, i.e. about 100 %. By counterbalancing the pressure loss over a wide part of the Metseal seal and then allowing the whole contact surface to have a relatively equal allocated contact pressure, one can reach a very low average contact pressure, and also a very low maximum contact pressure. Metseals piston seals always have full contact between the seal's outer wall contact surface and the surface of the cylinder bore. In consequence of the fact that the cylinder bore is elastic with the overpressure, the cylinder bore always will be slightly conical at the seal's position. The flank sides of the Metseal seal will be twisted to the same degree as the amount of taper in the cylinder bore. This will lead to the flank sides not always touching the side surface in the piston's groove to their full height. The sealing contact surface for a Metseal seal remains flat, with little wear, while the side surface may be worn towards a rounded surface. This is not a problem but rather an advantage. When the double acting piston seal, type D, works as a single acting seal, the whole contact surface remains flat, even after wear. If type D works as a double acting seal, it will be worn to a slight arrow shape. Half the seal width will then be sealing surface in each direction of the pressure loss. After the wearing-in period, the pressurized width will be half the original. In consequence, the friction is also halved. Thanks to the half width, the pressure distribution will be more favorable and the maximal contact pressure on the low-pressure side will be lower.