A tandem hydraulic seal assembly includes a graphite-filled teflon inner seal and a molybdenum disulfide-filled polyurethane outer seal. The pressure drop across the inner seal limits the pressure on the outer seal to values low enough to prevent high friction. The outer seal compresses axially when pressure is applied to it to admit fluid to its grooved seat, and expansion of the outer seal when pressure is relieved pumps fluid back past the inner seal to the pressure chamber.The hydraulic seal relates to pressure seals, and more particularly to improved hydraulic pressure seals for use in hydraulic rams and like devices. The hydraulic seal has particular utility in high-pressure hydraulic actuators of the types used on lift trucks and other material-handling devices. A requirement to move heavy loads with small-size actuators in turn requires that high hydraulic pressures, which may vary from zero up to the order of 5000 psi, be used. A hydraulic seal interposed between a hydraulic cylinder and a movable piston or ram desirably would be very tight so as to minimize fluid leakage. On the other hand, provision of a seal which is extremely tight not only increases friction, wasting power, but also causes rapid seal wear and chatter, so that an engineering compromise has been required. It is one object of the hydraulic seal to provide an improved hydraulic seal arrangement wherein very little fluid leakage occurs, and where friction, chatter and wear are minimized.
In accordance with one concept of the present hydraulic seal, two dissimilar seals are used in tandem on a hydraulic actuator, one being made of a material such as molybdenum disulfide-filled polyurethane, and the other made of a material such as graphite-filled "teflon". Teflon is the dupont trademark applied to polymers of tetrafluroethylene. Polyurethane has superior sealing properties, and long-wearing polyurethane seals adequate for use at low pressures are easy to design. However, because of the elasticity of polyurethane, it is extremely difficult to design a polyurethane seal which will open at high pressures without causing great drag resistance, or moving friction, and chatter. In accordance with a further concept of the hydraulic seal, the teflon seal is used in tandem with the polyurethane seal to provide a pressure drop. Thus even if a pressure differential of 5,000 psi exists between the inside and outside of the cylinder, a substantial pressure drop occurs across the teflon seal, thereby reducing the pressure across the polyurethane seal to a value such that it does not create great moving friction or chatter due to static friction. Various arrangements of tandem hydraulic seals have been known in the prior art. A problem commonly encountered with such arrangements is that hydraulic fluid seeps past the first seal and becomes trapped between the pair of seals, and then some fluid eventually may seep past the second seal and leak from the cylinder.
