Why does anyone need a seal? There are two general reasons; to keep something in or to keep contamination out. On the “keeping something in” side of things, today we’ll talk about the very common hydraulic fluid, oil. Oil is a highly useful power transmission fluid in that it has very low compressibility giving it a volumetric consistency under pressure, and it is a lubricant to help components last longer. Though the term “hydraulics” can mean use of any fluid, hydraulic oil is far and away the most common fluid used in the hydraulics industry and seals designed to keep that particular fluid in is a core focus for many seal manufacturers.
Seals for High- and Low-Pressure Hydraulic Systems
In a very general sense, there are two conditions where a seal needs to function in an application. The first is when it is under pressure. Depending on the pressure rating of the system, various seal profiles might be used. All are designed to resist the pressure of the system extruding the seals into the gaps between mating components and causing the seals to prematurely fail. In a higher pressure application, seals are typically designed to fill the sealing cavity with a stable profile that gives a high installed load and little movement of the seal is desired in application. This can result in very high friction for the seal, but often the force of the pressure overcomes any negative impact of higher seal drag. For lower pressure applications where friction of the seal can be a factor, a lighter touch is desirable so pressure-activated sealing using a u-cup type seal can be better suited.
In those profiles using seals that feature a lip design, the length of the lip is directly proportional to how much pressure-activation can be expected when the seal is pressurized. A long lip lays down with a longer area, while short, stubby lips would remain relatively fixed when under load. In a very general sense, sealing at pressure is easy because the fluid pressure is helping press the seal into any gaps where fluid would escape.
Hydraulic Seals for No-Pressure Conditions
The second, and often more challenging condition for sealing, is when there is no pressure. When a machine or system is turned off, there is no pressure to engage the seals and ensure it remains compressed within the gland. Seals that accomplish their sealing through pure compression still work with no pressure due to the high load when installed. Lip style seals can have a bit more difficulty. When pressure is removed or in some cases during start-up, a vacuum can be achieved and the same lip design that gives flexibility for higher sealing can pull off the sealing face and cause a leak. This is overcome by using a loader behind the lip to keep the lip engaged. A Polypak is one style of u-cup that loads the u-cup with an o-ring or spring. Other lip seals use similar methods to ensure the seals remain engaged with the sealing surface in low or negative pressure case.
Hydraulic Systems Continue to Evolve
The seals are only one of the key components in a hydraulic sealing system. Seals for shafts and pistons are very important, but other components like wear bands, wipers, buffer rings, and the static o-rings make up a complete hydraulic sealing system so the components can function as intended without worry of oil or other hydraulic fluid leakage. Demands for zero leak systems continue to drive innovation in component design and material development for every type of sealing system piece. Each new system design brings with it new challenges and benchmarks, but the sealing industry is up to the task.