HYDRAULICS When people talk about hydraulic failures, they often think first of pumps, valves, or hoses, but in many cases, the problem can be traced back to how a seal has dealt with pressure and load over time. Getting this right is less about dramatic breakdowns and more about consistent, careful design and material choice. Pressure is at the heart of how hydraulic systems work, and it’s also one of the main challenges for the seals that contain it. Each seal has to hold back the force generated by pressurised fluid without allowing leaks or deforming under stress. This sounds simple enough, but the relationship between pressure, temperature, movement, and material properties makes it a delicate balancing act. Too little pressure on the sealing surface, and the fluid escapes. Too much, and the seal can be squeezed, extruded, or even torn apart. When pressure rises above what the seal can handle, the material can begin to flow into any small clearances between metal parts. This is known as extrusion, and it’s a common reason for seal failure in high-pressure systems. Once a seal starts to extrude, it may lose its shape permanently, and the damage quickly leads to leaks. Designers often use backup rings -typically made from a harder material like nylon or PTFE - to support softer elastomer seals and stop them from being forced into gaps. It’s a simple measure, but one that makes a significant difference in the life of the seal. The load applied to a seal doesn’t come from pressure alone. It’s also affected by the mechanical forces within the system, such as side loading on a piston rod or uneven surface pressures caused by misalignment. These forces can cause the seal to wear unevenly or create areas where the sealing contact is too tight on one side and too loose on the other. Once wear begins, small leaks form, and pressure loss becomes gradual but steady. For this reason, understanding how components move under load—and how that movement transfers stress to the seal - is just as important as choosing the right material. Material selection is where many of the practical solutions lie. Each type of seal material reacts differently under pressure and load. Nitrile rubber (NBR) is widely used because it offers a good balance of flexibility and resistance to oil, but at very high pressures or elevated temperatures, it can start to lose its shape. Polyurethane, on the other hand, offers greater resistance to extrusion and can handle higher loads without deforming. PTFE seals are another option, especially for dynamic applications where friction and wear are major considerations. The right choice often depends on how pressure varies through the system - whether it’s steady, fluctuating, or prone to sudden spikes. Pressure cycling Pressure cycling is another subtle but important factor. Systems that regularly move between high and low pressure create repeated expansion and contraction of seals. Over time, this can lead to fatigue, particularly in softer materials. The seal might not fail suddenly but instead begin to seep slowly, requiring more frequent maintenance. To reduce this, designers sometimes choose seals with energising elements such as springs or pressure-activated lips that help maintain a consistent sealing force even as pressures change. Installation practices also play a part in how seals handle pressure. Even a wellchosen material can fail early if it’s nicked or stretched during fitting. A tiny cut on the edge of a seal might not be visible, but under pressure it becomes the 14 HYDRAULICS & PNEUMATICS October 2025 www.hpmag.co.uk When pressure tests the limits of a seal Even the best seal is only as strong as the pressure it’s designed to contain. Understanding how pressure and load interact helps prevent wear, leaks, and costly downtime. H&P reports. weak point where damage spreads. Using the right tools and lubricants during assembly helps avoid this, as does checking that metal edges are properly chamfered or deburred. It’s a small investment of time that prevents many early-life seal failures. For maintenance teams, monitoring pressure and knowing the limits of each component is essential. Running a system slightly above its rated pressure may seem harmless, but it accelerates wear across the whole hydraulic circuit. Over time, seals that were perfectly adequate for their original design pressure can begin to struggle. Pressure relief valves and accurate gauges are the best defences against this, as they keep the system within its safe working range. Ultimately, dealing with pressure and load in hydraulic seals isn’t about adding complexity, it’s about consistency and awareness. A well-matched seal, properly supported and installed, can handle demanding conditions for years without trouble. But even small oversights in material choice, hardware design, or operating pressure can shorten that life dramatically. The key is to approach sealing as part of the whole hydraulic system, not as an afterthought. When pressure, materials, and design all work together, seals stop being a weak link and become what they’re meant to be: reliable, invisible, and quietly doing their job.
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