Pressure traps are an often-overlooked issue in hydraulic cylinders, yet they can have serious consequences for both equipment reliability and operating costs. Over time, trapped pressure damages seals, affects component performance, and ultimately results in unexpected failures, costly repairs, and equipment downtime. Understanding why pressure traps occur, and how they can be prevented, is an important part of ensuring the safe, reliable, and efficient operation of a hydraulic sealing system.
What is a pressure trap?
A pressure trap occurs when hydraulic fluid becomes trapped between two seals, or between a seal and a mating surface, and is unable to return to the main working pressure side of the seal.
How pressure traps form
Pressure traps commonly occur when the seal profile creates a one-way path for hydraulic fluid, then holds that fluid enclosed in the cavity between the seals or mating surfaces. During assembly or operation, hydraulic fluid can enter the cavity in one direction but is unable to return to the working pressure side. As the trapped fluid is compressed and more fluid fills the cavity, the localised pressure rises well above the working pressure.
Why pressure traps damage hydraulic seals
The trapped pressure acts on surfaces of the seal it wasn't designed to handle, such as the heel (back) or sides, producing forces the seal was never built to withstand. This can cause:
- Seal extrusion into a clearance gap on the working pressure side
- Nibbling or tearing of the seal as it moves
- Deformation, rolling, or twisting of the seal that increases friction
- Premature wear and leakage
- In severe cases, seal failure or blowout
It's also worth noting that trapped fluid isn't always pure hydraulic oil. Air or gas can become entrained in the fluid as well, and this adds another layer of risk. If that trapped, aerated fluid is suddenly relieved (during disassembly, a pressure spike, or once the trap finally vents), the rapid pressure drop can cause the entrained air to expand suddenly. This rapid decompression generates localised heat and shock, similar to dieseling, causing further damage to the seal on top of whatever mechanical wear the trapped pressure has already caused.
How certain seal designs eliminate them
Seal manufacturers address pressure traps primarily by designing a path for trapped fluid to equalize back into the working pressure side. Common approaches include:
- Pressure-relief grooves or vents that prevent fluid from becoming isolated
- Special seal profiles with geometry that avoids creating a closed fluid pocket
- Stepped or relieved gland designs that provide a leakage or equalization path
- Anti-extrusion and pressure-energized profiles designed so hydraulic pressure acts predictably on the seal rather than becoming trapped behind it
The Bottom Line
While these manufacturer designs go a long way toward addressing pressure traps, using the correct combination and profile of hydraulic seals for your application is still crucial to ensuring successful sealing and long cylinder life.
Prevention tip: When specifying or replacing seals, check the manufacturer's technical data sheet for pressure-relief features built into the profile, and avoid pairing seal geometries that could inadvertently create a closed fluid pocket at the housing design stage.
