The main function of a hydraulic cylinder piston seal is to maintain pressure on both sides of the piston while allowing smooth movement within the cylinder bore to the desired position.
The most common types of piston seals are U-cup seals and compact seals. Compact seals and polyurethane U-cups are relatively easy to install. However, Teflon U-cup seals can be difficult to install without damage. In the image below, the Teflon seal is in the foreground, and polyurethane seals are in the background.
Why Use Teflon Seals?
Teflon seals offer advantages such as excellent durability at high travel speeds and low friction. A good example is the extension cylinder in a crane boom, where fast yet smooth movement is essential. Compared to rubber or plastic blends, Teflon is more sensitive to sharp edges and contaminants, which must be carefully considered during installation.
Stretching the Teflon Seal
At this stage, the piston and seal groove should already be secured on the rod, so you don’t need to tighten the piston after seal installation. Before placing the Teflon seal, an O-ring is typically installed underneath it. This should be done with a rounded installation tool to avoid damaging the groove surface.
The Teflon seal must be gently stretched to fit over the backup ring grooves. This is done using a stretching cone or rounded tool, such as a soft-edged screwdriver, to carefully roll the seal over the sharp edges. Warming the seal slightly can help make stretching easier.
Once the seal is stretched to the desired size, it should be installed immediately. Teflon has a tendency to shrink back to its original size quickly, so this step must be done without delay.
Shrinking the Teflon Seal Gasket
During final assembly, the Teflon seal can easily chip or crack if it’s too loose or poorly seated—especially when inserting the rod-piston-guide assembly into the cylinder tube. A misaligned or loose seal may tilt inside the groove and break under pressure. Sharp edges or burrs in the tube’s internal threads can also scratch the piston seal, allowing oil to leak past it over time. That’s why it’s crucial to deburr all edges and ensure precision machining of the cylinder tube.
To prevent seal damage, use a seal compressor tool. The compressor gently and evenly presses the seal into shape without causing scratches. The tool is wrapped around the seal, the seal is centered, and any slack is rolled out using the tool’s roller before tightening it firmly with the handle.
Let the seal retract for approximately 10–20 seconds before removing the compressor tool. The seal compressor is a very handy tool that stretches easily even around large pistons. For larger cylinder models, it’s recommended to allow about one minute for the seal to settle.
Next, the backup rings are installed onto the piston, the piston is lubricated with hydraulic oil, and the assembly is carefully inserted into the cylinder bore—guiding it gently past any threads or internal steps.
After that, the next steps are attaching the cylinder end cap, pressure testing the cylinder, and reinstalling it. When commissioning the cylinder, it’s best to begin by slowly moving it back and forth to ensure the new seals are properly lubricated and don’t overheat or become damaged from running dry against the bore.
We hope these tips were helpful for installing Teflon seals. If you need further guidance, assistance with seal installation, or are interested in purchasing a seal compressor, we’re here to help—and we stock tools specifically designed for hydraulic cylinder maintenance.
