Hydraulic cylinder seals

Hydraulic systems only perform as well as their ability to maintain pressure. This is particularly evident at the interfaces between the oil side and the outside. This interface is usually formed by the extending piston rods of linear actuators. To prevent hydraulic fluid from leaking during this movement, appropriate seals are used. Although these seals are very small, they bear the greatest load in the entire system. For this reason, hydraulic cylinder seals for large applications are designed in multiple stages.

Types of hydraulic cylinder seals

The larger a hydraulic cylinder becomes and the higher the system pressure, the more complex the sealing of the component becomes. While a single seal is sufficient for small applications, such as brake pistons in vehicles, heavy-duty cylinders use up to seven different sealing elements. These are:

• Piston seals
• Guide elements
• Combination elements
• Rod seals
• Wipers
• O-rings
• Back-up rings

Design and function of hydraulic cylinder seals

Piston rings seal the hydraulic piston. They prevent pressure from passing the piston from the pressure side to the suction side. The guide elements keep the piston precisely centered in the sleeve so that it cannot tilt. Combination elements are piston rings with integrated guide elements. The rod seals ensure that no oil can leak to the outside. They seal the slender piston rod. To ensure that even the smallest amounts of oil cannot flow past the rod seals, additional wipers are used in suitably large linear actuators.

If this is still not sufficient for sustainable, long-lasting sealing of the hydraulic cylinder, a number of O-rings can provide the required tightness. O-rings are primarily used in multi-part housings. They ensure that the linear actuator can be disassembled for maintenance purposes. As long as the piston rod is not bent and the housing is not damaged, a hydraulic cylinder can be restored by replacing the seals.

Back-up rings seal single-acting hydraulic cylinders on the closed side.

The piston guide ring and piston seal are located on the outside of the piston and seal against the cylinder wall. The static piston seal is located on the piston rod inside the piston. The housing is sealed along the rod by the pre-seal, the rod seal, and the wiper. In addition, the housing contains a rod guide ring that prevents vibration or tilting of the piston rod. Between the head of the hydraulic cylinder and the outer wall there is also a static head seal. Back-up rings are used for this purpose, for example.

Requirements for hydraulic cylinder seals

Hydraulic cylinder seals must have the following technical and mechanical properties:

• Sufficient elasticity for installation, removal, and conforming to the hydraulic cylinder’s metallic components
• Chemical resistance to acidic or alkaline hydraulic media
• Resistant to abrasion
• Resistant to high and low temperatures

For particularly demanding tasks, these properties are not provided by a single material. Therefore, high-quality hydraulic cylinder seals are assembled from different materials.

Materials for hydraulic cylinder seals

Hydraulic cylinder seals are manufactured from the following materials:

• TPU: Thermoplastic elastomer based on polyurethane for normal temperature ranges
• PTFE: Polytetrafluoroethylene for particularly high temperature ranges
• NBR: Acrylonitrile-butadiene rubber for maximum swelling resistance
• POM: Polyoxymethylene for particularly stable piston guide elements
• EPDM: Ethylene-propylene-diene rubber for acid-resistant O-rings

This selection of hydraulic cylinder seals requires extensive expertise to source the correct spare part for a linear actuator due for overhaul. It is also important to investigate possible causes of premature seal failure. Innovative sealing materials may make it possible to upgrade a previously undersized hydraulic cylinder for new and more demanding tasks.