There are a few different types of seals within your hydraulic cylinder and they each have their own, specific, job requirements. Some must cope with extreme pressure, while others are simply there to keep dirt out of the system. Each of these seals needs to be specially selected according to their role - to ensure that it can deal with the kind of pressure, heat, and chemical elements it will encounter in day to day operation. Making the wrong choice at the time of hydraulic cylinder repair may limit the lifespan of your cylinder, lead to poor performance, increase operating costs or even create conditions for catastrophic failure. With this in mind, let’s explore what types of seals are used in hydraulic cylinders, to give you a better understanding so you can get maximum performance and longevity out of your equipment.
Dynamic Seals vs Static Seals
Before we go into the specific types of seals, we need to distinguish between the two main categories of seals. These are dynamic seals and static seals.
Dynamic seals are found between the parts that are in relative motion - such as the rod, piston and barrel. This includes piston seals, rod seals, rotating seals, wipers/scrapers and oil seals. With components in motion, dynamic seals must have more elasticity and are often designed to run on a film of hydraulic fluid.
Unlike dynamic seals, static seals create a barrier between parts that are fixed together and do not move. For this reason, static seals can be made from different material. Seal material may be less flexible and the material may be more heat and pressure resistant. Static seals are found in a number of locations within the Hydraulic cylinders depending on the design, but they are usually found between the piston and piston rod and between the head and cylinder bore tube.
Piston seals are located between the piston and the cylinder wall. Its job is to create a pressure barrier and prevent fluid passing the piston. This pressure is required for controlling the cylinder motion or maintaining the position of the piston when at rest. Clearances between piston and cylinder wall are extremely tight and the piston seal performs a vital role in maintaining the relationship between these reciprocating components. Thus, correct seal size and material is critical to the performance of the cylinder.
A rod seal is a ring of elastomer material located between the reciprocating cylinder rod and the cylinder wall. Its job is to keep pressurized fluid contained within the cylinder whilst the rod extends and retracts. It is therefore critical to the operation of the cylinder because it keeps the operating fluid from escaping and allows the system to build the operating pressure required to move the actuator. Rod seals are also designed to maintain a thin film of fluid as the cylinder rod extends and retracts, which is important to help lubricate the rod, wiper seal and inhibit rod corrosion.
Buffer seals are usually located in front of the rod seals and serve as an extra line of defence in the event of an extreme spike in internal pressure. Preventing that pressure from reaching the rod seal and potentially blowing it out. Buffer seals also have a general regulatory effect on system pressure, ensuring smoother operation, better rod seal performance, and longevity by ensuring more gradual pressure buildup. Buffer seals also reduce the number of system contaminants such as metal particles that reach the rod seal, where they can do damage.
You will find hydraulic cylinder wiper seals at the mouth of the cylinder barrel in front of the rod seal. This is the seal that is usually visible when you inspect a cylinder externally. As the name suggests, the role of the wiper seal is to clear contaminants from the rod as the cylinder extends and retracts. It moves ahead of the rod seal and prevents external contaminants from getting under the seal and causing leakage, or entering the system and causing premature wear. The wiper seal also serves to retain the internal fluid lubrication film on the rod as it retracts.
Know Your Hydraulic Cylinder Seals
Your hydraulic cylinder seals have a very important role and these relatively fragile components - made of polymers and elastomers - must deal with extreme heat, pressure, and exposure to chemicals and fluids. Therefore correct hydraulic seal selection is non-negotiable when it comes to designing a new cylinder or repairing an existing unit. We hope this article gives you a better understanding of the difference between each of these seal types and the roles they play. If you require more information, contact your nearest Berendsen Branch. Berendsen Fluid Power maintains close relationships with seal manufacturers who often custom manufacturer seals to our specifications when required.