Types of hydraulic seals and their applications

Seals for hydraulics stop fluid leakage and contamination from entering valves, cylinders, pumps, and motors. The most common types—O-rings, piston seals, rod seals, wiper rings, and static seals—are chosen according to temperature, pressure, and motion type and compatibility with fluids, instead of a standard design. A hydraulic system could be equipped with the right pump, the appropriate fluid, and an appropriately sized cylinder but be unable to function properly when the seals it contains are not compatible with the application. Knowing the various types of seals and the place they belong is among the most effective ways to decrease downtime and increase the longevity of components.

What are the reasons why seal selection is important?

Every seal in the hydraulic system has a purpose: to keep a boundary between the fluid and the external environment or between two chambers that have different pressures. However, the challenges the seals are exposed to vary greatly dependent on the location within the system. A rod seal inside an excavator mobile cylinder is faced with dust, temperature fluctuations, and side-loading. The piston seal inside a press cylinder has to deal with extreme pressure and a very low tolerance to friction. The wrong sealing geometry or material at any position can lead to excessive wear and tear, leakage inside, or even complete extrusion of the seal.

This is why seals for hydraulics are typically classified according to purpose first—whether static or dynamic—after which they are further classified according to their design and materials.

Dynamic seals

Dynamic seals are utilized when there is a movement between the seals and the moving surface, like the piston rod or bore. They are subject to continuous friction and must ensure their sealing performance and wear with minimal.

Rod seals

Rod seals are placed inside the cylinder head. They will seal against the piston's reciprocating rod, stopping the flow of hydraulic fluid out of the cylinder when the rod expands and retracts. They typically have an elongated lip that ensures a constant pressure for contact across various pressures within the system. The most common rod seal profiles are U-cups and compact seals and step seals. They are typically coupled with a backup ring to prevent extrusion when in high pressure.

Rod seals are employed in almost every hydraulic cylinder, including mobile equipment such as excavators and loaders up to lifts, presses, or stationary machines. They are the last protection against external leakage; the rod seals are often the first thing to be checked in the inspection of cylinders.

Piston seals

Piston seals are placed on the piston and are able to seal against the bore of the cylinder, which separates the two chambers of pressure within the double-acting cylinder. In contrast to rod seals, piston seals have to typically maintain the pressure both ways, and this impacts their shape. They are typically U-cup seals, compact piston seals that have backup rings and guide rings with O-rings that provide low friction, bidirectional sealing.

Piston seals play a vital role in situations where holding force under load is crucial. Hydraulic presses, lifting cylinders, and steering cylinders are all dependent on the seals of pistons to stop internal bypass, which is visible as a cylinder that shifts or decreases its force with time, even if leakage from outside isn't visible.

Wiper (scraper) seals

Wiper seals, sometimes referred to as scraper seals, are positioned on the outside on the head of the cylinder before the seal on rods. They are not designed to keep fluid out; however, they are there to keep dirt out. When the rod retracts into a cylindrical cylinder, a seal called a wiper scrapes dust, dirt debris, and moisture off the surface of the rod before it reaches the rod seal or even gets absorbed into the fluid systems.

Wiper seals are particularly important for outdoor and mobile equipment, such as construction equipment and agricultural machinery, as well as marine applications in which rods are exposed to extreme conditions. A worn or damaged wiper seal is one of the most common root causes of contamination-related seal and fluid degradation.

Wear rings (guide rings).

Wear rings, also known as bearing rings or guide rings, are not seals within the conventional sense but are often grouped together with dynamic seals due to their importance in cylinder performance. They are positioned on the piston and, sometimes, the rod to prevent contact between stationary and moving parts by absorbing side loads as well as the radial forces.

By preventing contact with metal wear rings, the shield protects the seal profile and the bore surface of the cylinder from being damaged, which increases the lifespan of the sealing components. Wear rings are commonly found in cylinders with loads that are off-axis, like arm and boom cylinders used on excavators.

Static seals

Static seals are utilized when two components are not moving between them, like a flange for the valve body or a joint for a pump housing or a port connection.

O-rings

O-rings are among the most commonly utilized static seals used within hydraulic equipment. Their circular cross-section design is cost-effective and makes them easy to insert into grooves that are machined into flanges and port fittings and housings. O-rings are made by compression between two mating surfaces in order to form the seal. They are judged according to material compatibility as well as temperature range and pressure class, rather than by distinctive shape.

O-rings are found throughout hydraulic systems, sealing ports, valves, pump housings, port connections, and even manifold connections. Because they're inexpensive and easy to install, they're also one of the most frequently repaired components in routine maintenance.

Gaskets and flange seals

Gaskets are often used on more extensive static joints, for instance, reservoir access covers, in which flat sealing surfaces are more suitable than an O-ring with grooves. Flange seals are used for the same function, specifically at the flanges of hoses or pipes that often include an alloy or composite reinforcement that can withstand tension and greater vibration.

Considerations regarding seal material

Seal material and seal type are two distinct but connected decisions. The same seal profile for a rod could be made of the nitrile rubber (NBR) for conventional mineral oil systems and fluorocarbon (FKM/Viton) for extremely aggressive or high-temperature fluids, or polyurethane (PU) for applications that require higher abrasion or resistance to extrusion. The PTFE-based seals are typically chosen due to their low friction and extensive chemical compatibility, specifically for wear rings in which minimizing drag is an important consideration.

The wrong material selection that is not suitable for the operating temperature or type is the most common reason for premature seal failure even if the geometry of the seal is suitable for the purpose.

The type of seal that matches the application

In the real world, sealing selection is not about choosing the "best" seal type—it's about matching design and material to the particular requirements of motion, pressure, temperature, and the environment at every point of sealing in the system. A cylinder for mobile hydraulics that is exposed to temperature and dirt requires a durable wiper seal as well as an abrasion-resistant rod seal, and a high-pressure industrial press cylinder will be able to benefit from the resistance to extrusion of piston seals as well as bidirectional sealing with low friction. All static joints, for instance, rely on properly measured and material-specific O-rings and gaskets to stop the kinds of leaks that are slow and easy to miss, which can reduce the efficiency of the system as time passes.

A regular inspection of the seal, looking for signs of cracks, hardening, extrusion, or surface glazing—is one of the best ways to identify failures prior to they trigger unplanned interruptions or events of fluid contamination.

What's the distinction between the rod seal and the piston seal?

The rod seal can be placed within the cylinder head and seals against the rotating rod to stop liquid from spilling out of the cylinder. A piston seal is attached to the piston and seals the bore of the cylinder, separating the internal chambers of pressure.

What is the purpose of wiper seals? need to be sealed if they aren't hydrofluid?

Wiper seals are used to keep contaminants such as water and dirt off the rod's surface prior to the time it retracts into the cylinder, thereby protecting the rod seal as well as the fluid from wear caused by contamination.

Does it use the same sealing material in all hydraulic applications?

The material selection is based on the type of fluid as well as operating temperature and chemical exposure. NBR is suitable for the standard mineral oil systems, and FKM and PTFE are better suited for extreme temperatures or aggressive fluids.

What is the cause of leakage inside the cylinder, despite not having apparent outside leaks?

Extruded or worn piston seals permit fluid to flow between chambers of pressure, which reduces force output and can cause rod drift, with no fluid being visible outside of the piston.

Wear rings are considered to be sealing components?

Wear rings do not make a seal in a direct manner, but they block contact between metals and absorb the side loads, protecting the actual sealing elements as well as the bore of the cylinder from premature wear and tear.