Hydraulic Cylinder Seal Selection

How to Select the Right Hydraulic Seals for Cylinders

Hydraulic seals are a critical component to any hydraulic cylinder and yet they are often the thing given the least consideration in a hydraulic application. Hydraulic seals are often called "packings" because they are packed into the grooves machined into the piston and rod gland of a cylinder.

Seals used in fluid power devices are usually elastomers. This means that they are made from materials that are elastomeric or flexible like rubber and soft plastics such as polyurethane. This elastomeric quality enables the seal to more effectively flex and close off the small gaps between machined metal components thus containing the pressurized hydraulic fluid.

Early hydraulic seals were made from leather or heavy weaved cotton material. Modern hydraulic seals are usually made from high durometer rubber or polyurethane.

Hydraulic seals are made in a wide variety of shapes, combinations, configurations and sizes to suit every conceivable application. This presentation shows cross sectional views of Hydraulic Cylinders and the different types of Seals used in the different areas on cylinders.

Often, the designer or user simply shops for the cheapest type of seal and the least expensive supplier. This is a recipe for failure.

Quality hydraulic seals that are carefully selected to suit the cylinders application will give the end user a long trouble free service life.

The 4 primary considerations in selecting proper hydraulic seals are:

seal function
service and shock pressures
service temperature
chemical compatibility

Seal Function

Hydraulic seals are selected to operate under either static or dynamic functions.

Static seals remain stationary or do not service a moving surface. A common example of a static seal is one that seals the area between a rod bearing and the cylinder end cap. Another example is between the end cap and the cylinder barrel. A third example is the seal used at the connection area between the piston and piston rod.

Once installed, these seals do not move. They simply seal off the gap between the two assembled stationary components and prevent the hydraulic fluid from leaking past.

O-rings are the most common seal used in static areas but other seal styles can also be used including square rings, gaskets, and even liquid thread sealants such as Locktite.

In high pressure systems, static seals may be configured with back up rings to prevent the pressure from extruding the seals, excessively compressing the seal, and nibbling away at the material. In critical systems that will not tolerate leakage, static seals are often doubled up in series to ensure that all of the fluid is retained.

Dynamic seals service moving surfaces. Examples of dynamic seals are piston seals and shaft seals. Because they service moving surfaces, dynamic seals are subject to wear. Good dynamic seals, therefore, must resist and compensate for this wear so that service life is not adversely affected.

Some dynamic seals are specially designed to accommodate a moving surface on their inside diameter such as shaft seals and rod wipers. Other dynamic seals are specifically designed to accommodate a moving surface on the outside diameter of the seal such as with piston seals.

U-cup and V-cup seals are often used as dynamic seals. As cup seals wear, the pressure continues to inflate the seals and thus compensates for the wear. The advantage of cup seals is that they provide a very tight leak free seal.

Another effective dynamic seal design is the slipper seal. Slipper seals use a combination of a wear band on top that provides the sliding or dynamic surface. Underneath the wear band, a compressed O-ring is positioned. In very high pressure applications, the O-ring may have back up rings on either side. This seal design provides a cost effective and long life dynamic seal solution. It is not as leak tight as cup style seals but is able to endure heavy side loading. The slipper seal design is also very space efficient allowing for a very short piston design or enabling the piston designer to incorporate large bearing surfaces on the piston.

Cast iron rings are also used as dynamic seals on pistons. They provide a very heavy duty seal solution but have a high cross piston leakage rate. Iron rings are effective in very high temperature applications.

There are other types and styles of dynamic and static seals but the ones discussed above are the ones most commonly used in hydraulic actuators.

Service and Shock Pressures

Seals are designed to service a range of pressures. At low pressures, softer (low durometer) seals are used as these provide tight sealing and are easy to install. At higher pressures, harder (higher durometer) and stronger seal materials are used to contain the pressures involved.

Some seal designs are better suited to high pressures. For example, an O-ring by itself is generally not effective at high pressures. At high pressures, an O-ring by itself will compress and deform and even extrude out of the seal area. Repeated deformations will result in the material wearing away and small pieces breaking off.

The shock pressure of a hydraulic system must be considered. Whereas, the hydraulic pump may be designed to produce 3000psi continuous pressure to accomplish the work and forces required, this is often not the maximum pressure that the cylinder seals will experience. Shock pressures or pressure spikes can be experienced due to external factors and the laws of hydraulics. Even if the system is equipped with over-pressure prevention devices such as relief valves or accumulators, very brief pressure spikes can still echo through the hydraulic system causing seal damage.

Shock pressures can be encountered when a cylinder is moving a heavy load in one direction but then the directional control valve is shifted to stop or reverse the direction of the load being carried. The same happens if the cylinder pushing the load suddenly hits an external stop.

System over pressures can be experienced due to pressure intensification. Picture a hydraulic cylinder with a large diameter piston rod. The cap end of the cylinder may have a piston area twice that of the rod end. If the oil flow from the rod end of that cylinder is blocked when the cap end is pressurized, the rod end pressure will spike to twice the system pressure due to hydraulic pressure intensification. It is a simple application of the basic hydraulic law P=F/A . The pressure on the cap end is exerting a force on the rod end where the piston area is 1/2 as large. Therefore the pressure will be intensified to twice that of the cap end. A 3000 psi system will see 6000 psi pressure exerted on the rod end seals.

With these scenarios in mind, actuator designers must carefully consider both service and shock pressures when selecting seals.

Service Temperature

Hydraulic actuators encounter a wide spectrum of temperatures and these must be considered when selecting the proper seal materials.

A seal material such as natural rubber may be very effective between 0 and 70 degrees Celsius. At higher temperatures, however, the material may become to soft and weak to contain the hydraulic pressure. At lower temperatures, the material may become brittle or shrink so that it fails to seal the pressure.

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Fortunately for designers and users, a wide variety of seal materials have been developed to accommodate the various temperature ranges encountered by hydraulic equipment. Some of these materials commonly used are natural and synthetic rubbers and elastomeric polymers such as PTFE and polyurethane.

It should be noted that seal material should be selected based not only on the ambient temperature that the equipment will be immersed in but also the temperature of the hydraulic fluid. A hydraulic system working on a cool day may produce hydraulic fluid with a much higher temperature than the ambient temperature. Therefore both ambient and fluid temperatures must be considered when selecting seals for an actuator.

Chemical Compatibility

Some hydraulic fluids contain special chemical additives that may adversely affect standard seal compounds. An example of this is fire resistant hydraulic fluids often used in areas such as metal foundries. These fire resistant additives will harm natural rubbers.

Hydraulic seals often encounter other chemical compounds besides the system hydraulic fluid. These chemicals may deliberately or accidentally applied to the outside surface of the actuator or introduced into the fluid through the rod gland or at the reservoir via the breather. These foreign chemical compounds can swell the seals, shrink the seals, cause the seals to become brittle or dissolve, or otherwise change the qualities and capabilities of the seals and reduce their service life expectancy.

A common example of an application with potentially harmful compounds applied to hydraulic cylinders are the caustic wash down fluids employed in the food industry to clean the equipment and kill bacteria. Also, the chemical industry has a virtually unlimited host of chemical compounds that cylinders used in production, handling and transportation will encounter.

Besides just affecting the seals themselves, seal failure is often carried to other parts of the hydraulic system causing valve or pump failure.

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Seal Storage

Hydraulic seals must be stored properly to ensure that they do not prematurely degrade in storage.

Because seals are made from rubber or polyurethane plastic, they do have a limited shelf life. Over time and with exposure to heat, light, humidity, ozone, oxygen and other factors, the physical and chemical properties of seals will slowly change. Seals may become cracked, brittle, hardened, less flexible, glazed, and shrink or expand. For this reason, care should be taken to properly rotate seal inventory. A "First In, First Used" procedure should be followed.

Label seal packages with the purchase or manufacture date so that this will be clearly seen in future by maintenance personnel using the seals. This will enable them to determine if the seals are suitable for use or indicate to them why the seals are degraded. If it is known, also label the seals package with the estimated expiry date.

Seals should be stored in sealed airtight packages. If seals have been removed from the original supplier packaging they should be resealed and relabelled.

Do not store seals where they will be in direct sunlight or under strong artificial lighting. Ultraviolet light will contribute to the breakdown of seal compounds. Storing the seals inside cardboard packages will often protect them from light.

Do not store seals where they will be exposed to water or high humidity.

Do not store seals where they may be exposed to chemicals especially cleaning agents and solvents.

Store hydraulic seals at room temperature at approximately 20 degrees Celsius (70 degrees Fahrenheit). The storage temperature should never exceed 50 degrees Celsius (120 degrees Fahrenheit) or drop below 5 degrees Celsius (40 degrees Fahrenheit). Do not store seals near a furnace, a heating vent, or base board heaters.

Do not store seals in an area where they will be exposed to air circulation. Circulating air will increase the exposure to ozone which is particularly harmful to rubber compounds. Ozone is also produced by some electrical appliances such as printers, photo copiers and air cleaners. Seals must be stored away from these appliances.

Seals should be stores in a way that they are not deformed by squeezing or crushing. They should lay in their natural relaxed condition. This will prevent them from becoming permanently deformed in storage.

When seals are removed from storage for installation, they should be warmed to at least 20 degrees Celsius (70 degrees Fahrenheit) before attempting to install. The higher temperature will facilitate greater seal flexibility which will ease installation.

Conclusion

With these matters in mind, it is clearly seen proper seal selection is critical to the life of the whole hydraulic system. Seal cost is often a relatively small component of the overall cost of an actuator but has a direct impact on the life expectancy and effectiveness of a hydraulic cylinder. Proper seal selection should therefore include: applying the correct seal function whether static or dynamic; considering both the service and shock pressures that will be encountered; selecting the right material to suit the service temperatures that will be seen; and finally choosing a material that is compatible to all the various chemicals in the final application.