Every operator is aware of how important proper seals are in fluid power systems. Just as important, however, is ensuring appropriate seal-fluid compatibility.
There are dozens of different types of hydraulic systems in common use, each of which may utilize a different type of noncompressible liquid to generate motion. The various fluids interact differently with the different materials commonly used to create fluid-tight seals, and in some cases, they may cause degradation. When this occurs, it can lead to leaks and premature wear and may ultimately cause the machinery to fail.
Below, we’ll examine how different seal materials interact with the most common hydraulic fluids and provide guidance on seal-fluid compatibility. The right combinations will help ensure long-term system performance and reliability.
Creating a Strong Seal
There are two main goals of an effective seal in a hydraulic system: First, to prevent liquid from seeping out. If the hydraulic fluid escapes from the container as a result of a poor seal, it will cause a drop in pressure that will weaken the effectiveness of the machinery.
The second goal is to prevent air ingress and other contaminants from entering the system. Contaminants that enter the fluid through a damaged seal can also damage the equipment. Air bubbles, dirt, and broken-down bits of the seal material can hamper the liquid’s ability to reduce friction, causing the system to overheat. The resulting wear and tear will eventually lead to an expensive breakdown. To prevent these issues from damaging your hydraulic machinery, it’s crucial to create a proper seal using a material that is rated for the stresses that it will be subjected to.
Various types of rubber are among the most widely used materials. For example, nitrile is popular due to its flexibility and relative inexpensiveness. Polyurethane is a similarly flexible material that provides excellent resistance to the stresses that most hydraulic systems endure. In other cases, when more rigid materials are required, thermoplastics may also be used to create seals. Polytetrafluoroethylene (PTFE), also known as Teflon, is an excellent low-friction material often paired with elastomeric backup rings to provide flexibility in systems facing extreme conditions. Additionally, ethylene propylene diene monomer (EPDM) is a synthetic rubber known for its excellent resistance to heat, ozone, and certain chemicals, making it a popular choice for seals in systems exposed to harsh environmental conditions.
Common Stresses a Seal Must Withstand
Hydraulic machinery is ubiquitous in modern manufacturing, construction, aviation, and many other industries, each of which provides its own set of stresses. For example, machinery used in manufacturing may be subjected to corrosive chemicals, meaning that any material used to create a seal must be chemically resistant. Hydraulic systems used in aviation will be subjected to environmental extremes, such as heat, cold, and the ultraviolet radiation of the sun. In the construction industry, seals will be subjected to massive pressure as the systems are used to move large amounts of weight.
When selecting a material for a seal, operators must take the above factors into consideration.
Which Materials Work Best with Which Hydraulic Fluids?
The other major concern operators have when installing seals is whether or not the hydraulic fluid itself will degrade them. There are many types of hydraulic fluids, some more commonly used than others, and each will have a different effect on different seal materials. There are too many potential chemical interactions to cover them all in a single blog post, but we’ll discuss some of the most common ones here:
Petroleum-Based Fluids
Many of the oils used in hydraulics are petroleum-based. The hydrocarbons in petroleum tend to have a degrading effect on most rubbers. However, some rubber materials can effectively resist the degradation caused by petroleum, including nitrile. Polyurethane is also largely resistant to petroleum-based oils, as is PTFE, thanks to being largely chemically inert. However, polyurethane and PTFE may lack the flexibility that nitrile provides and therefore not be as effective for certain applications.
Non-Petroleum-Based Fluids
If the machinery in your hydraulic system requires an oil that isn’t petroleum-based—which is commonly the case in machines subjected to high temperatures or at risk of fire—propylene glycol is another common choice.
This fluid can degrade some of the most common seal materials, such as polyurethane. Glycol also damages nitrile, meaning it’s incompatible with this type of seal material. If you are utilizing a system that has a glycol-based fluid, PTFE, or, in some cases, EPDM, is often your best choice for seal material, as it can resist the damaging effects.
Synthetic Fluids
Synthetic fluids containing polyalphaolefins, organophosphate esters, phosphate esters, and polyalkylene glycols are also frequently used in hydraulic equipment. These are designed specifically for extreme conditions, such as high temperatures and environmental stresses. However, these same harsh conditions can be harmful to many seal types. High heat conditions can damage nitrile and other rubber types, and ultraviolet light can damage polyurethane.
Once again, PTFE is a good choice for seal-fluid compatibility with synthetic oil. Fluoroelastomer (FKM), a fluorocarbon-based synthetic rubber, is suitable for many synthetic fluids, though phosphate esters often require EPDM for optimal compatibility.
For Further Information
Above, we’ve discussed the most basic seal materials and the most common hydraulic fluids. While this list is by no means exhaustive, these combinations do cover the vast majority of applications in most industries. If you are considering a seal-fluid combination that has not been discussed here, you can usually ascertain its viability by consulting the manufacturer’s chemical compatibility chart or contacting the manufacturer or supplier directly. If you have questions about what will work for your purposes, have a specific need that must be addressed, or are looking for a customized solution for your operations, contact the team at Aberdeen Dynamics. Our experts will help ensure seal-fluid compatibility in your machinery so that you can ensure maximum productivity, avoid unnecessary downtime, and extend the lifespan of your equipment.