In a fluid power system, fluid compatibility is often discussed with respect to seals. This means, does the chemical composition jibe with the fluid composition? Chemical composition is the composition of the seal itself. What type of rubber, essentially, is it made from? Generations ago, fluid power seals were made of things like leather and natural rubber, which had poor composition—completely across the board.

Also, seal compatibility must be considered with respect to whether the seal performs properly across the required temperature ranges, which could include extremes. These are things that the system designer and operator need to be mindful of. If you have machinery that’s grooming ski slopes, that needs to have a different type of seal set than with something that only works in high temperature applications like a steel mill, where there’s tons and tons of heat.

If you look at the difference between, say, Buna Nitrile and polyurethane, you will see that the temperature range tends to be different. The polyurethane one’s usually good for colder temperatures. Some of them are good for higher temperatures. We’ll see improved polyurethanes but also look at the chemical composition. For some urethanes, like polyurethane compounds, are not so good with some of the HE fluids.

With EPDM rubber, it’s generally good for water but not so good for mineral based oils. It’s also good for low temperatures. You have fleurocarbons which are Viton, these are good for oil but not for water … but are also good for high temperatures. Buna Nitrile, again, is good all around material. It’s the most common seal material for fluid power applications. It’s good for oil or water but also poor with synthetic and fire resistant fluids. Also, it has a medium temperature range.

Polyurethanes, these are high durometer, meaning they’re a higher hardness. They’re also low friction. A lot of polyurethane U-type seals or cup seals are good for low friction applications like flight simulators. They also have a medium temperature raise.

For plastics, they have good chemical compatibility—they’re really hard and resistant to a lot of things, so they are good for wide temperature ranges but they’re typically very hard. Sometimes, they’re not so good at sealing. The softer rubbers are better at sealing because they deform easy, they take that shape easily. Teflons feature good wear and chemical resistance and a very wide temperature range. It goes very high but also can result in poor sealing because they’re very tough and very rigid.