Why is seal compatibility important in fluid power?


Compatibility for fluid power seals means—does the chemical composition jibe with the fluid composition? Chemical composition is the composition of the seal itself, so what type of rubber, essentially, it’s made from. Back in the day, fluid power seals were made of things like leather and natural rubber, which had poor composition completely across the board. Also, compatibility, does the seal perform in required temperature extremes, those are things you need to be mindful of. If you have machinery that’s operating grooming ski slopes, that needs to have a different type of seal set than something that only works in high temperature applications, like a steel mill, where there’s tons and tons of heat.

If you’ve ever flipped through a Parker catalog they always have a lot of good engineering data. The Parker Seal Catalog shows the chemical compatibility of different rubber materials and different fluid applications, as well as what the temperature ratings are and durometer. They give examples of their different rubber compounds, and they have the codes on the left side. The polymer base, MBR is nitrile bunyl rubber. Other columns show hardness, so that just means the durometer, so that means literally how hard the seal is. If you have an O-ring, for example, or any kind of seal, the higher number means it’s harder, so that means it’s more stiff rubber.

Even though, for example, you would imagine you would put a seal like an O-ring into an application, you would imagine it would stay circular under pressure, but in reality, a rubber circular O-ring turns into like a rubber jelly. It turns to nearly liquid looking if you’re able to get it under a microscope while it’s under pressure. They tend to take the shape of whatever they are, almost like a liquid, of course still sealing, and the hardness of that rubber dictates how much it resists that change in shape.

There are applications say high pressure hydraulic, you need to have a higher shore hardness, or higher durometer than you would on low pressure applications, or air applications. The color, that typically doesn’t matter that much other than applications. Usually nitrile is almost always black, but they tend to make, say, Viton and other materials like brown or green, or you can have a red polyurethanes and such.

Next set you have your temperature range. This will be the range at which the rubber will work, not just for nominal, so for example the first one here, that N3571 nitrile bunyl, that one’s good for minus 35° C, to +100, which is 100° is the blowing point. It also has the short temperature range there, where it tells you how much it can handle for a short period of time before it starts to lose its stiffness. Starting here, it showed a little better in the previous slide, the list of chemicals you can see at the top here. They did a different list of fluids. Those are listed all the way across the top, and it has a little dot to let you know that it’s good for those applications. In this case, mineral oil, which is your standard hydraulic oil, that’s good for almost everything. Polyolphins are synthetics, and it gets to these HE type fluids, which those are your bio oils, so those are like say vegetable oils, or other natural based oils.

If you look at the difference between say bunynitrile and polyurethane, you can see that the temperature range tends to be different. The polyurethane one is usually good for colder temperatures. Some of them are good for higher temperatures, but mostly improved polyurethanes, but also look at the chemical composition. You can see that at the first set of urethanes, that polyurethane compounds, not so good with some of those HE fluids, so those could be synthetic esters, or environmental fluids and such. They’re not compatible with those, whereas if you go back and look at the nitrile ones, this isn’t a complete list, but it shows better chemical compatibility.

Below that you have some plastics. There’s different types there. You can see it will vary completely. There’s Teflons, and other things. Also, you look there on the fluid types, you have dot three or four, that’s a brake fluid. Obviously not a lot of seals are good for brake fluids. They’re kind of a special, and it can be a little bit more corrosive than others. All important things. Are there reasons that we’d use these kind of fluids? Yes. For example, next to the dot three four you have the HFAE fluids. Those are all of the water-based glycol type fluids. You would use those for fire resistant applications. For example, if you’re in a steel mill, and there’s high heat, there’s some flammability around, obviously mineral oil is flammable. If you had a leak of hydraulic oil, or a hose break, or some kind of failure where fluid would be gushing everywhere, you would not want to be flammable in a steel mill. That would be very dangerous, and literally adding fuel to the fire.

Some of the considerations for different rubbers here, these are some of the choices you have. 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 fluorocarbons, which are Viton. These are good for oil, but not for water, but are also good for high temperatures. Bunylnitrile, like we talked about, it’s a 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. It also has a medium temperature range.

Polyurethanes, these are higher durometer, means they’re higher hardness. They’re also low friction, so a lot of polyurethane U-type seals or cup seals, they’re for low friction applications like those flight simulators. They also have a medium temperature range. Then you get into your plastics. They have good chemical compatibility. They’re really hard and resistant to a lot of things. They’re good for a wide temperature range, but they’re typically very hard. Sometimes they’re not so good at sealing. The softer rubbers are better at sealing because they form easily, they take that shape easily. Then you get Teflons. Those are good wear and chemical resistance, also very wide temperature range. It goes very high, but also poor sealing because they’re very tough, they’re very rigid.


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