Why are the location of seals important in fluid power?


It matters where you put a seal. Every part of a cylinder has seals, and they’re in there for a reason. The rod wiper is the one that is most exposed to the elements in any hydraulic cylinder application. It has a groove that pushes against the rod itself. Anytime the cylinder retracts, anything on the rod wants to be scraped off of it. You can also upgrade from rod wiper seals—which tend to just get off dust and other loose debris—to a metal scraper. If you’re in an application such as rubber or plastics where there is the opportunity for melted junk or other garbage to get on the rod, you can get metal scrapers that will remove that.

Rod wipers are not so good at sealing in the opposite direction. This particular one has a little bit of a u-type application, where it probably could seal a little bit, but you wouldn’t count on those as your primary rod seal. The rod u-cup seal in this case, you can see that the direction of the U is the direction that it seals from. This is going to seal fluid from the right side. It’s off to the middle here. There’s a seal in front of it, which we call a buffer seal. This one right here is a special type of seal. Not a lot of cylinders use it, except for heavy duty applications where there tend to be high shock loads, so an excavator might use it. You might see it in steel mills, as well. A buffer seal acts as a buffer, because it’s really just a good word for it, between the fluid that’s in the cylinder, and protects the U-cup from pressure spikes.

One of the disadvantages of rod U-seals, they have to be pliable enough to be in a softer type rubber, and they can cause extrusions and damage from high pressure spikes. They can cause early failure, so you put this loaded O-ring in front of it, and it acts as a buffer. The U-cup has to have some dynamic sealing, so it’s good for a constant seal but slides back and forth, but any of those pressure spikes are absorbed by the buffer seal. After that, you have the guide ring, or the wear ring. This acts as a bearing on the rod. Not all cylinders have this type of configuration. Some of them might have a bronze bearing that is also the gland. It might not have a wear ring at all. It could have the gland itself be the bearing.

Then the end seal, so this is the seal that would seal the gland from the rod side of the barrel, so the gland being in this case it’s the head side. Though this is just a static seal, this is a heavy duty one that has an O-ring and a backup, so it can handle very high pressure between the differential, which is atmospheric pressure, and the high pressure that would be in this cylinder.

With static seals in the piston, if you were to energize the right side of it, on the piston side, fluid enters outside, fluid will easily work its way across those threads, so the threads of the rod are attached to the piston itself, and the fluid can easily bypass there. If you didn’t have this static seal here, the cylinder would still work, but it would be terrible at load holding. There would be some leakage. In fast/dynamic applications, it would be okay. A lot of places will just use an O-ring here. This one has an O-ring plus two backups. Once again, very heavy-duty application, good for high pressure.

Wear strips, so these are two guide rings, wear strips that help support the load of whatever is on the rod, or the differences in load between the cap and the rod side, and also took the wear of the cylinder. It’s easier to replace two wear strips than it is to replace the whole piston, or replace a barrel. You put these in here, they do all the wearing, and easy to replace.

The piston seal, this one is a loaded T seal again, so the energizer is below the black part, is the O-ring type part. It could be an O-ring with a couple of backups on either side. It could be a locking backup washers. Pretty effective seal all around.

There’s a bearing here in this case. It’s probably just a bronze bushing. It’s a little insert that would support the rod. It kind of acts as a wear strip, as well, so going back and forth rather than having the rod wear against the head, it wears against this bearing. This cylinder probably costs you $40 anyway, so it kind of would be a throwaway, but at least this little bearing would give you a little bit of rebuild ability. You could change that once or twice, and change those O-rings out around the piston, and rod seal, and she’ll be good as new. Finally, you have an end seal. This particular one I believe is just two pieces for the body and the head are one combined piece, with ports drilled in that one body for air in and out, and then one little cap that has those bolts that hold it to the body, one little O-ring, very simple.

Inside this valve, here, you have a spool valve. It’s pretty straightforward. One thing people don’t really know about some spool valves is the spool is so finely machined as well as the housing that the oil molecules have a hard time fitting between the gaps in that machining, so the tolerances are so tight that oil has a hard time fitting through. Those rings you see in the center of those spool are actually for holding fluid. Because the fluid has a hard time passing that gap, those rings will hold it. It will allow it to lubricate the spool between the body. It’s actually the oil does the sealing in this application. Across the whole spool, up to high pressure, it is literally an oil seal. Just because of these gaps in the clearances, it seals itself. The only O-rings you’re going to see in this type of spool valve are the spool seals on the end. Those ones stop fluid from leaking out from inside the valve.

In this particular valve, which has auxiliary ports at the top, things like counterbalance valves, or port release valves, there’s going to be a port seal up there, as well. It could be an SAE type plug, or an SEA type cavity, and it really is that simple. Like I said, the fluid seal is what occurs in this, so literally the gap and the nature of the molecules in oil allow it to seal that spool.

Moving on we have the pump. This one is pretty straightforward—we have a mechanical shaft seal, which may or may not be spring loaded. We have a static seal that seals that end head from the body. That was it. There’s literally just this particular pump, which has the same type of housing that is machined from one end, there’s really only one static seal for the flange, and the housing, and then one shaft seal. There’s a reason that gear pumps are very effective, but also very inexpensive.


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