An important consideration for seals in fluid applications is installation—literally how a cylinder seal is installed, and what you need to know when you’re picking a seal. How we describe a seal is the cross section, which is looking at a seal from the side. The cross section is the width across the U-cups, or from one side to the other. The depth is how far it is in linear length, and you also are concerned about the ID and OD. The cross section is half of the OD minus the ID, which makes sense because when you cross section a seal it’s a circle, so there’s going to be two halves. It’s pretty straightforward.

With a U-cup seal, it’s energized, it’s got a little O-ring inside the U part of it that helps keep it energized at low pressure. There’s a minimum dimension between those two. The cross section should never be wider than the length. It tends to want to roll inside its groove when you do that. Also, what’s shown at the top is an extrusion gap, also known as an E gap. The gap is very critical. Every seal manufacturer will list in their catalog what that E gap should be, and that’s the dimension behind the seal, between in this case the barrel and the piston. If that gap is too big, the O-ring, or the seal, will want to get extruded pass it. Like I mentioned, seals under pressure can act like a jelly. They get very soft, and it will just get extruded right past that extrusion gap. Too tight, and you’ll have excessive wear, and you may not be able to get your piston inside your barrel.

The extrusion gap works on the rod side, as well. You’ve got to make sure that that extrusion gap, in this case it’s mentioned between the bearing of the gland and the rod, is big enough so that it doesn’t seize up, but also that gap is small enough so that the seal doesn’t extrude at that gap. Once again, the width of the seal should be longer than the cross section so that it doesn’t want to roll inside the groove.