Hydraulic Sealing Basics
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The function of hydraulic seals goes far beyond preventing leaks. Hydraulic applications require seals to withstand high pressures, extreme temperatures and transverse forces within the cylinder.
As an introduction, Jesse Thomas breaks down the different types of seals, glands, and considerations when designing your cylinder.
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This video is intended for first-time hydraulics users, but we’re going to be diving into each of these topics in much more detail in future videos.
Anatomy Of A Hydraulic Cylinder
To begin, we have the anatomy of a hydraulic cylinder. Starting with this chamber, the hydraulic fluid that is pressurized is provided through an inlet/outlet port. That pressurized fluid exerts its work on the piston and the rod to which it is bolted – which allows it to actuate linearly. To take full advantage of that pressurized fluid we are going to need to stop leaking across the cylinder piston.
We’ve got a leak path here between the piston and the bore and between the piston and rod. But these are two different problems. Between the piston the rod there’s no relative motion so we can just use an o-ring, but between the piston and the bore, we’ve got the opposite. We’ve got a sliding movement and for that reason, we’re going to need special geometries and materials to make a seal that can last.
This component here is the wear ring – simply is a strip, usually thin of plastic often nylon – that reduces the chance for metal on metal contact and damage. You could also see wear rings appear on the rod head gland side.
The head gland on this cylinder has been threaded in. Giving us similar problems to what we saw on the piston and the rod. We have a zero dynamic motion application up here. We can just use an o-ring again between the bore and the head and then like the piston seal we’re going to need a rod seal between any sliding rod and static piston. This is especially important in a bi-directional cylinder where both chambers are occasionally pressurized.
Another component you’re going to see is the wiper. The wipers are particularly important in applications where the environment has debris that could enter your cylinder and damage your components.
To keep these components in the right place we machine glands into our metal parts.
Types Of Glands
The most basic of which is the open gland. Open gland often used for wipers far at the front of the head. It is just a counterbore into the head and then you press in a metal OD wiper. The press-fit provides for retention and stability.
Split gland begins as an open gland and it’s used for difficult to install components in the head and also used for very small diameters – 5 mm / 6 mm. Instead of bending it into a certain shape and then installing it and allowing it to expand like you would for a typical rod seal or wiper, you just install it and it’s round shape before bolting on a faceplate to achieve a closed gland.
A truly closed gland is the most important of the glands because this provides the most control when you’re trying to seal something. That’s because you know the distance between the diameter where the gland ends, and the diameter of your rod bore – whatever you’re sealing against. As you’ll see in future videos this is extremely important for increasing the pressure rating of your system.
Considerations When Designing Your Cylinder
Speaking of other design considerations, you want to look at the temperature. If you are in Arctic conditions or are using farm implements out in Iowa, you’re going to need to be aware of the temperature rating of your elastomer materials. A lot of these rubbers are going to lose their elasticity at low temperatures. You need to be aware of that.
SPEED & Ra/Rz/Rq
Speed of the rod, as well as the surface, finish your seals are sliding against. If it’s too rough, too fast, you’re going to be wearing out in no time.
ASSEMBLY & SIDE LOAD
– Have you understood the tools used to install these components?
– Do you understand that people are going to be stretching piston seals over a piston? You need to be aware of that to build a realistic hydraulic cylinder.
– Is there going to be a weight causing a cantilever motion that’s going to be forcing your piston against your bore? You need to be designing your wear rings to handle that.
MEDIA & CONTAMINATION
A lot of these elastomer materials do have chemical compatibility issues. You need to be aware of what fluids you are using, fluids you are going to be seeing out here, and what your customers going to be cleaning the cylinder with. Perhaps, it could have an impact on the lifetime of your seals. Contamination, same thing. If there’s solid ice on your rod when you start up the cylinder, you’re going to need a specially designed wiper for that application.
PRESSURE & GAP
The pressure is the first thing we think of. A lot of these seals come with catalog entries that have a pressure rating for the seal. However, like we’re going to see in future videos, the pressure rating is only as good as the gap – the extrusion gap. The difference between the diameter of the component with the gland and the diameter of whatever you’re sealing against. If that gap is too large, it doesn’t matter what pressure is going to be coming at it. It could easily extrude the component even though it’s pressure rating said it was to something much higher.
As I said, we are going over these in more detail in future videos, but that is hydraulic sealing basics.