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  • O-rings are the epitome of elegant engineering: The ring itself costs only a few cents, and

  • the groove it goes in is simple and easy to manufacture. But despite this simplicity,

  • the resulting seal is able to reliably hold many thousands of psi of pressure. O-rings

  • are definitely a machine design component youll want to be familiar with, and in

  • this video, were going to tell you all about how to design seals with them.

  • An O-ring forms a seal when it is squeezed between two adjacent surfaces. As the ring

  • is squeezed, a contact stress between the O-ring and the surfaces emerges. If the fluid

  • pressure is lower than the contact stress, then the seal prevents the fluid from escaping.

  • In general, as the fluid pressure increases, the O-ring is compressed even tighter into

  • the groove, further increasing the contact pressure, and hence helping the O-ring to

  • seal even better. This positive feedback loop of increasing pressure leading to increased

  • sealing is calledself-energizing”.

  • Seals can be generally classified as either static or dynamic. This hydraulic cylinder

  • has examples of both. The seals between the cylinder and the end cap, the gland and the

  • end cap, and the piston and the rod are static seals, since these components don’t move

  • relative to one another after the cylinder is assembled. The seal between the piston

  • and the cylinder, and the rod and the end cap are dynamic seals, since these components

  • slide when the cylinder is actuated.

  • The most common type of seal is a radial seal, which can be designed in one of two ways.

  • If the groove is on the ID of the housing, this is called a rod seal. If the groove is

  • on the OD of the shaft, then this is a piston seal. If you have a choice between a rod and

  • piston seal, it’s better to go with a piston seal, because the grooves are much easier

  • to machine and inspect.

  • The biggest weakness of radial O-ring seals is that the clearance between components creates

  • a path for the O-ring to extrude due to the pressure acting on it. Components called backup

  • rings can help alleviate this. Backup rings are designed to spring out of the gland and

  • block the extrusion gap. Where an O-ring alone could withstand perhaps only 2000 psi, a backup

  • ring can help it hold 5000 or more.

  • Backup rings are very cheap, and effective. They are made of a plastic like PEEK or Teflon,

  • and they usually have a scarf cut to help you install them in the gland. Technically

  • speaking, if you only had pressure in one direction, you could get by using only one

  • backup ring. However, it’s very easy to put the ring in on the wrong side, so as a

  • design-for-assembly precaution, if designing with backup rings, you should always design

  • for two.

  • Another configuration is a face seal, which you might use when trying to seal an enclosure.

  • These are really a type of gasket, and they require a clamping force, usually provided

  • by fasteners, to compress the O-ring. Face seals are actually really tricky to get right,

  • because squeezing the O-ring requires a great deal of pressure. This first lid design is

  • far too thin, and in the middle, there is virtually no squeeze on the O-ring, and hence

  • no sealing. We can fix this design by adding a lip around the perimeter.

  • A third type of design is a boss seal. You pretty much only see them on hydraulic fittings,

  • but they have a lot of advantages for other applications. In this configuration, the O-ring

  • sits in a triangular space that is usually made with a special form tool. Boss seals

  • are really easy to manufacture, since there aren’t any undercuts, and if the gland gets

  • damaged, they can be reworked by just machining the profile slightly deeper.

  • Regardless of the gland design you select, youll likely need to choose an off-the-shelf

  • O-ring from a catalog. O-rings are available in standardized sizes. The most common standard

  • is AS568, and each size is assigned a “dashnumber.

  • O-rings conforming to these sizes are available in many different materials. The primary considerations

  • for selecting a material is the working fluid and temperature. Design tables, like this

  • one in the Parker O-ring Handbook, are the easiest way to select a material that is appropriate

  • for your application.

  • The same material is often available in a range of different hardnesses. The hardness

  • is typically expressed as the Durometer Hardness. 70 is a fairly typical hardness and is good

  • for most uses. 55 Durometer is much softer, and is a good choice for pressures below maybe

  • 1000 psi because it is easier to install and less sensitive to surface finish.

  • 90 Durometer is extremely hard, and consequently, is more resistant to extrusion. For higher

  • pressures, exceeding maybe 6000 psi, youll definitely want to consider using 90 durometer.

  • However, the better high-pressure performance comes at a price. 90 durometer O-rings can

  • be really difficult to install, particularly in small sizes. A good tip is to drop them

  • in hot water for a few minutes to let them soften. They will be a bit easier to install.

  • Another important installation tip is to apply a high-quality O-ring grease before assembling

  • the parts. In addition to lubricating the rubber and helping it slide in easier, most

  • O-ring grease is designed to cause the O-ring to swell slightly, helping increase the squeeze

  • after installation.

  • When you assemble the components, you need to squeeze the O-ring quite a bit to create

  • a seal. It helps to have a shallow entry angle of about 15 degrees. This surface should be

  • totally smooth and free of burrs so that the ring isn’t inadvertently cut.

  • Surface finish on the components is extremely important. As a general rule, the side of

  • the gland, and the bore or rod, should have a 32 rms surface finish for static seals.

  • The walls of the gland can be slightly rougher, at 64. This is where the piston seal really

  • shines, since it’s typically more difficult to verify the dimensions and finish on a rod

  • seal gland.

  • In a dynamic application, O-rings can work, but there are much better options to consider.

  • This is called a T-seal and they are specifically designed for dynamic applications. They are

  • packaged with two backup rings as a unit, and their primary advantage is that they have

  • a wide, flat bottom to keep them from rolling around in the groove. They don’t cost much

  • more than O-rings, and in our experience, are very reliable in dynamic applications.

  • Up to this point, we haven’t mentioned where the dimensions for the components come from.

  • There are three seal parameters that will define the dimensions: squeeze, stretch, and

  • percent gland fill.

  • Squeeze is how much you radially compress the O-ring when it’s installed in the gland.

  • In general, 18-25 percent squeeze is appropriate for most static seals, but as high as 30 percent

  • is sometimes used, particularly for cold-service applications.

  • Stretch corresponds to how much the O-ring is tangentially stretched AFTER it is installed

  • in the groove. It doesn’t directly have to do with how much you stretch the O-ring

  • during installation, though with small sizes of rings, the installation stretch creates

  • other problems. Stretch should be below 5 percent, because high values of stretch cause

  • the cross section to become smaller, decreasing the squeeze.

  • Volumetric gland fill is the final parameter. The O-ring and backup rings are incompressible,

  • and if you don’t have enough space, you won’t be able to assemble the components

  • because there will be nowhere for the O-ring to go. You also have to watch out for thermal

  • expansion, because the O-ring will get bigger with temperature, and it can actually crush

  • the metal components and yield them if you don’t have enough room. A good guideline

  • is to keep the gland fill below 85%.

  • While you could use these equations to calculate the gland dimensions yourself, there is a

  • source for pre-calculated values called the Parker O-Ring Handbook. A digital copy is

  • available for free from Parker’s website, and weve provided a link below. Well

  • show you a quick example of how to design a static piston seal.

  • Well flip to theStatic O-Ring Sealingsection of the Parker book, on page 4-9. There

  • is a figure defining the different dimensions for both rod and piston seals. Well select

  • the number of backup rings were planning to use, then find the row for the 200-series

  • O-ring were using in the table. Working across that row, we can read off the limits

  • for the groove width.

  • Then well turn to page 4-13 where the 200-series O-rings start. Well find the 210 O-ring,

  • and then work across the row to read off diameters A, B-1, and C. Youll notice that the tolerances

  • for these dimensions are given at the top of the table. Then, all we have to do is transpose

  • this information onto our drawing.

  • Unless you have a very unusual or demanding application, the guidelines weve given

  • you, and the tables published by Parker should allow you to confidently design reliable O-Ring

  • connections. If you found this video helpful, we’d really appreciate it if you shared

  • it. We have a long list of engineering topics that well be covering in future videos,

  • so be sure to subscribe so you don’t miss these. If you have feedback, a question, or

  • an idea for a future video, be sure to leave a comment. Thanks for watching!

O-rings are the epitome of elegant engineering: The ring itself costs only a few cents, and

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O-Rings? O-Yeah! How to Select, Design, and Install O-Ring Seals

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    小鐘 に公開 2022 年 08 月 16 日
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