Dimensions for rotary shaft seals in vacuum applications

[zaillian]

The task at hand is to seal a rotating shaft with high vacuum (10^-5 tor) on one side using a STEFA rotary shaft seal.

The STEFA is basic and look reminiscent of the one below.

Are there guides for choosing the dimensions of the enclosure, similar to the O-ring guide used for O-ring seals? The STEFA is marked as an having inner diameter of 6 mm and an outer diameter of 16 mm. The shaft has an outer diameter of 6.3 mm.

The current enclosure has an inner diameter of 16 mm and there is no compression against the face of the seal, only a stopper. The seal works when the shaft is not rotating, but leaks small amounts of air when rotating. The intuition is to apply more compression.

 

[Baluncore]

Stefa® High-Pressure Radial Shaft Seals are a Trelleborg proprietary product.
They are lip seals designed to operate under differential pressure.
They are not specified as high vacuum seals.

O-ring seals, with axial compression, will offer a better seal for slowly rotating shafts, as the contact path length is longer, and the contact pressure is greater.

You will need to polish any shaft, and lubricate any seal with a vacuum grease.

 

[zaillian]

Stefa® High-Pressure Radial Shaft Seals are a Trelleborg proprietary product.
They are lip seals designed to operate under differential pressure.
They are not specified as high vacuum seals.

O-ring seals, with axial compression, will offer a better seal for slowly rotating shafts, as the contact path length is longer, and the contact pressure is greater.

You will need to polish any shaft, and lubricate any seal with a vacuum grease.

Thank you. The seals are not from Trelleborg, but from Eriks. For some reason the term is used here for the seals.

For now the issue was fixed by applying additional pressure to the face of the seal and using vacuum grease generously. The vacuum pump cover uses similar technique for sealing, although the cover of course does not rotate.

O-rings were considered first and Parker has an excellent documentation for them which recommends using two in series for rotary application, but the radial seal makes things more straight forward in terms of designing the enclosure.

 

[Baluncore]

O-rings were considered first and Parker has an excellent documentation for them which recommends using two in series for rotary application, but the radial seal makes things more straight forward in terms of designing the enclosure.

O-rings seal against pressure, by allowing that pressure to get behind the o-ring, forcing the o-ring against the material junction. The dimensions of the o-ring and groove, need to be correct, to allow the o-ring to move and make the seal.

With a vacuum, o-rings would be operated more like a stuffing box or gland, where axial pressure, mechanically applied by a gland nut, adjusted from outside, is used to make the compression seal against the shaft. There is a diagram here.
https://en.wikipedia.org/wiki/Stuffing_box

For a vacuum chamber, lip seals would appear to be mounted backwards, with the atmosphere against the hollow inside of the seal. The elastomer and lips would then be pushed against the shaft by atmospheric pressure, working with the helical spring, to make the seal. The helical spring is then exposed to the external atmosphere, rather than being enclosed and protected in an oil space.

Machining the recess or groove to house a lip seal is less critical than an o-ring groove, partly because the external diameter of the lip seal, is covered in elastomer. For a high vacuum, I would push the lip seal into the recess, lubricated and sealed with vacuum grease, as most other sealants would release volatiles.

 

[zaillian]

O-rings seal against pressure, by allowing that pressure to get behind the o-ring, forcing the o-ring against the material junction. The dimensions of the o-ring and groove, need to be correct, to allow the o-ring to move and make the seal.

With a vacuum, o-rings would be operated more like a stuffing box or gland, where axial pressure, mechanically applied by a gland nut, adjusted from outside, is used to make the compression seal against the shaft. There is a diagram here.
https://en.wikipedia.org/wiki/Stuffing_box

For a vacuum chamber, lip seals would appear to be mounted backwards, with the atmosphere against the hollow inside of the seal. The elastomer and lips would then be pushed against the shaft by atmospheric pressure, working with the helical spring, to make the seal. The helical spring is then exposed to the external atmosphere, rather than being enclosed and protected in an oil space.

Machining the recess or groove to house a lip seal is less critical than an o-ring groove, partly because the external diameter of the lip seal, is covered in elastomer. For a high vacuum, I would push the lip seal into the recess, lubricated and sealed with vacuum grease, as most other sealants would release volatiles.

Yes! This looks like it avoids the problem of having to make a groove inside the enclosure, from the link by Baluncore.

Question with this design is whether there should be additional o-ring somewhere to cover the outer hole. In the picture, the o-ring is covering two holes at the same time, the one against the shaft, and one between the compression piece and the outer edge.

 

[Baluncore]

The contents of a stuffing box, or compression gland, are under pressure. That pressure forces the stuffing against both the inner and the outer diameter of the box, so the seal is complete. The solid black shown in the diagram is not empty space, but is probably NBR, or a graphite impregnated material.

When o-rings are used in a compression fitting, they will initially fit lose on the shaft and in the box. As the gland nut is tightened, the o-rings are deformed to become more elliptical in section, then slightly flattened, until they contact and apply pressure to the inner and outer diameter walls. The seal is made and maintained by the elastic o-ring material, yet there may still be some grease within the box, in corners between the o-rings. Careful adjustment of the external gland nut, will adjust the pressure, to balance between a reliable seal and low-torque shaft rotation.

 

[Averagesupernova]

There are many places in industry that are doing this.
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Here is one: https://www.google.com/search?clien...jQIHekBH7cQtKgLegQIGxAB&biw=360&bih=464&dpr=3

The info given by @Baluncore is good. Apply it to what you see in other applications as shown in the link.

 

[Averagesupernova]

This thread may point you in a few directions to source the seal you want.

https://www.physicsforums.com/threa...om-one-place-to-another.1067639/#post-7139586