Seal design

[JeanMicheling]

Hello folks,

I'm designing a new spindle and I'd like to have a great seal design. I got some information with skf but i'd like to have something more relevant. Does someone have an idea where I can find more information?

Thanks for any help

 

[Tmoose]

Get all the bearing manufacturer's precision bearing catalogs. Each has some valuable information. Clearances on labyrinth seals are very important. Splashing coolant shoould not be allowed a direct shot at the first seal gap. Put a spinning deflector in front. Think like an evil raindrop, provide continuous downhill drainage, and don't allow any bird baths or cesspools to form. If you use air purge, use it on both ends of the spindle to avoid air flow thru the bearings. The air must be managed very carefully to ensure uniform flow thru the air gap.

 

[ejc]

Seals on spindles can be very difficult to specify. These shafts usually rotate at a very high speed, so lip seals are not able to be used. Bearing isolators are another option. However, as these are usually non-contact seals, care must be taken to make sure that the bearing lubrication does not “flood” the seal. If this happens, the seal will leak, and the bearing will be subjected to premature failure. There are hybrid type seals on the market that utilize both a contact-type seal with the labyrinth type isolator.
Most bearing distributors also sell seals. I would start there. They can at least give you some company names to further your research.

 

[EnglishMuffin]

See following :

http://news.thomasnet.com/fullstory/13381/954

May only be available on SETCO spindles or under licence.

 

[JeanMicheling]

ejc, It is realy interesting what you said about overflooding the seals, I think it might explain some of the premature failures I saw. Some of the bearings I opened had lubrication issue. The grease deteriorated too fast and there was wear tracks so maybe the bearing ran too warm because of the seal leakage.

EnglishMuffin, I looked for those setco seals. They seem to work really good. I'll let you know if they sell it apart.

 

[Clyde38]

Have you considered FerroFluid Seals?
See -

http://www.roymech.co.uk/Useful_Tables/Seals/Rotary_Seals.html

 

[JeanMicheling]

The ferrofluid seals look good but maybe a little too tricky for my application.

I called setco en unfortunately, they don't sell the seals apart.

And what about the axial clearance on labyrinth seals? I designed some axial labyrinth seals and the axial gap between the seals should be, according to skf recommandation, 3mm. Why this gap is so big? Is it because of the thermal expansion or to have a room for the grease? I wonder if I have to be so precise?

 

[Tmoose]

Axial clearance in labyrinth seals is necessary for thermal expansion, and to allow reasonable manufacturing tolerances on covers and housings, and to allow rebuilding the spindle with off-the-shelf bearings in the future. For instance, the width tolerance even on superprecision bearings used to be about 0.005 inch (+/- I think). They generally do a lot better, but potentially the "length" differences of the new bearings at manufacturing or rebuild will require adjusting covers and othere components for "trap" or "pinch". Having to check and correct axial lab clearance would be a nightmare.

My recommendation is not to have labyrinths or other metallic non-contact seals embedded in big housings. In the event of a touchdown the heat generation forces the seal to expand inward, quickly turning into a brake gripping or even welding to the shaft. Using various bearing bronzes for the lab material does not help. Bronzes' high thermal expansion just makes it grip harder faster.

 

[JeanMicheling]

I know that even precision bearing has a big length variation. The ones I'm using have -0.015'' max deviation. But I think that the length is exactly the same for both outer and inner ring of a same set. So there is no risk for the seals to loose the axial clearance because as we keep the same gap between the cover and the housing end face, the cover and the nut will have the same relative position. By the way, what would be the best tolerance, is it necessary to have it tight?

 

[ejc]

Remember, with labyrinth seals, the larger the gap between the two parts, the easier it is for lubrication to enter the seal. Once the seal is comprimised, failure usually follows.
Many companies sell unitized seals, just press on shaft & bore and don't need to worry about setting the clearances.

 

[Tmoose]

Ball bearing widths are pretty well controlled. Roller bearing widths are much less so. If a taper bore roller bearing is adjacent to a pair of ball bearings the roller adjustement (if done via fitted spacer) will move the ball bearing location at assembly. Some cover/flinger/slinger fitting will be required.

Sometimes it makes sense to machine part of the labyrinth right in the bearing side of the spindle flange. Decreasing bearing width then stuffs the labyrinths deeper axially. There will be a radius or chamfer at the "bottom" of the laby groove. The mating laby lip would need a generous chamfer to ensure no rubbing. The chambers resulting from axial clearance can be used as collection areas for pressure relief and leakage, especially when combined with grooves, drip edges, and drainage troughs or weep holes.

Axial labyrinth clearance is my friend.

 

[JeanMicheling]

Tmoose, what do you mean by machining part of the labyrinth right in the bearing side of the spindle flange. Could you be more clear or a picture would be great.

 

[EnglishMuffin]

With machine tool spindles, except for very low speeds, you should avoid contacting seals because they generate so much heat. So that means designing some sort of labyrinth, or in some cases a flinger. You can buy ready made labyrinth seals from GMN (see web site). There is a very comprehensive paper put out by the University of Aachen, which goes into the optimum design of labyrinth seals, particularly applicable to machine tool spindles, and there are a number of tricks to it. If you are going for a labyrinth, I would recommend that you follow that if possible - but don't ask me for the exact reference/title etc. If you have air pressure available, a very reliable way of keeping contaminants out is to pressurize the inside of the spindle (which you get for free if you happen to be using oil/air lubrication). That is basically what SETCO have done, with the added feature that they have something like a Forsheda V-ring seal which continues to keep out the contaminants in the event the air is turned off. The lip of the seal is lifted from the rotating surface when the air pressure is turned on. They have patented this, but I am not sure how strong the patent is, because things similar to that have been done for a very long time. I don't think they can patent the air part of it at all.

 

[unclesyd]

Here is one of the original bearing isolators. We use these in numerous high speed spindle applications as well as several expensive ones in centrifuges.
Give them a call.

http://www.inpro-seal.com/apps/machine.phtml

 

[EnglishMuffin]

We (I won't say who) have had rather bad luck with those Inpros. Maybe it was our fault.

 

[unclesyd]

EnglishMuffin,
Like everyone else we had some failures, mainly from the use of "hardcore unemployable mechanics" making field adjustments. As you probably know the Isolators requires a little finesse.
I can’t recall any on the spindle side though I know there had to be a few due to bearing failures.

 

[EnglishMuffin]

Yes, I think bearing failure caused most of the Inpro failures in our case. But in addition, as so often happens, if something isn't applied correctly from the start, it fails and gets a bad rap, which can then never be overcome regardless of who is running things. But as I said, I think using air as an excludant is the best method I have personally come across - the labyrinth can then be made very simple with quite a bit of clearance.

 

[Tmoose]

Find, Follow, and, even better, work to understand the AACHEN guidelines. (Only use the design details labelled good, not the ones called bad or even acceptable).

Mid 90s FORD Europe (Lima?) received a batch of new spindles for an engine component machining tranfer line. FORD, against the spindle manufacturer's vigorous protestations, had specified the then-current spindle version of INxxO seals. Shortly after the line was commisoned spindles started failing. Each time the spindle manufacturer claimed coolant contamination had been the cause. After many coolant contamination bearing failures, and receiving the report from the spindle manufacturer's in-house testing, FORD payed the spindle manufacturer to substiture their own proprietary labyrinth with AIR purge (which is what the spindles would have been built with). The spindle failures stopped.

The coolant environment on machining transfer lines of the time was EXTREME. Coolant sprayed everywhere inside an enclosure where the workpieces, tooling, (and spindle noses) lived.

As of about 3 years ago I believe at least one of the INPRO designs for spindles included, (if not relied on) a certain amount of AIR purge, probably along with their belov-ed vapor blocking flying o-ring.

Please don't get me wrong. I am sure bearing isolators have their place.

I checked out the INPRO performance gurantee.

http://www.inpro-seal.com/guarantee.phtml.

It is signed by Mr David Orlwoski, the inventor and patenter of bearing isolators.

"If you are not satisfied that it is working to your satisfaction, Inpro will either modify the Isolator to your satisfaction or refund your purchase price. "
They promise, by golly, to keep re-designing that isolator until it actually works, or maybe you will just get the isolator price back.

Each Machine tool Spindle rebuild cost several 1000s of $ (American bucks). Machine tool spindles are typically sold with a 1 year warrantee. Defective bearing, ineffective seal, poor workmanship or flawed materials, the spindle maker gets to eat it all, for a year.

Several years ago an acquaintance who builds race engines professionally had a valve spring retainer fail, and a valve wandered into areas it did not belong. Many expensive parts were destroyed. The manufacturer of the cam/lifter/valve/ spring/retainer system paid to fix the engine. I don't think many companies did that then, and fewer would today.

As of about 3 years ago The SETCO seal included a fully developed (by design and verified with tests) AIR purge with carefully managed AIR delivery.

Air, air, air.

If I suspect something is going to try to get into my spindle bearings I'd include some air purge.

 

[JeanMicheling]

I looked those inpro seals. The design is quite good but what about the limiting speed. Can this seal work with air purge? That would be a great mix. If I understand, you just have to press fit the seal on the housing? By the way, can someone help me to find this famous paper from the University of Aachen?

Also, I read something interesting on seals. It says that spiral seal design should be avoided when the spindle is designed to rotate in both directions. Can someone tell me what is a spiral seal design? Is it a labyrinth seal?

 

[ejc]

A spiral seal is one that has a directional feature. Usually, this is a type of helix, or spiral along the shaft direction. This feature works with rotation to pump fluid back into the bearing cavity. However, if the seal direction is reversed, this feature pumps the oils straight out of the cavity. The bearing is quickly starved of lubrication, and will fail.