turbine bearing types
 

Hi.
I have an old TREFZ KJ66 clone turbine and lately i want to experiment a little with the bearings.
It looks very difficult to find cageless full ceramic, full complement, angular contact 608 bearings so i am willing to experiment with normal radial full ceramic bearings.
My question is if anyone knows or has experience with full ceramic NON angular contact bearings ie normal bearings but full ceramic.
I want to try them for a change, the preload system will remain the same and i have calculated for the different temperature expansion coefficient.
I have searched the internet extensively but i found no interesting information about different bearing usage and modes of failure.
Of course i only ask for any experience or opinion, if any damage occurs during my experiments is on me, i am prepared for it.
Chris

I have run normal steel caged SKF 608 bearings in a homebuilt KJ66.

I limited rpm to 100.000, and even so they lasted only a matter of minutes.

The biggest problem is going to be the bearing cage though. It's a lot to ask from a bearing cage, the rpm we have these things running at.

The non angular contact full complement bearings have filling slots, I believe these will cause trouble with the bearing preload.
Also the precision of the bearings supplied by Boca Bearings and the likes cannot be compared to real precision bearings.

Your best bet really is to get a set of proper bearings from GRW, and try to make them last by getting your engine balanced.

You can get them at http://www.ukpraezitec.de/default_e.html

The angular contact races are your shaft stops, with regular bearings you have no stops to hold the shaft in position, even the pre-load could push out the outer race from the rear bearing, not a good idea…

https://cimg4.ibsrv.net/gimg/www.rcu...c558adaba.jpeg

Now that is the information i want to know, thank you very much both!
It looks like my only chance is to try and find full complement angular contact full ceramic bearings if my test has a chance of success.
I understand that the bearing precision is not always good, i have some cheap ceramic bearings for shaft balancing that their dimensions are all over the place, some will not slide in to the shaft!
Chris

Yes but even deep groove bearings can take up to 25% of the maximum radial load as axial load.
A radial bearing should hold the shaft of a KJ66 with 8Kg of thrust in position, shouldn't it?
I am asking because i only have theoretical knowledge not actual experience...

There must be some balance between of the axial loads on the bearings. The compressor is pulling forward and the turbine is pushing out the back.
I've never taken a turbine apart, do they have thrust bearings?

You’ll be placing a load on the bearing it is not designed for, it will fail…the grooves on the races are to keep the surfaces on the balls and races aligned to avoid uneven and premature wear, not to support axial loads. The bearing may be able to take an axial load at low rpm with no short term problems, but microturbines run at a median of 70-80k rpm of their rpm range (the small ones even higher), why would you risk it? Makes no sense

Yes i think you are right.
Anyway i found some cheap Hybrid bearings to try....
It looks like i am stuck with the Hybrid bearings, although i did found full ceramic full complement angular contact bearings but the manufacturer says they will not hold.
On the other hand i found someone advertising radial PTFE caged full ceramic bearings who says that he use them on a KJ66 with success
https://www.worthpoint.com/worthoped...amic-114876373 and https://www.ahrinternational.com/full_ceramic.htm
I have those and i don't think that they will hold on a kj66 turbine.
Chris.

Caged deep groove bearings?

No they are GRW clones branded as "Yugos Jet Technologies"
It's a calculated risk but for the sake of trying something different i will try them, https://www.ebay.com/itm/393574608037?hash=item5ba2dfeca5:g:ayIAAOSwZzlhRNh 5

Thank you for explaining the difference.
Always wondered, thanks for dumbing it down so a non-engineering person could get his head around it. :cool:

I think this is what you want : D608/602C GRW by Boca Bearings :: Ceramic Bearing Specialists

I know but i want to try something different at least at the compressor side.
Chris

But why?

Well it is difficult to explain but i like to experiment with mechanical things like my turbine, my motorcycle, my bicycle etc.
I don't know why but i need it, also it would be rather boring if everyone had the same house, the same car, the same clothes, the same furniture etc.
Of course some times experiments fail....

I share that notion. In small turbines the rear bearing takes near all the axial load. Bad because it really hot already, good because the overhang and turbine mass do not need as much radial support. Front bearing caries a lot of radial load and almost no axial load, due to overhang and precession forces of the compressor. I think it may be worth a try using a deep groove up front with some preload.

I will just to see what will happen, the static friction of the full ceramic PTFE caged bearings is almost non existent...
I will report my test results here when i am done testing.

You can buy German bearings from jetjoe for a very good price, they still in the business and the shipping all parts, I have some stuff from them and they bearings was made in Germany

I have lost 1700 dollars when i bought a JJ1400 if i remember correctly many years ago, it never worked more than 5 minutes and it ended up in the garbage bin because there is no way i could sell it to an unsuspected buyer and live with this thought.
https://www.rcuniverse.com/forum/rc-...thread-36.html
I only hope that the bearings i bought are not from them....

Hi Chris

As many have already said, caged bearings in this application are just a no no because the cage cant handle it for long and would fail without warning and fast. Since the cage is what keeps the whole inner race central, when it fails its catastrophic and sudden stop to put it mildly.
Unforthnately there are alot of cheap full complement ceramic 608s but most are not good due to poor design or build or both (for this application), few are of okay design but consistency in quality control is poor therefore you can get a good one some times and other times is worse than a good SKF steel 608.

So I believe the question you need to ask yourself is are you prepared to put your engine on the line for someone else poor design or build quality in manufacturing bearings that at best will be okay for a skateboard ??

i am prepared but i don't want to install just any bearing but something that has a chance of working well.
I guess that your experience applies for both sides?
I was thinking to use a full ceramic full complement radial bearing like this one https://www.aliexpress.com/item/3281...44937989%22%7D
at the compressor side at least but i do understand what you are saying about quality...
Chris

Let us know how you get on with the those bearings

Of course i will write about it!

Also i am really worried about those two notches for inserting the balls in to the races, one will never come in contact with the balls due to preload but the other certainly will.
I can't understand how the balls get over it without hitting it repeatably and get destroyed during high speed rotation....
Anyway i am waiting for a price quote on full ceramic, full complement, cageless angular contact bearings...

A few actual facts:
Full compliment is not the best option in high rpm applications. We get away with it due to reduced life expectancy.
AMT uses angular contact bearings with machined brass cages. The do exist and are actually rated for our use rpm. Expensive and rare in our sizes.
Ceramic inner race is very susceptible to cracking due to hi difference in thermal expansion. Requires an inner spring centering bearing on shaft to accommodate this.

Excellent information, THANK YOU VERY MUCH!
Chris

With ceramic balls or steel?
Chris

Ceramic Silicon Nitride

Bearing replacement
 

I just bought these bearing from xcioy a month ago.
https://cimg5.ibsrv.net/gimg/www.rcu...92199961f5.jpg
https://cimg6.ibsrv.net/gimg/www.rcu...4d5f1f53b2.jpg
https://cimg7.ibsrv.net/gimg/www.rcu...2547c07313.jpg

Yes i know Gaspar, he is very helpful and straight person.
If i decide to finally fit the GRW bearings after testing and hopefully surviving the tests i am going to buy them from him, did those bearings worked well?
So far i haven't found 8x22x7 angular bearings with brass cage it looks like a special order or a Unicorn but i also emailed AMT in case they have them.
Chris

actually I never have a smallest problem with my jetjoe 1400 turbine specially after making my own program they usually sales with blank or not complete fadek program I’m pretty sure that this turbine still working perfect with the jet that I sold with , at the beginning of those turbines sales in USA not too many people knew about making the new program and they usually ended with blowtorch instead of a good working turbine, I’m pretty sure that if someone with better knowledge will take over the USA sales back days this could turn to today Kingtech class brand, I have like 3 of them and all of those turbines have a perfect working history.

You should have seen inside mine, screws with damaged threads, threads were stripped, the NGV was out of true etc.
It was like i bought a used one, i am talking about what i found the first time i stripped it down.
When i contacted Jetjoe i only got a cold answer. when i compared it to a used one i got (TREFZ TT66 from Germany) it was like i was comparing a Rolls-Royce with a Lada.
I decided that it was not worth it so i accepted my loss and moved on.
Btw i am about to buy 2 ANGULAR CONTACT bearings with brass cage and silicon nitride balls directly from AMT...

https://cimg2.ibsrv.net/gimg/www.rcu...5ad9b792e3.jpg

Not come across these in Model turbines yet. Are they 608 size ? How much do these go for ?

Cheers

Yes they are 8x22x7 mm
I am waiting a reply for the cost and if those are good at least for testing in a KJ66 turbine.
As soon as i get the response i will post it here.
Chris

AMT Turbine arent cheap, I suspect these bearings are not either however at least you can rest assured when it comes to quality for sure.

We will see!
I am waiting for their reply which probably will come tomorrow.
I will try one at least on the hot side!
Chris

OK the bearings cost 30 Euros each plus shipping, they are rated to 140,000 rpm and their precision is ABEC7
I will order two for testing them.
Chris.

I also found those from the manufacturer but no way to order them.
Notice the rpm limit and the wider ball grove.
Attachment 2271046

There was a lot of diy articles in Jetpower magazine, back in the days. They showed how to make hybrid bearings, from regular 608, and filling them up with balls from cheap ceramic bearings. The GRW bearings doesn't look a lot deeper than an ordinary deep groove bearing, with one of the shoulders turned down. Axial forces is not that big.

Now if i tell you that i also did exactly the same by using regular 608 angular contact bearings and buying ceramic balls from Alixpress?
I didn't test them but they should be fine!