|
Post by racket on Oct 22, 2020 16:05:54 GMT -5
Hi Patty
What was the ID of the "bearing spacer" ??
Did you check the squareness of the bearing seat at the turbine??
I think you'll need to balance your rotor to a finer limit , Smithy use to balance mine to very low limits of ~4 mg/in on the comp and ~14mg/in on the turb , though the Schenck guy recommended a 35mg/in on a 2" grind radius as suitable for our sized rotors when spun at between 1200 - 1700 rpm to keep it out of any resonance.................Standard Garrett balance with brass bush bearing for our size rotors would be ~50mg/in minimum.
Any inbalance is only going to exacerbate the resonance issues.
Cheers John
|
|
|
Post by madpatty on Oct 22, 2020 17:44:24 GMT -5
Patty, as John said he used a "poor man's squeeze film damper" on the outer race of his bearings. The best squeeze film dampers are sealed on both sides with oil injected between the seals and drained from the opposite side. The seals can be O-rings preferably or piston rings. The O-rings give a square pressure profile while the piston rings only have a peak pressure in the middle due to leakage on both sides. The unsealed damper on the other race does help to get through the critical speeds. Thanks, Ron Hi Ron. I am planning on integrating “squeeze film dampers” at some point in my current engine but again that will require some major redesign to my shaft tunnel. Whilst looking at Ball bearing turbos, they don’t have any O-ring or piston seal on the outer race of their bearings. They just have a clearance fit between the outer ring of the cartridge and the CHRA body(much like a clearance between the journal bearing and chra) where an oil pressure is maintained. What do you think about THAT kind of squeeze film damper? Regards. Patty
|
|
|
Post by madpatty on Oct 22, 2020 17:50:05 GMT -5
Hi Patty What was the ID of the "bearing spacer" ?? Did you check the squareness of the bearing seat at the turbine?? I think you'll need to balance your rotor to a finer limit , Smithy use to balance mine to very low limits of ~4 mg/in on the comp and ~14mg/in on the turb , though the Schenck guy recommended a 35mg/in on a 2" grind radius as suitable for our sized rotors when spun at between 1200 - 1700 rpm to keep it out of any resonance.................Standard Garrett balance with brass bush bearing for our size rotors would be ~50mg/in minimum. Any inbalance is only going to exacerbate the resonance issues. Cheers John Hi Racket, The ID of the bearing spacer was same as the shaft OD, 16mm(sliding fit). Yup Turbine side bearing seat’s squareness was checked. Regards. Patty
|
|
|
Post by madpatty on Oct 22, 2020 18:19:50 GMT -5
Hi Patty I think you'll need to balance your rotor to a finer limit , Smithy use to balance mine to very low limits of ~4 mg/in on the comp and ~14mg/in on the turb , though the Schenck guy recommended a 35mg/in on a 2" grind radius as suitable for our sized rotors when spun at between 1200 - 1700 rpm to keep it out of any resonance.................Standard Garrett balance with brass bush bearing for our size rotors would be ~50mg/in minimum. Any inbalance is only going to exacerbate the resonance issues. Cheers John Hi John. These unbalance numbers always seem to confuse me. Usually any unbalance tolerance is given in g.mm ( gram x millimetres) or oz . Inch so when you say ~4 mg/in on the comp and ~14mg/in on the turbine does it mean 14mg at 1 inch of grinding radius which will thus mean 14mg at 25.4mm equating to (14 x 25.4/1000) = 0.36g .mm Also the Schneck guy recommendation is even more confusing to me. 35mg/inch at 2” radius will be 70mg of total unbalance at 2”(50.8mm) radius. This total unbalance of (70mg/1000g x 50.8mm) = 3.556 g.mm Now the Holset service manual that I have specifies maximum tolerance of 7.2 g.mm for turbine end and 12.3 g.mm for compressor end for journal bearing operation.which(if I am getting it all right) means at 1inch radius we can have(7.2 g x 1000mg)/(1inch x 25.4mm)) = 283.5 mg at 1” radius ( or 283.5mg/inch) which is like 6 times what Garrett recommended. I think I am getting it wrong somewhere maybe? Regards. Patty
|
|
|
Post by racket on Oct 22, 2020 19:40:39 GMT -5
Hi Patty
I mentioned 50mg/in MINIMUM , in other words it'll generally be far greater than this, maybe up to your 283mg/in , turbocharger balance is very crude with brass bushes , generally a quick grind of a turbine blade tip on the workshop grinder , Smithy balanced my rotors to half the Allison C20 compressor balance limit , they were super finely balanced , .........thanks Smithy :-)
But with ball bearings the crude turbo balance limits aren't applicable , it needs to be much more finely balanced unless theres "flexibility" built into the bearing mounting , eg , oil squeeze or O'ring.
Cheers John
|
|
|
Post by finiteparts on Oct 22, 2020 21:59:21 GMT -5
Hi Patty,
I struggle to believe that such bearing damage could be done due to imbalance when you are only getting to 24000 rpm. My personal belief is that the damage is due to angular misalignment or centerline offset.
Looking close at the bearing race it is hard to determine if there is spallation. It appears that there might be a spot of spallation the second photo (right hand side of the melt zone). If this is indeed spallation, then the contact stresses on the inner race must be very significant and the only reason that I can think of that would drive such stresses is misalignment or centerline offset.
I cannot determine from the photos how well the shaft has been machined, but it does not appear to have been centerless ground. My concern is that the shaft may have been machined in such a way that you have a centerline offset or even a bearing centerline angular misalignment between the forward and aft bearings, which would cause the bearing internal radial clearance to close up (and then ultimately thermal lock-up).
Turbocharger shafts are machined to very tight tolerances by centerless grinding. When they use journal bearings, they can handle much larger bearing centerline deviations. Ball bearings require much tighter control of the shaft to bearing centerlines. You need to check your bearings alignment, front to back, and make sure that when they are installed, the centerlines are within a very tight tolerance (+/- 0.0005 in) and collinear. If you can measure any eccentricity, I can help you determine what those imbalance loads might be. At 24krpm, with a large eccentric mass (1 oz-in), the imbalance force is less than 4 lbf, which doesn't seem like enough to cause the damage that you are seeing.
My second thought is that the aft bearing is conducting too much heat from the turbine wheel, which could also lead to thermal lock-up. Do you have any pictures of the aft bearing mounted on the shaft from the side?
I do agree that it is a weird wear pattern. I would have expected to see two or more spots. I can only assume that there was some severe inner race contact which led to a local spall. As the spall progressed and fed contamination into the preceding contact areas, the contaminants are churned up and melted. Maybe...
Good luck!
Chris
|
|
|
Post by racket on Oct 23, 2020 0:27:07 GMT -5
Hi Patty
I've been looking through my FM-1 journal and all its "disasters" , but most were up ~50,000 rpm , my first spoolup used standard bearings and ran at 35,000 rpm .
Then I started making up hybrid bearings with SiN balls , the first "malfunction" was at 48K when the preload spring jammed , for the next test with another shaft I added extra preload to ~25 lbs
A month later another failure but I had severe heating of the raceway area , again ~50K before bearing rumbling .
After another failure I found the spacer sleeve ends weren't quite parallel so it was fitted to a mandril and ends skimmed .
Another failure with severe heat marking near turb hub/bearing/shaft , hot gases getting down "front" of wheel ....................we MUST have bleed air across the NGV to cool the hub area and prevent hot gases getting near the bearing.
Another test another bearing problem caused by too much lubrication causing ball skidding .
And the list went on and on and on ........until I started using the Allison C20 ball races in bronze cups with oiljet lube.
As Chris said , 24,000 rpm is a modest speed for such damage ,..........theres very little air pressure axial loads and even a fair bit of inbalance should be handled by a bearing of that size , I'd be taking Chris' advice and check alignment and squareness of everything .............sumthin' ain't right :-(
Cheers John
|
|
|
Post by madpatty on Oct 23, 2020 8:19:29 GMT -5
Hi Chris and John.
Thanks for your valuable inputs. If you think a normal unbalance cannot produce such a catastrophic failure every-time, that too at a low rpm of 24k then definitely we should be looking somewhere else.
I always thought these shafts are ground between centers and that's why those machined centers on either side of the turbine shaft and the inconel wheel are there for. I also read somewhere that these centers in the nose of the turbine wheel are located at the centre of mass of the wheel and not the geometric centre.
If I may ask, what do you think should be the best way to check for any misalignment or eccentricity in the shaft? I have some basic tools only like a lathe and a dial gauge which may or may not be able to measure this angular misalignment or centreline offset.
I did a small checkup last time when this thought that this shaft maybe bent crossed my mind. I held the shaft between the 4 jaw chuck and the tailstock dead centre(turbine wheel's outer 'hex nut like' portion held between the jaws and the machined centre on the compressor side of the shaft in the tailstock dead centre). I then Dialed the shaft to near zero on the turbine end (jaw side of lathe) and then checked for any "bend" in the shaft at various points across the length of the shaft.
All the points were within 0.01-0.02mm.
what should be the other way of doing it for checking all angular alignments and collinearity. Should I be holding the turbine shaft between it's factory machined centers on both sides.(By machining a dead centre in the lath first and a dead centre on the tailstock length). I can also grind the shaft between its factory machined centre so that everything is true and straight between the factory machined centers.
What you guys think? I need your suggestions on how this system should be checked for trueness and how further improvements can be made in case we need to be sure there isn't any misalignment.
Regards.
|
|
|
Post by jetjeff on Oct 23, 2020 13:49:30 GMT -5
Hi Patty,
Supporting the shaft on V blocks at the bearing journal and checking the center of the shaft with a dial indicator, is one way. If you don't have the V blocks a local machine shop should be able to check it for no cost.
Regards
Jeff
|
|
|
Post by turboron on Oct 23, 2020 15:53:42 GMT -5
Patty, a circumferential channel in the housing to feed the "squeeze film damper" would be an improvement. The channel width should be 3 or 4 times its depth.
Your failure reminds me of a engine we destroyed by locking the rotors together. When we tried to force it through the critical it broke the engine mounts in less them 15 seconds. Seem familiar? I think that your maybe due to rotor dynamics.
Thanks, Ron
|
|
|
Post by racket on Oct 23, 2020 17:10:12 GMT -5
Hi Patty Maybe we should also check the shaft tunnel for trueness, you said it'd been CNC'ed , but mistakes do happen :-( I had to do this with my 12/118 shaft tunnel , pics half way down the page jetandturbineowners.proboards.com/thread/612/fat-boy-118-thrust-engine?page=6 . It requires machining up a solid jig in the lathe and without removing the jig, mounting the shaft tunnel in various positions ( front /back) using the bearing bore/s to position the tunnel radially and the tunnel end face for any "tilt" Cheers John
|
|
|
Post by racket on Oct 23, 2020 19:53:32 GMT -5
Hi Patty
Could you please draw up a diagram of your bearing mounting and preload spring arrangement , I need to know which way the bearings are mounted and their respective load surfaces as I can't seem to understand your explanation.
Also what sort of "endfloat" distance is available
Cheers John
|
|
|
Post by racket on Oct 23, 2020 20:09:15 GMT -5
|
|
|
Post by madpatty on Oct 23, 2020 21:24:57 GMT -5
Hi Patty Could you please draw up a diagram of your bearing mounting and preload spring arrangement , I need to know which way the bearings are mounted and their respective load surfaces as I can't seem to understand your explanation. Also what sort of "endfloat" distance is available Cheers John Hi Racket. I have attached very crude drawing pics which I made long back of the bearing arrangement in this engine when I originally bought it. I carried the same shaft tunnel design to my new redesign engine. LEFT side in both pics is the compressor side. Regards. Patty
|
|
|
Post by madpatty on Oct 23, 2020 21:33:51 GMT -5
Hi Racket. This is the front bearing that was damaged in one testing. That’s the only compressor wheel bearing damage that I had (possibly due to out of balance compressor wheel which once it was balanced the bearing stopped failing) MAYBE. The 6 rectangular slots are machined in the shaft tunnel which are aligned with 6 holes drilled at an angle in the diffuser which take air from behind the diffuser (casing area) to feed into the shaft tunnel. There’s a small plate that goes on top of it bolted to the shaft tunnel (through the diffuser, 1 empty bolt hole is visible in bottom right of the picture). The golden debris is when the compressor bearing failed, the brass spacer contacted that mild steel plate. I hope it all makes some sense. Regards. Patty
|
|