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Post by finiteparts on Dec 9, 2018 22:44:01 GMT -5
Hi Anders,
Sorry to hear about you engine trouble.
I have a theory that I was thinking about when you said that you had dropping oil pressure and now that you have experienced this event, it may make more sense.
So initially, I was thinking that if your pump had good clearances, then why would the oil pressure drop over time? As already stated, there could have been a change in the oil viscosity due to temperature or aeration...and this would change the pressure required to push the same flow rate through the system. But there is another way the system pressure would be modified and this is if the controlling orifice area changed.
My thought is that the controlling area of the oil system is at the bearings and it is this area that is the primary driver on the system pressure requirements. How might this controlling area change? When you look at your center housing, it is in almost direct thermal contact with the hot gas flow. The NGV is bolted directly to the aluminum center bearing housing so the high heat transfer that occurs on the NGVs can conduct very quickly to the bearing area. Also, the center housing is essentially wrapped with the combustor that can radiate a lot of heat to the bearing housing very readily. The end result of this is that the bearing housing likely comes up in temperature very quickly.
If the center housing increases in temperature by 300 F on the aft bearing end, and we assume a typical thermal expansion coefficient of 12.4 in/in*F for a cast Al, then the bearing bore opens up by 0.0045 in. If the bearing housing increased by 400 F, then the bearing housing bore diameter would increase by 0.006 in/in*F.
Most turbocharger manufacturers use cast iron for the center housings...cast iron has half the thermal expansion that cast Aluminum does...they also take great pains to isolate the aft bearing boss from the turbine heat. This includes using heat shields and dead air spaces, they also cast large cavities in the turbine side of the housing to allow the oil to carry away the heat before it can be conducted through the metal around to the bearing bore diameter...in fact, Garrett likes to add and oil jet that sprays on the inner surface of the turbine end of the housing for the sole purpose of carrying out some turbine end heat. Additionally, in general, cast iron has half the thermal conductivity of cast Al, depending on the specific flavor of cast iron or cast Al, so the ability of the heat to move from the NGV plate into the bearing housing has been enhanced by the use of a cast Al for the bearing housing. By design, the standard turbocharger center housing would keep the bearing bore cooler, but for the sake of arguement, let's say that the cast iron housing increased in temperature by the same 300 and 400 deg F that we used for the previous discussion on the cast Al bearing housing...this time the cast iron housing would only increase by 0.002 and .003 inch respectively...i.e. half the thermal growth.
This was my concern prior to you having the most recent event...I was concerned that the use of the cast Al bearing housing would have higher bearing clearance growth as compared to the original cast iron turbocharger housing that had more features designed into it to minimize the heat transfer from the turbine gas to the aft bearing bore. My original concern was that this increase in clearance would lead to reduced oil pressure for a given flow rate as the engine heated up. But in light of you most recent event, I do wonder if the engine may have experienced an increase in rotor orbit or even a rotor whirl instability due to an increase in outer bearing clearance. An increase in outer bearing clearance would reduce the bearing stiffness and as such the rotor orbit due to residual imbalance would increase in amplitude. If the amplitude was sufficiently large, the compressor could have rubbed the housing.
This is just an idea I was thinking about and I thought I would share. If this is the case, then it might be possible to increase the oil flow in the bearing housing near the turbine end to combat the heat transfer from the NGV plate...or maybe you could use a low conductivity material as a gasket between the NGV plate and bearing housing to "break" the thermal conductivity.
Good luck!
Chris
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Post by racket on Dec 9, 2018 23:04:40 GMT -5
Hi Monty
Anders and I were thinking more about the oil slinger and piston ring holder components when contemplating if oil on them during assembly might cause problems , they're ground surfaces that are simply clamped together by the comp wheel being forced up against them by its nut , hopefully the Locktite will provide sufficient "lubrication" during the tightening of the nut to give us the desired stretch in the quill, and once it "goes off" does produces a join that needs a lotta torque to undo , I generally heat the nut before disassembly to soften the glue, but those nice shiny ground parts will slide pretty easily against each other and the surface area at the end of the piston ring holder is pretty small , I'd think its the "weak link" when it comes to its ability to provent spinning between parts , mmmmmm, maybe a lap together with some fine grinding paste to "roughen" the surfaces might be in order .
Cheers
John
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monty
Senior Member
Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Dec 10, 2018 0:45:54 GMT -5
Hi Monty Anders and I were thinking more about the oil slinger and piston ring holder components when contemplating if oil on them during assembly might cause problems , they're ground surfaces that are simply clamped together by the comp wheel being forced up against them by its nut , hopefully the Locktite will provide sufficient "lubrication" during the tightening of the nut to give us the desired stretch in the quill, and once it "goes off" does produces a join that needs a lotta torque to undo , I generally heat the nut before disassembly to soften the glue, but those nice shiny ground parts will slide pretty easily against each other and the surface area at the end of the piston ring holder is pretty small , I'd think its the "weak link" when it comes to its ability to provent spinning between parts , mmmmmm, maybe a lap together with some fine grinding paste to "roughen" the surfaces might be in order . Cheers John John,
Ahh yes....silly me. I see now. Let's run some numbers. What is the OD of the piston ring holder where it interfaces the comp wheel, and the pitch of the thread on the nut? I failed to measure all that before I sent it off to you.
Monty
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Post by Johansson on Dec 10, 2018 3:16:05 GMT -5
Hi guys!
Thank you all for your posts, I'll digest them and sit down and answer them as soon as I find a moment.
Right now it feels like the nut should be able to do its job unless the comp is touching the cover, I dont remember the clearances but they were following Johns recommendation but not excessive. Might be a low spot that I missed somewhere around the cover, hard to tell now.
So, adding a keyway, locking washers or any other locking contraption will be a last resort since it might produce some other cause for problems. A thorough clearance check/adjustment for the compressor, a good clean of the rotary parts behind the compressor wheel, a bit more torque on the nut and I think I am fine.
Since John stated that I am already running in the 70.000rpm region I don't need to go much further, so if I can make the nut sit a little bit tighter I am all good.
Cheers!
/Anders
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Post by racket on Dec 10, 2018 3:20:12 GMT -5
Hi Monty
The piston ring holder at the end that sits inside the oil slinger only 17 mm OD with an ~11mm bore , not much metal , the thread is 20 TPI 7/16 UNF .
What I might do is rig up some sort of jig using a dead turb wheel and see how many foot pounds is required to "spin" a securely tightened "comp" off.
Cheers
John
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Post by jetjeff on Dec 10, 2018 4:13:18 GMT -5
Hi Anders,
Looking at page 14 of this build the compressor securing nut looks like it's right hand threaded, if so it's wrong,,,,it needs to be left hand thread if your compressor turns clockwise.
Regards
Jeff
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Post by Johansson on Dec 10, 2018 6:43:44 GMT -5
Yep, we always lubed the threads and the nut contact face on the C20 and C30 comp nuts during assembly, it's in the latest RR manual. Anders, I wonder if there's a "transient imbalance" which may have had the comp wheel touching the inlet housing at certain rpms and then causing the nut to loosen. We often noticed a balance issue at certain RPM algorithms on the balance machines. Some of the manufacturers mention to not run constantly at certain rpm's....and to either stay under or go right through to a higher rpm. Doesn't take much for the shaft to "bend" enough to cause grief. Smithy. Hi Smithy, That might be a reason for this, it might pay off to open up the exducer clearance 0.1-0.2mm just to make sure that any vibrations in the compressor won´t make the blades touch the housing. If I can give a suggestion... To clean the aluminum deposit on the shaft, you can put it into sodium hydroxide solution. Aluminum are destroyed by it, steel are only deeply cleaned. If the shaft isn't damaged or bent, but the problem it is only the aluminum deposit, it can solve the problem. Be careful to wash the shaft accurately after the treatment, and oil it to avoid oxidation. Can't you add a second nut after the main one? It is a simple way to avoid loosening. For sure it add some weight, but not too much, expecially if you use a low profile nut. Hope this helps. Davide Hi Davide, Good idea, I took the rotor with me to work today and used a cleaning liquid known to eat away aluminum. It is boiling as we speak and I can clearly see aluminum disappearing. Thanks for the idea! A second nut is certainly an option, but as stated in the previous post a single nut should be enough if the comp isn´t touching the cover and everything is torqued properly. I don´t want to add anything to the rotor unless absolutely necessary to avoid upsetting the dynamics. Anders, a couple of points on centrifugal compressors based on my experience: 1) All centrifugal compressor outside diameters moved forward toward the stationary shroud/scroll during operation. This is confirmed by finite element analysis and observation. This is one of the reasons aircraft gas turbine engines use abradable coatings on the shroud/scroll. 2) Adding a bore to the center of the impeller for a shaft doubles the stress and deflection compared to a impeller without a bore. These factors point to a couple of possible reasons for your nut coming loose. The first is that the impeller to shroud/scroll was not sufficient to prevent contact. The second is that the material strength of the billet impeller was not to specification. You may want to check the impeller bore to see if it has opened up. Thanks, Ron Hi Ron, John has a theory about the engine cover flexing from air loads which pulls the rim of the cover back effetively closing the clearance gap, this theory in combination with what you say about compressor exducers moving forward sort of indicates that the tip clearance might have to be a bit larger than it is now. The bore is scrap so no way of measuring it now. CRAP!! Anders, I was so excited to see your post, and then my heart sank with that flame out the back... It looked like you could see aluminum filings flying around in the air right before the "big" event. How rigid is the area where the front bearing is located/supported? could that have flexed, and allowed the wheel to contact the front shroud? If you had good oil pressure everything should be fine, unless the actual bearing support area flexed and allowed the wheel to move forward. Once that wheel contacts the housing, all bets are off with a right hand nut. How do the thrust washers between the circlips and bushings look? How does the front face of the turbine facing the NGV look? Anders => glad you fixed the oil pressure issue, sorry about the difficulties. Monty Hi Monty, The thrust washer seat is very rigid, it cannot flex unless the whole casting has broken and I don´t think that is the case. I´ll disassemble the engine core tonight or tomorrow and check the bearings closely but I cannot feel any excessive axial play in the thrust bearing so I think it is ok. Cheers! /Anders |
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Post by Johansson on Dec 10, 2018 6:48:06 GMT -5
Hi Anders, Sorry to hear about you engine trouble. I have a theory that I was thinking about when you said that you had dropping oil pressure and now that you have experienced this event, it may make more sense. So initially, I was thinking that if your pump had good clearances, then why would the oil pressure drop over time? As already stated, there could have been a change in the oil viscosity due to temperature or aeration...and this would change the pressure required to push the same flow rate through the system. But there is another way the system pressure would be modified and this is if the controlling orifice area changed. My thought is that the controlling area of the oil system is at the bearings and it is this area that is the primary driver on the system pressure requirements. How might this controlling area change? When you look at your center housing, it is in almost direct thermal contact with the hot gas flow. The NGV is bolted directly to the aluminum center bearing housing so the high heat transfer that occurs on the NGVs can conduct very quickly to the bearing area. Also, the center housing is essentially wrapped with the combustor that can radiate a lot of heat to the bearing housing very readily. The end result of this is that the bearing housing likely comes up in temperature very quickly. If the center housing increases in temperature by 300 F on the aft bearing end, and we assume a typical thermal expansion coefficient of 12.4 in/in*F for a cast Al, then the bearing bore opens up by 0.0045 in. If the bearing housing increased by 400 F, then the bearing housing bore diameter would increase by 0.006 in/in*F. Most turbocharger manufacturers use cast iron for the center housings...cast iron has half the thermal expansion that cast Aluminum does...they also take great pains to isolate the aft bearing boss from the turbine heat. This includes using heat shields and dead air spaces, they also cast large cavities in the turbine side of the housing to allow the oil to carry away the heat before it can be conducted through the metal around to the bearing bore diameter...in fact, Garrett likes to add and oil jet that sprays on the inner surface of the turbine end of the housing for the sole purpose of carrying out some turbine end heat. Additionally, in general, cast iron has half the thermal conductivity of cast Al, depending on the specific flavor of cast iron or cast Al, so the ability of the heat to move from the NGV plate into the bearing housing has been enhanced by the use of a cast Al for the bearing housing. By design, the standard turbocharger center housing would keep the bearing bore cooler, but for the sake of arguement, let's say that the cast iron housing increased in temperature by the same 300 and 400 deg F that we used for the previous discussion on the cast Al bearing housing...this time the cast iron housing would only increase by 0.002 and .003 inch respectively...i.e. half the thermal growth. This was my concern prior to you having the most recent event...I was concerned that the use of the cast Al bearing housing would have higher bearing clearance growth as compared to the original cast iron turbocharger housing that had more features designed into it to minimize the heat transfer from the turbine gas to the aft bearing bore. My original concern was that this increase in clearance would lead to reduced oil pressure for a given flow rate as the engine heated up. But in light of you most recent event, I do wonder if the engine may have experienced an increase in rotor orbit or even a rotor whirl instability due to an increase in outer bearing clearance. An increase in outer bearing clearance would reduce the bearing stiffness and as such the rotor orbit due to residual imbalance would increase in amplitude. If the amplitude was sufficiently large, the compressor could have rubbed the housing. This is just an idea I was thinking about and I thought I would share. If this is the case, then it might be possible to increase the oil flow in the bearing housing near the turbine end to combat the heat transfer from the NGV plate...or maybe you could use a low conductivity material as a gasket between the NGV plate and bearing housing to "break" the thermal conductivity. Good luck! Chris Hi Chris, That is an interesting theory, in combination with a tight compressor clearance this bearing bore growth might be enough to make the blades touch the cover and spin the nut off the shaft. An aluminum shaft tunnel has sucessfully been used on JU-01 and numerous other engines, so with the 24V pump feed I will have more than enough oil flow to cope with a bit increased flow through the bearings. Hi Anders, Looking at page 14 of this build the compressor securing nut looks like it's right hand threaded, if so it's wrong,,,,it needs to be left hand thread if your compressor turns clockwise. Regards Jeff Hi Jeff, Yup, pretty darn stupid if you ask me. I have not thought a bout this before now but it feels natural to have a left hand thread on this rotor. |
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elventu
Veteran Member
Joined: October 2018
Posts: 122
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Post by elventu on Dec 10, 2018 6:56:22 GMT -5
I'm happy that you can solve the issue on the shaft chemically dissolving aluminum.
From my experience it is a viable option, excluding in the case of some anticorodal alloys like 6068/6082, that it is a lot more resistant to sodium hydroxide.
I tried this system after making some experience doing bottle explosion 😂
We use some alu chips playing on it in the job parking -with the employer-, and we see that the anticorodal one aren't good for the reaction.
I used the system to save my air compressor crankshaft, I had for free after the previous owner forgot to put oil in it...
I pull the connecting rods out of the crankshaft with a press, and after the treatment the crankshaft were still intact.
After pushing a machined brass bushing in the connecting rods to save them I have a fully functional twin cylinder compressor fot my workshop 😁
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Post by turboron on Dec 10, 2018 8:32:55 GMT -5
Anders, as the compressor exducer moves toward the shroud/scroll under load the heel of the compressor bore on the turbine side opens up. I have seen industrial compressor bores with steel shafts and impellers that run a long time with rust in this area. All this is to say that the compressor exducer to shroud/scroll clearance can close on the order of 0.010" at high loads. Compressor efficiency drops approximately 1% for each 0.001" cold clearance increase. Compressor efficiency is probably not a key parameter for your application.
Thanks, Ron
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monty
Senior Member
Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Dec 10, 2018 9:40:08 GMT -5
Hi Monty The piston ring holder at the end that sits inside the oil slinger only 17 mm OD with an ~11mm bore , not much metal , the thread is 20 TPI 7/16 UNF . What I might do is rig up some sort of jig using a dead turb wheel and see how many foot pounds is required to "spin" a securely tightened "comp" off. Cheers John John,
I just did a quick calculation. With that thread pitch 130 degrees of turn gives .01805 in of stretch to the quill. Using
F=(E*A*DeltaL)/L
I get about 1250 lbf for clamping load.
Assuming a coefficient of friction of about .6 for aluminum on steel and about a 0.2767 mean radius for the seal holder works out to about 17 ft lb. Assuming that you get about the same from the nut.....34 ft lbf of torque transfer capability.
Not a lot of margin there. Lapping is not a bad idea...
Monty
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Post by Johansson on Dec 10, 2018 9:47:40 GMT -5
Hi Monty The piston ring holder at the end that sits inside the oil slinger only 17 mm OD with an ~11mm bore , not much metal , the thread is 20 TPI 7/16 UNF . What I might do is rig up some sort of jig using a dead turb wheel and see how many foot pounds is required to "spin" a securely tightened "comp" off. Cheers John John, I just did a quick calculation. With that thread pitch 130 degrees of turn gives .01805 in of stretch to the quill. Using
F=(E*A*DeltaL)/L
I get about 1250 lbf for clamping load.
Assuming a coefficient of friction of about .6 for aluminum on steel and about a 0.2767 mean radius for the seal holder works out to about 17 ft lb. Assuming that you get about the same from the nut.....34 ft lbf of torque transfer capability. Not a lot of margin there. Lapping is not a bad idea... Monty
Hi Monty, Ouch, that is a bit too close for my taste. How much more do I have to torque the nut to get enough clamping force? What would 140 degrees produce? /Anders |
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monty
Senior Member
Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Dec 10, 2018 10:00:54 GMT -5
Hi Monty, Ouch, that is a bit too close for my taste. How much more do I have to torque the nut to get enough clamping force? What would 140 degrees produce? /Anders Anders,
Let's not panic just yet. I made quite a few assumptions. It would be good to do like John suggested to double check. One experiment is worth a thousand calculations. I would also like to double check my calculations.
If they prove to be correct, and John's experiment confirms the problem:
I would lean towards using a larger diameter seal holder. Increasing the mean radius of contact is the best way to get a higher torque transfer capacity. That, and perhaps a machine washer under the nut on the compressor face to accomplish the same thing.
Something doesn't add up though, because lots of turbos use similar arrangements with no problem.
Monty
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Post by turboron on Dec 10, 2018 13:57:44 GMT -5
Anders, I need to add to my previous post about the compressor impeller heel lifting off under load. As a result, the toe of the compressor clamps the shaft which helps take the aero torque in addition to the nut clamp force. I used this knowledge to build a rotor dynamics rig using asymmetrical flywheels that could be easily assembled with a slip fit between the flywheel and the shaft. The design worked great.
Thanks, Ron
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Post by Johansson on Dec 10, 2018 14:51:44 GMT -5
Anders, Let's not panic just yet. I made quite a few assumptions. It would be good to do like John suggested to double check. One experiment is worth a thousand calculations. I would also like to double check my calculations.
If they prove to be correct, and John's experiment confirms the problem:
I would lean towards using a larger diameter seal holder. Increasing the mean radius of contact is the best way to get a higher torque transfer capacity. That, and perhaps a machine washer under the nut on the compressor face to accomplish the same thing.
Something doesn't add up though, because lots of turbos use similar arrangements with no problem.
Monty
I totally agree with the last bit, it is designed to work and there are thousands of TV94 rotors out there spinning at top revs all around the world. It should be doable even for me, especially if I add a little extra torque to the nut to compensate for the larger compressor wheel. Anders, I need to add to my previous post about the compressor impeller heel lifting off under load. As a result, the toe of the compressor clamps the shaft which helps take the aero torque in addition to the nut clamp force. I used this knowledge to build a rotor dynamics rig using asymmetrical flywheels that could be easily assembled with a slip fit between the flywheel and the shaft. The design worked great. Thanks, Ron Ron, I get the feeling that you have lived a very interesting life and worked with lots of fashinating stuff. Far more interesting than shoveling dirt and repairing worn out conveyors, that is for sure.😁 |
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