gidge348
Senior Member
Joined: September 2010
Posts: 426
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Post by gidge348 on Jul 27, 2014 20:44:08 GMT -5
Bugga
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Post by finiteparts on Jul 27, 2014 21:39:58 GMT -5
Anders,
Besides the melted cage, do the races look heated, fretted or spalled?
Just an idea, but do you think that you might need to get more air than what leaks through from the oil mister, into the cavity behind the turbine? If you have a dead region where the exhaust gases can hang out just behind the turbine, it might be tough to keep the bearing temps down. A lot of older engines used a leakage path from the outside arrange such that the flow momentum would draft air through and into the main gas flow. That might be tough to do here, but you could tap some of the compressor discharge air through a pipe to set up a positive pressure region and thus keep the hot gas away from the outlet shaft area...you would then only be battling the heat conduction through the shaft. If you had the compressor discharge air impinge on the shaft, you might be able to reduce the heat flow conducted through the shaft and then to the bearing.
Another question...if you are dribbling the oil flow, do you think that it will get to the inner race? Usually, the oil jet is directed to the inner race because centrifugal forces work against the oil actually getting in there. Just looking at the pictures, I can still see a nice chrome appearance on the inside of the inner race, which makes me think it is not getting too hot. How does it look on the ball race area?
Also, I was looking over your build of the exhaust ducting and I am still wondering if the area ratio of your exit might be contributing to the higher temps. Because you have the cone in the transition area, you might check the area at the minimum plane in there to make sure you are not constricting the flow. It appears from the pictures that the min area might be formed from a line 90 degrees to the cone surface extended up to the cylindrical section just aft of the turbine exit plane multiplied by the length of the exhaust ducts. Its just like the exhaust cones on the turbojet, but now you have much less pressure ratio to push the exhaust out from behind the power turbine. Additionally, the sharp 90 degree turn could be creating large flow separation regions...you could check for them by smearing a little oil on the ducts and watching to see if it gets "blown" off or if it creates regions of recirculation.
Can't wait to see you up an going again! ~ Chris
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Post by pictsidhe on Jul 28, 2014 11:54:27 GMT -5
Just caught up on this. I bet there are many people in your locality who think you are nuts, tests rides will enhance that; keep it up! Most of my thoughts have already been voiced by others, but that gearbox bearing is probably also going to suffer from heat soak after you've shut down after a run. Ordinary bearings lose their temper at 150C. As well as keeping temps under that while running (could you get a thermocouple into the housing near it?) some good post run cooling seems prudent if you don't want to keep swapping bearings. How much shaft and of what material do you have between the turbine rotor and the bearing?
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Post by pictsidhe on Jul 28, 2014 11:57:16 GMT -5
Some oil jets onto the shaft behind that cooking bearing could be helpful for heatsoak.
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Post by Johansson on Jul 28, 2014 16:56:32 GMT -5
Hi Anders Whats the oil flow rate into the bearing ?? Interesting how the "curvic" lugs on your coupling piece seems to be running cooler than the section closer to the gearbox ,........ is there a chance that the coupling was rubbing something , does it need some cooling air in that region to stop hot exhaust gases heating things up ? How do the gears look after their run ?? Cheers John I haven´t measured the oil flow but I can see oil flowing at the bottom of the pneumatic hose from the oil dribbler into the gearbox so it shouldn´t be too little at least. There might be a chance that the narrow gap seal clogged up with burned oil, it had lots of deposits on its sealing surface when I disassembled it. The main reason for that was because I used regular gear oil used in lathes and mill gearboxes so it isn´t supposed to handle temps very well. I have several cans of Mobil Jet2 turbine oil that should be much better for this. I could close off the gearbox vent in the filler neck to force all bleed air through the narrow gap seal and open the seal area up somewhat to make sure it won´t rub the shaft, that should increase the cooling of the front bearing a fair bit. The gears look just fine, no signs of wear other than that I can see where the teeths have engaged each other. Anders, Besides the melted cage, do the races look heated, fretted or spalled? Just an idea, but do you think that you might need to get more air than what leaks through from the oil mister, into the cavity behind the turbine? If you have a dead region where the exhaust gases can hang out just behind the turbine, it might be tough to keep the bearing temps down. A lot of older engines used a leakage path from the outside arrange such that the flow momentum would draft air through and into the main gas flow. That might be tough to do here, but you could tap some of the compressor discharge air through a pipe to set up a positive pressure region and thus keep the hot gas away from the outlet shaft area...you would then only be battling the heat conduction through the shaft. If you had the compressor discharge air impinge on the shaft, you might be able to reduce the heat flow conducted through the shaft and then to the bearing. Another question...if you are dribbling the oil flow, do you think that it will get to the inner race? Usually, the oil jet is directed to the inner race because centrifugal forces work against the oil actually getting in there. Just looking at the pictures, I can still see a nice chrome appearance on the inside of the inner race, which makes me think it is not getting too hot. How does it look on the ball race area? Also, I was looking over your build of the exhaust ducting and I am still wondering if the area ratio of your exit might be contributing to the higher temps. Because you have the cone in the transition area, you might check the area at the minimum plane in there to make sure you are not constricting the flow. It appears from the pictures that the min area might be formed from a line 90 degrees to the cone surface extended up to the cylindrical section just aft of the turbine exit plane multiplied by the length of the exhaust ducts. Its just like the exhaust cones on the turbojet, but now you have much less pressure ratio to push the exhaust out from behind the power turbine. Additionally, the sharp 90 degree turn could be creating large flow separation regions...you could check for them by smearing a little oil on the ducts and watching to see if it gets "blown" off or if it creates regions of recirculation. Can't wait to see you up an going again! ~ Chris I will get some closeup pics of the bearing and the power turbine shaft tomorrow and post here, the shaft has a blue tone to it all the way back to the rear bearing so heat soak is probably the reason why the ball cage melted. 5-600°C after shutdown and no bleed air going through the bearing. As I wrote to John, closing off the gearbox vent should force all of the air through the front bearing and the narrow gap seal acting just the way you described a positive pressure region behind the turbine wheel hub. I cannot think that the exhaust path is restricting the flow, but I will soon find out when I start testing the engine. I´ll add one freepower piece at a time to find out just what is making the temps so high, and then I will see if the exhaust duct is the one to blame. Just caught up on this. I bet there are many people in your locality who think you are nuts, tests rides will enhance that; keep it up! Most of my thoughts have already been voiced by others, but that gearbox bearing is probably also going to suffer from heat soak after you've shut down after a run. Ordinary bearings lose their temper at 150C. As well as keeping temps under that while running (could you get a thermocouple into the housing near it?) some good post run cooling seems prudent if you don't want to keep swapping bearings. How much shaft and of what material do you have between the turbine rotor and the bearing? I think you´re spot on the main problem, heat soak. I can´t see what I can do about it though since at the end of a run I will be 2 km´s away from both crew and any kind of equipment that can be used to cool the engine down... Finding a bearing that can handle the temp is probably the only solution, perhaps in the end I´ll have to modify a steel bearing into a full compliment ceramic bearing to get rid of the cage. Hopefully the steel bearing will prove to survive. There is only a centimeter or two between the turbine wheel and the bearing that is filled up with the narrow gap seal disc, so no room for any modifications I am afraid. Shaft is unhardened steel, nothing fancy, and the curvic coupling that holds the turbine wheel is 304 stainless.
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Post by Johansson on Jul 28, 2014 16:59:57 GMT -5
Some oil jets onto the shaft behind that cooking bearing could be helpful for heatsoak. Sure would, the problem is that the configuration I am running now relies entirely on bleed air from the engine both for cooling and for supplying oil from the "oil dribbler" (off-the-shelf lubricator for air tools) to the turbine wheel shaft bearings. The output shaft bearings has a wet sump thanks to the <5000rpm they are subjected to.
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Post by racket on Jul 28, 2014 18:51:16 GMT -5
Hi Anders
OOOOOOOPs , bummer , I was sick in hospital when you did the oiler, sorry I didn't pick this up earlier , I didn't realise you weren't running an oil jet lube :-(
You need to get a lot more lubrication to the bearing , not only to lube it but mainly to cool it, that input bearing could also be subjected to radial loads from any balance issues with the turbine wheel , at high rpm that could generate considerable extra heat in the bearing above any soakback heat .
Pneumatic lube systems can have greater cooling due to a larger pressure drop in the airflow , the air can be very cold when expanded, I think you might need to make some major mods .
To minimise heat soakback on both of my freepower bike setups I endevour to have fairly large amounts of bleed air blowing onto both sides of the turbine wheel disc , the gearbox side can be accomplished by having an air seal on the shaft that exhausts against the disc , whilst on the exhaust side a bleed air line into the "bullet" fairing over the disc, with air discharging against the disc .
The shaft air seal will discharge some air back into the bearing but not strongly enough to overcome an oil jet squirting into the bearing from the other side .
We might need to make the mods first , then think about how to get the extra oil out of the gearbox :-)
Cheers John
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Post by Johansson on Jul 29, 2014 0:00:25 GMT -5
Hi John, That is probably the only way to get around this, if I connect the gearbox to the main oil system I can keep the oil pumping through it after shutdown and keep the heat soak at bay. It shouldn´t take that that much work to modify it, I could use the bleed air entry for the oil and fit a T-piece inside the shaft tunnel that aims the oil at both bearings. A milled slot at the bottom of the rear bearing race drains the oil back to the gearbox sump to keep the oil from pooling up. A bleed air line can be routed to keep the turbine hub cooled. An AN8 hose connected where the upper level of the wet sump is drains excess oil back to the oil tank through the filler neck where the tank vent is placed now. Just a quick idea, but something like this might work. Cheers! /Anders
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Post by racket on Jul 29, 2014 0:47:57 GMT -5
Hi Anders
Yep , just some small changes , that's R and D , make'em , break 'em, fix 'em :-)
Cheers John
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Post by Johansson on Jul 29, 2014 1:00:08 GMT -5
I am actually looking forward to modifying the gearbox, must be the machinist in me speaking. *LOL* I know we discussed this while you were in hospital, despite your voice of doubt I went on the oil dribbler route just to try it out before I did any heavier modifications to the gearbox. The oil system is capable of the increased flow through the upcoming oil jets, I have regulated it down a bit as is and still get >5kg oil pressure. I have even bought a set of piston rings to seal the freepower shaft properly in case I needed to build a circulating oil flow through the primary shaft bearings so I must have anticipated a failure. Cheers! /Anders
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Post by racket on Jul 29, 2014 4:11:02 GMT -5
Hi Anders
LOL.......you were just in a hurry to have a ride ;-)
Its not much of a job fitting oil jets , you'll knock it over in no time.
Cheers John
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Post by pictsidhe on Jul 29, 2014 14:03:12 GMT -5
Ceramic bearings that take heat use M50 or similar tool steel races. Putting ceramic balls in a normal bearing won't help much for temperature resistance as the races start losing their hardness at 150C. A diy ceramic would take more revs, but not more heat. Is it much work to change that bearing after a run?
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ashpowers
Veteran Member
Joined: February 2011
Posts: 207
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Post by ashpowers on Jul 29, 2014 18:46:37 GMT -5
Hi Anders, The #8 bearings in the RR Allison 250 have M50 tool steel races and designed to take a good bit of heat. Might want to get your hands on one of those.. I *know* a guy. ;-) PM me. -Ash
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Post by pictsidhe on Jul 30, 2014 12:56:17 GMT -5
Or take the budget option and go brass!
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Post by Johansson on Jul 30, 2014 13:32:49 GMT -5
Hi Anders LOL.......you were just in a hurry to have a ride ;-) Its not much of a job fitting oil jets , you'll knock it over in no time. Cheers John Yup, now I can relax and get back to the usual pace when I have made the maiden ride. Ceramic bearings that take heat use M50 or similar tool steel races. Putting ceramic balls in a normal bearing won't help much for temperature resistance as the races start losing their hardness at 150C. A diy ceramic would take more revs, but not more heat. Is it much work to change that bearing after a run? Thanks for the explanation about the bearings, I was unaware of how temperature sensitive the races are and thought only on the ceramic balls and what they can handle. It is a couple of hours of work, I have to drop the oil tank to get to the gearbox mounting bolts, pull the entire freepower section out of the frame and disassemble most of it to get to the bearing. I´d rather not have to do it after every run. Hi Anders, The #8 bearings in the RR Allison 250 have M50 tool steel races and designed to take a good bit of heat. Might want to get your hands on one of those.. I *know* a guy. ;-) PM me. -Ash That sounds great, I´ll send you a PM right away!
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