|
|
Post by Johansson on Feb 25, 2024 15:33:53 GMT -5
According to Chris the bolts are not the first things to fail anyway, so it is perhaps overkill to spend a couple of hundred bucks on titanium bolts for the 5k rpm wheels.
|
|
richardm
Senior Member
Joined: June 2022
Posts: 413
|
Post by richardm on Feb 25, 2024 15:53:50 GMT -5
|
|
|
|
Post by finiteparts on Feb 25, 2024 16:07:05 GMT -5
I was able to find a few moments today to do a few calcs. to get a better picture of when you will experience plastic deformation of your components. First, if I look at the stainless steel blade portion, due to the fall off of material capability of most stainless steels, it is limited. Not knowing which variant of SS you are using, I just made a guess of 304L. I got some properties from here: nickelinstitute.org/media/1699/high_temperaturecharacteristicsofstainlesssteel_9004_.pdfIf we look at Figure 3 on page 7, we can see that the 0.2% yield strength at your design point of 1050K, is below 30 ksi and the tensile strength is below 45ksi for almost all of the stainless steels. Setting the yield to be below 30ksi, the bolt holes will reach that level of stress at about 3500 rpm. They will exceed the tensile capability of the blade material by the time they hit 4200 rpm or sooner. I used a stress concentration factor, Kt ~ 1.2 from Peterson's "Stress Concentration Factors" Figure 147a, which is for a pinned or riveted joint. This is an approximation of course, but it should be roughly the order of magnitude expected and representational. If you haven't already, make sure that you use shoulder bolts in the bores of the through holes, because if you let the threads push on the inside of the holes, you will get larger peak stresses that could lead to local damage and potential failure. Also, making the bolt a tighter fit to the through hole will allow it to have better contact and distribute the stress somewhat better. If the bolt diameter is small relative to the through hole, there is less contact and it becomes more like a line contact with a sharp increase in local stress. As for the disk portion, the bore will likely hit yield first at around 7500 rpm and exceed UTS at 8450 rpms. A perfectly smooth through hole would approach yield at around 8500 rpm, but if they are tapped, then they will have even higher stress concentrations. I do not have a good estimate of how the stress concentration factor changes when the load is aligned 90 degrees to the thread centerline,so I can't estimate it. The smooth through hole radial stress would pass the UTS at around 8500 rpm. I would suggest that you take really good measurements of the bore, the o.d. and the bolt hole diameters, so that after you runs, you can the disk for any plastic deformation, indicating that you have exceeded the yield strength of the material. Remember, these are all "fuzzy" numbers with a lot of assumptions baked in to get you a rough order of magnitude idea of where the problem areas might be. Good luck, Chris |
|
richardm
Senior Member
Joined: June 2022
Posts: 413
|
Post by richardm on Feb 25, 2024 16:44:32 GMT -5
|
|
|
|
Post by Johansson on Feb 26, 2024 13:41:44 GMT -5
I was able to find a few moments today to do a few calcs. to get a better picture of when you will experience plastic deformation of your components. First, if I look at the stainless steel blade portion, due to the fall off of material capability of most stainless steels, it is limited. Not knowing which variant of SS you are using, I just made a guess of 304L. I got some properties from here: nickelinstitute.org/media/1699/high_temperaturecharacteristicsofstainlesssteel_9004_.pdfIf we look at Figure 3 on page 7, we can see that the 0.2% yield strength at your design point of 1050K, is below 30 ksi and the tensile strength is below 45ksi for almost all of the stainless steels. Setting the yield to be below 30ksi, the bolt holes will reach that level of stress at about 3500 rpm. They will exceed the tensile capability of the blade material by the time they hit 4200 rpm or sooner. I used a stress concentration factor, Kt ~ 1.2 from Peterson's "Stress Concentration Factors" Figure 147a, which is for a pinned or riveted joint. This is an approximation of course, but it should be roughly the order of magnitude expected and representational. If you haven't already, make sure that you use shoulder bolts in the bores of the through holes, because if you let the threads push on the inside of the holes, you will get larger peak stresses that could lead to local damage and potential failure. Also, making the bolt a tighter fit to the through hole will allow it to have better contact and distribute the stress somewhat better. If the bolt diameter is small relative to the through hole, there is less contact and it becomes more like a line contact with a sharp increase in local stress. As for the disk portion, the bore will likely hit yield first at around 7500 rpm and exceed UTS at 8450 rpms. A perfectly smooth through hole would approach yield at around 8500 rpm, but if they are tapped, then they will have even higher stress concentrations. I do not have a good estimate of how the stress concentration factor changes when the load is aligned 90 degrees to the thread centerline,so I can't estimate it. The smooth through hole radial stress would pass the UTS at around 8500 rpm. I would suggest that you take really good measurements of the bore, the o.d. and the bolt hole diameters, so that after you runs, you can the disk for any plastic deformation, indicating that you have exceeded the yield strength of the material. Remember, these are all "fuzzy" numbers with a lot of assumptions baked in to get you a rough order of magnitude idea of where the problem areas might be. Good luck, Chris Hi Chris, So the first thing to fail are the blade holes, good to know. I will use shoulder bolts and keep a close eye on the hole diameters. Just curious, if the bolts are a little smaller in diameter than the holes causing local stress and deformation wouldn´t the bolt "seat" itself and eventually get full 180 degree contact? If I can remove say 5% weight from the blades by profiling them when they have been given the right twist, how much would that affect the hole tensile capability rpms? 8000rpm is the usual max revs for a snowmobile clutch, so I think this will be my goal to eventually reach. Makes finding the right weights and springs easier when I run the clutch like it is supposed to. /Anders |
|
|
|
Post by wannabebuilderuk on Feb 26, 2024 14:07:06 GMT -5
Remember turbos are usually balanced to a high level since even a tenth of a gram at high rpm is a lot of force so any weight you can shed that is further away from the axis of rotation will benefit you, plus profiling makes it better for flow so win win.
You could probably profile before twisting, use an end mill or HSS radius cutter after you do the slot cut on each blade segment?
|
|
|
|
Post by Johansson on Feb 26, 2024 14:10:56 GMT -5
Good find, it looks like 12.9 steel shoulder bolts are the way to go. |
|
|
|
Post by Johansson on Feb 26, 2024 14:11:36 GMT -5
Remember turbos are usually balanced to a high level since even a tenth of a gram at high rpm is a lot of force so any weight you can shed that is further away from the axis of rotation will benefit you, plus profiling makes it better for flow so win win. You could probably profile before twisting, use an end mill or HSS radius cutter after you do the slot cut on each blade segment? True, but since the blades are laser cut to .1mm tolerances the balance in the un-profiled wheel is probably as good as it gets, profiling the blades and then statically balancing the wheel will most likely make it worse. I have to do any profiling by hand, flap disc style. Milling the blade profile would awaken vibrations from the outer regions of Hell...  |
|
|
|
Post by wannabebuilderuk on Feb 26, 2024 15:29:46 GMT -5
Remember turbos are usually balanced to a high level since even a tenth of a gram at high rpm is a lot of force so any weight you can shed that is further away from the axis of rotation will benefit you, plus profiling makes it better for flow so win win. You could probably profile before twisting, use an end mill or HSS radius cutter after you do the slot cut on each blade segment? True, but since the blades are laser cut to .1mm tolerances the balance in the un-profiled wheel is probably as good as it gets, profiling the blades and then statically balancing the wheel will most likely make it worse. I have to do any profiling by hand, flap disc style. Milling the blade profile would awaken vibrations from the outer regions of Hell... Shame you can't just get it red hot and squish it into shape with a few tonnes haha save the grinding nightmare. Sure you can rig up some setup to keep the grinder in the same position for each blade, like a diy surface grinder |
|
|
|
Post by Johansson on Feb 26, 2024 15:43:55 GMT -5
True, but since the blades are laser cut to .1mm tolerances the balance in the un-profiled wheel is probably as good as it gets, profiling the blades and then statically balancing the wheel will most likely make it worse. I have to do any profiling by hand, flap disc style. Milling the blade profile would awaken vibrations from the outer regions of Hell... Shame you can't just get it red hot and squish it into shape with a few tonnes haha save the grinding nightmare. Sure you can rig up some setup to keep the grinder in the same position for each blade, like a diy surface grinder I don´t think it will be that much of a headache to profile the blades, if I weigh each segment before and after profiling I should be able to make them pretty identical. If this proves to work it is still a very easy way of making a turbine disc compared to turning a big chunk of inconel into a disc and manually grind out each blade from solid. Just need to improve on the lowest hanging fruit which seems to be the blade segment holes, until I can get a decent performance out of the design. |
|
|
|
Post by wannabebuilderuk on Feb 26, 2024 16:25:22 GMT -5
Perhaps instead of just bolt shear strength holding it maybe it's worth doing a hybrid approach of a modern turbine disc where you use the bolts but mill a lip in the disc and corresponding slots in the blades like a 2nd version of the fir tree slots? So major of the stress is between the overlapping surfaces instead of just the bolt small surface?
|
|
richardm
Senior Member
Joined: June 2022
Posts: 413
|
Post by richardm on Feb 26, 2024 18:39:00 GMT -5
You can grind the blades to shape and then use a precision scale to weight each segment With some patience you can bring each segment to to the same mass within a fraction a gram
Milling a lip in the disk will only make it weaker as its get thinner.
|
|
|
|
Post by Johansson on Feb 27, 2024 1:40:18 GMT -5
We´ll see how fast this contraption can be spun before things go downhill, for the gen2 version I am tempted to try a single blade approach with hollow sheet metal blades locked by pins (similar to fir tree but with round holes in the disc instead).
With a series of 3D printed press jigs I think it would be possible to make them, a 1mm folded blade tig welded together along the inducer and exducer edges. Holes radially drilled in the disc to supply cooling air to the blade internals.
The compressor wheel shouldn´t have the same problems with the holes since it is in one piece, holding itself together and not pulling on the bolts like the segmented turbine blade arrangement.
|
|
|
|
Post by Johansson on Mar 2, 2024 3:29:14 GMT -5
For some strange reason my Onedrive account is blocked, so all images posted in the thread so far are gone. Hopefully it will solve itself but until then I upload the images to JATO.  Yesterday my youngest daughter had her friends over for a birthday dinner, a strange feeling to be kicked out of my own house. In Sweden there is a children story about Emil i Lönneberga where a kid who makes lots of unintended pranks gets locked in the wood shed until his father calms down. I got those vibes last night when I was ordered to go to the workshop and stay there for 3 hours.  The turbine blades are all profiled and twisted now, closest to the root the twist is not as good as I had hoped for but it is what it is. The main portion of the blade has the 30 degree exducer angle I was aiming for.  I didn´t take a picture of it but I also evened out the gap around the compressor wheel.  Cheers! /Anders |
|
|
|
Post by racket on Mar 2, 2024 4:51:19 GMT -5
Hi Anders
LOL.............can you bribe your daughter to have more friends over more often :-)
Rotor looking amazing ..............now thats BIG .
Cheers
John
|
|
