After a hiatus of writers block we are back, and by we I mean me. Lots going on in life and the blog timeline doesnt follow real world timeline but what does. Ok So we know the turbine was taken apart and potentially broken, and in the last post we now know the motor generator seems to be working as it should, but we kept hearing that this shaft was a problem. The shaft above was in our unit and you can see that there was a nubbin spray welded on, why? We arent sure but we can postulate later.
The Elephant in the Room
Posted on: August 21, 2025
As you can see here is a second shaft that was either never installed or in short service. It has this cast nubbin on it that is sorta a feature but not really meant to be, but it should have been.
If you remember this video, this is the bearings and planetary gearset. This whole contraption set about a 39 degrees inclination. The other side of this is the generator that has a female splined coupling that matches this intermediate shaft. If you noticed on this intermediate shaft there are no constraints, no way to keep it in one place, it sort of just floats in between the motor and the planetary. Also the planet carrier which weighs about 40lbs has no contraints. You can see and imagine in this video that the plate that holds the planets spins at 535rpm, but the shaft spins at 1800, and the only thing holding up the planet carrier is this shaft. And the shaft and the faceplate of the carrier are spinning at different speeds, so over time the nubbin you see gets ground down. We hypothesize that one failure mode of the generator is a chunk of iron from this process gets in just the right spot and eats the windings before it gets picked up by the oil filter in the pump unit.
So what do you do? Do you just clean up the spray weld and see how long that lasts? Or do you fix the problem.... Well you need some sort of bearing, right? One that goes into the end of the shaft and can take the thrust and the speed. In our case the thrust isnt terribly high, but the speed is semi significant. So you need something called a Thrust Bearing for Shaft Ends on Mcmaster, and click and its on our doorstep before we wake up the next morning. Now you must be thinking to yourself just as I did rewriting this, Aaron that bearing is only rated for 1,500 rpm, isnt your generator 1,800rpm? But if you remember the end of the shaft is really only seeing 1,800rpm of the generator MINUS the 535 or so from the propellor, its almost like we thought about this!
Here is a photo of the spray welded shaft after a cleaning pass to see whats going on. You can see this weld is hard as your nipples in Antartica, makes a hell of a noise too as you will see. But you can see all that internal stress that comes from the spray welding from differential heating and cooling, lots of cracks. Ive never dealt with spray before but its intersting to see how homogeneous this stuff is. Our plan is simple face off this weld to the nice gear face, then bore a hole the diameter of the bearing. You must be saying well Aaron, didnt you see in the spec sheet that it needs lubrication? you can't make it a one step bore the size of the bearing! And you would be correct, its a two step bore with a small gap on the front part of the bearing, it serves two purposes lubrication and the ability to turn differentially as you will see. We only want this bearing sticking out about 50 thou.
I know all of you wanted to hear how this sounded when I was machining this, and its horrible. With the powered vested with me from the overlords of AI, if you click the video you will hear the audio, then click again to mute. This stuff is harder than it is for your MOHther-in-law to resist giving unsolicited advice. But you can also hear in the video that the harness of the shaft itself atleast on the inside is no where near as hard as this spray weld.
Here is a pic of just a test fit after measuring with a internal bore gauge and a micrometer, the deeper part of the bore is the right diameter as its a nice fit. Now we can cut the step and bob's your auntie.
Austin through another friend aquired a machine shop from a estate sale from a man who lived to be 103! The guy was a Grade A Certified hoarder, but he did have some good stuff in the mix of terrible old broken stuff, in it was a box of this grease. I dont often get to use this tube of ancient Moly D grease, but when I do I am excited. For those who are not aware molybdenum disulfide is a excellent lubricant in high pressure situations, like thrust bearings. The more you push the more slipperier it is, or thats how I understand it. Anyways whenever I can use the D I do.
Here you can see it all finished and happy in its new home. Ok I forgot we didnt machine off all the spray weld, but we took off an amount and measured all the parts to see the tolerances of the bearing spacing as well with measuring the non worn out shaft. We then voted on a number in which the bearing sticks above the face of the shaft, and bingo whatever the magic number was thats what it ended up being, it was thought about and I did indeed have a drawing. While putting everything back together we really didnt find anything wrong other than this. Well atleast in terms of the generator section, but that will have to wait for the next episode on here.
Here is a sneak peak of the next fun thing. Austin brought this back to Arlington and assured me that it was very heavy, and being the manly man that I am I didn't believe him. I was indeed wrong. You would think that this steel hub that holds cast aluminum blades that are the least hydrodynamic thing ive seen, that it wouldnt weigh that much. Jeebus its gotta be like 400lbs atleast. Its an absolute unit. Oh did I mention it was a MILLION Zimbabwe Dollars, see I was totally telling the truth.