Sunday, December 15, 2013

Jet Engine - Part 7

This morning I painted and cured the exhaust scroll after grinding off a few spatters and bumps, and filing the flange flat. It turned out reasonably well considering how pitted the original part was.

Then it was assembly time. There's really no secret to this: all the parts go together one way only, except for the shield under the turbine. At first I thought it should cup away from the base of the turbine, since that seemed to match the existing curve better. But after assembling the core I heard the shield rattling around. A quick Internet search showed it should be mounted the other way, curving towards the turbine. This made for a snug fit. The photo shows the correct orientation.

I should mention that it is a good idea to replace the retainer ring on the shaft under the turbine. Since the old one was a pain to remove it was tempting to avoid this step, but under was some coked oil that needed to be carefully scraped out with a dental pick. Cleaning up little details like this can make the different between having the seals last (because of lower friction and the ability for cooling oil to get in there), or blowing them out fast.

A few drops of oil on the shaft and it all slid together beautifully. There were two circle clips in the rebuild kit and I'm not entirely sure what they're for. One could be a replacement for the expansion ring at the bottom of the core, to prevent the brass journal bearing from falling out the bottom. Since there are two replacement journal bearings, it's probably a new clip for each. I just left the original clip in place since it seemed fine. It is unlikely to fail and would be really hard to remove anyway since it's a coil-style and not a circle-clip with ears for a tool.

Admittedly, the washers to hold on the turbine scroll look a little hokey. The originals are too corroded to reuse and are an unusually thick material. I'll have to look for some decent replacements. The seal on the turbine scroll is my main concern with this turbo. I'm not sure how gas-tight it'll be. It wouldn't be good for hot combustor gases to blow out around the perimeter.

The shaft spins nicely and nothing seems to be interfering, so I think things are good to go to the next step: designing and fabricating the combustor. Before I can start that, I need to finish my thermo calculations and come up with a design spec. There are some online tools to help locate the air holes for optimal flow of shield air and combustion air. However, some experimentation will be required before I'm confident to attach the combustor to the turbo.

As you can see, I've rotated the compressor outlet so it's pointing the same way as the turbine inlet. The combustor will mount to the flange I welded to the turbine inlet, and a U-shaped hose will connect the combustor inlet to the compressor. It's looking pretty industrial already and I need to start thinking about how to make this look steampunk with appropriate details for the overall assembly. Lots of brass and stained oak, I think.

Update (April 4, 2014): Work and the need for a MIG welder have delayed my progress. But I promise: more to come!

Saturday, December 14, 2013

Jet Engine - Part 6

Welded the flange this morning. Well, actually, Chris Branje (John Branje's son) stick-welded it for me. He earned a case of Bud for his mighty fine job. Check it out:

The procedure was to pre-heat the parts to 800F in the kiln, giving them a good soak before welding. Chris used a nickel flux rod for filler, laying down a remarkably nice fillet. Penetration looked good all around except deep in the scroll crevice where it was hard to reach. We decided to flip the scroll over and just fill that entire area with rod to ensure a gas-tight fit for the flange. The inside of the flange was perfectly aligned and no cleanup there is needed. 

Thanks to the preheating, Chris said the welding was like butter. He's successfully welded cast iron before, but this was the easiest he's seen yet. 

After welding he peened the welds lightly to knock off the dross and I stuck the whole shebang back into the kiln. After re-heating to 800F, I let it cool slowly to about 160F over 6 hours. No cracking observed in the finished product. Oxidation is mild and cleans up easily with a wire wheel on the Dremel. After a thorough cleaning, I'll paint it with the black VHT paint and fire it again in the kiln. 

After Chris finished he let me have a go welding some scrap steel. I had a hard time sparking it up and not getting the rod to stick, never mind dragging a puddle and actually welding. This was my first time trying stick welding. Gives me all the more respect for skilled welders! Hopefully I have better luck with a MIG. 

Thursday, December 5, 2013

Jet Engine - Part 5

Time to test that Very High Temperature paint. It's actually a ceramic material, although you need a microscope to read that detail on the label. Prep was pretty straightforward: thorough cleaning with degreaser and water to rinse out all the sandblasting dust, then quick shot of WD40 to displace the water, then another degrease with acetone and then a quick shot of compressed degreaser. The parts came out nice and clean after that, with no apparent oxidation even on the cast iron. You can see why it's called gray iron. Compare the appearance of these parts now with the rusted and filthy junk at the start!

Then it was time to mask and spray. I chose black for the hot turbine parts and red for the compressor. There's not much choice in this temperature range anyway, and these colors should look suitably Victorian. Two light coats and then one medium coat all done within the recommended one hour max, and then into the kiln for a three stage cure at 250, 400, and 600F with a 30 minute soak and cool down between each heat. That red sure looks bright right after painting.

I put the cast iron turbo core and the compressor cover plate under a kiln shelf so it wouldn't be exposed to direct heat from the elements and the little bit of crap that comes off the elements as they expand and contract. I was also curious to see how direct and indirect heating would affect the paint cure. The next morning showed an interesting contrast: the scroll cover which was under the shelf was a nice bright red (although not as bright before curing), whereas the scroll case left exposed to the direct heat was a dull red. Either the direct heat did what it should, or the indirect heat did what it should--but one of them isn't quite right. Thumbnail scratch tests of the three parts showed that the paint is more fragile than I anticipated. Oh well, we'll see what happens when it's really cooking. Hopefully it slows down the oxidation at least a little bit.

The turbo core also took on a blue sheen from surface oxidation due to prolonged heating. Not sure why it would've been so hot for so long: I double-checked my kiln program, but maybe my ramp-down rates were too low. Anyway, this oxidation is cosmetic and should have no impact on the turbo performance since the moving bits are supported by the bronze bearing and an oil film, and there's nothing left to flake off.

Next step was to rig a method of holding the flange to the turbine scroll for welding. I used a scrap of 1/4" plate for a base and then wired everything in place, engraving a groove around the scroll case where it contacts the flange so I can relocate it precisely in case something moves. John Branje, our local expert welder, will attempt to MIG these pieces together using a nickel filler and by peening the welds between passes. Before that I will pre-heat the parts to about 800F in the kiln so there's less risk of heat shock (which can crack the iron casting or weld), and then post-weld slowly cool the parts in the kiln for the same reason. As you can see from my set-up, it'll be easy to lift the parts out of the kiln and onto a ceramic fibre sheet using a long steel rod inserted through the scroll case. One of the benefits of working with kiln-formed glass is I have refractory materials just lying around!

If it all holds together, I'll clean up and paint the scroll case black as well.

The turbo rebuild kit arrived today. All the parts appear to be of high quality and fit perfectly. As a bonus, the replacement o-rings smell like cinnamon oil. Given the kit cost only $78 delivered from Amazon, I should've bought it long ago and just moved on with the project. It's exciting to see this crappy old turbo start to show some promise here! Fingers crossed that by the end of the weekend I can have it all back together and ready to plumb for oil pressure. Then the project really begins.