Tuesday, January 15, 2013

Jet engine project - Part 2

I hadn't really inspected the main journal bearing closely up to now, so when I finally did I was dismayed to discover that in fact it shouldn't be in two pieces. Funny what a little grimy oil can obscure!

That little ring on the right is supposed to be attached to the piece on the left. I thought maybe I'd snapped it off somehow when I took apart the seals, but closer inspection suggested that it probably torqued off last time the turbo operated. The brass in the portion of the journal inside the smaller piece was galled, probably from heat. I suspect the bearing lost oil pressure or maybe had a blockage at this end, and allowed the hot end of the bushing next to the exhaust turbine to overheat. Not a happy donor car at all. Hopefully there isn't a problem with the end seals around the thrust bearings. Visually they seem fine.

Close inspection of the shaft suggests it's still serviceable. I took a zillion measurements to get a sense of the dimensions and tolerances I'm dealing with. It's all metric apparently, but most of my tools are in inch so that's what I'm going to use, although some of the numbers seems weird. There's a good 0.001" of clearance to work with between the shaft and journal which would be filled with the pressurized oil that serves as the working bearing. The shaft still measures consistently within 0.0001-0.0002" along its bearing points, so any deviations above the surface won't penetrate the oil skin into the brass proper. Any microscopic grooves in the shaft bearings shouldn't have any impact on performance. They certainly pass the fingernail test. I will need to measure run-out as well, but I don't anticipate it being a problem. 

After searching the internet for a replacement journal bearing, all I could find were complete rebuild kits with parts I don't think I need (famous last words?). I can't afford to pay $120+ for this part. There's a proliferation of el-cheapo options from some guy named Rothenbacher who runs various internet parts stores which all seem to actually be the same business. However, online reviews suggest that parts quality is extremely poor and the businesses are awful to deal with. Two apparently reputable parts companies that I contacted never got back to me. 

That leaves me the the sole option of making my own journal. It's not a technically complicated part--it's just brass, after all--but it'll be vital to get the inside journal dimensions just right. 

So, I spent an entire evening making careful measurements and thinking about a strategy to make this piece. 

First off, my lathe wasn't up to snuff for these tolerances. It took an entire evening to tear it apart, polish the ways, and generally tune it up to eliminate unwanted play. It was due for some TLC anyway since I moved shops. Some test cuts in scrap brass showed I could consistently achieve cuts within a few ten thou, so I know I'm now in the ballpark. Yay Busy Bee. 

Second, I lacked some important tooling to work brass at this scale and achieve smooth finishes and the profiles I needed. To make tooling, I needed to use my grinder--and it's had a chronic wobble on the fine stone that I needed to use. So another evening was spent truing the grinding wheel and carefully carving out a cutter for fine surface work, and a roughing-out tool--both in HSS. Roughing out the journal with these tools showed they worked perfectly and well within the tolerances I sought. A few bucks of type 360 brass rod later, and I'm on my way to fabricating this critical part.

The hardest two cuts will be first the inside of the journal between the actual bearing surfaces (critical to allow just the right amount of bearing oil to pass through the journal), and then the milled-out dimple on the end which takes an insert to prevent the bearing from spinning inside the turbo. I don't have an inside cutter small enough for the former, and I'm nervous about rigging the finished bearing in a four-jaw chuck to cut the latter. Then I had the idea of making an inside cutter from an HSS drill bit, and modifying another HSS drill bit as an end mill. I'll experiment extensively with both tools as well as using the four-jaw chuck before committing to the procedure. 

I'm fairly confident I can make this part. Yes the tolerances are tight, but I the think film of pressurized oil will be somewhat forgiving within the tolerances I can achieve. As long as there's plenty of oil and enough freedom to prevent metal-on-metal contact, even my home-made journal should work fine. And I don't expect it will look any worse the the OEM one.  

Now it's off to make more tooling!  

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