The ATIS Stationary Engine Mailing List is an on-line, on-going, worldwide engine discussion. Each month, one of the subjects of discussion is taken by an English woman and turned into an article for GEM. It is checked for technical content by a Texan, all via the internet. The advice comes from a variety of levels of experience and so gives interesting diversity to the methods of tackling the problem, which this month is on the subject of a damaged crankshaft:
The flywheels on my engine were loose at one time and damage has been done to the crank as well to the inside of the flywheels, of course. Along with a good bit of normal wear and some rust (it sat outside) on the crank-bearing surfaces, the crankshaft doesn't look very good. Been thinking, why not make a new crankshaft? Rebore the flywheel hubs, oversize, then create the crankshaft to fit those hubs, which are split hubs. Has anyone had experience making a crankshaft? Sounds like a lot of trouble and expense, but I think it may be worth some trouble.
I would be willing to bet that you could grind the crank down ?' and still have PLENTY of 'meat' to run your engine safely! These old engines were 'overbuilt' if nothing else!
Why not just grind the crankshaft and pour new bearings? The flywheel problem could be solved by enlarging the center hole and using a spacer. The spacer could even be welded to the crank and then machined to size. This sounds simpler than building a crank to me!
Above would be my first choice of a fix. Try Loc-tite on the flywheel before getting too involved. Certainly building a crank would be a fun challenge.
I've used both Loc-Tite and Araldite, a two part epoxy which is much thicker than Loc-Tite and a great filler on loose fits, and Loc-Tite on our marine engines which use only a gib key to hold the flywheel on and have never had a failure. Removing a friend's flywheel several years after Loc-Tite was used required a press and heat after the key was removed. This is a flywheel that would not stay tight with the key alone. It's common to see these engines with broken flywheels from driving the gib key in, trying to keep the flywheel on.
I had a crank once that was severely pitted. I explored options of grinding, welding up and turning, etc. Finally, I decided to experiment a little. I went to a local supply company and purchased a material that they said was used to build up shafts. I think it was originally designed for electric motor shaft applications. I don't think it was an epoxy, like JB or some of those. It was silver in color and looked just like sanded metal when applied. My dad had a large old lathe with a 4 jaw chuck that I was able to use to get the throw centered. I had sandblasted the throw to clean all rust and debris. I applied about 3/16' of the material to the throw, let it set up, then turned it back down true on the lathe, then sanded it with 400 grit paper using a light oil (WD 40 as I remember). It turned out perfect and worked fine, as the old engine never ran over about 150 to 200 rpm and the throw never developed any heat.
I have a 3 HP Domestic that I turned the crank down and made a sleeve; both ends all the way into the journal. I shrunk it on, put several holes in the sleeve, button welded through the holes then reground it and recut the keyways. This was close to 20 years ago. Never a problem.
Having made several crankshafts for model engines, I would suggest welding up the existing crank and having it re-ground. You are talking about a very slow and tedious project for just one piece. The flywheels could be bored oversize and an insert made to bring it back to standard size. It is also possible to chrome plate them and then regrind to size.
I had a connecting rod journal welded and reground to get rid of some very bad rust pits. The shaft came out fine but the person who did the work had some choice words for me when I picked it up. It seems that the heat distorted the crank and the guy had a real hard time getting it straight.
The 'crack' shaft of our vertical Blackstone has been welded in the past. Judging the weld I guess it's over 50 years old. The crankshaft is perfectly straight.
I did think about spray welding or even real welding. I have some concern about localizing some stress at the weld area, with the flywheels hanging on the outside, if it did fracture the consequences could be dangerous. It would certainly be easier though. An obvious method could be to turn down the crank and bush the flywheel bore but with split hubs I doubt that the bushing would be clamped tight enough onto the shaft. To weld a bushing to the shaft is another thought. I am not aware of anybody having welded a crank.
I suspect this can be done but even a small crank would take quite a large lathe plus some tooling to hold it off center to turn the rod bearing.
It would be quite a challenge to make a new crankshaft. I think the biggest problem is to keep it perfectly in line. If it isn't perfectly in straight it will give lots of stress when clamped in the main bearings.
On the 20 HP Bovaird & Seyfang engine I restored, the crankshaft was broken in two pieces. The only part of the crankshaft that was re-used was the throw. The throw was bored, a 7/8' key broached in and then the new shaft was pressed in with 7/8 keys and a 'J' weld was used to weld the new shaft to the throw. I am sure this crank shaft is better then the original. A 'J' weld is nothing to do with J-B weld. If you were to cut a cross section through the weld, it looks like a 'J'. As a matter of fact, it almost looks like a large radius with a tail even before the crank gets turned and ground in the corner against the throw.
As this is a long-term project, it may be some time before we learn which method of restoring the crankshaft was chosen, but it certainly sparked an interesting discussion.