Crankshaft went kaboom, please help

[borna]

Hello all,
This is the engine I am working on. It is my own design with 5.5" flywheel and 1" bore x 1.5" stroke.

http://cdn.homemodelenginemachinist.com/attachments/f31/59459d1357071718-my-single-cylinder-engine-002f.jpg


As I was tuning the engine, yet another disaster stroked. This time the crankshaft sliver soldering gave up between the rod in the center and the web .


Now I am thinking 2 things I can do

1. Use a two piece clamp-on shaft collar and screw that into the web with 2 6-32 screws. Just like what I did with the flywheel.

[URL="http://www.homemodelenginemachinist.com/external-link/?c=2&site=www.homemodelenginemachinist.com&url=http://cdn.homemodelenginemachinist.com/attachments/f26/59518d1357355483-flywheel-becoming-loose-setscrew-used-003c.jpg"]http://cdn.homemodelenginemachinist.com/attachments/f26/59518d1357355483-flywheel-becoming-loose-setscrew-used-003c.jpg[/URL]

2. Take this to a shop and have it tig, or mig welded.
What do you guys think is the best approach.


First one is easy and I can do that myself, but not sure if it will hold.
I am assuming the second solution is a better but I need to find a shop to do the welding for me since I don't have the tool and don't want to get involve in that.

This is the picture of what I am thinking to do.

Thanks
Borna

 

[kuhncw]

Borna, if the fit is still good between shaft and disk, you might consider drilling through the disk and shaft and pinning it, possibly with a tapered dowel or pin. A fix like that would be nearly invisible and would not change the look of your engine. The crank in my quarter scale Galloway was built with Loctite and tapered pins and has stayed together with no problems.

Regards,

Chuck

 

[MuellerNick]

Silver soldering, if done properly, has a very high strength. Often near the strength of the base material.

How does the broken joint look like? Picture?


Nick

 

[goldstar31]

It will be tricky to resort to Mig/mag or Tig with something silver soldered. Maybe Nick will confirm or correct my fears.

Norman

 

[MuellerNick]

Maybe Nick will confirm or correct my fears.

It is not just tricky, it is impossible. You have to throw away the parts and start from new. If you try to remove the solder, the part will be out of dimensions.
(Silver-)solder makes an alloy at the soldering surface. That means, it penetrates the base material. Impossible to get it out without removing base material.


Nick

 

[goldstar31]

I was being cautious in my comments. My thoughts were that Mig/Tig heat would cause-- an explosion of the silver content. I've had hard solder explode before.

 

[mwilkes]

You could maybe bore a wider hole in your crankshaft web and solder (or press fit?) in a bush. Then use solder or grubscrews to attach the bush to the crankshaft?

 

[MuellerNick]

an explosion of the silver content. I've had hard solder explode before.

It starts bubbling and bursting. Not a real explosion where all goes off in a big kaboom, but small bangs where the solder gets too hot.
Don't ask!

Edit:
My guess is, that the solder joint was bad. There was no room for the solder to flow in, it just sat in the chamfer.
I guess!
Pictures of the broken joint would allow a more educated guess.


Nick

 

[borna]

Thanks all for the great comments. I will provide a picture of broken part. Actually no damage to either rod or the web. Only the soldering broked. So I guess at this time tig/mig is out of question. I may have to go with the other suggestions you guys provided. But what about the first solution I have with using two piece clamp-on shaft collar?

Thanks
Borna

 

[t.l.a.r. eng]

Simple is better, why not make the crank parts press fit, would last forever.

Nice looking engine.

 

[MuellerNick]

why not make the crank parts press fit, would last forever.

Honestly, a good press or shrink fit in this dimension is a bit of a metrological and machining challenge. I wouldn't suggest that to a beginner.
A solution that was used for crank shafts in the old days quite often was a conical bore and a matching shaft. Shrinked and then secured with a nut makes a solid connection. Quite easy to make.

Nick

 

[rleete]

A simple light knurl will turn a slip fit into a reasonable press fit. Easy-peasy.

 

[t.l.a.r. eng]

Red loc-tite stud and bearing mount can also work quite well in this application. Leave .002 or .003 for clearance to let in the material.

 

[borna]

[ame]http://www.youtube.com/watch?v=rQSRTw5B10Q[/ame]

 

[robcas631]

I believe press fit can be accomplished by beginners. I'd practice on scrap materials first. Also, I believe in redundancies.....so besides locktite, I'd use a pin.

 

[MachineTom]

According to MHB a shrink fit is 7 times stronger than a press fit. Maybe a point of reference, a 125cc honda dirtbike the old 2 strokes, have a press together crank, the webs are no bigger than yours, although the crankpin is larger. These engines put out 40HP, and a loose pin is almost unheard of. You appear to have some meat around the crankpin area on the web disk, first bore out a bit to clean up the bore, then use a reamer to ream a size, Now make a pin that is .0015-.002 over the reamed size, heat up the web to 5-700° work quickly and drive the pin in. If you want a bit more security make a chamfer on the back side of the web, make the pin about .010 longer than the thickness of the web, and using a 4-6oz hammer peen the pin over to fill the chamfer. I've done this on both the crankshaft and crankpin on my Corliss. Looks Old School.

 

[robcas631]

I have read that as well.....

 

[borna]

Well I took the easier route and seems to holding very well now. I had the engine running none stop for 20 mins today after all tunings were done. I used 2 piece shaft collar clamp around the crankroad and screwed into the web.

Thanks
Borna

 

[robcas631]

Borna, Did you want that part of your crankshaft to be adustable?

 

[jpeter]

What a beautiful engine. Its a sweetie.
Silver solder will work. You need a couple of thousands clearance for the solder. Flux the parts well. Flux will not flow into the joint too well so you'll need to coat them both well before assembly. I'd ream the hole for an exact fit then over bore part way through from one side for solder clearance. Another method I've used is to file some flats in the shaft for solder space. Press fitting is a solution but given the small diameter of the main shaft and the way your starter is torquing it I doubt it'd hold up. Some soft solders are labeled silver solder. Don't be fooled by that.