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Hit & Miss Gas Engine Discussion

straightening bent crankshaft


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  #1  
Old 01-19-2013, 07:47:34 AM
Harry Terpstra Harry Terpstra is offline
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Default straightening bent crankshaft

I need some advise on how to straighten the crankshaft of my 4hp Wichterle sideshaft engine. It has a little whobble in it, I meassure a little less than 1/4" at the rim of the flywheel. At the end of the crankshaft I meassure 1/8" so it's not allot.
However I want to see if I can get it straight. I build a frame around the engine
The initial plan was to take of the main bearing cap and put a block of wood on the top of the crankshaft and jack up the shaft just in front of the flywheel. It is a 3" diameter shaft.
I now wonder if it is better to put the bearing cap back on and put a jack on the cap, or the other option to press the crankshaft down and let the lower bearing support the pressure in the lower bearing.
What would be the best solution?
Any advise is appreciated.
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  #2  
Old 01-19-2013, 11:03:42 AM
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Elden DuRand Elden DuRand is offline
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Default Re: straightening bent crankshaft

Harry:

I think the best way to do the straightening would be to press down on the end of the crankshaft and let the engine frame and lower bearing take the strain.

Now - don't run right out and try it! Wait for someone more knowledgeable to either correct me or agree.

DISCLAIMER: I could be wrong and probably am!
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Old 01-19-2013, 11:17:17 AM
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Default Re: straightening bent crankshaft

I would be really nervous about trying to straighten the crankshaft while it was still in the engine like that. Although its alot of work, I would feel alot safer taking the crankshaft out and doing it in a press. I would rather take the time to dissassemble the engine than to hear something go "BANG" that you didn't want to hear go bang.

Another thought, what if you rolled the crankshaft over so the high spot of the indicator readout was facing up, then used a little heat where the bend was and let the weight of the flywheel pull it down with the aid of a jack? I dont know if that would work and if the heat would hurt the crankshaft, but its a thought. I would be worried about effecting the crankshaft if too much heat was applied.
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Old 01-19-2013, 11:22:57 AM
Harry Terpstra Harry Terpstra is offline
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Default Re: straightening bent crankshaft

Tanner, there is absolutely no room to heat the crankshaft next to it´s bearing as there is a sleve on it for the clutch. Also I would hate to put a torch so close to the bronze bearing.
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Old 01-19-2013, 11:23:02 AM
Charlie Mc Charlie Mc is offline
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Default Re: straightening bent crankshaft

I had good results by by removing the bearing cap and putting a piece of channel iron on top of the crank supported by the crosspiece of the frame .
july 2001 of gem magazine has a picture and info a man put a flat piece of iron ontop of the bearing and he used a 4x4 piece of timber .
good luck.
charles
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Old 01-19-2013, 11:58:38 AM
uglyblue66 uglyblue66 is offline
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Default Re: straightening bent crankshaft

That wobble is probably very annoying.But mistakes made trying to fix it,would be WORSE.
I aint so sure I couldnt learn to live with before I broke something very costly.
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Old 01-19-2013, 12:25:27 PM
Ed Sparks Ed Sparks is offline
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Default Re: straightening bent crankshaft

Do you know precisely where the problem area is?

Is it the crank or the flywheel runout?

If not you could cause worse wobble trying to fix it .That is the kind of fix for a press and someone who's done it before.
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Old 01-19-2013, 12:44:47 PM
beezerbill beezerbill is offline
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Default Re: straightening bent crankshaft

Hi,

I take it you do not have a press sufficient to straighten the crank. For a quick and dirty field repair pressing against the engine frame would probably work. But I would be somewhat nervous about trying to straighten it in a precious antique engine. As noted, there is the chance you might break part of the engine. Also there is the chance you might get rid of the wobble by creating another bend somewhere else that compensates for the bend causing the wobble. The worst thing you can do is end up with the crank journal distorted in a way that the rod tries to wobble and put undue stress on everything from the rod bearing to the wrist pin, or even cause the piston to try to rotate as it moves up and down. Best is to find out where the bend really is (lots of time with V blocks and an indicator and height gauge in different configurations but all nice, quiet patient bench work) and then straighten the crank where it needs straightening. I would straighten it cold and use soft aluminum or brass saddles that conform to the crank as bearing points when pressing it straight. There is also issues with stress releiving both before and after straightening that I have not really looked into this but it might come up and is worth looking into.
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Old 01-19-2013, 01:13:38 PM
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Wayne A Thackery Wayne A Thackery is offline
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Default Re: straightening bent crankshaft

Harry, hope all is well and say high to the wife for me!

I had to straighten the crank in the Western and done so by putting it into the press and making saddles to fit the shaft for support and where I pressed. The crank was fairly soft and easy to straighten cold. When I started the flywheel rim had 3 inch run out. If you don't have access to a press I would feel confident you can straighten it in the engine by rolling the crank with the high side up and pushing down on the end of the crank using the base as the support. You can always peen the spokes to get it perfect. The dial indicator is your friend!

Good luck and let us know how it comes out.
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Old 01-19-2013, 01:27:53 PM
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Brian Lynch Brian Lynch is offline
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Default Re: straightening bent crankshaft

If this were my engine, I would first take it down to just the bare crankshaft. I would first have the crank Magnafluxed by a certified shop to find any cracks or other anomolies. Then a plan can be set forth for straightening it and repairing anything the Mag'ing shows up. After the work is done, run it thru the Magnaflux again to make sure nothing else has cracked. May be time consuming and expensive but worth it in the long run.
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Old 01-19-2013, 02:41:55 PM
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Default Re: straightening bent crankshaft

Whatever is done, just make sure its running when George and I get there
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Old 01-19-2013, 03:05:26 PM
Larry Anger Larry Anger is offline
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Default Re: straightening bent crankshaft

Harry - I think like others have said you need to strip the crank down and get it on a bench in V-blocks so you can see exactly where it is bent. Otherwise when straightening in the engine frame you might be correcting the wobble by putting another bend in the crank to compensate for the original bend. That wouldn't be good. Like Tanner says wait till June to correct this problem because I want to see this engine running at Nuenen!!!!
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Old 01-19-2013, 03:18:00 PM
George Best George Best is offline
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Default Re: straightening bent crankshaft

Quote:
Originally Posted by Larry Anger View Post
Like Tanner says wait till June to correct this problem because I want to see this engine running at Nuenen!!!!
What?????
Where did wait till June come about
We're all going to be there in May.
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Old 01-19-2013, 03:43:20 PM
Merv C Merv C is offline
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Default Re: straightening bent crankshaft

If the bend is slight it can be straightened by by applying heat on the inside of the bend to the point where it reaches cherry red with a oxy torch, then douse it with water to shrink it straight. Like it has already been said take it out as the bend usually occurs at the mains. you will also need some means to determine when it is straight.
I haven't done this with a crankshaft but have straightened a side shaft with this method.
merv.
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Old 01-19-2013, 03:45:06 PM
oldjdinterest oldjdinterest is offline
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Default Re: straightening bent crankshaft

Hoi Harry, be Careful doing that! I don't like to put pressure by a jack on the engine base with bearings, is hij krom net buiten het lager of ook vanaf de kruk, (sorry in dutch laguage), Get the crankshaft out of the engine and measure with a gauge the position where it is bend, then make a construction for under a hydraulic shoppress, the most important thing is then to ondersteun het op de juiste plek, of verwarm en buig het ofzo. Then measure between the centers in a lathe.

groeten Gerrit
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Old 01-19-2013, 04:44:55 PM
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Default Re: straightening bent crankshaft

Hi Harry. This worked for me. Used a dial indicator to find low spot, put hydraulic jack against brace and measured how much I lifted with jack. Then measured run out again and deflected more, and kept checking untill I had zero run out
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Old 01-19-2013, 07:13:01 PM
Larry Anger Larry Anger is offline
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Default Re: straightening bent crankshaft

Quote:
Originally Posted by George Best View Post
What?????
Where did wait till June come about
We're all going to be there in May.
George I don't mind seeing it run in May with a very slight wobble! Wait until after the Nuenen show to mess with it!!! Just in case things go wonky!!
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Old 01-19-2013, 07:51:13 PM
Harry Terpstra Harry Terpstra is offline
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Default Re: straightening bent crankshaft

Thank you all for your advise. Sofar I have tried this:
I found a 3" split bronze bearing in my shop so that made a perfect support on top of the crankshaft. At least now I don't need to put pressure on my main bearing cap.
I put a jack under the crankshaft and put another jack on the main bearing.
I have put quite allot of tons on it and it is moving. However every time it comes back to it's old possition. I have pressed it even 3/8" to far but it springs back every time. To be honest I am scared to go any futher.
We'll see how we continue......
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Old 01-19-2013, 08:08:27 PM
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Default Re: straightening bent crankshaft

Stay at the 3/8 "too far" and apply heat to what was low side, and is now being stretched. That will relax metal in that area.

Some people heat the high side and quickly cool it to make it shrink, but I do not like that - think it induces stresses and can cause cracking.
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Old 01-19-2013, 09:45:16 PM
Henry Macomber Henry Macomber is offline
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Default Re: straightening bent crankshaft

I have straightened many crankshafts with a portapower and a hardwood wedge, no heat, and crankshaft and flywheels left right on the engine. I haven't damaged anything in any way at all. A few were bent pretty good too, but, to each there own!
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