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crankshaft straighteners


indypower's Avatar
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05-22-08, 07:12 AM   #1  
crankshaft straighteners

Just looking for input about crankshaft straighteners such as the "mighty midget". I bought a Scotts 3-in-1 self propelled mower with a Tecumseh OVRM120 engine on Ebay for 99 cents. The mounting base of the engine was broken. Both parts were there and since the broken piece had one of the 3 mounting bolts, I had it welded for $50. While I was waiting for the blade & mounting parts, I started it. Runs smooth & GREAT!!!
I put on the blade and started it. Vibrates like hell. Disappointing, but not a big surprise considering the engine damage.
Crankshaft is bent.
I know the OEM's condone straighten crankshafts due to liablity concerns.
Anyone used a crankshft straightener and had good results?

 
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05-22-08, 02:45 PM   #2  
I'd be a little hesitant to straighten a lawnmower engine crankshaft. From an engineering perspective, materials will bends elastically (return to their original form when the force causing them to bend is removed) until the internal stress in the metal reaches the material's yield stress. At that point, plastic deformation occurs and the material is permanently bent.

Plastic deformation leads to stress hardening (ever bend a coat hanger back and forth until it breaks? you are left with a little hook on one side) so portions of the crankshaft will be harder than others. Given that the crankshaft is spinning at > 3000 RPM, and it has a sharp metal blade attached to it, I wouldn't risk trying to straighten it.

Just my $0.02

- Joe

 
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05-22-08, 03:17 PM   #3  
The possible results sound way too dangerous for me, you shpuld be able to buy a new crank for around $75.00, however there was a lot of stress on other internal parts also, it could be the old fix one then the next one goes. My $0.02. Have a good one. Geo

 
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05-22-08, 04:21 PM   #4  
50 yrs ago I saw lots crankshafts straightened and never saw one break later on. Could it be the cranks back then were softer and therefore didn't get as hard as the ones today?
Mike

 
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05-22-08, 07:45 PM   #5  
mla2ofus;
In the last 50 yrs it has worked just the opposite for me. Sorry everyone, but I couldn't resist. Have a good one. Geo

 
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05-22-08, 08:46 PM   #6  
Good one,George !!! I opened myself up for that one. However, I must remind you there are a lot of comics around trying to land a gig !!!
Mike

 
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05-22-08, 11:45 PM   #7  
I don't have a straightener, but I know a lot of folks use them with success. I don't generally reccomend it, but mainly for liability reasons. There is always that "what if..." that makes me shy away from some repairs (like plastic welding on gas tanks). If it's your mower, and you're going to use it, and you're comfortable with the risk (probably not a big risk), then go for it. I've seen very very few flat out broken crankshafts on small engines. In all my experience, I could probably count them on one hand. I guess there's always that chance though.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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