Bent shaft. How do you guys generally deal with it?

[nbpt100]

I am putting this in the Briggs section but it could be under any brand. How do you folks generally deal with a bent shaft?

There are the 4 ways that I know of repairing it:

1. Replace the Crank shaft
2. Use a long pipe and try to bend it back to something usable.
3. Hit it with a Big hammer. Hope you do not break anything and make it usable
4. Use a fixture to bend it back. most people do not have access to such a tool.

Is there another way I am missing? In most cases I would bet most have walk away and junked the mower or the engine.

But I may be wrong?. share what you do....

 

[ILENGINE]

Option 1 is not cost feasible in most cases. And the other 3 are a liability risk for most shops and won't happen

 

[Tinkerer200]

I doubt that you will find anyone here recommending any of the procedure you listed except junking. I have straightened them but *only* using a friends straightening fixture made for that express purpose. I made my own specifically for straightening Lawn Boy F series crankshafts which always seemed to be bent but I would not recommend that anyone else do either. You will likely hear of all kinds of terrible results from straightening, I never had first hand knowledge of any disaster in yea many years of experience.

Walt Conner

 

[bertsmobile1]

And here is tech number 3 saying the same thing
Once the crankshaft bends, its microstructure is compromised so it will forever be weak & highly prone to bending again
Straitening jigs rarely get a crank 100% true and a crank that is not true will shake like Elvis's hips if it is bad down to a light tingle in the bars if it is good .
Even a good one , unless it is true to within 0.0005 ", the bent crank will gouge out the bearing surface at either end so down the track a bit the engine will do a seal then seize from oil loss before you notice it .
DIY then replacement cranks are feasible remembering it will be new seals & gaskets as well
Taking it to a workshop the repair will be around 75% or more of the cost of fitting a new engine.

 

[nbpt100]

Once the crankshaft bends, its microstructure is compromised so it will forever be weak & highly prone to bending again
Straitening jigs rarely get a crank 100% true and a crank that is not true will shake like Elvis's hips if it is bad down to a light tingle in the bars if it is good .
Even a good one , unless it is true to within 0.0005 ", the bent crank will gouge out the bearing surface at either end so down the track a bit the engine will do a seal then seize from oil loss before you notice it .

It depends. If there are cracks it is a bad idea to bend it or use it. Bending it with out any visible cracks will work harden the steel and make it harder and less prone to bending in the future. At least in the same place.

I have bent 3/4 diameter shafts so that the vibration is not noticeable by the average end user. Not perfect but usable and back in service with no issues. Will it take some life out of the bearings? Very likely. But you are giving some life back to something that was dead. It is all bonus life. That is my practical point of view. Getting it back to .0005 TIR or close to it is not going to happen.

I know of two people who have bent their shafts and still use the mower as it is. It vibrates a bit more than normal but they live with it. Nothing bad has happened so far. One is a $600 Honda and the other is a cheap Poulan.

I know most larger shops or dealers will not do it. They want to sell you a new machine. They will take your junk mower off your hands.

I know of one independent shop in my area who will not do it for a customer, but he does it for himself. He will resell mowers that had bent shafts that he has straightend. How do I know this? He as much as told me.

Stens sells a fixture and the last time I looked at it the price was around $350.

 

[sgkent]

I browsed the post. If the crank is bent replace it or junk the engine. The next post will be "it only vibrates when it is running... "

 

[StarTech]

Personally I haven't seen but a handful of bent shafts in the 13+ yrs I have been servicing mowers for others. No way I would ever recover the cost of a straightener jig. Now of course fabricate my own jig but why as it would just take valuable space in my shop collecting dust after taking a week to it designed right.

I once tried refurbishing mowers and sell them with a 90 days warranty. All I got was insult offers so I will not waste my time or money doing it. I just part them out as I am a repair only shop now with a few parts only sells.

 

[ILENGINE]

It depends. If there are cracks it is a bad idea to bend it or use it. Bending it with out any visible cracks will work harden the steel and make it harder and less prone to bending in the future. At least in the same place.

I have bent 3/4 diameter shafts so that the vibration is not noticeable by the average end user. Not perfect but usable and back in service with no issues. Will it take some life out of the bearings? Very likely. But you are giving some life back to something that was dead. It is all bonus life. That is my practical point of view. Getting it back to .0005 TIR or close to it is not going to happen.

I know of two people who have bent their shafts and still use the mower as it is. It vibrates a bit more than normal but they live with it. Nothing bad has happened so far. One is a $600 Honda and the other is a cheap Poulan.

I know most larger shops or dealers will not do it. They want to sell you a new machine. They will take your junk mower off your hands.

I know of one independent shop in my area who will not do it for a customer, but he does it for himself. He will resell mowers that had bent shafts that he has straightend. How do I know this? He as much as told me.

Stens sells a fixture and the last time I looked at it the price was around $350.

The question are you willing to risk several million dollar lawsuit if something goes wrong. And yes Stens sells a straightener. But take into account that most mowers that end up with bend cranks are less than $200 for the average customer. And take into account that using the straightener requires removing the engine and putting it into the jig. and the going through the straightening procedure which by this time has taking up to a couple hours. With in shop time could be $150-200 so you have exceeded the replacement cost of the mower. For the $600 mowers they would be more worth just replacing the crankshaft on a new mower, not one with several years of run time

And for the record I don't sell new or used mowers. Too many shops around that sell new stuff, which I will work on after the warranty. And like somebody else say used stuff just gets you lowballed on selling price. How many straightened crankshaft mowers are willing to sell for $15-20 each. Because that is the backyard selling price for people to put on the curb with a for sale sign in my area. Or pick them up at the local community auction for $5 a piece.

 

[bertsmobile1]

Well I will put my metallurgists hat on and tell you that you are so wrong it is not funny
There are 2 common crankshafts , forged steel or cast malleable iron
Both are used in mower engines
Forged steels do not work harden unless they are a molly grade as used in tractor blades and some mower blades .
Cast cranks will recrystalize where it bent originally
It was spinning when it bent so it bent in a high energy bi-axial load situation somewhat similar to spin forming
The straitening jigs are static load so they put a lot of stress into the crank
In either case the worked region will have a different crystal macro structure so it will no longer be uniform top to bottom or around the circumference or radially .
If you were to cut through the crank, polish then etch & look at the macrostructure the damage would be quite evident .
I have done this sort of stuff for near a decade to establish if what the driver of the vehicle that caused the collision was telling the truth ( usually not ) or the recient repairs had been done properly ( again usually not ).
Just because some one managed to force the crank back into reasonable shape and the mower did not immediatley disentergrate does not mean it will not happen in the future
Some times you can be lucky and some times you are not lucky but the consequences of the crank shaft breaking while you are walking behind it do not stack up comparred to the cost of replacing the crank.
Or as I often say to customers the price of the part is less than the taxi fare back from the ER let alone the ambulance to get you there in the first place .

The purpose of this forum is to give sound honest advice that is in the best interest of the poster and that is what they got from 5 techs who have nothing to gain from them replacing the crank or the mower so on the face of it should be good advice
IF nbpt want to try his luck he is more than welcome to do it but 5 techs have all advised against it ,

Now You tube is full of ego maniacs desperate to prove they have more than 2 functioning brain cells so it will be awash with "how to straiten a bent crank shaft " videos but you can be sure none will be posted by engineers or metallurgist who actually know & understand why it is not a good idea .

 

[StarTech]

Gee Bert you give them a lot more credit. I was thinking they barely had one working brain cell. I know they get on my last working nerve here. If I could would depot to the Sun where at least they would of some use.

Usually with walk behind the replacement costs are just as low as the repair or less because the way they dump the mower on the market. Besides the customer get a warranty and it least has all new parts.