Flame Straightening A Machine Shaft

It is possible to straighten a shaft with heat.  Some folks call it flame straightening.

The shaft, in this case, is the output shaft on a small lawnmower.  Hitting a hidden obstruction caused the shaft to bend.  The resulting vibration made the mower a candidate for the scrap pile, it was virtually unusable.

Runout is determined by mounting a dial gauge in such a manner that the plunger rides against the shaft while the shaft turns.shaft straightening1The magnetic base works nicely.


Runout was determined to be .030shaft straightening2Rotate the shaft, watch the gauge, and zero the gauge on the low spot.  Rotate the shaft again, and read the maximum runout at the high point.

After you have determined the “high” point, heat is applied to the opposite side of the shaft at the “low” point.  The shaft seal is protected by a rag soaked in water.  You do not want heat to ruin the seal.  I used a standard Acetylene cutting tip (not a rosebud) turned down to a low flame.  Don’t melt the area, just gently heat it by moving around a bit to spread the heat.shaft straightening3The area is heated to a barely red color.  Dull red, as seen in the shade.  Working in full light will sometimes hide the color change, and you can go too far, heating beyond the proper range.

The principle:  Because the shaft has stretched on the outside radius of the bend, you have to shrink it back to original dimension.

Method:  Heating alone will not accomplish the job.  The metal has to be restrained.  Otherwise the heated area would simply expand, and return to it’s bent state when it cools.  Force must be applied to restrict any expansion, and help the metal contract when cooling.  Apply the force in the direction you want the metal to bend.  Just enough force to put some strain on the shaft, not tons of force.  The shrinkage as it cools will substitute for tons of force.

When force can’t be applied:   Sometimes, as in this case, you can’t find a way to efficiently apply force.  Obstructions, location of the part, etc……  You can substitute peening for the force.  Heat the area, allow it cool below the dull red state, then immediately peen it.

A word about metals:  Always consider the metal you’re heating.  Is it mild steel, or some sort of high carbon heat treated steel?  Mild steel is very safe to heat, as long as the temperature is kept within limits.  Some steels can’t be heated without damaging the molecular structure.  In this case I assumed the shaft to be, at the very least, case hardened or forged steel.  I also assume that I have possibly weakened it to some degree by heating it.  Carefully weigh the alternatives, and dangers.  In this case, I’ve accepted the risk against throwing the mower on the scrap pile, or replacing the crankshaft (probably as expensive as an entire mower).  My risk threshold may be lower than yours.  This is not a recommended repair unless you are willing to accept the risk.

misc,_stuff_020Any day above ground is a good day

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