Sunday, June 9, 2019

Should I buy it or not?

MIG welding of heat-treatable steels is usually considered a no-no because the welds tend to be brittle and have high residual stresses unless they receive special strain relief heat treating.
I am mulling over whether to buy a finish mower that is not working. The person selling it says that the problem is a gear that broke loose from a shaft. The photo clearly shows the gear was welded onto the shaft.


The dilemma is that used, working finish mowers cost $500. The owner wants $200 for this one and the replacement part costs $350.

I am twisting the Captain's arm. If the end of the shaft is too beat up I will pass on this offer. If the gear fits on snugly then I have a nephew who can grind off the old weld, chamfer the edges and weld them together using by preheating the parts and using high-nickel rod.

Back when I worked for a large automotive firm, the transmission division had a similar geometry and they friction welded the two parts together. They had a huge bank of flywheels and enormous, three phase electric motors. They clamped onto the shaft with a chuck, spun the flywheels up to speed and jammed the torque converter housing onto the shaft. They had something like a fifteen second cycle time.

The process ran like a champ. Eventually they brought in an electron beam welding process. The new process was never as fast, as reliable or as tolerant of dirt as the old process.

Sometimes brute force is the best answer.

If you look at problems as opportunities in work clothes, then a small shop could probably churn these out for $20 a pop and sell them for half the price of the major parts suppliers. That would be a 10X mark-up.

5 comments:

  1. I'd not spend more than half the price of the new part on the whole thing. Look for the price of a new gearbox before you make the decision. Might be a better choice than trying to fix the gear, and you'd know what it would cost worst case.
    Bearings and other shafts already have the wear, and you would have a new part in a used mower (gearbox), even if the gear could be welded correctly. (Or you could pin it, but still...)

    Buying stuff like that and fixing it usually doesn't work out. I doubt that the original part was welded, leaving me to believe that there is either something else wrong, or that it was defective from the factory.

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  2. While I understand the nickel rod, I would probably braze it.

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  3. I've seen that friction welding process (at least on video). Wow. I was amazed

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  4. Brazing is good.

    Unless you have a soft spot in your head for that particular kind of mower, or already have several and need another parts machine, it doesn't sound like a good deal.

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  5. Surplus Center might have an appropriate gearbox, either at this link or on one of their other power transmission pages.
    https://www.surpluscenter.com/Power-Transmission/Gear-Reducers-Gearboxes/Gearboxes/

    No connection other than a satisfied customer.

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