Metal Fatigue

The other day, when I was clearing the driveway of snow for the Christmas Dinner, the snow-blower quit working. Oh, it would push snow but the auger would not turn. I could not see any jam or lump of ice but it just would not engage.
 
Picture showing broken bracket
The cable to the auger became loose and I had tightened it but it was no longer engaging the auger. Today, I got a close look with a flashlight under the cover of the drive-belts and saw the problem. An “L”-shaped piece of steel which anchored the sheath of the cable controlling the idler wheel on the drive-belt for the auger had snapped, right at the corner. Shucks. The thing is about 20 years old but metal fatigue in a cable-retainer? It must have been cracked since it was made and after thousands of cycles the crack grew. The belts seem in fine condition for hundreds of hours of use. The engine is easy to start and purrs. There have been several recent failures: last year, the snow chute broke and I had to buy a replacement. The year before the spring for the deflector at the top of the chute broke. I tied it to a fixed position with a rope. The universal joint for the direction of the chute failed too. I wired that up… The year before, a tire went flat but I was able to reinflate it and it’s holding up well today. The thing is old but still worth repairing IMHO.

This is a prime candidate for a buzz with “farmer’s rod” but it will require removal of the auger from the engine to do a good job. It has to be welded on both sides but only one is accessible and not at a good angle. That “L”-shape had been welded to the frame. No one had anticipated it would ever fail… :-|, so I can’t just unbolt it and do a perfect job of repair/replace. Nope. I have to break the whole machine in two to get at it and it’s one of the coldest days of 2015 here… It can wait until Tuesday when 10C warmer temperatures prevail. The machine is in the workshop but the welding will be done outdoors. Well, if it lasts another 20 years, that’s a better deal than buying a new machine for ~$1000.

UPDATE It took some time and effort but I finally figured out how to get the thing apart. Besides the obvious removal of 6 bolts holding both parts of the frame together, I had to remove the driving pulley for the auger. It was on tight but a clamp on the pulley, a wrench and a variety of prying tools did the deed. I waited for some warmer weather and welded the two parts of the bracket back together. It was made of about 2.4mm painted steel. I ground off some of the paint so that I could make good contact with a 3.2mm E6011 electrode at +85A. I’m not used to welding such thin stuff so I burned through at the edges but I had good penetration and welded both sides. It should last a few years. Now I just need to thaw my fingers, clean it, grease it and put it all back together. That’s the trick. When I was a boy, I often took things apart and could not put them back together. Failure is not an option when there are piles of snow on the driveway… 😉

About Robert Pogson

I am a retired teacher in Canada. I taught in the subject areas where I have worked for almost forty years: maths, physics, chemistry and computers. I love hunting, fishing, picking berries and mushrooms, too.
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2 Responses to Metal Fatigue

  1. DrLoser wrote, “Basically what we have here is a failure of elasticity. It almost certainly has nothing at all to do with this “original flaw” that you posit.”

    Uh, a single steel strap is bent 90º and cracks at the bend 20 years later… Sounds like classic metal fatigue to me. The original bending created a crack which grew after thousands of cycles of engaging the belt-drive for the auger. “failure of elasticity” may be a euphemism/synonym for that but it matters not. The single piece ended up being two pieces and prevented clearing my driveway at my convenience.

    I write this comment nearly three years later and the damned chute broke again. Sears used a plastic collar to connect that rattling chute in cold weather so that it could rotate to spray snow in arbitrary directions. Damned thing costs less than $20USD but my delivered cost will be $58CDN or more if a broker is engaged at the border… I may be better off to pound and cut a piece of sheet steel to the required shape. Damned SearsPartsDirect.ca does not ship to Canada… even though their site has the part and they sold the snowblower in Canada. Go figure… eReplacementParts.com charges more for shipping than the value of the part. It’s strange I can buy pounds of tree seeds from USA and the shipping is minimal… Can you say “RIPOFF!”, boys and girls?

    Chuckle… Apparently MTD made this machine as I can see the parts are identical in this video: https://www.youtube.com/watch?v=osq7WUW8FXI

    Sigh… MTD doesn’t sell parts in Canada either and neither does their agent, Atlasparts.ca… Finally ordered from discountonlineparts.com. Price is still a ripoff… but at least they ship to Canada.

  2. DrLoser says:

    An “L”-shaped piece of steel which anchored the sheath of the cable controlling the idler wheel on the drive-belt for the augur had snapped, right at the corner. Shucks. The thing is about 20 years old but metal fatigue in a cable-retainer? It must have been cracked since it was made and after thousands of cycles the crack grew.

    That’s not actually how shear via “metal fatigue” works, Robert. Let me quote from a random Wikipedia page:

    it would be absurd to use the linear theory to describe the failure of a steel bridge under a high load; although steel is a linear material for most applications, it is not in such a case of catastrophic failure.

    Basically what we have here is a failure of elasticity. It almost certainly has nothing at all to do with this “original flaw” that you posit.

    BTW, how’s the dirt-cheap Chinese “tractor” going?

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