Spring question

daTeacha · Oct 16, 2007

I bought a new spring for the idler pulley on my mower over the weekend. Got it all installed, remounted the deck, mowed about 15 minutes, and the spring broke. I called the dealer and they gave me a new spring Monday. The spring I installed over the weekend broke in exactly the same place as the original. The dealer parts guy said he figured it was a defective spring that they gave me and so didn't charge me for the new one.

While installing the new one, I noticed a bump on the inside of one pulley that turned out to be a bit of oak twig that was jammed in the bottom of the groove. Could something like this cause the spring to break? It made the belt tighten and loosen just a bit with every revolution of the pulley stretching the spring same amount each time, so I'm thinking that repeated motion might have induced metal fatigue in the same spot on the two springs leading to the premature failure.

Alternately, I thought spring steel was pretty resistant to metal fatigue. Comments and discussion will be appreciated, but not enough to get me to come and mow your place.

Farmwithjunk · Oct 16, 2007

Consider that piece of wood MIGHT have been even bigger at one point. Possibly, it's already worn down some from what it was in the beginning. What makes you suspect a problem now might have been very much evident as a problem. For some reason, the original lasted quite a while, where #2 lasted only a short time. Better chance than not, something other than "a bad spring" caused your problem. If spring #3 works well, it's probably safe to assume the piece of wood was AT LEAST a contributing factor, IMHO.

daTeacha · Oct 16, 2007

Everything rotates smoothly, there is nothing binding on the spring in question. The break was in the hook on the end of the thing, but in area that is free of contact with other metal. I just thought it was odd that both the original and the first replacement broke in the same place and am trying to find some explanation.

I got the deck in a trade just recently. I mowed quite a few hours with it before the first spring broke, but didn't notice the twig until last night. I just never heard of metal fatigue in a spring being a problem, but can't think of any thing else that it might be. The break is pretty clean, with no marks on the outer portions of the spring. Even the paint is intact up to the edge of the break.

SPYDERLK · Oct 16, 2007

Its failing on the hook of the end that is oscillating because of the twig - - right? That would be the highest stress area. Its ashame the system is so close to the springs failure limit, but if you get rid of that twig it should help. If the spring is tighter than necessary maybe you could also add an extension to loosen it a little.
larry

chrisjbell · Oct 16, 2007

There are a lot of things that can cause problems with pullys and belts on a mower deck. One of the big rough mowers at work just threw a belt because the idler pully was out of alignment. That'll take some welding to fix. Anything that causes the pullys to oscilate can contribute to things breaking.

I am a bit surprised that your spring is breaking, though. The point of the spring is to allow some movement in the idler pully. Get rid of your stick and see what happens...

zzvyb6 · Oct 16, 2007

I would guess that the twig is NOT the cause of the spring to break. More likely a heat treatment on the spring(s) problem.

But, now we are gonna have to know: How is spring #3 doing? I'm also willing to bet the wrong belt size (too tight) is a culprit.

Check the new spring to see if there are any metal forming marks in the wire in the viscinity of the breaks, cause by the winding operation.

If so, consider an alternative source for springs. If you have a torch, heat an old broke spring up and put a new tail on it.

daTeacha · Oct 17, 2007

The twig piece is gone, but I haven't run it yet. I got a new drive belt for the tractor (JD 240) and figure I'll put that on before mounting the deck again.

The area of the break is just after the windings as the steel forms the hook that holds the spring to deck frame. The other end, attached to the idler pulley arm via a second swinging arm, is fine.

I looked the thing over pretty carefully and could see no damage to anything other than the break, which is pretty clean. Like I said, even the paint isn't chipped.

The belt is the correct one. The dealer parts guy checked it. There is not a whole lot of tension on the spring. It's pretty easy to install by grabbing it with channel locks and pulling it into place, and no, I didn't damage it with the pliers since I grabbed it in a different place than the break occurred.

The physics teacher down the hall figures the twig caused enough repeated stretching in a short time to cause the failure at that point, which is the weakest part since it's where the steel is bent and twisted to form the hook that holds it in place. That sounds reasonable, but it still seems like they could have left a larger safety margin on this particular part.

ccsial · Oct 17, 2007

It seems odd that repeated stretching would casue that breakage but I suppose is could. I'd guess it is not as good of a spring as say, a valve spring on an engine. That is really repeated stretching. Then again, maybe springs last longer in compression instead of tension.