2.2.5 – Bolt Fails causing loss of Blade

This is something that can never happen, as it could cause serious injury or property damage. This was a new prototype BrushCutter-Blade that I had asked a commercial customer to thrash as hard as he could to see how it could withstand the punishment from a high powered brush cutter since my 22cc brushcutter did not have the power to really work this new larger blade.


A blade had separated from the Hub at what appeared at first sight to be a sheared bolt, but I could not understand this as the bolts are more than capable of handling the shear load.

I asked the customer to pull the blade apart to examine the remaining two bolts and send me a picture.

Ware to bolts

You can see from the photo that the two remaining bolts have worn almost halfway through, so the bolt that gave way must have been worn even more than these two. I suspect this is the case because the customer said the BrushCutter-Blade was running rough for some time before it failed. This would have resulted from one blade protruding out slightly more than the others, causing a slight balance issue and this one blade would have been biting in deeper than the others. The customer reported that the blade separated from the hub when he was cutting through an old dead piece of hardwood, so this blade probably bit in a bit hard and was flicked back so violently that it sheared through what left of the bolt.

Studying the photo lead me to the following conclusions; that the spring bush was rotating on the bolt instead of the blade rotating on the bush, but the only way this could happen was it the lock nut was not done up tight. I then tried torquing up the nut very tight and yet the spring bush still ended up rotating and wearing the bolt. I tried marking the lock nuts to see if they had come lose, but they had not. It was studying the photos for a second time that gave me the clue; the spring washers were flat instead of a spring spiral. The only thing that would cause a spring washer to lose its spiral was heat; it was the friction between the blade and the bush that produced enough heat to cause the spring bush to lose its spring. It was this heat that also allowed the highly tensioned stainless steel bolt to stretch, thus allowing the spring bush to rotate. This problem only really presents itself when pushing the blade hard with a high powered brush cutter, that causes the Flail-Blade to flick back and forth a lot for a long period of time.


Was simply a matter of changing from a stainless steel button head bolt to a high tensile socket head bolt.


This high tensile bolt is not so affected by the heat, the larger hex drive allowed the bolt to be torqued up much higher to ensure that the spring bush will not rotated and wear the bolt.

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