Chipper blade snapped

[Khriss]

A good range of technically informed and working experience here, if i were Forst, i would be getting a new blade design out, pronto, and, god forbid before someone is killed. 

 

 Seen a few blades let go and frankly two instances where a bloke could have been hospitalised. I understand a corporate position on this but look at Boeing and thos Max aircraft.......... K

[Gray git]

This is why you NEVER get in the back of the van with the chipper running, just never know when something could let go!
Had a old farmi PTO machine spit out one of the twig breakers which went straight through the alley headboard, back window of the van and lodge itself nicely in the dashboard, would easily have killed someone if they were taking a piss in the chip.

[Ratman]

Once went on a torque course at work about 15 yrs ago at a place called “christie” and think it was Leeds way, they showed us all sorts of variations in torque complications when using added “lubes” they suggested using no lubes in most applications or if you “had” to, then only a very light oil.

A big no no in their opinion, and they proved it, was to never use copper applications or grease.

With these added to threads you have to increase the torque given to reach the recommended required torque needed for said fastening to do what it is required to do.

For example, they had two large 10mm thick pieces of steel about 2 foot by 1 foot square with holes all the way around the outside. To demonstrate they firstly started with fresh dry un-oiled/greased bolts, and just torqued them one by one in a clockwise rotation to the same torque on each. (We all know they should of torqued opposites or to a set sequence but they did it this way to highlight what actually goes on in the day to day out there in the real world with us human beings)

Typical example being to compare what happens when you remove and refit a wheel.

Once the plate was torqued they offered up a debate on what would happen next when we went back to the first bolt? This passed about 5 mins of time.

He went back to the first bolt and it clicked off at the same torque as initially tightened too.

Next trial was role repeat but with grease added to the threads, “cos we dont like seizure in our world do we” he said.

Torque sequence repeated, all the way round one by one in a clockwise direction, then again next up was the debate...... what happens when we go back to number one?!

 

I was gobsmacked at the outcome!!! Every bolt moved, and not just a little, but a lot!!! The grease disperses under pressure slightly and so the bolt becomes no longer fully tight.

They also said this is a major issue when using dedicated stretch fasters (stretch bolts, one time application bolts) this being because, when you tighten the stretch bolt to say 100NM or whatever, to achieve 100NM a the mating surfaces of your bolt and work piece you actually need to apply more than 100NM of force, otherwise the bolts will not be fully tight due to the grease as stated above, so to make it achievable because you are having to force the grease out of the way on all your threads, using more than 100NM recommended force, this in turn takes the bolt past its “elastic limit” therefore becomes..... stretched and unable to achieve its goal.

They recommend using the lightest oil possible in all applications to reduce the amount of friction that is produced on the threads that could end up giving the over torquing scenario.

I still personally use oil/grease/copper etc myself because i work in the real world as you guys do, and we like things to come off/undone again but it was a real eye opener for me, and so every time i do a torque on anything i ensure i go opposite every time and if i use anything above an oil, grease/copper etc i try to use least amount possible and torque up with a little extra nudge [emoji6] (rightly or wrongly) and for one time applications they get a dusting of tipping oil from my squirty can, nothing more.

Only cos i dont like things dry of course! [emoji4]

 

[Khriss]

I think people equate ' tight' with 'safe' you only have to get yr wheel changed at the local tyre fitters to find that . I torque wrench  wheel nuts now so if I or anyone else has to change a wheel - it will actually come off !! Had to stand on a wheel wrench once after car had new tyre fitted , to crack the bolt . K

[PeteB]

1 hour ago, Ratman said:

Once went on a torque course at work about 15 yrs ago at a place called “christie” and think it was Leeds way, they showed us all sorts of variations in torque complications when using added “lubes” they suggested using no lubes in most applications or if you “had” to, then only a very light oil.

A big no no in their opinion, and they proved it, was to never use copper applications or grease.

With these added to threads you have to increase the torque given to reach the recommended required torque needed for said fastening to do what it is required to do.

For example, they had two large 10mm thick pieces of steel about 2 foot by 1 foot square with holes all the way around the outside. To demonstrate they firstly started with fresh dry un-oiled/greased bolts, and just torqued them one by one in a clockwise rotation to the same torque on each. (We all know they should of torqued opposites or to a set sequence but they did it this way to highlight what actually goes on in the day to day out there in the real world with us human beings)

Typical example being to compare what happens when you remove and refit a wheel.

Once the plate was torqued they offered up a debate on what would happen next when we went back to the first bolt? This passed about 5 mins of time.

He went back to the first bolt and it clicked off at the same torque as initially tightened too.

Next trial was role repeat but with grease added to the threads, “cos we dont like seizure in our world do we” he said.

Torque sequence repeated, all the way round one by one in a clockwise direction, then again next up was the debate...... what happens when we go back to number one?!

 

I was gobsmacked at the outcome!!! Every bolt moved, and not just a little, but a lot!!! The grease disperses under pressure slightly and so the bolt becomes no longer fully tight.

They also said this is a major issue when using dedicated stretch fasters (stretch bolts, one time application bolts) this being because, when you tighten the stretch bolt to say 100NM or whatever, to achieve 100NM a the mating surfaces of your bolt and work piece you actually need to apply more than 100NM of force, otherwise the bolts will not be fully tight due to the grease as stated above, so to make it achievable because you are having to force the grease out of the way on all your threads, using more than 100NM recommended force, this in turn takes the bolt past its “elastic limit” therefore becomes..... stretched and unable to achieve its goal.

They recommend using the lightest oil possible in all applications to reduce the amount of friction that is produced on the threads that could end up giving the over torquing scenario.

I still personally use oil/grease/copper etc myself because i work in the real world as you guys do, and we like things to come off/undone again but it was a real eye opener for me, and so every time i do a torque on anything i ensure i go opposite every time and if i use anything above an oil, grease/copper etc i try to use least amount possible and torque up with a little extra nudge (rightly or wrongly) and for one time applications they get a dusting of tipping oil from my squirty can, nothing more.

Only cos i dont like things dry of course!

 

Thanks for the heads up and clarification. Tbh, I'll still use copper grease myself.....

 

Back to the OP, the bolts are in situ and still tight, 'nothing other than timber went through' but they bust and someone is quoting an inordinate amount of money for two bearings and a mornings work. What caused the failure he asked.....

[josharb87]

I read on Facebook the price went from 1500, to 1300, to 900+vat 

 

seems legit ?

[Gray git]

So adding lube decreased how tight they are so am I getting this correct should lesson the risk of overtightening and causing the countersunk heads cracking the blades apart but surely not leaving them slack so flapping around in the breeze

[Ratman]

So adding lube decreased how tight they are so am I getting this correct should lesson the risk of overtightening and causing the countersunk heads cracking the blades apart but surely not leaving them slack so flapping around in the breeze

“If” lube was used, i.e a heavy grease or copper application, then “potentially” it could have led to the bolts becoming not fully tight, which in turn could of allowed movement in the blades mountings, altering their chipping angle, that in turn could of made the blade want to bite off more than it could chew, resulting in the stress/impact/vibration/point of digging in, causing the blade to shear. “If” this was the case then the likeliest place of fracture/breakage in my mind would be from around the counter sunk bolt holes, the thinnest and weakest area of the blades.
Thats a theory but not defo the cause, as others have stated, it could just have been a simple flaw in the making or tempering of the blades? I just posted the torque scenario including lubes cos it was a real jaw dropper for me when i attended the course and its always stuck in my mind when doing things.

[Mesterh]

I guess the moral of this thread is its best to follow what the manufacture states regardless.  Lubricating a bolt affects the torque fairly massively from what I  can see, something I never realised before reading this thread. I always thought it best to lubricate the bolt, looks like I need to be more careful in the future. As previously mentioned I do give the blades a tap to check if they are cracked.

 

 

[openspaceman]

8 hours ago, PeteB said:

Tbh, I'll still use copper grease myself.....

 

FWIW I only use it on nuts I think may get corroded in harsh conditions, like shackle bolts, which won't need looking at for a long time. Blades need changing fairly regularly...