Chipper blade problem i can't fathom

[rowan lee]

I bought a second hand pto chipper recently from a garden machinery dealer, who was selling on a trade in. 'A negri R220' chipper/shredder'.

 

It came with brand new blades and new bolts. I watched a demonstration before i handed over the cash. All seemed well. it was chewing away on thick Laurel branches.

I took the machine home and before i used it myself, i discovered that both of the new blades where badly damaged. each of the two blades was sheared in half through their short axis and looked like this:

 

Interestingly the damage occurred to both blades on the same side of the drum?

 

this could just be coincidence.

 

Additional points:

1) the last owner of the machine didn't have the correct flanged bolts fitted and had welded on standard hex bolts instead.

2) the branches used for the demonstration didn't appear to have foreign bodies embedded in them

3) A shear bolt protection shaft was not fitted during the demonstration,

(the negri pto chipper - runs on a direct drive from the pto to the drum through a speedup gearbox with out the use of v-belts)

 

Not knowing alot about chippers in general - just wondering if anything like this has happened to anyone before?

Is it possible the that the heat generated during the spot wields and the angle grind removal damaged the strength of the steel?

Was it lack of a shear bolt protection?

Is there something else wrong with the internals of the machine like bearings or something?

How could a couple of branches have ruined brand new blades?

Something not meeting the eye here.

 

The dealer has purchased me new blades, but as the machine is located 150miles away now, I'll have to install and test myself. I'm just concerned the same thing will happen again.

 

Cheers in advance for any enlightenments!

[Mesterh]

Possibly didn't clean the surface before during the blades. Or could be a set of poor/dodgy blades.

 

Did you mean that they have welded then ground down some nuts directly onto the blades? If so that would be my guess as to the prob. The heat can harden steel but make it brittle.

[Stephen Blair]

where the blades sit into must be perfectly clean, if not its just like a slab with not enough hardcore under it, they will snap. I think something has been rammed in there with great force to do this, if the edge of the blades are all chewed then its been metal, if not it must just of been too big a stick with too much force i reckon. Not familiar wit these machines but i have had a blade break once.

[treequip]

Assuming the leading edges of the blades aren’t severely chipped (which would indicate a foreign body) there are a couple of potential causes for this type of break. Either, (as others have said) the blade bed wasn’t clean, which I can see with a single failure, or the bolts were over tightened which is the more likely scenario on a double failure.

 

Andy

[rowan lee]

Possibly didn't clean the surface before during the blades. Or could be a set of poor/dodgy blades.

 

Did you mean that they have welded then ground down some nuts directly onto the blades? If so that would be my guess as to the prob. The heat can harden steel but make it brittle.

 

Nah - the bolt heads had been spot wielded to the blades, and before the correct bolts could be fitted the spot wields were cut.

Could this have been enough heat gneration to cause brittleness?

[rowan lee]

where the blades sit into must be perfectly clean, if not its just like a slab with not enough hardcore under it, they will snap. I think something has been rammed in there with great force to do this, if the edge of the blades are all chewed then its been metal, if not it must just of been too big a stick with too much force i reckon. Not familiar wit these machines but i have had a blade break once.

 

I never got to see the broken ends which were in the pile of chip at the dealers yard. the remaining blades were chewed a little, but this was probably from the broken bits flying round inside momentarily.

 

Appar. this machine is rated for 3inch (max), so yeah if the dude had tested with too big a stick, and without the shear bolt protection this is a strong possibility.

 

Heres a look at the machine in question, not a bad wee machine for a hobbist like myself, and at 1100 quid i kindnt turn this childhood dream of mine down.

[ame=http://www.youtube.com/watch?v=BsProBexev0]YouTube - R220[/ame]

[Mesterh]

Depends on how well it was welded ie good penetration:blushing: and how thick the steel is. But any welding and heating wont do it any favours tbh plus if the bolts had been tighten up after they had been welded then that could have caused a stress fracture.

 

If there is no damage to any of the anvil, casings etc in the machine ie the blades arent hitting anything then I wouldnt worry and just fit some new blades imo. As long as the bearings are good with no play at all and the surface the blades bolt to is even and not damaged.

[rowan lee]

Assuming the leading edges of the blades aren’t severely chipped (which would indicate a foreign body) there are a couple of potential causes for this type of break. Either, (as others have said) the blade bed wasn’t clean, which I can see with a single failure, or the bolts were over tightened which is the more likely scenario on a double failure.

 

Andy

 

this is an interesting note actually - didnt realise the potential for over tightening

must ring the negri dealer for proper torque readings.

 

Also as others have mentioned - the bed on which the blades sit - perhaps its bent or something? must have a closer look.

 

 

Note to all - thanks for your quick efficient and informative answers.

Great to have it answered on here

[treequip]

Ooooer,

 

A dealer machine with bolts and blades welded together, you have hardened and tempered high carbon blades, hardened bolts and plain mild steel mig wire (presumably).

 

That’s not what you would call best practice.

 

Take it back and ask (firmly) for a new set of blades and bolts.

[rowan lee]

Depends on how well it was welded ie good penetration:blushing: and how thick the steel is. But any welding and heating wont do it any favours tbh plus if the bolts had been tighten up after they had been welded then that could have caused a stress fracture.

 

If there is no damage to any of the anvil, casings etc in the machine ie the blades arent hitting anything then I wouldnt worry and just fit some new blades imo. As long as the bearings are good with no play at all and the surface the blades bolt to is even and not damaged.

 

Aye cheers pal - probably was tightened straight after wielding. I'd say this is probably a strong possibility as well.

 

Yeah i must thoroughly inspect the drum, castings etc before i fit these new blades, and maybe just do a hand test rotating the drum, before i attach it to a tractor. cheers Mesterh