Chipper blade snapped

[Khriss]

21 minutes ago, Gray git said:

I remember questioning that as my 25" torque wrench didn't go high enough for it. Our bandit 90xp is if I remember correctly 85ft pounds which is 115nm and that's fairly
F T!
Was chatting to a engineering friend and she reckons that over 250nm you need some form of thread lube to get a truer reading as any imperfections in surface can throw out being accurate and driving something with a tapered head into hardened steel plate like that is asking for problem especially if you add vibration and impact into the mix.

Thankyou - I couldn't remmbr wot our one was, those are abt 16mm bolts I think. Taper heads also.  And big wheel giving plenty of smack.  On a slant . Think Forst have seriously f#cked up this one.  K

[GA Groundcare]

21 minutes ago, Khriss said:

Thankyou - I couldn't remmbr wot our one was, those are abt 16mm bolts I think. Taper heads also.  And big wheel giving plenty of smack.  On a slant . Think Forst have seriously f#cked up this one.  K

They are M16 bolts, 10.9 grade (I think)

Its the same setup as Jensen, the Forst blades look identical to a Jensen. Whether they actually fit I have not tried.

We sharpen quite a few Jensen blades, never seen one crack though. Again the Jensen torque is 310nm. In the past I have torqued them to 250nm and even then it feels damn tight. 

[butters]

They are M16 bolts, 10.9 grade (I think)
Its the same setup as Jensen, the Forst blades look identical to a Jensen. Whether they actually fit I have not tried.

We sharpen quite a few Jensen blades, never seen one crack though. Again the Jensen torque is 310nm. In the past I have torqued them to 250nm and even then it feels damn tight. 

I’ve had 2 Jensen blades crack so not just a forst problem and only use genuine blades.

This blade on this machine if everything else is fine probably been sharpened incorrect and cause stress cracks.

[aspenarb]

Piss taking aside just for a minute, the design of the blade fixture as a whole looks a bit suspect. In my mind the base of the blade could sit on a machined ledge on the rotor so the bolts are only used to hold the blade back. As is the bolts and countersunk holes are taking an unnecessary hammering, they would take even less of a hammering if the blades were not canted away from the rotor like they are. If left like they are I would consider closing up the anvil/blade clearance to minimise blade stress. Take smaller bites.

 

Bob

[josharb87]

[Mick Dempsey]

Just saw that myself @josharb87  .

 

I don’t change the bolts as standard, but I do clean with a wire brush, then WD40, then blow them dry with an airline before retightening with the torque wrench.

Edited July 10, 2020 by Mick Dempsey

[josharb87]

Changing (blades) and refitting are two completely different meanings imo.
 

Easy to miss interpret

[doobin]

On 09/07/2020 at 14:03, aspenarb said:

Piss taking aside just for a minute, the design of the blade fixture as a whole looks a bit suspect. In my mind the base of the blade could sit on a machined ledge on the rotor so the bolts are only used to hold the blade back. As is the bolts and countersunk holes are taking an unnecessary hammering, they would take even less of a hammering if the blades were not canted away from the rotor like they are. If left like they are I would consider closing up the anvil/blade clearance to minimise blade stress. Take smaller bites.

 

Bob

Does sound like a shit design with a needless weak point built in.

[PeteB]

I got admit, I rarely changed the bolts or nylocs on my two GreenMech or Arboreater units back in the day, 150nm was tight enough! I over sharpened (depth wise) a couple of the Arboreater blades and they let go and made a mess, I just learned not to grind them back too close to the bolt holes. I never had a problem with the GM kit. 

 

Having worked for GM for a bit, I've only seen or heard of a couple of instances of blade/bolt bother. I think the copper slip thing is bunkum myself. A split or belleville washer may increase the torque value but I truly doubt that copper slip would. My opinion only and please correct me if I'm wrong. A true engineer to aircraft standards may change the fixings at every turn, on less critical items, punch a dot on each nut face then bin after 6 uses. A Ag engineer would use it until he could do the nyloc up by hand, but even so, FT is FT.

 

According to the OP, the bolts are still good, it would be interesting to see the bolt holes etc to see if a crack runs outwards. At first glance, the blade has folded. An experienced user told me that the blade angle is too keen, thus leverage comes into play. While he had these units, he experienced failures that got expensive so turned the infeed speed down- this effectively reduced the performance but greatly reduced the chance of a blade folding.

 

The models of chipper that incorporate a hydraulic tank as part of the wrap can experience significant damage which could result in massive oil loss, damage and cost if the blade punctures that and badgers the chassis too. Maintenance is critical, make time for it before it makes the time necessary and expensive.

[Mick Dempsey]

In fairness, everything else has gone on my ST8, but the blades have never let go.