2.2.17 – Failure of Model-19 Pivot Bolt

PROBLEM

Failure of Model-19 Pivot Bolt

It is now 9th August 2019 and the Model-19 blade has been on the market since April 2019. At some stage in June – July 2019 I started using a new batch of Pivot Bolts and in the first couple of weeks of August I have received feedback from two customers of the bolt head breaking off from the shank.

The first customer reported noticing some vibration, not long after starting to use his new blade and upon inspection noticed that one of the bolt heads had broken off. Below are the two photos he sent me, one showing the missing bolt head and the other the teeth of the Flail-Blade showing almost no wear. This was the first report, ever of a bolt head breaking off.

I received a second report a couple of weeks later, with these photos showing that one bolt head was found broken off when the customer unwrapped his blade.

This is indeed a serious problem and I immediately stop selling all blades fitted with this new run of bolts. I then had to determine if there was a problem with the bolts themselves, or if I was over torquing the bolts on assembly. These bolts are custom made to the highest grade 12.9, which is as strong as you can get.

For the Model-14 Blades and earlier. There was a need for a high strength bolt, as that design required the bush be squeezed up tight between the top and bottom plates to prevent it from rotating. But the current design, with the stepped bush that is designed to rotate freely, the need for such a high strength bolt is not really required. However I decided to stay with the high strength bolt, as I expect that many of my customers, would not have a torque wrench, to torque up the bolts when replacing the Flail-Blades and with a standard 200mm (8″) long wrench (bottom wrench in picture below) it would be impossible for anyone to over torque this bolt.

I was able to over torque and snap the bolt using a 500mm long torque wrench and even then it took all my effort to break the bolt as can be seen in the picture below. Unfortunately I was not able to measure the exact torque at which this bolt broke, as my current torque wrench had rusted up inside and no longer works. but I expect it would have been many times the maximum recommended torque for this size bolt. I have placed an order for a new digital torque wrench, for future testing of future bolts that come in.

To see if I could replicate this failure, I have replaced the old pivot bolts and nuts on this blade with new ones selected at random from the latest bolt run. I have over torqued the bolts as hard as I can with the 200mm wrench and test in hard use to see if the bolt fails. If they should fail, I will have a record of how long before they failed.

This photo shows the testing of pivot bolts to see if they fail from over torquing.

I forgot to take a photo of the timer, but the first bolt failed at around the 2.3 hour make.

The second bolt failed at 2.6 hours and as you can see it failed half way through the pivot bush.

The third bolt, after a further six hours, has not failed so this bolt may possible one of the old bolts.

Below left, is all that is left of the second failed bolt, next to the first failed bolt, compared to a new bolt. In both cases, the failure happened along the threaded part of the bolt.

Next I tried two bolts from the same new run, but only very lightly torquing them up. This was to determine if the thread locker will prevent them from coming undone and also to see if the bolts still break. I have now done over six hours of hard testing and have had no bolt failures.

To give you an example of how strong these bolts should be, I have commercial customers who are still using the Model-15 blade with this same design pivot bolt assembly. They go through many sets of Flail-Blades a year and are still using the same pivot bolt assembly four years later.

CAUSE

At this stage the cause is undetermined, but I suspect that the metal treating process was not controlled properly, or the zinc treating process which has gone through a lot of changes in China to meet new pollution control requirements. I will update this, when I find out more on possible causes.

SOLUTION

I have stopped all new sales of blades fitted with these bolt from the date of receiving the second customer feedback of the bolt failure on arrival. Since then, I have been selling reassembled blades with bolts that had been put aside for some rework. This just involved some hand filing with a diamond file to remove some burrs to the bottom edge of the bolt head. Unfortunately, I am getting low on these bolts (plastic jar on the right) and am saving the rest of these to send out to any customers that might have a bolt failure. The larger container on the left is mostly new bolts, but about 25% at a guess, were the old run bolts and since there is no way of telling the difference between the old and new bolts, the whole container may have to be thrown out.

I will also notify customers, who might have one, two or all of the substandard bolts in their blades and request them to stop using the blade as soon as they feel any vibration. To notify me and I will send them out a set of new cleaned up bolts from the jar on the right.

After becoming aware that the bolt heads had broken off, continued to use the blade with a missing bolt head and in both cases the Flail-Blade did fly loose. It depending on how hard you are using the blade, as to how quickly the Flail-Blade will fly loose, in one case it took about 30 seconds when chipping away at a larger tree and about a couple of minutes when clearing some soft regrowth.

Me loosing a Flail-Blade is not a problem on our large property, as there is no one around where I am clearing. 

However I do not recommend that anyone should run their blade, with a missing bolt head.

I have ordered more bolts from a different supplier, unfortunately I have not yet received a confirmed delivery date, but I expect it to be at least late September early October 2019.

Very frustrating for me and you as well, but if you like you can send me an email saying you would like to go on the waiting list to be the first to get a blade when they are ready to go again.

When I get the new digital torque wrench, I will fit a blade with a new set of bolts, picked at random from the top of the box and torque them up to Grade 8.8 specification. If these bolts survive with no failures, then they should still be OK to use as Grade 8.8 would be more than strong enough for this design configuration.

In future, I will test a random sample of bolts from each new delivery, to ensure that the meet the standards for a grade 12.9 bolt.