2.2.11 – Customers Blades break and testing to try and replicate failure

I have received these damaged blades from one commercial customer and feedback from a second commercial customer, both have somehow managed to crack and bend blades to the point where they break off.

Flail blades bending and breaking 1

As you can see, one blade has broken away at the end and the other blades are bent upwards with cracking easily visible at the front and trailing edge. The second customer which I have not yet received samples of the damaged blades, says this happened when he was clearing areas with lots of hidden rocks that he could not avoid hitting. Both customers have high powered commercial machines that can feed a lot of power into the blade.

Flail blades bending and breaking 2

I personal have not cracked or broken any Flail-Blades, even after many hours of testing with my 35cc Honda before and after releasing this latest model. So to see if more power was required to break a blade, I dragged out BRUTUS MAXIMUM, a 92cc piece of Chinese junk, that is difficult to start, smokes like an Old Russian Trabant car, vibrates your hands off and guzzles fuel like an old American V8. I could go on and on about all its faults and that will be for another article, but what it does do is feed a lot of raw power into one of my blades and is the only reason I brought it. If you see it on eBay and think you might want to buy it, DON’T BOTHER.

Brush Cutter Brutus Maximus 1

Br4ush Cutter - Brutus Maximus 2

In all the testing I did, I was unable to simulate the bending and breaking of the Flail Blades, but I did end up bring to light other problems. The testing was carried out with the above brush cutter, while trying to bash my way through hardwood saplings with a blunt blade and shear brut power over about a three hour period which was all my body could handle.


At this stage I have been unable to replicate the cracking and breaking of Flail-Blades as shown above, but I am certain the cause is a combination of, high powered brush cutters, the blades striking a glancing blow across the top of a hard object bending it up and flicking back against the Anti-Fouling Blade stop. Looking closely at the blades, I am suspecting that the bending must have happened very quickly, as there are no marks on the back of the blade where it would have impacted the hubs when it is flicking back at high velocity.


At this point in time I have only have feedback of the problem from commercial operators as commercial operators use high powered brush cutters and keep pushing ever harder to get the job done. I would like to point out here that I do not blame the commercial operators; they are paying a lot of money for my blade to do the job quickly and efficiently and shouldn’t have to worry about pampering their blade. In fact it is the needs of the commercial operators that have gotten this blade to where it is now and for the average home operator it should last a lifetime.

To give you an idea of how hard I have been pounding this blade, the testing has brought to light further weaknesses that I had not expected and will also need to be address in future models.

The photos below show the early signs of cracking to the bottom hub and crushing to the bushes after three hours of hard testing. My body also took quite a pounding, I ached all over and my fingers where white and tingled for days from the pounding and vibration. The way the bushes have been deformed, makes it look like they are made of soft lead, but this steel is very hard and requires a very expensive cobalt steel tap just to tap the M8 thread in the bushes so this should give you some idea of the forces involved to easily crush such a hard steel.

Cracks Appearing in Bottom Hub

Crushed Washers

If your blade looks OK, yet seems to vibrate more than you remember, than you may also have one or more bushes that are starting to collapse. It is obvious in the photos above that the bushes are not crushing evenly and I suspect this is due to one blade at some point impacting an object very hard slightly crushing its bush. When a Flail-Blade hits a hard object, it is flung back very hard against the Anti-Fouling Stops, which are now very strong and can handle the high impact loads. So now the weakest point is the bush, which now starts to crush because there is so much mass of the blade past the impact point, this impact point now becomes the new pivot point that the blade rotate around resulting in a hard hammer blow to the back of the bush crushing it. The steel used in the bush is way to hard to be crushed by the 1.3 tonne centrifugal load that the blade at full revs places on the bush.

BrushDestructor - How Bush Crushes

This crushing means that this blade sticks out a little further than the other blades and as a result impacts hard objects a lot harder than the other blades. This results in that bush getting more hard impacts and results in getting impacted and crushed more that the other bushes.

I am reluctant to make the bushes harder which I could do, as this would than mean they are harder than the bolts. This would make the bolts the weakest link and they would than crush now being the weaker material. If the bolts started to crush in, it would be impossible to undo the bolt to replace a worn blade and they could eventually reach a point where they would shear and release the blade which must never happen.


Remove the trailing edge sharp corner as any sharp internal corners can be a seed point for a crack and add a large radius as shown on the bottom right image. As can be seen from the image below, the longest crack comes from the sharp trailing edge internal corner, but at this stage I am not sure why a I would have expected to peel the front edge up and back instead of the back edge down.

Cracks seeded from sharp internal corners

Below you can see an image of the current and new Flail-Blade. Adding this large radius will also makes the blade wider at this point and hopefully the extra strength gained from this will reduce the likelihood of future failures. This change will be incorporated into the next run of blades.

Compare 2014-15 Flail-Blades

I have also addressed the problem of the crushing bush, with the totally new design of the bush, bolt and nut.

New stepped BrushDestructor Bush to handle extreme shear loads 1

The improvements with this design are;

Increased bearing surface, instead of trying to prevent the bush from rotating as was the case up till now. With this new design, the bush is free to rotate so the flail-blade can rotate on the bush or the bush can rotate in the hubs.

Saves money, Because the bush is now made from a steel that is much harder than the blade and should last as long as the hubs, this mean that replacing the Flail-Blades will be half the cost that it is now as it will no longer be necessary to replace the nut, bolt and bush each time you replace the Flail-Blade.

Solves the problem of extra drag and vibration caused by snagged grass, as the custom made bolt and nut have tapered heads to prevent grass snagging on them. These new bolts and nuts are made to the highest grade 12.9 engineering standard.

I will also try and resolve the issues of the bottom hub cracks and crush bushes and the results of further tests will be the subject of a future article.

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