2.2.13 – Failure of Model-15 Bottom Hub

In early September 2015, I received two separate notifications of major Bottom Hub failures of both versions of the latest Model-15 Blade. Failure of a Hub is something that must never happen as it could lead to the total blade flying apart, with potential dire consequences. Below are pictures of failed blades from the users.

BrushDestructor Bottom Hub Failure 1

BrushDestructor Bottom Hub Failure 2


I believe this is a combination of two issues; blunt flail-blades resulting in pushing ever harder into the job and possible inferior run of the high strength steel that has caused this problem. Fortunately, the BrushDestructor blade comes with two hubs and if one hub fails the other hub can carry the load long enough to warn that there is a problem. I have tried this scenario where one of the hubs is broken and the imbalance and vibration caused by this is so extreme that it is impossible use the BrushDestructor blade.

I asked the customers to clean up the top hub to check if there was any signs of failure and at that point the top hub was still sound. Although if you look carefully at the photo below, you can see what looks like the start of seed crack in the bottom of the top slot on one of the top hubs.


I also asked these operators to send me as many photos of the failed blade as possible and from talking to them and studying these photos I have came to the following conclusion.

That in the effort to make the blade as strong as possible, I have made it too stiff. This design has resulted in the creation of new weak points and when these points fail it fails quickly. In simple English, the blade had become too stiff and as a result losing some of its flexibility. With a blade like this, flexibility is toughness and it is toughness that makes a blade like this safer. I would like to point out that the Model-15 blade is an amazingly tough blade, but as it turns out, it is not indestructible.

As a result, I have finally come to the realisation that I cannot make the blade any stronger, yes it is possible to use more exotic steels used in the mining industry, but the minimum thicknesses these steels come in is 8mm and even if thinner sheets were available the cost would make the blade unaffordable. I could make the blade from 3mm instead of 2mm plate, but this would make it way too heavy to hang off the end of a brush cutter and would also add a lot to mailing costs.

Firstly, I would like to point out that the bush, bolt and nut developed for the Model-15 blade, has proved to be pretty much indestructible and their design will remain unchanged from now on.

Sample hardened bush and fasteners

But unfortunately in making the bush much harder, this has meant that some of the shock load that was once absorbed by the softer bush as shown below is now being transferred to the flail-blades and then on onto the hubs.

Old and New Bushes (600x376)


The older Mode-14 flail-blade had a very wide body, making it very stiff and unyielding. This lead to a number of failures of the blade, where a crack would be seeded from the sharp back trailing edge and the top third of the blades would crack and eventually come off. This was the point where the blade widened out and so all of the bending of the blade happened at this point.

BrushDestructor Flail Blade Failure

By putting a large curve to the back of the blade, has solved the problem of seeding of a crack at the back of the blade. You can see the large curve on the back of the flail-blade in the second image below. You can read more about this here This prevented the blade from cracking and failing, but all it did was transfer the bending loads on down to the hub.

The extension to the bottom of the flail-blade was in anticipation of increased wear of the pivot hole because I was going to a much harder steel bush than the blades are made of. When the blade steel was harder than the bush, the bush used to wear and needed to be replaced each time the blades were replaces, but when I made the bush much harder than the blade I expected the blade to wear. As it turns out I have now gotten the mixture of steels and hardness just right and there is no wear to either parts if used in conjunction with a PTFE Dry Lubricant spray.


This extension was also prevent the flail-blade from moving up and down within the hubs. This gave a narrower and quicker cut when cutting through hardwood saplings where before the cut was 5 to 6mm wide so a lot more wood had to be removed and the cut was slower. This also unfortunately took more flexibility from the blade and forced the hubs to flex more.

What makes the BrushDestructor blade so effective, is the sharp teeth, learning how to protective the teeth from being damaged and keeping them sharp are what make this blade so effective.


However let the teeth go blunt and this blade becomes no more effective than all the other metal blades out there on the market. Leaning into anything other than grass or watery weeds with a blunt blade, with ever more power is a recipe for disaster, sooner or later something has to give. I can remember when I was young and doing wood work class at school, the teacher always went on and on and on, that a sharp tool is a safe tool, the same applies with my blade.


With the Model-14 & Model-15 blades, I had doubled up the thickness of the anti-fouling blades, because with earlier models the flail-blade had punched their way through the anti-fouling blade stops. This happens when the teeth become blunt and the operator leans harder into the job to continue cutting, causing the flail-blades to fly forward and backwards against the stops with considerable force. Now that the double thickness anti-fouling blade stops could take the hammering without shearing, more of this high impact shock load was transferred to the hubs. If you look carefully at the photo below, you can see that the metal behind the anti-fouling blade is gone, as are the anti-fouling blades themselves. This would have then compromised the hub allowing a larger portion to break away.

BrushDestructor Bottom Hub Failure 3



I have not been happy with the variations in the quality of the steel for a while now. To date I have been ordering the steel in Australia through a specialist steel importer, but unfortunately I had no say in where the steel was coming from, because I had not been in the position to order large enough quantities. This has meant that there has been some variations in the quality of the steel, even though it is all supposed to be the same steel.

I have noted that some flail-blades wear better, than others from different runs. I have also noted that there is a difference in the sound the diamond ream makes when I ream out the laser corrugation for the pivot holes in flail-blade and hubs from one run to the next.


As a result of this problem, and the need to make the blade a little bit lighter to reduce international mailing costs, I have redesigned the blade which will now be known as the Model-16 Blade.

Below is a comparison between the Model-15 and Model-16 blades, the differences between the two models are explained below.

Compare 2015 and new 2016 blades-3 (580x281)



The new Model-16 Flail Blade, will have a curved uniform width over its whole length, to give the blade more flexibility to bend up and down over its whole length. When it hits a hard object the increased flexibility means, it will no longer put as much bending load on the top and bottom hubs.

20150707-6 - Blade,Flail,BC,BrushDestructor16 - 2mm. (580x267)

The new model will also have more area behind its centre of rotation, this will mean that it will fly smooth and flat through the air as the blade rotates. The old flail-blade had more area in front of the centre of rotation and this means that when you think of it as a wing it was very unstable. This meant that as the blade rotated, it would flap its way around and the only thing stopping it from flipping over and over was the top and bottom hubs. This creates another source of vibration and stress on the hubs as they have to retain the flail-blade in its horizontal position.

The extension at the bottom of the blade has been removed, which saves weight and gives more flexibility for the blade to move up and down between the hubs, thus reducing the bending loads on the hubs.

Since keeping the teeth sharp and balanced is important for efficiency of the BrushDestructor blade, this new model will be supplied with a sharpening jig, to help rebalance and sharpen the teeth after multiple sharpening touch ups or after an impact with a stone that has chipped away one of the teeth.

Sharpening Jig 2 (580x359)

A video showing how to use this jig will be put up on the How To Page as soon as possible.

The anti-fouling blades

The Model-16 blade will again now only come with a single layer anti-fouling blade stop, this will give the operator warning that they are pushing the blade too hard if they are sheared in half and lost. If the operator continues to push the blade hard, they will then start to crush the main lock nut guard as shown below.

Problem - Nut guard becoming severely deform ed.

They will no longer be designed to last the life of the hubs, but will become disposable items like the flail-blades and will be able to be purchased as spare parts like the flail-blades. Like I said before, the aim for this Model-16 Blade is not to make the blade yet stronger again, but to make it safer. Losing some anti-fouling blades is a much safer option than losing a hub.

Top and Bottom Hubs

The largest change with the Model-16 blade is the shape of the top and bottom hubs.

First off, the slots are only wide enough to accommodate one thickness of anti-fouling blade stop, thus reducing the size of the opening in the hubs and adding that little bit more metal to make them stronger.

The location of the rear anti-fouling blade has been moved back, so that the flail-blade strikes it further back along its length. This reduces the impact load, as the leverage effect of having half the weight of the flail-blade past the impact stop point has been eliminated, which in effect halves the impact load on the anti-fouling blades and the bushes well.

Compare 2015 and new 2016 blades-4 (580x522)

I have not been able to move the location of the forward stop, but if operators don’t cut on the backward stroke, the flail-blade shouldn’t be hitting the forward stop very much. You can read more about cutting on the backward stroke here.

The large bulk of metal behind the flail-blade pivot point has been removed. The idea for this was to help guide the flail-blade back in between the hubs as it was flicked backwards, which it did very well. What it also did was to absorb the energy from a bent flail blade, as it straightened the flail-blade out as it was being flicked backwards. The new design will reduce the leverage effect of a whip lashing flail-blade to the hubs as it rotates backwards in between the hubs.

The new design also adds more metal where it is needed around the anti-fouling blade slots to improve this weak point.

There will be only one 25.4mm arbor size, as only about 1% of sales are for the 20mm arbor. The larger Stihl and Shindaiwa brush cutter are the only brands that have a 20mm arbor on some of their larger models and Stihl has done such a good job of convincing all their outlet around the world that flail type blades are dangerous, so sales to Stihl owners has dropped through the floor. This has left me with a small stock of about 20 odd 20mm arbor Model-15 blades that I will cannibalise for spare parts and just throw away the top hubs.

The Steel

From this next run onwards, I will be sourcing the steel direct from the steel mill, where it will be run specifically for me and to my specifications. This will mean that there will be no variations between runs as I had to order many tons of the stuff. This mill will only deal with customers who make large orders, so for me it has been a very large investment to order and pay to store such a large amount of steel. Dealing direct has allowed me to stress to the supplier that for me to stay in business and to continue to order more steel, they must supply steel of the highest quality.

It is hoped that the design of the BrushDestructor blade will remain the same for some time now, to allow me to focus all of my time and energy on a BrushDestructor Clearing Machine.

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