This model has slight improvements to the Pivot-Bolt Assembly and spanner set and also comes with a totally new sharpening jig.
Is an improved variations of the Model-20, 19 & 17 blades.
Please read down through this page, to see the evolution of this blade, from the Model-17 through to the current Model-21 blade.
Model-21 – What’s Changed
The only improvement over the previous Model-20 blade, is the Pivot-Bolt which has gone back to a three piece design, that again uses a lock nut to prevent the bolt from coming loose.
Due to a couple of cases of the bolt coming loose and one reported case of a blade flying loose with the Model-20 Pivot Bolt design, (which is something that must never happen). This failure has necessitated the need to again redesign the Pivot-Bolt, to ensure that this does not happen again.
One of the main reasons for the previous design Pivot-Bush bolt for coming loose, was the low co-efficient of friction of the nickel plating, making it very easy for the bolt to rotate and come undone. This means that the Model-21 Pivot-Bolts are not plated and will need to be sprayed with a dry lubricant after each use to prevent rusting.
Model-20 – What’s Changed
This new Model-20 Blade, although at first glance, looks identical to previous Model-19 & 17 blades, has a major design change to make it easier to change worn or chipped Flail-Blades out in the field.
Model-20 – What’s Changed
The Pivot-Bolt is now made of two parts, instead of the previous three parts and the bush-nut and bolt are now made of the same material. This means that the bolt is no longer a high tensile bolt, but a high impacts and wear resistant bolt.
The aim of this design, was to make it easier to for those who spend long hours out in the field, to easily change a worn or damaged Flail-Blade. This was to be achieved through three design features;
The first being – A very low profile head, so that an impact with it and a stone or wood was less likely to rotate the bolt.
The Second being – A left hand threaded bolt, where any impacts creates an anti-clockwise turn that wants to do the bolt up, not undo the bolt, as with the previous right hand threaded bolt that required loctite to keep it from coming undone.
The Third being – A super fine M10 thread, which means it takes less turning effort to tighten the bolt enough that it will not come undone.
However, although the non plated prototype worked well, the production run nickel plated version does not work so well and as a result, the bolt still needs to be done up with loctite. See below for the reason and development of a new pivot bolt design.
A big problem I have now is that at a guess, almost half of my sales are now coming from word of mouth from satisfied customers and I do thank them for that.
But what is happening, is a person gets rave reviews from a neighbour and then goes and orders a blade without reading any instruction on the web site on how to best use this blade to get the best results. They then go out and used this blade, just like any tri-blade and very quickly lose the sharp cutting teeth. I have had customers think, that the pivoting flail blade is to protect their machine from impact shocks. However this is not the case, the flail-blade design is to protect the blade from impact fatigue so that it does not fail and fly apart sending out bits of metal in all directions.
However as sales has increase to United States, I have had some feedback of Broken Flail-Blades and in one case where a broken Flail-Blade went through the plastic guard and grazed that operators leg. Loosing a Flail-Blade, is something that must never happen and over the last ten years, I have continually focused on how to make the blade stronger and safer. But they have now reached the limits of what can be achieved with metals available, weight limits and design.
All failures have been with those customers that use powerful brush cutters and crews of commercial operators where those operators have little respect for their tools. From the feedback I have had, all of these operators are using bicycle handle brush cutters and with these machines it is much harder to have quick control of the cutting head. This means that it is harder to avoid hitting an object and taking the edge off the cutting teeth. It then become natural to lean harder into the job to compensate for the less efficient cutting and these higher powered machines are able to maintain high revs under these increased loads. I know, I have done it myself.
High impacts at high speeds, will eventually result in failure of the impact-stop (as can be seen below). This rubber impact stop I am told, failed within the first hour of use and yet with all the hours of testing I have only put a few small chips in one of these impact stops. This is the difference between a 35cc and 50cc machine and why I no longer recommend fitting this blade to 50cc machines.
Please scroll half way down this page, to see what went into developing this Impact-Stop.
This blade has been designed to use sharp teeth to cut through material, not brut force, That is what a tri-blade is for. A blunt blade, pushed hard with more higher powered machine has now proven to cause eventual damage to the Rubber Impact Stop and metal fatigue and failure fo the Flail-Blade.
So, if you own a very powerful brush cutter and don’t think you can adapt you’re cutting technique to use sharp teeth, not rev and bash to do the cutting. And to run the brush cutter a lower revs, that best suit this blade. Then this is not the blade for you.
In one case, someone managed to get the Flail-Blades so hot, that the metal must have lost its hardening to take on a permanent a bend as you can see below. I have not been able to replicate this, so I guess my brush cutter is not powerful enough to put that sort of energy into the blade.
This blade get its efficiency from sharp teeth, but the very nature of brush cutting means that sooner or later your blade will hit something. So the next option is to make it quick and easy to swap out blunt Flail-Blades out on the job.
However, to achieve this goal, has taken a year and five different prototypes you can see below, to get to this new Model-20 blade.
Prototype-1. Simple two diameter oval shaped pivot hole design, that relies on centrifugal force and gravity from preventing a Pivot Bush from coming out.
This actually worked quite well, but wet grass would get compacted into the oversize hole, resulting in vibration. Eventually I lost a bush, when I had the brush cutter upside down trimming the top of a small tree. So this designed failed.
Prototype-2. A simple oval shaped pivot hole in both the Hubs and Flail-Blades. This design required that the Flail-Blade could only be in one position for the oval head bush to come out. Actually worked very well, until the pivot hole gets gunked up with plant sap and fibre. Resulting in blades out of position again and vibration. And if you forgot to clean it after use, the dried sap makes it very difficult to remove the pivot pin.
Prototype-3. I really thought that this would be the one, a pivot bush held in place with a cir-clip. It had big washers top and bottom, to prevent plant mater getting into the pivot bush and was made eccentric, so as the washer mover back and forth as the blade spun up and down. The idea being that this movement would scrape any gunk away from under the bush. There was also an unexpected benefit, in that the eccentric washers positioned themselves to absorb vibration a bit like how a washing tub self balances itself in a washing machine making for a very smoother running blade. However, plant sap still got under the washers, gunking everything up and the first time I went to replace a Flail-Blade in the field, the cir-clip flipped off the pliers, never to be seen again.
This design also proved to be twice as expensive, with the large eccentric washers and a custom made cir-clip as there was no off the shelf cir-clip that would do the job and I would need to supply a cir-clip pliers as well as not many people have these laying around.
Prototype-4. Now I though I was really clever with a two way cir-clip, that I could find no record of existing anywhere. A cir-clip that when closed in, was fitted into an encapsulated nut and when opened out, released the whole nut from the locating shaft. The design worked perfectly, except for that dreaded gunk clogging up the cir-clip so that it became a major exercise to clean it out before you could open the cir-clip to separate the two parts.
The other down side was the large head, would snag grasses and branches. Creating drag and vibration like what was caused with the early use of, off the shelf lock nuts and this design stilled required you to carry a cir-clip pliers with you in the field. So back to the drawing board.
Prototype-5. So now the only thing left was to go back to a threaded design, but how to make it simple and easy to undo out in the bush. It actually took some time, before some unconscious thought whacked me around the head saying, (why do you have to go to so much effort to stop the bolt from undoing)
When it finally came to me that most of the impacts the bolt head has, in on the outside edge and since the pivot-bush is mostly locked up due to the high load centrifugal load from the Flail-Blade. The bolt wants to undo itself, with each strike. This has meant that the bolt had to be torqued up tight, locked with a lock nut and also fixed with loctite for good measure. So why not make the bolt with a left hand thread, since it is custom made anyway and would not add any cost.
Because of Covid 1984 t took three months to get my fat little hands on the first prototype and woh all I can say is why didn’t I think of this before. My shins are still sore, from kicking myself for not thinking of this sooner.
I then did quit a number of hours with the bolt loose by half a turn to see if it would come undone and it never did. But what I did find, was that that dreaded sap would get in between the bolting faces. Because the bolt was loose, a slight knock on the inside edge would loosen it a bit more and more sap would get in between the bolting faces, until it would build up to about 2mm thick. However it would never get any thicker than this, as the G forces, would at some stage throw it out and then it would start building up again. This is why the bolt still needs to be lightly done up and the lever length from the supplied Jig Wrench is all that is needed.
I have also made the heads lower and of the same 19mm (1/2″) socket size as the supplied brush cutter wrench. So that if after a long period of time the Jig-Wrench does not give the leverage to undo the bolt, the brush cutter wrench will.
Below you can see how the supplied tools to make it easy to undo and change Flail-Blades out in the field. I have found through quite a bit of use, that if the bolt is done up with the thumb using the Jig Spanner, it is easy to undo with your thumb.
This allows those, that go out into the bush for many hours, to quickly change Flail-Blades from a pouch with sets of spare sharp blades using these tools.
Unfortunately Murphy has had to have the last word and put a spanner in the works. The prototype was supplied black, as the time delay to get it plated was too long and as I have now found out the nickel plating has changed everything.
The nickel plating, has changed how easy the bolt threads into the bush-nut and of cause how easy it undoes and I have had a report back from one customer that a bolt has come out and disappeared.
I guess I should have known this, as the metal used in the bush has a high nickel content to give that metal a lower co-efficient of friction to minimise wear.
This means that these nickel plated bolts will need to be loctite-ed in as with the previous design and so not as easy to undo out in the bush.
I have contacted all the customers who have received a blade up to this point, stating this and told them that they will need to get loctite for their blade and loctite the bolts in. To do regular inspections to see if the bolt has come loose and to refit bolts with loctite at the first opportunity. As can be seen in the photos below, it is very easy to spot a loose bolt by stopping the blade and looking down at it.
I have also asked them if they would have any issues with the Pivot-Bush being supplied in future blades, being non plated and those that have got back to me all said that that they did not.
With the next order I put in, the pivot-Bush will not be nickel plated but left black from the heat treatment. This will mean that it will need a spray with a dry lubricant before putting the brush cutter away for storage.
The other new feature to the Model-20 blade, is the sap clearing channel to both Top & Bottom Plates. Most of the time it will clear out the gunk from around the bush in general use, but some sticky saps it will not.
What I do now, is every time I refuel and before putting away at the end of the day, I put the bush cutter socket on the nut and spin the blade around several times which spins the pivot bush quickly. This shaves out dried plant matter in the bush and frees up the Pivot-Bush so that it rotates freely again.
Model-19 – What’s Changed
To allow this blade to be fitted to new high-powered brush cutters, that have deeper mounting arbors to centre thicker metal blades to handle the higher power. The standard depth of the locating arbor use to be 2mm, but some are now 2.5mm and even 3mm deep.
This photo shows how a deeper arbor, popped off the Nut-Guard, when a customer went to mount it to a larger brush cutter.
As you can see below, most brush cutter had an arbor depth of a little then 2mm to allow a 2mm thick saw blade to be done up tight.
These photos below, were sent by a customer who had to make a 1mm thick spacer, so that the blade can be done up tight against the arbor mounting face. Without this spacer the blade would free spin, as even when the lock nut was done up tight, it could not lock up the blade.
This resulted in the blade not being able to be done up tight enough to prevent slippage, which the photo below shows the scratching away of the writing on the blade. You can also see the new arbor fitted to this old blade and see the difference between the old and new Nut-Guard in the photos above and below.
Below you can see the difference between the Old and New Nut-Guard. The New Nut-Guard can handle 25.4mm arbors up to 2.5mm deep and 20mm arbors up to 3.5mm deep.
With an increase of radius to the back edge of the blade to help reduce the chance of a crack seeding and leading to the Flail-Blade breaking off. You can read more about this failure by clicking – here
As you can see by the crystalline structure of the break, the steel is very hard, but in the interest of safety, I have made the steel slight softer. This will make it tougher and springer, but will also mean that the teeth will not hold their edge quite as well. Sadly there is always a compromise.
Plus there is a slight increase in size of the front shredding teeth, to extend the ability to shred leaves and branches, even when these teeth start to lose their edge.
One way to reduce stress and shock loading to the Flail-Blade, is to keep the teeth sharp, because to keep expected cutting efficiency up, you end up pushing the blade harder into the job. Blunt teeth have two bad side effects, Firstly because they do not cut as well, they tend to bounce away from the cutting surface, resulting in more kick back and shock loading to the blade and secondly blunt teeth generate a lot more heat and this heat can quickly build up to the point where it can soften the steel causing a permanent deformation of the Flail-Blades. Yes really!
To help solve this problem, I am including a small Diamond Hand File, in Options 3 to 8
This will help make it much easier, to touch up the teeth in the field, keeping them sharp. I have specifically chosen to keep the file small, to make it easier to carry it with you.
But one word of warning, if you live in an area of Australia, that has Bower Birds, don’t leave it lying around. I have already lost one file, that I left sitting on the back of my Ute, to a very quick Bower Bird.
Model-17 – What’s New
Below are details on the development of the new Model-17 Blade, compared to the Model-16 blade. I do recommend you read down this page to see the amount of work that went into developing the Model-17 blade.
This model-17 had a lot of changes, compared to previous Model-16 blade, all these improvements, are a direct result of lots of feed back from the many current users of BrushDestructor Blades.
This feed back has resulted in a new blade designed specifically to meet the users requirements, a true bottom up design. Meeting all the many requested requirements has resulted in a new model that is;
To withstand extreme punishment required by commercial operators with high powered brush cutters.
Smaller in diameter
By 34mm, due to the majority of sales of this blade now going to residential property owners who are buying this blade and who’s brush cutters are generally 30cc or less.
Being smaller in diameter will also allow it to better withstand the extreme stresses from high powered machines, that can maintain high speeds under while under high loads.
By 22% then the previous model-16 blade, again making it more suitable for the lower powered machines and to make it that little bit easier on commercial operators who have to swing a brush cutter around all day.
Below is a drawing comparing the new Model-17 blade to the older Model-16 blade and below that is a more detailed description of what has changed and what is new.
Top and Bottom Hubs
Have been made smaller to reduce the centrifugal stresses on the hubs. This also makes the blade lighter and more easily powered but smaller brush cutters. The number of Anti-Fouling-Blades have been reduced to three, the slot opening size have been reduced by a third and the slot location have been moved to put more metal where all the high tension loads and bending stresses are. This results in a hub that is considerably stronger than the previous model.
Have been made smaller and lighter to help reduce the overall weight of the Blade and the back edge redesigned to reduce the impact loads to the rubber impact stop.
Are considerably more efficient at clearing away stringy grass and vines, even though they are a third the size and there are now only three anti-fouling-blades instead of the six found on the previous model. After carefully studying exactly how these long grasses start to wrap around the gear head, this new design shreds the grass as it is starting to pull tight around the gearhead and directs it over the top of the gearhead and guard. The previous design tried to wind the grass away from the gearhead and under the guard.
Below you can see how long wet grass, has not been able to wrap around the gearhead and stall the brush cutter, which used to be a major problem when I was using a line triller.
New Nut Guard
Is now included and become an integral part of the design improvements. The inclusion a custom designed nut guard, has allowed the removal of the large hole opening in the bottom hub, that has been the weakest link in the design right from the very first Model-10 Blade.
This nut guard, is made from the same grade and hardness of steel as the flail-blade pivot bush, to give a very long life.
New Rubber Spring Washer & Impact Stop
Replaces the steel spring washer and the need for the Anti-Fouling-Blades to act as impact stops for the flail-blades. The design and search for materials has been one of the biggest challenges faced to make the model-17 blade stronger and tougher than any blade before it.
This model has taken more time, more prototypes, more testing, more punishment to machine and more costs than all the previous models before it combined.
I recommend that you scroll further down this page to read more about the development and testing that has gone into making this blade what is is now.
Due to the extra two expensive parts that have been added to this model blade and the shear time and number of prototypes involved, to meet all the goals to make this the best brush cutter blade in the world. Unlike all earlier models, where I personally carried the cost, this will the first blade, where the actual development cost are being amortised into the cost of the blade. The recouping of this cost, will then go into the development of the BrushDestructor Clearing Machine, which is going to require considerable capital to make a reality. Your support, by buying this blade, will help make the new BrushDestructor Clearing machine.
As I have said elsewhere, it is the blade that is the business end of any brush cutter. All the brush cutter does, is provide the power and direction. Pick any brand of brush cutter and you will get more done with a 25cc model & one BrushDestructor Blade than with a 50cc model using the manufacturers multiple range of blades and still be cheaper off.
Right from the very start, the ability to do the job the BrushDestructor blade was design to do with ease and safety has always taken priority over cost and that requirement has not changed with this blade. If you should have doubts that this blade cannot possibly be worth this price, please take a read through the Customer Reviews Page.
I would like to add here, that I make no apologies for the high price of this blade. It has been designed and built to do a very demanding job, while requiring very low maintenance and with the exception of the replaceable flail-blades should out last most brush cutters.
This blade does not mind if it is being driven by a cheap Chinese brush cutter that cost less than it does, or by a $1000 plus commercial machine, the results are the same and only the life expectancy of the brush cutter will be at question here, not this blade.
Development & Testing
The development of this Model-17 blade, has taken the longest time, with the most number of prototypes and is the most expensive so far to date. Below is the story of all the prototyping and testing I did until I was 100% happy with its performance and toughness.
One of the greatest weaknesses right from the very first model-10 blade, has be the large opening in the bottom hub to accommodate most the different sizes of nut guards that come standard with the wide range of brush cutters.
It is this large opening that has created a weakness in the bottom hub for all the models till now and although the number of failures of the bottom hub has been extremely small. There has never been a failure of the top hub which would mean the completer failure of the whole blade. However, as far as I am concerned, any failure of a hub is unacceptable.
The obvious solution here was to supply a much smaller nut guard as a standard part of the blade and so reducing the large opening in the bottom hub. Move the Anti-Fouling blades so that they no longer act as an impact stop and include a rubber impact sheet sandwiched between the top and bottom hubs as can be seen below to absorb the energy of the flail-blade flicking back.
I spent a lot of time researching all the different types of rubbers that could withstand high impacts and found one that was used to line the blades of snow ploughs and tip trucks to prevent ice from sticking to the steel in the northern US States and Canada. Below left, you can see how the rubber was supposed to prevent metal to metal contact between the Flail-Blades and the Anti-Fouling blades that results in high shock loads transferred to the hubs.
Unfortunately, as tough as this rubber is at withstanding high impacts, it proved to be very weak under tension loads. It took less the 30 seconds at full revs for the rubber to rip itself apart from its own weight under the high centrifugal loads as you can see above, before I even had a chance to test its ability to withstand the impact loads. Never did find the other bits of rubber.
Once the rubber was gone, the flail-blades quickly sheared their way through the anti-fouling blades and started impacting the custom designed nut guard.
The flail-blades were now free to hit one another, with the resulting damage after a very short time of use and rendering the blade inefficient at cutting anything.
Below you can see the damage that was caused to the nut guard now that it was acting as a stop for the flail-blades.
This guard was made from the same steel and heat treated to the same hardness as the the grade 12.9 bolts, so it gives you an idea of the forces involved when a flail-blade is flicked back after an impact with a solid object. It should also give you an idea of the energies involved that a standard supplied fixed metal blade is also subjected to and which has to be absorbed by the brush cutter.
I then tried a fibre reinforced conveyor belt rubber, but because of the design of the reinforcing fibre weave, all these rubbers are very strong in tension in one direction and not in the other. Under high centrifugal loads, this resulted in no stretch in one direction and a lot of stretch in the other as can be seen in the photo below where the stretching rubber polished away the plant sap. What was even more amazing, was how much vibration the stretching rubber caused.
I then made a mould to press my own reinforced rubber with three layers of fibreglass cloth laid at 120 degrees to each other. The fibreglass cloth turned out to be a too tight a weave and it quickly separated into three seperate layers after only a couple of minutes.
I then found a loose weave dyneema fabric, again laid in three layers a 120 degrees to each other. It actually worked very well, but eventually started failing after a couple of days of hard use.
In the close up below, you can see how the compressive impact shock waves were blowing the rubber away from the reinforcing fibre.
Finally I found a rubber that would do the job, it is the hardest version of synthetic rubber that is used in suspension and shock absorber bushes in cars. It easily pours into a mould as a liquid and can be cast into any shape, but to get the hardness I require the rubber has to cure under very high pressures and with this first prototype only the first sample came out of the mould to match the thickness tolerance I required. Because of the large flat surface and pressures involved the mould was not strong enough and quickly distorted from the high pressures.
The video below shows the extreme testing this blade was put through, to see if this rubber could withstand the punishment and as you can see in the video and close up photos it survived the test unscathed. Unfortunately the brush cutter did not fare so well.
I can tell you one thing, I would not want to be standing anywhere in the vicinity of this test if I was using a standard supplied three star blade as I am fairly certain that it would shatter and explode parts of the metal blade in all directions.
It was at this point that I thought the design was finalised and was gearing to go into production when I received three emails in close succession about how amazing the blade was but it would be better if it was a just that little bit lighter.
So I decided to make the blade a bit smaller in diameter, which would reduce the stresses on all the parts. Making it lighter, would also make it better suited to lower powered brush cutter as I found that most of my customers now, have brush cutters under 35cc.
Going smaller however created another problem, where to put the anti-fouling blades? This was a part that I did not want to remove, as it is what prevents long stringy grass from wrapping around the gear head. So the only place for it was as shown below. Enter a new shape rubber impact hub form a new stronger and more expensive mould.
The problem with this design, is the area that takes the hardest hit from the flail-blade has the least amount of travel and even this very hard rubber was blowing itself apart from the shock of the impact. Unfortunately I do not have a close up lens to show you how the rubber is tearing itself apart, but if you look closely you can see the spiral tares in the rubber.
This rubber is more than capable of withstanding very high loads without any loss of memory for its original shape and theoretically can easily withstand the load forced placed on it from the flail blade. However what does the damage is the speed of the impact, as the velocity of the shock wave increases the rubber is no longer able to give way quickly enough and that energy blows the rubber apart.
To give another example, I can remember as a kid shooting at tyres. a 22 slug would often bounce off a tyre, but a 222 slug would punch right through both side walls of that tyre.
Putting it simply, the higher the velocity, the more the rubber starts to behave as a solid, keep increasing the velocity and it will behave as a piece of ice. This might mean that this blade may not be suitable for very high powered brush cutters that can be pushed really hard without losing speed. The steel can handle the punishment, but maybe the rubber cannot, only time will tell me this as I get feedback from the commercial customers.
This model has only very slight changes of the location and angle of the anti-fouling blade and the removal of rubber where the velocity of the impacting blade is the highest. I had hoped that the rubber would take a good portion of the impact shock and the anti-fouling blades would take the rest of the hit that the rubber could not handle.
Below is the red rubber sample B, which still showed signs for disintegrating in a very short time, although it is not visible in this photo. unfortunately as I found out later, adding colouring to the rubber makes it more susceptible to fracturing under high speed impacts even though it is the same hardness as the yellow sample.
Below is one of the original yellow samples that was too thick to fit in-between the top and bottom hubs. It was sanded down to the right thickness and then cut the the right shape.
This uncoloured yellow rubber is able to absorb the impact shocks, but even as hard as this rubber is, it was compressed 9mm and there was still enough energy left in the flail-blades to shear through the anti-fouling blades as can be seen below.
So the question here is, how do I maintain the success of prototype 17.2, but make it smaller and lighter and that is to move the anti-fouling blade away from the impact area as in the Model-17.2which can see below. Unfortunately to keep the weight and hub diameter down, the anti-fouling blades are so small that I expected the be near useless.
Below you can see that I am continually shaving away the hubs to reduce weight and test that the strength is still there. I had also hoped to keep the flail-blade unchanged for longterm ease of parts supply, unfortunately the only way to protect the rubber from high point impact zones and failure of the rubber was to redesign the back of the flail blade.
Another problem that I encountered with this design, using the Model-16 flail-blade, was the amount of spring back from an impact. This meant that the flail-blade would bounce back and forth who knows how many times as you cannot see it, you can only feel the extra vibration it causes. This bouncing back and forth would only stop as a result of either slowing or speeding up the blade until it settles back into its normal cutting position. Keep the brush cutter at the same speed and the flail-blade would continue bouncing back and forth, causing a lot of vibration and strange sound.
Anther added cost, was to get another high pressure mould made, this time just a round disc, which allows me to freely adjust the outer profile until it is able to withstand the impacts and at the same time cause minimal bounce of the flail-blades.
Below is the model-17.5 fitted to the brush cutter and if you look carefully you can see how small the anti-fouling blades are. I had thought that these very small anti-fouling blade would be near useless, but infact they work better than the much larger multi anti-fouling blades as found on the model-16 blade. An unexpected win, after so many failures.
After a couple of tanks of very hard use, the bolts that clamp the gear head to the shaft had stripped the aluminium thread allowing the gear head to work its way off far enough to disengage from the drive shaft.
Here you can see the angle of the gear head, another 10mm and the whole gearhead, guard and blade would have come off.
This is what I was testing this blade on, cutting up an old fallen gum tree where the wood is dry and very hard. But this is the most extreme conditions that I think anyone would ever put a brush cutter through.
Below are the two heavy duty Stihl gear heads I have gone through in the precess of extreme testing the Model-17 prototypes. With one, all the threads in the aluminium body were stripped out and the other the gears were worn away enough for them to no longer mesh. The problem was not the hardness of the gears, but that hypoid gears should not be lubricated with grease when the meshing speed exceeds 200ft/min. This results in the grease being compacted against the casing wall and not enough on the gears to lubricate them.
I like pulling them apart to look for the problems and weak points so that the gear head I design in the future will be able to handle this sort of punishment. The seals need to improved so that a high pressure hypoid gear oil can be used as in the diff of a car, instead of grease.
So before you Armchair Generals out there, who have to point out that this is not what a brush cutter is supposed to be used for, or as one very intelligent General put it, USE A CHAIN SAW MATE! (actually he said dickhead, which I found very offensive) It is sad, that the only contribution some people can make, to making the world a better place, is to run down someone else that is at least trying to make a difference.
If you are not happy with the contribution I am trying to make, please don’t email me about this, just leave this site and go and vent your frustration somewhere else.
And yes I do get emails like this and worse, which is why I deselected the comments section on the pages of my web site.
As far as I am concerned, I am ecstatic that the brush cutter breaks before the blade, but that is beyond the scope of a (smarter then the average bear) General who only looks at the pictures and does not read or take in what has been written on these pages.
Below is the next version, the hubs are the same but the flail-blades rear edge and the outer profile of the rubber has been changed to see if I can reduce the high speed impact load to the point that the rubber will last a long time.
Below is a another new gearbox attached to the brush cutter to continue testing the the revised model.
Below is the blade after about 15 hours of really hard clearing of rainforest undergrowth, ranging from long stringy grass to sapling regrowth and fallen dry and hard branches up to 200mm. Even though the rubber has abrasion marks on the side, which was from silver paint so that I could see how the rubber was compressing. Close inspection showed that there is no signs of failure of the rubber.
Unfortunately, there was still some acceleration induce vibration, caused from the flail blades swing back and forth, that was not a problem with earlier heavier version. After a lot of observation to try and determine the cause, I found out that the weight ration between the hub and flail-blade is critical and so I determined that I needed to add extra weight back onto the hubs as I did not want to remove any more weight from the flail-blades. I am guessing that it was this same weight ratio issue that caused vibration, with the two bladed Model-15 blade.
Below are some pictures of the final Model-17 blade, which will be released in July 2017. It will sell initially alongside the Model-16 blade, until the Model-16 blade sells out. Unfortunately, due to the extra parts in this new model, it will have to sell for at least $60.00 more than the Model-16 blade.
For the last couple of months now, I go out early every morning to burn up a tank of fuel putting this blade through it paces and every time I use it, I am amazed how well it cuts and mulches up everything that gets in its way. Even though this blade is smaller in diameter and lighter especially the flail-blades compared to the Model-16 blade, as you can see below. It runs that little bit smoother, cuts that little bit better and clears grass that could grind the blade to a holt just that much better. In fact, with the exception of the price, everything about this blade is better than the model before it.
Perhaps I should add here, that all the testing has been done with a modified long shaft brush cutter, with a Stihl gearhead and guard and powered with a 35cc Honda 4 stroke engine. This brush cutter is so good, that I couldn’t bring myself to waste money on any other brand of brush cutter to further test this blade. I am not saying that these other brands are no good, just that they have all focused all of there development on bicycle hand models, to the detriment of loop handle models, which I think is a big mistake as none sell higher powered loop handle brush cutters.
Below you can see the final shape of the rubber impact stop.
Below is the Model-17.7 fitted and ready for testing, everything on this blade is new, with the exception of the bolt, bush and nut. The same second generation, bush-bolt assembly has been used on the model-17.5, 17.6 & 17.7 to test the durability of the revised bush bolt assembly.
The most expensive addition to this Model-17 blade, is the addition of the nut guard, with the cost price for this part alone being $19.00, but it is worth every penny.
The first nut guard on the left, cost $5 to machine from a 35mm solid bar, of the same steel as the high tensile bolts. It was cheap and light, but the problem with it was that it allowed the blade to slip and the dirt would easily compact in around the lock nut and was very difficult to clear out to undo the lock nut. The thin edge also wore away easily on gravelly ground.
The second nut guard on the right, is CNC machined from a 50mm solid bar made from the same steel as the bush and heat treated to the same hardness as the bush for long life, which is why it cost four times as much. It locks the blade up so that it does not slip, it is easier to clean out to undo the locknut and the increase slope allows the blade to easily slide over branches laying on the ground without snagging as you move the brush cutter from side to side.
This twenty odd dollar cost, would for many, be the price they would have to pay anyway. As most lower powered brush cutters now, no longer come with a nut guard, spring washer and lock nut to fit a metal blade as standard equipment with the brush cutter.
With the exception of the lock nut, which may still need to be purchased, the nut guard is part of this new blade and the rubber impact stop also acts as the spring washer meaning one is no longer required.
It is amazing how quickly you forget about a problem once it is solved and you have moved onto new problems. With all the testing I have been doing, I was beginning to think that I could not longer keep this up as my hands were permanently tingling from all the vibration they have been subjected to. Yet yesterday, my partner Linda asked me how my hands were coping and I suddenly realised that they were fine, so I guess the latest model anti-vibration gloves really do make a difference, even though they don’t feel like they are making a difference.
I recently spent another evening with a good friend, who again harped on that I secretly have no desire to succeed in business and have endured this from him every time I mention an upgrade. Why for gods sake do I keep wasting my time and money continually improving the blade, when it is already 50% better than anything else on the market and probably 90% as good as it could ever get. Leave it alone and move onto another product.
And of cause in a lot of ways what he says is true, unfortunately the world is no longer a simple place and the quick answer to this is liability. If I knowingly become aware of a flaw in the design, sweep it under the carpet and continue selling it regardless it will eventually come back and bite me as many car companies have found out the hard way. Personally I do not want to loose my house and or spend the best part of my later years fighting law suits.
So for my thinking it is good business, because if I want to move the selling of my products over to the retailers, they have to know that the product they are selling is safe and will not come back to bite them.
And that if I want to continue living my current lifestyle on top of an unheard of plateau mountain that is green like Ireland and that most Australian’s can only dream of and or to develop a business that does not have the hassles of employing staff than it makes perfect business sense.
I would also like to mention that the current Model-16 blade is perfectly safe and there has never been a catastrophic failure where the blade has blown apart. However a very small number of people with high powered bicycle handle brush cutters that have used the standard brush cutter supplied 3 or 4 bladed knife blade for some time and have developed the bad habit of throwing the brush cutter into the job (like in my video above), to bash their way through a branch. It is this type of bad cutting habit that has resulted in four failures (that I know of to date) of the bottom hub, but not the complete blade.
Why spend so much time and money reducing size and weight?
After much feedbacks about the size and weight of the current model-16 blade, I decided to do a google search of the location of customers for the last year of sales and found that by the start of 2017, almost 50% of sales were now going to residential properties especially in the United Stated.
Up until then, I have been focusing all my energy on making this blade heavier and stronger to withstand the punishment dished out by commercial clearers. Whereas in fact they now only represent an extremely small percentage of the market.
I was also surprised to find that the number of woman using this blade has increase and may be, at a guess as high as 5%, based on email feedback and questions asked.
However this exercise of bigger and heavier has not been a total wast of time, the result is now a brush cutter blade, that for the back yard user with a 25cc Bruch cutter, is safe and indestructible in the backyard environment.
As I could see from the aerial photos, not everyone has a manicured back yards and for a lot of people (especially in the higher rainfall areas) maintaining a back yard jungle can be hard on equipment and especially time.
This new model blade will be designed to withstand the punishment from a commercial clearer with a 50cc high performance commercial brush cutter, but can also be easily driven by a 25cc consumer brush cutter.
It is especially suited to the slower revving Honda 4 stroke engine and eventually, the even slower revving electric brush cutter as more of them come onto the market with the ability to fit a metal blade. This blade does not need high revs to clear, even the thickest back yard jungle.
I would like to make the point here, that even though this blade is safe in itself, using any metal blade in a built up residential area comes with responsibilities on the part of the operator to use it safely, especially with respect to other peoples and neighbouring properties.
These responsibilities also include being aware of what direction debris is being thrown and planning your cutting to control where the debris goes. Plus the regular inspections of the blade and brush cutter before each use and when refuelling to ensure nothing is coming loose as cutting heavier material results in more vibration to the brush cutter than you would get form a nylon head.
The Model-19 Blade is available in four options, click photo below for more option and purchase details.
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