2.2.12 – Threaded Bush and Grade 12.9 Bolt Crushing

The photo below shows the extreme crushing of a high strength grade 12.9 bolt and a hard steel threaded bush. This extreme crushing, can even happen in quite a short period of time and if not detected could result in the bolt failing, allowing a whole Flail-Blade to fly loose which must never happen. The photo below gives the impression that these parts are made of lead, but they are not. It is a good example of the extreme loads that are place on a brush cutter blade.

Threaded Bush deforming under extreme loads 1

Below you can see photos of the old threaded bush and a new prototype stepped threaded bush. I both cases the blade and bushes were exposed to similar extreme clearing with a 92cc brush cutter for about three hours. As you can see in the close ups below, the new bush shows no signs of wear or crushing.

Old and New Bushes (600x376)


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 easily 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 as can be seen in the photo below. 

BrushDestructor - How Bush Crushes

The steel used in the bush is very hard and can easily handle the 1.3 tonne centrifugal load that the blade at full revs places on the bush. The problem arises when users with high powered brush cutters, who don’t keep the teeth sharp and to keep up cutting efficiency they keep pushing harder into the job. The other occasion when this can happen, is when clearing close to the ground where the blade hits a lot of hidden stones.


To the problem, was a combination of design and materials, as just making the bush harder would not solve the problem and would lead to more extreme wear of the bolt and or the pivot hole in the Flail-Blade. Getting the right steel with the right hardness that gives the lowest co-efficient of friction to keep the heat produced down is the only solution. In the past, it was friction generated heat that expanded the bolts allowing the brush to start rotating on the bolt and quickly wearing through the bolts. As I have said elsewhere, at full revs there can be a 5 to 90 degrees flicking back and forth of the Flail-Blade per second with a 1.3 tonne load on the bush. Plus with the high centrifugal loads, no liquid lubrication will stay in place, so the harder the blade is worked, the more friction heat is produced.

New stepped Bush to handle extreme shear loads 5

Here you can see when the material and hardness was not right, the wear was extreme and the stepped bush was deformed in only 5 hours of hard use.

Wear and crushing to stepped bush

Below is another prototype stepped threaded bush with matching custom made nuts and bolts. Unfortunately, these did not last either, with all three fatiguing and cracking under extreme conditions.

Sample hardened bush and fastenersFailure of new bush and fasteners (600x217)Finally I am very pleased to say that have got everything right, with bushes that should last the average person the life of the blade.

New BrushDestructor Bush and Fasteners Pass the 70 Hour Test 1 (600x401)

This blade was pulled apart after 70 hours of extreme clearing, with no signs of crushing or wear of either the bush or the blade. The deduced wear would is also due to the use of PTFE dry spray lubricant, sprayed around the bushes every time the weed wacker was refueled and before storing away in the shed.

New BrushDestructor Bush and Fasteners Pass the 70 Hour Test 2 (600x338)

One of the other problems, this new design solves is damage to the bolt head making it difficult to undo the fasteners.

Old Socket Head Bolts Hard To Undo (600x181)

The new tapered head solves this problem, by keeping the hex head faces out of harms way.

New BrushDestructor Bush and Fasteners Pass the 70 Hour Test (600x309)

Here you can see, a lot of abrasive damage to the bolt head, but the hex head faces are undamaged.

New BrushDestructor Bush and Fasteners Pass the 70 Hour Test 5 (600x337)

Plus the tapered heads also solves the problem of grass snagging on the bolt and nut heads, causing drag and vibration issues.

Grass dragged around gearhead by pivet bolta

The really great thing about having bushes and fasteners that now last a long time, is that I will no longer have to supply these with each replacement Flail-Blade, thereby halving the cost of replacing damaged or worn Flail-Blades. So I will be now be able to send out twice as many spare Flail-Blade for the same mailing weight and price, which will considerably reduce the operating costs especially to the commercial operators. Below you can see the old and new bush, bolt and nut side by side.

New & Old Bush & Fasteners (600x157)


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