Here are the answers, as there is more than one answer.
Having an inquisitive engineering mind, I quickly became aware of this problem and figured out the reasons why. Like everyone else, I was breaking the lines on my very first electric Weed Eater in the mid 1980’s and quickly worked out what was needed to prevent this from happening.
I bought my first petrol powered Honda brush cutter in 1999, with the initial intention of using a metal blade on long grass, bracken and blackberries on our first 10 acre block. The blade that came with the Honda was a four star knife blade which turned out to be totally useless against the grass and bracken, so I tried a saw type blade which was OK until I hit my first stone and then it was totally useless.
For the next eleven years I used my brush cutter a lot cutting everything that shouldn’t have been possible with a nylon line, first on the 10 acre block and then a 100 acre block. The photo below is the early days of starting to tame the hill side and around the dam. You would actually be amazed what you can cut with a nylon line, with the right technique.
After several years we had 50 odd acres of hillside and all around the dam manicured like a golf course, so I can assure you I have spent countless hours on the end of a brush cutter. I used to trim around every tree and rock, to give a crisp sharp line between it and the grass.
It used to be very satisfying to sit back and watch the shadows of the trees move as the sun set at the end of a hard days work.
I wasn’t till we purchased our current rainforest property that I again had a need for a metal blade. Over the years using a nylon line, I tried many different brands of line and different thickness, but eventually ended up settling with a medium thickness Husqvarna dimple line.
SO TO ANSWER THE QUESTION, which I have now been asked very similar questions on three occasions now.
Why does the nylon line on my line trimmer keep breaking at the head?
Can you tell me why my Stihl weed whacker line keeps breaking?
I have a Stihl 250 weed wacker and keep having problems with the nylon line, the 4mm line which is the heaviest gets destroyed quickly on heavy weeds. Any Ideas?
I will start by first describing how a nylon line cuts, for those who have not thought about. It relies on shear tip speed to cut its way through grass and weeds. It is the very end of the line that does all the cutting and the sharper the end of the nylon line the better it cuts. Unfortunately nylon lines do not hold this sharp edge for very long, quickly fraying and splitting which means instead of cutting efficiently it ends up thrashing and bashing it way through grass.
Line trimmers need to be pretty much run at full speed so the line becomes very stiff under its own weight, this keeps the line torte and straight to expose the end of the line to the material being cut. run at slower speeds, or push to hard into the job and the end of the line will deflect and slide past the material being cut.
There are five main reasons for why a nylon lines breaks so easily;
First is basically using too thick a line,
Second, is cutting on the backward stroke,
Third, Pushing too hard into the material being cut,
Forth, the line is old.
Fifth, The line tangles in the spool head damaging the line to create weak spots.
LINE TOO THICK; many people (me included) keep going to thicker and thicker line thinking that it will be stronger and last longer and yes in tension a thicker line is much stronger. However tensile strength is probably the least important requirement for a brush cutter line, flexibility is more important. The problem is that the thicker the line is, the stiffer it is, the less it likes being bent. Also a round line if very inefficient drag wise, and when you double the diameter you are quadrupling the drag. Make the line square or odd shaped and you can double and quadruple the drag again, which robs a lot of power from the brush cutter just to drive it. Now if you consider some of the aftermarket heads with 4 and sometime 8 thick lines hanging of it and you are sucking a lot of power just to spin it round.
Without the speed to keep the line straight, the line flexes more and tends to slide past the weeds instead of cutting it. If you are not getting efficient cutting, the natural reaction is to lean into the job more and you start bending the line back and forth more. When you consider that the average brush cutter head is spinning at 100 to 130 revs per second and that is enough bending back and forth to work harden the plastic (just like metal) to the point where it breaks and more often than not it breaks at the eyelet which is very frustrating as it will no longer self-feed out.
CUTTING ON THE BACKWARD STROKE; is another way you can very quickly break nylon lines. Backward stroke is when you cut from your left to your right, which means you are using the 12 to 4 o’clock area of the cutting head. When you hit an object with the 12 to 4 o’clock area of a blade or nylon line, it wants to dig in. With a metal blade it is flung forward with greater velocity than the blade is traveling and will knock the brush cutter back violently.
With a nylon line you do not feel this kick back, it is the line that absorbs this shock by compressing and sending a whiplash down the line where it is suddenly stopped by the eyelet. This sudden restraint of the whiplash at 100 odd times per second, will just shear the line off at the eyelet and the thicker and stiffer the line the easier it shears off.
On the forward stroke, this kickback and shock to the blade or nylon line is much reduced and you can normally cut cleanly through the job.
The science of why this happens is beyond the scope of this article, but when I get a chance, I will do an article on why this happens. All you need to be aware of at this stage, is that this is the way it is and a nylon line behaves just the same as the metal blades shown above.
I used to wonder why so many people do cut on the back stroke and now I think I know. If you have started out by owning a bent shaft line trimer before upgrading to a straight shaft line trimmer that has a gear box at the end, you are more likely to develop this habit. This is because the bent shaft trimmer has no gearbox and so the line spins clockwise in the same direction as the engine and the best quadrant for cutting is the 12 to 4 o’clock position. The straight shaft brush cutter with gearbox, results in the line spinning anti-clockwise in the opposite direction to the engine and the best quadrant for cutting is the 8 to 12 o’clock position. So the back stroke is on the opposite quadrants on the bent shaft and straight trimmers and if you own both types I imagine it is hard to control which way you should cut.
PUSHING TOO HARD INTO THE MATERIAL BEING CUT; will cause the line to start bending and sliding past the job instead of cutting it. The natural reaction is to push harder into the job, which creates more drag which slows the machine down and allows the line to become less stiff. This becomes a self defeating process, where the line wears thinner and thinner at the end and eventually it becomes so thin it has no mass to even bash its way through the the material. So you continue to push harder into the job, bending the line ever more until it becomes brittle and breaks. This process can happen slowly or in a mater of a few seconds if the line is rubbing against an abrasive surface or you push into the job really hard.
AGE OF LINE; if you are like me and buy large spools of line because it is cheaper, then unless you are using it up very quickly you will be wasting your money. The problem with nylon is it contains up to 20% water by weight, but as it ages it dries out and as it dries out it loses its flexibility. So a line that starts out appearing to be really tough and takes a lot of punishment will after a few years always seem to break easily. As I have mentioned before, the thicker the line, the less it likes being bent and the easier it breaks when subjected to high speed bending loads.
Fortunately, this problem is easily fixed, if you store that large spool of nylon line in a container full of water and only take it out long enough to cut off what you need it will stay strong and flexible almost indefinitely.
Unfortunately, having a thinner or well hydrated nylon line, is still no guarantee that the line will not break. In the case where the line is forced to flex at the eyelet 120 odd times per second, the internal bending friction in the line itself can generate enough heat that the line melts off at the eyelet. The smaller the radius of the eyelet, the more likely this is to happen and again it only bends at the eyelet because you are pushing too hard or cutting on the backward stroke.
LINE DAMAGED IN THE SPOOL
The very design of the spool itself causes the wound up line to loosen and tighten all the time. Eventually you will end up with lines laying at angles over the top of each other and when the line loosens and tightens this creates friction generated heat. The line can get hot enough where it can compromise the strength of the line or even heats it to the point that it melts apart. I am sure many of you have faced this problem, where the line appears to be melted together and will not feed out.
The problem with the current design, is that you wind a stiff line around a relatively small diameter spool and hope that it will stay tight. When the head spins up to the very high revs of that found in a line trimmers, the line expands out away from the spool rim under its own weight. But when it stops spinning it never fully retracts to its original position leaving it that little bit looser each time. It is this rubbing of the line against each other that creates friction generated heat that roughens the surface of the line and this prevents the weight of the cutting line that is outside of the spool from keeping enough tension on the line so that it always stays wound up tight. This is especially so if you let the cutting line wear to short so that it does not keep enough load on the spooled up line to keep it tight and or if you are constantly changing the revs of your weed eater.
In some ways, a better design would be for the line to be wound to the inside of a drum, instead of the outside. This way the centrifugal load would force the line against the drum and solve a lot of problems. However this creates another problem of how to feed out the line, but I am sure this can be worked around if some smart engineer put there mind to it.
BEST TYPE OF LINE; in my opinion, is what is called a beaded line, you can see a little what it looks like in the selection below. It is basically a round line that has been squashed flat in one direction and flat a little further along in the other direction, so that it basically looks like a plastic chain. What makes this design stand out against all the other profiles is that you have the weight and durability of a heavy line, with the flexibility of a thinner line. Where it is squashed flat, it is very flexible in that direction and where it is squashed flat in the other direction it is very flexible in that direction. It also has no sharp edges that can seed a crack.
Although I have personally not tried it, as it was not available when I last used a nylon line head. The Ellipse Twist line shown on the bottom left may be just as good and possibly even better than the beaded line although I am not sure how readily available it is. There are also dual core lines now for sale on eBay, with a soft core for flexibility and a hard outer surface for wear resistance. These types of lines may also solve many of the mentioned problems and is perhaps worth a try.
Another problem with all but the round line, is the resistance of the line to slide past each other on the spool as it is loosened and tightened every time you start and stop your line trimmer. I can just see these sharp edge lines getting damaged which leads the next problem with these lines.
Any line that is square or has a sharp edge, is a waste of time. A sharp edge may help cut grass a little better, but any damaged sharp edge leaves an opportunity to seed a crack and once a crack is seeded it will keep expanding until the line breaks off. As I have said before, it is the tip of the line that does all of the cutting and a line with a sharp end that is continually trimmed off by the guard knife will cut way better that any of these fancy lines.
Then there is a huge selection of after market heads where you feed a fixed length line in, with some of these being very expensive. As soon as the end of the line wears thin, cutting efficiency drops off and as the diameter reduces so does cutting efficiency.
THESE ARE MY RECOMMENDATION; based on memory, as I have not used, or needed to use a nylon line for six odd years now.
You are better off staying with a thinner lines and feed it out quicker than going for big thick line.
Only cut on the forward stroke, never on the backward stroke.
Learn to listen to the sound of the engine & cutting head and ease up to keep engine speed up.
Only keep the tip of the line cutting into the job.
Always store your line in water.
Only buy a beaded type line, although it might also be worth trying the twisted elliptical type line or dual core line.
Don’t let the fed out line get too short.
Once you start a job, keep the revs at half throttle or above and don’t change revs quickly to prevent the spooled line from unwinding.
After winding on new line in the spool, give the line a spray with PTFE Dry Lubricant, so that the beaded or twisted line will slide easier on itself to stay tight.
Don’t waste your money on after market fixed line heads, either clean & maintain your bump feed head to keep it working properly, or buy a good quality bump feed head.
Do not use any wet type lubricant on a bump feed head as it will attract dust and bind the bump head up very quickly.
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ALTERNATIVELY; if you are fed up with your nylon line breaking, there is an alternative. You can fit a BrushDestructor blade.
The BrushDestructor blade is just as happy in the back yard as it is out in the bush. If you are only cutting grass and watery weeds there is no need to keep the teeth sharp as it will cut grass and watery weeds better than any nylon line even when the teeth are completely worn away, as shown in the picture below. This is because the blade is only 2mm thick and has a lot of weight behind it to shear its way through grass.
Plus if you have a Honda brush cutter with its low down torque, you can trim most grasses and water weeds quite successfully at half revs saving on noise, vibration and fuel. You will never need to change a line again, you will never need to stop to feed out a broken or worn line and with a bit of practice you should never need to stop to untangle grass that has wrapped around the gear head.
If you decide to keep the teeth sharp, you can cut most grasses that you would normally cut with a nylon line at full throttle, you can cut with the brush cutter running a a fast idle. This will only work with a 4 stroke brush cutter, as a 2 stroke brush cutter does not have the low speed torque.
If you are only cutting grass and watery weeds, there is a simple way to improve cutting efficiency. Run your blade at a slight angle against a concrete path with the brush cutter at a fast idle. The drag against the concrete deflects the blade back 5 to 10 degrees and grinds a knifes edge along the end of the blade.
When you again run the brush cutter at a higher speed, the blade straightens up and you have an angled knife edge that very efficiency slices through grass and watery weeds.
Please note that this knife edge is totally useless on woody weeds and saplings. These require teeth to shave away the woody material layer by layer just like a saw.
I get many questions form people wondering if the two bladed model is still available and the answer to this is no. There were vibration issues with the two blades model that are not found when using three blades. The new model-17 blade is lighter and smaller in diameter and so can now be easily drive by the lower powered brush cutters.
The video below, shows how the new Model-17 blade can be used to replace nylon line head and all other metal blades.
I should also warn you, that when using a metal blade, any metal blade, this comes with the need for more responsibility on your part. A metal blade can do more damage to plants, pets and people if used irresponsibility.
Any comments left below, must be strictly related, good or bad, to your experience of using nylon line heads and should be worded such, so as to inform and educate other users to help them make more informed choices. – Off topic, comments will be quickly deleted.
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