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This article was originally published on the web at cranbrookengineering.co.uk however that website is defunct at April 2013. The copyright is with the original owners of cranbrookengineering.co.uk

Brushcutter and Line Trimmer Design

Some types of brushcutter and strimmer:

  • Bent shaft or swan-neck
  • Straight shaft
  • Loop or D handle
  • Double, bike or cowhorn handle

Let's begin with the different designs and why they are made like that. But first, what is a 'strimmer' exactly? The word is a shortening of 'string trimmer', arising from the fact that the cutting medium is nylon line whirling round extremely fast. A better name is perhaps 'line trimmer'. Then, what is a clearing saw? It's just a tough brushcutter, and many manufacturers don't distinguish them as a separate class. In this article we will discuss brush cutters, line trimmers and clearing saws all together.

Bent shaft or swan-neck

All these machines have a motor or engine at the top end of a long shaft, at the bottom end of which is the cutting tool. As you hold the machine sloping down at around 45 degrees, there has to be a method of ensuring that the cutter is caused to rotate parallel to the ground. The cheap and simple way is to have a bent shaft within which there is a flexible drive. These have improved but they are frankly not up to heavy work.

Straight shaft

The more usual design is a straight shaft with a gearbox which turns the drive direction with helical gears. Good gears last for years and bad ones don't. On the way to this gearbox is a remarkably thin steel shaft rotating at colossal speed. It does this to transmit a lot of power while keeping the machine reasonably light (power = torque x revs) and not simply because the blade must go fast to cut properly. This thin shaft could easily whip and set up a violent vibration, so the bearings down the length of the shaft must be adequate.

Vibration (and noise) are particularly important topics where brushcutters are concerned. The high-revving components would set up very unpleasant vibrations if nothing were done about it. Operators would quickly be left with tingling limbs and simply could not carry on working. Equally, noise emanating from an engine within 500mm of your ear can be highly damaging. Manufacturers all now damp vibration with a variety of devices, and all must publish their test figures to satisfy safety regulations and to justify the CE mark. Noise remains too high to be safe against the naked ear, so ear protectors must always be worn.

Safety

Safety is a particularly important consideration with brushcutters. First, always wear ear defenders as we have just said. Second, wear goggles, boots and long trousers to save yourself from flying debris. A helmet and gloves are also advised. Use the machine with commonsense: it can fling objects violently outwards, making a wide zone unsafe for people, pets or parked cars. And please don't think that tree bark is immune from trimmer line!

Another aspect of safety is how you control the machine. Steel blades can obviously cut tougher material, like brambles, than nylon line; but in the act of cutting they set up a torque reaction which needs to be resisted by the operator. It is easier to control a brushcutter with twin handles than with one loop handle. It is also important to wear a properly adjusted harness with all but the lightest machines so as to take the weight and keep the thing under control.

Handles bring us back to the design. Invariably, the controls must be easily worked without letting go of the handgrips. Moving up towards the engine, petrol engines need a centrifugal clutch to allow idling without turning the blade but then to connect the drive when accelerating. These clutches take a lot of punishment: contractors should go for a heavy-duty clutch.

Before discussing motors and blades, we should mention the split shaft type of brushcutter which can be taken in half to make it easier to fit in your car or shed.

Motive power

  • Petrol two-stroke
  • Petrol four-stroke
  • Mains electric motor
  • Cordless (battery) electric motor

Designers try hard to keep the weight of brushcutters down, which is one reason that most of them use two-stroke engines. Honda, however, have a tiny four-stroke which can cope with being tilted to any angle by using a dry-sump lubrication system. Two-strokes used to be considered hard to start, but really that is not true any more. But they do involve you in having to carefully mix the recommended ratio of two-stroke oil with unleaded petrol. Many machines come with mixing bottles to make that easier; or manufacturers like Stihl sell their oil in capsules, assuring you of exact quantities.

Electric motors are cheaper to buy and maintain, and they need no clutch - you just switch them on. On the other hand, they are not as powerful as petrol engines and they have the annoyance of a trailing mains lead. The way round that is to choose a cordless rechargeable model, perhaps also buying a spare battery. Batteries now tend to be lead-acid or Lithium-Polymer, which last well and which do not have the 'memory' problem of Nickel-Cadmium.

The cutting end

  • Nylon line (strimmer) head
  • Steel blades
  • Plastic blades

Nylon line (strimmer) head

The softest and least damaging cutting medium is nylon cord, whirling round very fast. A decent machine will mow grass and even quite tough weeds, but not brambles. Nylon line of less than 2.4mm thickness does not last all that long. Non-automatic heads are quite tiring because you have to keep stopping to pull out fresh line. Automatic or semi-automatic feeds are more convenient. A common method is 'bump-feed' where you strike the plastic head on the ground to jog out some more cord. Nylon line has many uses and is good for strimming against walls; but it will damage bark if you try to go too closely round a tree.

Steel blades

There is some debate about steel blades, but what is indisputable is that they are potentially dangerous and must be used carefully. As a guide the use of steel blades is as follows.

  • 3-tooth. Use for tall weeds, nettles, long grass, etc.
  • 4-tooth. For light brush and long grass.
  • 8-tooth. For brambles and light scrub.
  • 80-tooth. For heavy undergrowth, bushes and saplings.

However, the machine itself must be capable of driving the size and shape of blade: you cannot put a saw blade on a small, cheap brushcutter. The rule is not to buy bigger than the original blade size.

Plastic blades

Plastic blades are safer than steel but do not stand up to woody stems. They are for soft growth such as grass and weeds. They do mow more quickly than a nylon line head but are unforgiving if you touch a wall.

Judging quality

All the better garden machines have a good finish and a solid feel. Quality in a brushcutter can be judged by noting the design criteria mentioned above. Heavy duty clutches are best if you will be using a 4-line head. Steel blades should be thick and less than 2.4mm nylon cord will break often. Straight shafts are best, with ample bearings. Gears should be case-hardened. Sophisticated anti vibration measures are desirable. A harness which passes over both shoulders, with shoulder and hip padding, is superior to a simple loop over one shoulder - though the latter is acceptable for smaller machines. Brushcutters should be supplied with tools to change the blade and ideally with safety goggles.

Maintenance

All brushcutters come with maintenance instructions, but 'once a year' is a good rule for most people. Electric strimmers need hardly any maintenance and are therefore cheaper to run than petrol models. Even they are pretty easy to look after. Keep the air filter clean: a dirty one causes wear. Be careful to mix the fuel up correctly, and give the brushcutter a shake before using it, to ensure mixing of the petrol and oil. It is a great mistake to keep old fuel as it goes stale within a few months, making starting difficult and gumming up your carburettor. This can be avoided by adding a little fuel preservative, such as Briggs & Stratton make.

Other uses for the brushcutter

Some models can be adapted with accessories into pole pruners, long-reach hedge trimmers or hand-held cultivators. This extra use of the basic engine and shaft is economical but possibly results in a heavier tool than a purpose-made machine.

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