Engineering Report – Whipper Snipper/lawn Trimmer

Whipper snipper/ Lawn Trimmer
No one person or corporation is attributed with creating the lawn trimmer however the original lawn trimmers where developed from1968 to 1970. Prior to 1970, there were no line trimmers, as we know them today. Can you imagine edging sidewalks, driveways, parking lots and shrub beds with hand-held trimmers? Can you imagine how sore your back, arms and legs would be after a long day of pushing a manual edger along seemingly endless sidewalks and driveways? Can you imagine trimming any turf at all without a powered trimmer? Many of us take the advantages the line trimmer gives us for granted.


Line trimmers are one of the most important pieces of equipment in the wide array of landscaping tools. They put an edge on sidewalks, shrub and flowerbeds and parking lots. The time we save by using these tools is something people may take for granted these days. Since the early 1970s (when the first powered line trimmers appeared), the machines have improved with stronger line, more attachments and better engines. This is all in the aid of giving more time and quicker action in the increasingly hectic lifestyles of today.

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The basic trimmer works by the engine driving a multi-bearing supported hardened steel shaft housed in an aluminium tube through a centrifugal clutch this shaft is connected to a “head” that holds a nylon line that spun at high revolutions per minute (RPM). This nylon line then cuts the grass by hitting the blades of grass at high speed, this cause the grass to be severed at the point of impact. Thus trimming the grass.


The first models were petrol powered, as electrical lawn mowing devices had gone out of fashion due to the danger of electrocution after accidentally cutting the power cord. The original petrol powered trimmers where two stroke engines around 28 to 32 cubic centimetres in capacity powered by a mixture of “super” petrol and oil, today they are much the same with the exception of being powered by unleaded petrol and oil, but becoming more popular is the electric trimmer. A leader in engine technology Honda has taken the latest accomplishment, with Mini 4-Stroke engines. Powered by the only 360′ inclinable (can be turned on any angle and still run properly) 4-stroke engine available today, the new Honda trimmers deliver smooth, dependable power and unsurpassed fuel efficiency. They require no special gas/oil mixtures so there’s no chance for improperly mixed fuels. And they’re remarkably quiet, virtually smokeless and environmentally friendly.


The New 4-Stroke Honda engine
The two-stroke engine cycle produces lager amounts of smoke and noise than four stroke cycles, this is due the fact that there is less time for the fuel/oil mixture to combust completely and the fact that two strokes run at higher engine RPM (revolutions per minute) about 7000 RPM compared with around 4000 RPM of four strokes. This increased Rpm causes greater vibration of the engines cooling fins and in turn more noise. The increased RPM means that more air and fuel is going into the inlet port via the carburettor of the engine which means more induction noise, silencers are used but noise is still high, with all his extra air and fuel going in to the engine extra exhaust gases must come out meaning greater exhaust noise, mufflers do help but a high decibel reading still is present.


A typical 2-stroke trimmer;/b
The Carburetor is the device that mixes fuel and air for burning in the engine. The carburetor atomizes liquid petrol. Airflow carries the atomized gasoline to the engine’s cylinder, where the “aerosol” is ignited.


Those trimmers powered by Super petrol and oil mixtures have a quantity of lead in the petrol, once combusted this lead is expelled by the exhaust unit of the engine and falls to the ground in around 12 feet. This lead (allegedly) causes harm to humans and the environment however no evidence of lead poisoning due to engines has been produced. Many people campaigned to have this lead removed from the fuel, they succeeded and unleaded petrol was soon produced. However unleaded petrol has no lead, which acts as a lubricant so more oil would have to be used, this means more smoke will be produced as the oil burns less efficiently. Also unleaded petrol contains high quantities of Benzene, Xylene, heptane (add more) these are known carcinogens and as carcinogens cause cancer. It has been medically proven that these chemicals are carcinogenic and there is growing support to the theory that the increased amount of cancer sufferers is closely related to the increase in use of unleaded petrol.
Heads and line feeds in order of invention
Four types of spin-trimmer heads are available and they are classified based on how they dispense line:
Fixed-line heads do not hold additional line. You must use pre-cut lengths of line for replacements. This is how the original trimmer head worked.

Manual heads require you to stop the machine (turn it completely off) and manually pull or unwind the line that is wound inside the head. Was soon developed to save some time so that the work could be done more quickly.

Semi-automatic heads require you to tap the spindle head on the ground (preferably not on concrete) during operation when the throttle is engaged. The tapping and open throttle causes the line to advance and feed out the guide holes in the spindle head. This is due to the creation of more sophisticated components that can withstand the high rotational forces.

Automatic heads feed the line automatically during operation when the throttle reaches a certain speed. The most recent advancement but is yet to become wide spread.


Statistically speaking, tap-and-go (Semi Automatic) heads are the most popular variety. Most users view them as more convenient because you can control line length without stopping the machine. However, manual heads remain popular as well.


String types
String diameter varies greatly and comes in round-cut and edged varieties. Much debate occurs over which type and diameter trims grass the best. Unfortunately, there is no definitive answer to this debate. Diameter plays an important role in how long most string tends to last. Typically, the thicker the diameter, the longer the string will last. Diameters range from 0.066 mm to 2.60 mm. You must refer to the owner’s manual to know what size of string will fit inside the trimmer’s head.
Manufacturers market many different types of string. Some say that their string material is more durable, due to stronger and higher quality nylon others propose the shape (smooth or edged) enables the string to trim turf more efficiently. It should be recognised that some edged strings do not feed well in automatic-feed heads. Other types of string may “float” erratically while spinning, resulting in an uneven cut. Some string may cut extremely well, but also wear more quickly.


High-quality string-specifically an anti-weld or anti-fuse line. Because the string pulls when it hits vegetation, heat builds up inside the head and less-expensive string may fuse together and clog the unit. If you must use a softer string, spray it with a silicone spray after you’ve re-loaded the spool. This, although not a complete solution to the fusing problem, helps reduce string fusion.
Popular nylon line lawn trimmers are now the fifth leading cause of penetrating eye injuries. Operating at speeds up to 7000 revolutions per minute, the trimmers may spin off tiny fragments of the nylon line, which can enter the eye along with dirt and grass debris, causing corneal lacerations and fungal infections severe enough to threaten sight. This is why it is recommended to wear appropriate safety gear.


Drive Shafts
Shafts are a major component of the trimmer and you must pay particular attention to their durability. Most shafts are made of hardened steel. Flex shafts generally have been ignored by industry because of their high proliferation. As long as the shaft is labelled as a strong, reinforced unit, it should have no problems with breakage. Benefits you gain with flex shafts are the anti-vibration and shock-absorption characteristics that offer you comfort and help prevent gear and clutch wear and damage. In addition, if a drive housing on a flex shaft is bent, you can continue to operate it. However, if the drive housing gets bent on a solid-shaft unit, you must immediately replace it.


Fuel mix
With 2-cycle machines, such as trimmers, most manufacturers recommend using the highest-quality gasoline and oil in the proper ratio. However, it is also important to use only those products specified for trimmers and 2-cycle engines. For example, although there are high-quality oils designed for outboard engines, you shouldn’t use those oils for 2-cycle engines. Another thing you should be aware of is that some manufacturers double the life of their warranties if you use oil that they sell or produce.


Another benefit of higher-grade oil is that it produces fewer emissions. This issue certainly will affect all of us before the end of the next decade as new emission regulations take effect.
With regards to fuel, some manufacturers believe that you should stay away from alcohol-based petrol. They argue that this type of gasoline is hard on fuel systems and disintegrates fuel lines and petrol cap lids. It causes vapour locking. When oil is added to such fuel in 2-cycle engines, they don’t stay mixed as well and tend to separate over time.
The latest trimmers have relatively similar designs but can be petrol powered or electric depending on consumer preference, such as these one made by stihl.


Specifically electric trimmers are also made that have the motor mounted very low to the base having no need for a long drive shaft just the head connected straight onto the engine’s spindle.


Makita Electric Trimmer
The materials and components:
Engine: Is made up of a variety of materials,
Metals:
Aluminium is used in the piston of the engine as it is light and is a good conductor of heat.
Cast iron is used as the cylinder face, as it is hard wearing. This cast iron is an insert that has been pressed into the Aluminium barrel.
Platinum has been used in the spark plug electrode as it is very chemically inert, hard wearing and can withstand high temperatures. However Platinum is more valuable than gold.


Copper: is used in the electric motor as a coil to generate an electric field and rotate between two magnets so that rotation occurs.


Iron: magnets for use in the electric motor.


Ceramics: have been used in the spark plug as an insulator to keep the spark were it is wanted and to prevent heat damage to the electrode.


Plastics: such as in the engine cover; is made of a thermosetting plastic since it must withstand the high temperatures generated by the motor or by sitting in the sun for long periods.
Rubber: is a hydrocarbon and is used as an insulator in the sparkplug lead as it is flexible and can withstand vibration.
Throttle: is made of steel or of a hard plastic depending on the model and manufacturer it really make no difference except that steel with last longer and plastic may be more comfortable to hold.


Handle: Is usually made out of bent aluminium tubing covered in a coating of foam with a vinyl layer on top for good grip and comfort. Foam is basically latex that has been aerated with millions of air bubbles when being set. Polymerising vinyl chloride gas makes vinyl.


Drive Shaft: Is made of hardened steel. So that it will withstand the twisting forces of the engine trying to rotate it. The basic process of hardening steel by heat treatment consists of heating the metal to a temperature at which austenite is formed, usually about 760 to 870 C (about 1400 to 1600 F) and then cooling, or quenching, it rapidly in water or oil. Such hardening treatments, which form martensite, set up large internal strains in the metal, and these are relieved by tempering, or annealing, which consists of reheating the steel to a lower temperature. Tempering results in a decrease in hardness and strength and an increase in ductility and toughness, this is done to prevent factures.


Shaft housing: Is made of rolled Aluminium as this is very light and thus has an excellent strength to weight ratio. This is important as users do not want to have to carry around a very heavy piece of equipment nor do they want a material that will fail easily. Hans Christian Oersted, a Danish chemist, first isolated aluminum in 1825. Aluminium is refined from bauxite.
Head: The head of the trimmer is used to hold the nylon line. The head consist of several different components depending on the type of head.


Plastics: a thermo setting plastic that is very resistant to abrasion and is not brittle so that fractures and wear is minimal. Sometimes melamine formaldehyde is used but wide ranges of plastics are used because of the diversity of manufacturers.


Metals: Springs are used in semi automatic heads to return the bump button’ back to its original position. A steel bolt is also used to attach the head to the drive shaft.


Line: The cutting line is made of nylon. Nylon is a polyamide, they consist of highly ordered molecules, which give high tensile strength. Polyamides are made by reacting dicarboxylic acid with diamines (carbon molecules with the ion -NH2 on each end). Other types of nylon are synthesized by the condensation of amino acids. Polyamides have mechanical properties such as high abrasion resistance, low coefficients of friction (meaning they are slippery), and tensile strengths comparable to the softer of the aluminum alloys. Therefore, nylons are commonly used for mechanical applications, such as in lawn trimming.


All these materials have improved over time due to advancements in manufacturing and design techniques such as CAD and CAM (Computer aided Design/Manufacturing)
With the complexities of the internal combustion and electric engine the presence of simple machines can also be found.
Lever: the way in which the trimmer is held gives the action of a third order lever when holding the trimmer stationary to keep it upright. The action of a third order lever is repeated when moving the head of the trimmer from side to side to cut the grass.


The wheel action is used in the rotational motion that is created by the engine and transferred down the drive shaft through the head and rotates the nylon line at high velocity to cut the grass, pretty simple really.


Bibliography
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