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How Does a String Trimmer Work?
Although string trimmers can differ from model to model they all operate on similar principles. In this article, we will address how string trimmers work as well as common problems you may encounter.
String trimmers can be powered by an extension cord battery or gasoline most gasoline-powered string trimmers use a two-cycle engine which requires the gasoline to be mixed with oil for the engine to operate properly. Attempting to run a two-cycle engine without a suitable oil and gas mix will cause the engine to seize due to insufficient lubrication.
The main benefit of a two-cycle engine is that it allows for a high output of torque while being relatively lightweight. Since string trimmers are intended to be handheld and operated at different angles, a two-cycle engine is ideal for this application. To start the engine, the ignition switch must be set to the start position.
As the starter rope is pulled the starter engages the drive shaft on the fly wheel and rotates the crankshaft. The rotating crank shaft connects to the piston which moves up and down within the cylinder, and the ignition process begins.
The flywheel has permanent magnets built into it, and as it rotates past the ignition coil, a magnetic field is created. The magnetic field induces electricity allowing the ignition coil to send voltage to the spark plug.
As the piston travels down the cylinder, it exposes an intake port and pumps fuel oil and air into the cylinder.
As the piston travels back up the cylinder, a vacuum is created, and gas oil and the air are drawn through the carburetor into the crankcase.
When the piston reaches the top of the cylinder the spark plug ignites the compressed fuel and air mixture which forces the piston back down exposing an exhaust port where the spent fuel exits. A split second later the intake port is exposed again. Fresh fuel is drawn in, and the process is repeated with every revolution of the crankshaft.
The exhaust from the engine travels through a muffler and spark arrestor. The muffler reduces engine noise, while the arrester prevents burning carbon deposits from exiting the engine housing and potentially starting a fire.
Engines will have a choke primer bulb or both to assist with starting especially when the engine is cold. The choke will temporarily restrict airflow through the carburetor, so more fuel can enter the cylinder. Likewise, a primer bulb draws additional fuel through the carburetor which enters the cylinder before the engine is started.
If the engine starts but loses power or stall shortly after, the spark arrestor may be clogged which prevents exhaust from exiting the engine. The clog is often caused by using too much oil or the wrong type of oil in the fuel mixture.
If the engine doesn’t start at all, the most likely cause a defective spark plug or a restricted carburetor. The carburetor can be cleaned, but it often needs to be replaced.
Be aware that small engines are designed to use gasoline with no more than 10% ethanol. Gasoline with higher levels of ethanol can be corrosive and attract water which can cause starting or running problems and may damage the fuel system or engine. You should store the gasoline and oil mixture in a clean sealed plastic container approved for fuel storage. If equipped close the vent when not in use, and store the container away from direct sunlight. If you anticipate storing the fuel for longer than 3 months, consider adding a fuel stabilizer when you fill the container.
The crank shaft of the engine extends out the side of the crankcase and connects to a centrifugal clutch. The clutch engages one or more Drive shafts that attach to the trimmer head. When the engine runs at idle speed, the clutch prevents the drive shaft from rotating. When the trigger is pulled a cable opens the throttle on the carburetor which accelerates the engine. The clutch flyweights extend outward engaging the drive shaft which rotates the trimmer head. As the trimmer head rotates the trimmer line spreads out and can be used to cut down weeds, tall grass and light brush along edges and other hard-to-reach areas.
The size and thickness of trimmer line vary from model to model. But the most common diameters for residential use range from 0.065″ (1.65mm) and 0.095″ (2.4 mm). The trimmer line may be wound on a spool or be small individual strips loaded into the head.
Most trimmers have a bump head that can be tapped on the ground to feed additional line out of the head, as the line is worn down. If too much line extends out of the head, a blade located on the guard will trim the line as the head rotates. If the trimmer runs out of line, the head can be re-spool or the entire head can be replaced. If the head does not rotate when the engine is accelerated, the clutch may have failed, or the drive shaft may have stripped out. If the engine fails to accelerate the throttle cable may be broken. All of these parts can be replaced.
Grass Trimmer - WORX WG163 GT 3.0 20V Cordless Grass Trimmer/Edger with Command Feed, 12", 2 Batteries and Charger Included. See this How Does a String Trimmer Work Video from Repair Clinic for more details ☞ String Trimmers | Best String Trimmer Buying Guide.