How does a chainsaw work?
What is a chainsaw?
The clue is in the name! A chainsaw has two main parts: a saw blade built into a chain, wrapped around a long metal guide bar, and a small, one-cylinder gasoline (petrol) engine (sometimes an electric motor powered by a cord or battery pack). The chain is a bit like a bicycle chain, running around sprockets (gear wheels designed to turn a chain) only with about 30 or so sharp teeth (made from a hardened steel alloy) mounted around it at intervals. Inside the engine, as the piston moves in and out of the cylinder, it pushes a connecting rod that turns a crankshaft. The crankshaft turns gears that are connected (through a centrifugal clutch, explained below) to one of the sprockets on which the chain is mounted—and the chain spins around.
What happens inside a chainsaw?
Yes, crudely speaking, that's what a chainsaw does: in scientific terms, it converts the chemical energy locked in gasoline into mechanical energy you can use to "do work," turning a tree into logs, sawdust, noise, and heat. Here's a very simplified explanation:
The fuel you put in a chainsaw's gas tank contains, in chemical form, all the energy you'll consume cutting down and chopping up logs. To keep it nice and light, a typical chainsaw tank holds just 0.5 liters (1.1 US liquid pints) of gas (a car's gas tank holds maybe 45–55 liters or 12–15 US liquid gallons, which is roughly 100 times more).
The fuel feeds through a carburetor to mix it with air.
The air-fuel mixture passes into a cylinder, which works much like the ones in a car engine but with only a simple push-pull (two-stroke) action instead of the more complex (four-stroke) cycle used in a car. Inside the cylinder, the air-fuel mix is ignited by a spark (sparking) plug, burns, releases its energy, and pushes a piston back and forth. The piston in a chainsaw engine has a bore (diameter) of about 45mm (1.75 in) and a stroke (traveling distance) of about 33mm (1.3 inches)—so it's less than half the size of a typical car engine piston and moves only half as far.
A connecting rod and crank convert the back and forth motion of the piston to rotary motion.
A drive shaft takes power to the centrifugal clutch.
A chainsaw engine runs all the time, but you don't want the chain spinning unless you're actually cutting wood, like using it as a brush cutter machine: that's dangerous and it wastes energy. The clutch solves this problem. As explained in more detail below, the centrifugal clutch connects the engine and the chain when the engine speed is fast (when the operator pulls on the throttle) and stops the chain from spinning when the engine speed is low (when the chainsaw is just idling).
Gears carry power from the clutch to the sprocket that holds the chain.
The chain spins around the edge of a long-steel plate called the guide bar, spitting out wood dust as it goes!
Advantages and disadvantages of chainsaws
The main advantage of using a chainsaw(like when used as a hedge trimmer)—speed—is fairly obvious. It would be hard to spend an entire day chopping your way through a forest with a handsaw, but you could certainly do that with a chainsaw.
A little crude math shows why a chainsaw is maybe 5–10 times quicker than an ordinary hand saw. Think how many planks of wood you could make from a single, trimmed tree trunk: maybe ten or fifteen? Now think how laborious it is to saw through a single plank with a handsaw; cutting through an entire tree is going to take you at least 10 times as long, assuming you don't run out of energy or melt your saw blade first.
Let's try a more sophisticated estimate. Suppose you have a tree that is 30cm (roughly 1ft) in diameter and a chainsaw that makes a cut of 0.5mm (0.02in) into the wood with each pass of the chain. That means the chain needs to pass through the wood 600 times (30cm = 300mm and it takes two chain passes to remove each mm). If you're using a powerful chainsaw with a rotational speed of about 2800rpm (call it 3000rpm to make the math easy), the chain will (theoretically) make 600 passes in just 20 seconds. In practice, it'll take somewhat longer. Let's say a minute.
How long would it take with a handsaw? Suppose your saw has teeth the same size as the chainsaw's and suppose it's roughly the same length as the chainsaw (and therefore half as long as the chain). You still need to make those 600 passes through the wood. Maybe you're superhuman: suppose you can make one complete pass of the saw each second and keep up that pace constantly. Then it's still going to take, as a minimum, 600 seconds—or 10 minutes. In practice, it's going to take quite a bit longer as you get tired, as the saw slips out of its groove from time to time, and so on. These are only guesstimate, back-of-envelope calculations—but you get the idea: using a chainsaw is certainly several times faster (and probably 5–10 times faster) than using a good handsaw.
The two biggest drawbacks of chainsaws are safety and maintenance. Although chainsaws can be used by any fairly strong adult (after suitable training) and many practical uses like earth auger, they are nevertheless inherently dangerous. (I have the manual for a Stihl MS270 chainsaw open beside me as I write this and it's interesting to note that about 16 of the 64 pages—fully a quarter of the text—is devoted to warnings and safety precautions.) The biggest risk comes from a problem called kickback, where the chain catches on something but the engine keeps turning, so the whole saw flies up and backward toward your head (think action and reaction—Newton's third law of motion), potentially causing fatal injuries. Chainsaw helmets with visors offer some protection; so too do chainsaw uniforms (made from synthetic fibers such as nylon, which snag up the chainsaw teeth and bring the machine quickly to a halt).
The other big problem with chainsaws is the amount of maintenance they need. A handsaw is delightfully maintenance free: the sawdust you produce simply falls out of the groove you're making. In a chainsaw, the super-fine dust can get caught up in the mechanism and mix with the chain's lubricating oil to make a gungy mess that has to be cleaned out regularly. Again, looking at the manual for the Stihl MS270, it's interesting to see there's a full-page chart spelling out a couple of dozen different checks and maintenance jobs you have to do before starting work each day, or on a weekly, monthly, or annual basis thereafter. So while it's true to saw that chainsaws save you time in the actual chopping of wood, some of that time is, unfortunately, lost in maintenance!