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How Chainsaws work

How Chainsaws work

How Chainsaws work

Chainsaws

Two men saw through a large tree trunk with Stihl chainsaws.

Trees are pillars of living majesty that can take hundreds of years to grow; chainsaws are violent, hungry machines that bring them tumbling to the ground in minutes. If it seems like a crime to fell something so fantastic just for its wood, remember that trees usually die from the inside out and often become dangerous toward the end of their life. Sometimes fallen trees block roads or railroads and need to be chopped and cleared in a hurry: in emergencies and natural disasters, there’s often no more welcome sound than the buzz of a chainsaw. If you think chainsaws are damaging and destructive, remember that they’re very useful for routine forest management—for thinning and clearing trees to ensure the overall, long-term health of woodland habitats and the creatures who live in them. What exactly is a chainsaw and how does it work? Let’s take a closer look!

What is a chainsaw?

A photo of a Stihl chainsaw with all the main parts labelled.

Photo: A chainsaw is a simple and amazingly effective idea: put your saw on a chain and let a small gasoline (petrol) engine do the hard work! Here are some of the main parts of a typical Stihl chainsaw. Photo cropped from a picture taken by Jon Hyde, courtesy of US Navy, with annotations by explainthatstuff.com.

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.

How a chainsaw turns gasoline into sawdust!

A numbered artwork showing, in very simple terms, how a chainsaw works.

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:

  1. 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).
  2. The fuel feeds through a carburetor to mix it with air.
  3. 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.
  4. connecting rod and crank convert the back and forth motion of the piston to rotary motion.
  5. drive shaft takes power to the centrifugal clutch.
  6. A chainsaw engine runs all the time, but you don’t want the chain spinning unless you’re actually cutting wood: 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).
  7. Gears carry power from the clutch to the sprocket that holds the chain.
  8. The chain spins around the edge of a long-steel plate called the guide bar, spitting out wood dust as it goes!

This is a hugely simplified diagram of a chainsaw. You can see a much more detailed and faithful representation in this chainsaw cutaway from Popular Science, August 1951, which I used as one of my references. That shows much more detail of the air-cooling mechanism and the gearbox, but (as with all cutaways) it’s a little harder to follow

Centrifugal clutch

Let’s talk a bit more about the clutch. A clutch is a device that makes and releases a mechanical connection between an engine and the machine it’s driving: it’s a kind of “switch” that allows an engine to run all the time (which is quicker and easier than switching it on and off) without always driving the machine it’s connected to.

In a chainsaw, the clutch means you can keep the engine running all the time and simply disengage the chain (so it stays safely motionless) when the saw isn’t actually being used. Chainsaws use a centrifugal clutch, which automatically engages when the engine spins at high speed (when the saw is actually chopping wood) and disengages when it’s running more slowly (idling, in other words). A centrifugal clutch is really simple and consists of just a few parts. There’s an inner drive shaft (linked directly to the engine and spinning all the time) and an outer rim positioned around it (connected to the chain sprocket). In between them are mechanical arms, connected to the inner shaft, that fly outward at high speed (with centrifugal force—or with the lack of centripetal force, if you prefer) locking the inner and outer parts of the clutch together. Centrifugal clutches are also used in high-spec yo-yos.

Artwork showing how a centrifugal clutch is disengaged at low speed and engaged at high speed.

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