How A Chainsaw Works

This is an excerpt from the Book called “How A Chainsaw Works“ by Brian J. Ruth & Jen W. Ruth. Continue reading to learn more about How A Chainsaw Works, thanks to the author.

Though a toothed chain does the cutting, the heart of any chainsaw is the gas-powered engine or electric motor, so we will start there. It gives the chain the power to travel at speeds up to 4,000 to 6,000 per minute.

Engines And Motors

Gas-powered saws are started with the pull of a starter cord that turns the crankshaft. This, in turn, moves the piston in and out of the combustion chamber (unlike an automobile, chainsaws have a single piston and a two-stroke cycle.)

In the combustion chamber, as the piston rises, it compresses the fuel, the spark plug fires, and the fuel ignites. The combustion pushes the piston downward, allowing the exhaust gases to escape and forcing some of the fuel-air mixture from the crankcase into the combustion chamber. And then a new cycle begins. At full throttle, a chainsaw engine can run from 6,000 to 14,500 revolutions per minute (RPM).

Electric-powered chainsaws simply have a standard electric or battery-operated motor that powers the chain. They are generally sized from 1hp to 3 hp.

Clutch And Drive Sprocket

When the throttle trigger is depressed, power from the chainsaw’s engine or motor spins the drive shaft. Mounted on the drive shaft is a clutch. The clutch plays an important safety role since it permits the chain to spin only when the operator pulls the trigger throttle enough to start cutting.

The entire assembly works on the principle of centrifugal force, so that only when the engine speed reaches a specified number of RPMs (usually 2,500 – 3,500) do the clutch shoes spinning on the drive shaft push outward enough to engage and turn the clutch case or drum.

Attached to the cutch case is a gear called the drive sprocket. As the clutch case spins at full throttle, so does the drive sprocket, and, consequently, the chain that is wrapped around the sprocket.

The reverse happens when the operator releases the throttle. The drive shaft slows, the centrifugal force decreases, and the clutch shoes are drawn together by a circular spring. The shoes disengage from the clutch case, and the drive sprocket and chain stop spinning.

Chain

Small teeth on a linked chain, like a bicycle chain, carry out the cutting action. The chain travels around a steel guide bar.

The chain is constructed of three strands of hardened-links. They are riveted together to form one continuous loop. The middle strand consists of a series of drive links whose hook-shaped bottom edges fit into the slot along the edge of the guide bar. The thickness of the drive link is called the chain gauge. Most homeowner saws have a .050 inch gauge chain. The hooks on the bottom of the drive links also engage the teeth on the drive sprocket, so it is their job to move the chain along its course.

On both sides of the middle strand are strands of steel that include the actual cutters, spacers, and tie straps that hold the chain together.

Every other cutter is sharpened the opposite direction. The distance from the outside of a left-hand cutter to the outside of a right-hand cutter will determine the width of the cut (the kerf). The other important chain measurement is its pitch. It’s usually measured with a caliper spanning three rivets; then, divide this measurement by two. Most homeowner saws come with 3/8 pitch (.375 pitch) chain.

It is usually assumed that the cutter teeth ride tight along the guide bar at all times, but that is not actually true.

All saw chain cuts have a rocking motion. When cutting properly, a saw chain resembles a dolphin swimming in the ocean. As the cutter enters the wood, the leading (or cutting) edge starts to bite (1) causing the cutter to rock back as far as the depth gauge will allow (2). The cutter is now in the attack position and jumps off the guide bar and into the wood (3). Next, chain tension and power from the saw pull the cutter back out of the wood and the severed chip exits from the underside of the cutter (4). The cutter then returns to its original position (5).

There are two important parts on each cutter tooth. First is the cutting (leading) edge. But just as important is the depth gauge. The depth gauge is the rounded forward edge that sits in front of the cutting edge and determines how far the cutter will rock back in position and ultimately how large a bite the cutter will take.

The top of the depth gauge must be lower than the leading (cutting) edge. If it is not, the cutter will not be able to take a bite. But if the depth gauge is too low in relation to the leading edge, the rocking motion will become very rough and will force the cutter to take an over-sized bite. The cutter can grab the wood and the engine might stall or the chain might break. It can also lead to severe pull-in, pull-back, and kickback. It is important that as you sharpen the leading edge and the top plate you also lower the depth gauge in the same proportion. (For more about sharpening).

Guide Bar

The guide bar guides the chain so it can do its cutting. Bars are made from one solid piece of hardened steel or constructed of three laminated pieces.

The nose (tip) of the guide bar is the rounded area where the chain reversed direction and returns to the drive sprocket. This tip can be either solid with Stellite (a chromium cobalt alloy) welded on it to reduce wear or it can have a sprocket in it to further reduce bar tip wear. Since touching the tip to the wood can be a major cause of kickback, come manufacturers make make a special covered tip guard.

A slot goes all the way around the bar. The drive links of the chain ride in this slot, and therefore the long, thin sides of the slot are subject to wear. (For more about checking wear in the bar,)

Oil Pump

The rivets and links of the chain and guide bar need constant lubrication and that is the job of the oil pump. (For more about bar and chain oil,) Nearly all saws today have an automatic lubrication pumping system. On older saws and some smaller saws, oiling is manually activated through a push button or bulb you work with your thumb. If this is true of your saw, you should press the button at least once before each cut and check the oil level often.

No matter which system your saw has, the oil is pumped onto the chain and bar through a slot or hole in the guide bar. It is important to periodically remove the sprocket cover and make sure the slot or hole is clean of debris.

Chain Brakes

Any discussion of how a chainsaw works would not be complete without a mention of the brakes. The best chainsaws have two types of brakes: a manual and an inertial brake.

The front hand guard acts as a manual brake with a hair trigger, not unlike a hand pistol. With even slight pressure on the back of the hand guard, a cocked linkage and powerful spring mechanism goes off and clenches a binder strap on the clutchdrum attached to the chain drive sprocket. The chain is seized almost instantly. It all happens in a matter of milliseconds.

The inertial brake that’s built into the front handle/manual brake senses any rotational motion of the saw (such as a kickback) and sets off the cocked linkage that stops the chin automatically. You do not even have to think about doing it yourself. It is like an air bag in a car, just waiting for a chance to prevent an injury (or save a life).