||Chainsaws have been an important part of the life of Alaskans for
the last generation. Prior to that, heavy motor-driven saws were scarcely
faster than hand saws. Previous to that, we used Swede saws and two-man
saws. For the amount of gas burned, a modern chainsaw can do more
work than any other engine. Most houses can be heated on five gallons
of gasoline a winter for chainsaw operation. That represents numerous
hours with a Swede or two-man saw.
A 6, 8c,
B 1, 3
D 1, 3
At high speed
have expanded out,
turning the drum
If the chain on a chainsaw was directly connected to the engine,
it would be very dangerous. The chain would turn as soon as the
The clutch allows the engine to turn freely at low rpm [rpm=revolutions
(turns) per minute] without moving the chain. They are separate.
Unlike a four wheeler or truck, the chainsaw operator doesnt
have to engage the clutch or put the chainsaw in gear. It happens
automatically when the engine gets up to a certain speed.
How does the chainsaw clutch work? It uses two physical principles:
circular motion (inertia) and friction.
chainsaw clutch is made of two main parts that are separate when
the engine idles slowly:
- The drive shaft, shoes, and spring
- The drum, sprocket, and chain
The drive shaft is connected to the shoes.
The drum and sprocket are connected to the chain.
At zero or low rpm, the engine shaft, shoes, and spring turn together.
The drum, sprocket and chain do not.
At high rpm, the engine is connected to the drum and chain. They
How does this work so simply? Science.
When the engine spins slowly, the shoes are held tightly against
the engine shaft, and away from the drum by the spring wrapped around
When the engine is turning fast, the inertia of the shoes overcomes
the force of the spring. The shoes move outward, pressing hard against
the drum. Now the engine and chain are firmly connected.
When the shoes are out against the drum and the chain is turning,
there must be enough friction between the drum and shoes to allow
little or no slipping.
As soon as the engine speed drops, the spring overcomes the inertia
of the shoes, pulling them away from the drum. The engine is separate
from the chain again.
If the chain is stuck in a tree, and the operator tries to power
it out with the engine, heat from friction turns the clutch drum
blue and warps it out of shape.
chain has tremendous potential for friction and power loss as it
travels around the bar. Energy used overcoming friction is energy
that isnt available to remove wood fibers from the log.
There are deep grooves in the bar to keep the chain from flying
off the bar and to keep the chain cutting straight. There is much
For this reason, there is a constant stream of oil pumped through
a hole in the bar to the chain.
the oil is too thin, it flies off the end of the bar by inertia.
This leaves the underside of the bar and chain dry of oil. There
will always be a little oil coming off the nose of the bar. However,
if the oil is too thin, large amounts will splatter from the nose
of the bar. To determine this, the operator revs the engine with
the bar pointed at a clean surface (like snow), looking for the
If the oil is too thick, as it might be in the cold of winter,
the bar and chain dont get enough oil. There is great wear
and power loss due to friction. In subzero temperatures we often
thin bar oil with gasoline or kerosene. The operator should listen
if the bar sounds dry. If the oil level hasnt gone down considerably
when the gas tank is empty, thin the oil until it flows freely at
If the oil is the proper consistency, it will pump freely, and
will stay with the chain all the way around the bar.
Many saws have an adjustment for the oil flow. Longer bars require
more oil than shorter ones. The location of the adjustment isnt
always obvious, so refer to the owners manual.
While many people use 30w motor oil, professional chainsaw bar
oil is more effective because it is sticky.
All chainsaw manufacturers discourage the use of old crankcase
oil from generators or cars for bar oil. I used to think they just
wanted to make money when I bought their expensive bar oil, so I
used crankcase oil. What I didnt realize is that there are
many small iron filings in crankcase oil that cause excessive wear
on the bar and chain. Filings can also plug up the oil filter. Once
I understood this, I immediately started using bar oil or 30w oil
from large containers. Bars and chains have lasted longer with commercial
Many chainsaw bars have a roller on the nose. This is to reduce
power loss to friction. The greatest tension on the chain is between
the wood and the sprocket. Most of the time, cutting is done on
the bottom of the bar.
When cutting is done on the top of the bar, there is tension along
the full length of the bar and chain. Friction on the nose is great.
Without a roller nose there would be considerable power loss. Avoid
cutting on top of the bar when possible.
Most professionals turn their bars over daily so the bar will
wear evenly on both sides.
Chainsaws and Fishtraps
Sometimes we use chainsaws to cut river ice when setting fishtraps
through the ice. It doesnt hurt a cold saw to be splashed
with cold river water. However, icy water can crack a hot cylinder.
There are often rocks in the ice that will ruin the chain. This
is particularly true if the river has frozen, then thawed again.
During the warm spell, the ice that was frozen on the gravel bars
breaks free, carrying rocks from the bar. It drifts down the river
to freeze in a different place.
- Put a weight on a string. Attach a strong rubber band or soft
Bungee cord to the string. Hold the loose end of the rubber band
and spin the weight around the head. (The dangers of this should
be fairly obvious.) Try this with different weights and different
strengths of rubber bands. Can the students feel the difference?
- If students havent tried the old water-bucket-around-the-head
activity, let them try it. This illustrates inertia quite well.
- There are two types of chainsaw clutches: those with the sprocket
on the outside of the clutch and those with the sprocket on the
inside. Find one of each kind in the village and draw a top view
- Do this outside. Get an experienced person to run a saw that
has the sprocket on the inside and clutch on the outside. Take
the cover off and observe the operation of the clutch. Increase
and decrease rpm. Can you see the shoes go in and out? Describe
what you see to someone who didnt see the demonstration.
- Look on the chainsaw in the above activity for the oil flow
adjustment. Observe the oil coming out of the saw to supply the
bar. Adjust the oil flow. Is the difference obvious? Find the
hole in the bar that allows the oil to flow to the chain. Is there
a similar hole if the bar is turned over?
- Get a chainsaw clutch from an old chainsaw. Look at the drum.
Does it look blue from being overheated? Look at the shoes. Test
them for friction with other materials. Do they look like they
are made of high friction material? What do you think that material
is? To get the clutch off you will probably have to follow these
- Remove the spark plug.
- Put a screwdriver in the spark plug hole and turn the engine
until you feel the piston is at the bottom of the cylinder.
Remove the screwdriver.
- Cram the cylinder full of nylon or other plastic rope.
Now you can put a wrench on the nut holding the clutch and the
rope will hold the crankshaft from turning.
- Test the clutch spring for tension. Is it a strong spring?
Does it also look blue from overheating? Does it stretch evenly
or is it dysfunctional?
- Imagine what would happen if oil got into the clutch. What
problems do you think would occur if this happened when the saw
- Get some 30w oil. Heat it by putting the plastic container
in very hot water. How thin does it get? Do you think it would
stay on the bar well when it is that thin? Cool it in a freezer.
Feel it. Do you think it would pump well at this temperature?
- Get some commercial bar oil. Put a little on your fingers.
How is it different from regular 30w oil? Why do you think this
- Get some crankcase oil. Wrap a magnet in thin plastic wrap.
Immerse the tip of the magnet in the old oil and see if you can
pick up the iron filings that are supposedly in the crankcase
- Get a chainsaw bar with a roller nose. Does it have a hole
to grease, or is it permanently lubricated?
- Get several bars and chains, new and old. Test them for side
motion and wear. How sloppy is the chain in the bar? Does it tend
more toward one side than another? Which part of the bar is worn
the most? Does it look like the owners turned the bar over often,
or is one side worn more than the other?
- Draw or trace a side view of a chainsaw chain.
- Ask people in your village what is the best saw they ever owned
and why. What is the favorite bar length?
- Ask people in the village how they cut wood before chainsaws.
If you can, fall and buck a tree using that method.
- Ask people in the village the names of the chainsaws they know
or remember. Make a class list. Find on the map the locations
where they were manufactured. (You will need a world map.)
- Ask in the village if someone can demonstrate how to splice
a broken chainsaw chain.
- Gently file the materials of the bar, clutch drum, clutch shoes,
sprocket, chain dogs, and teeth. Which are hard and which are
softer than a file?
- Have a contest to see who can untangle a chainsaw chain the
fastest. (This is often a challenge!) Let students tangle a chain,
and pass it to the next person to untangle.
- Get an owners manual. Draw a picture of two dangerous
activities that should be avoided.
- What might happen if the chain and engine of a chainsaw were
- What are the two main parts of the clutch?
- How does inertia work in a chainsaw clutch?
- What is around the shoes that keeps them from flying outward
at low rpm? What would result if it were too loose? Too tight?
- Draw a chainsaw clutch where the engine is turning at low rpm
and the chain isnt turning.
- Draw a chainsaw clutch where the engine is turning at high
rpm and the chain is turning.
- If there were low friction between the shoes and drum, what
would happen when there was a load on the chain?
- What happens to the shoes when the engine is slowed down after
running at high speed?
- What is happening when the chain is stuck in the tree and the
saw is being run at high rpm?
- What happens when chain oil is too thin? Explain or draw it.
- What happens if the chain oil is too thick?
- How is professional bar oil different from 30w oil?
- Why is the use of old crankcase oil discouraged?
- What is a roller nose on a chainsaw bar? What is the purpose?
- Draw a chainsaw bar and chain as they are cutting a block.
Identify the place on the bar where the chain is loose. Identify
where it is tight.
- Commercial bar oil is $3 per quart. Henry bought eight quarts
and figured that his bar lasted two times longer than if he used
free crankcase oil. A new bar is $30. Did he save money?
- Would it be cheaper if he ordered an extra bar from a discount
place for $22 or commercial chain oil for $2.25?
- Henry can get bar oil for $3 a quart, or order in bulk, 5 gallons
for $32, plus $13 shipping. Is he saving money, and if so, how
much? The answer can refer to quarts or 5-gallon buckets.
- Jesse wants to sell cordwood. He figures that each cord for
his chainsaw takes 1 quart of bar oil at $2.75, 3/4 gallon of
gas at $3.50 per gallon including two-cycle oil. Snowmachine costs
are about $21 per cord, including wear and tear. It takes 5 hours
to cut and haul a cord. How much should he charge to make the
equivalent of $10 per hour? $12 per hour?