Village Science

Chainsaw Clutch & Chain

  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, 15
B 1, 3
D 1, 3







At high speed shoes
have expanded out,
turning the drum

The Clutch

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 engine started.

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 doesn’t 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 partsThe 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 turn together.

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 them.

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.

bar oil hole


grooveThe chain has tremendous potential for friction and power loss as it travels around the bar. Energy used overcoming friction is energy that isn’t 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 metal-to-metal contact.

For this reason, there is a constant stream of oil pumped through a hole in the bar to the chain.

oilIf 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 oil splatter.

If the oil is too thick, as it might be in the cold of winter, the bar and chain don’t 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 hasn’t gone down considerably when the gas tank is empty, thin the oil until it flows freely at that temperature.

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 isn’t always obvious, so refer to the owner’s 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 didn’t 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 bar oil.



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.

chainsaw    chainsaw


Chainsaws and Fishtraps

Sometimes we use chainsaws to cut river ice when setting fishtraps through the ice. It doesn’t 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.



  1. 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?
  2. If students haven’t tried the old water-bucket-around-the-head activity, let them try it. This illustrates inertia quite well.
  3. 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 of each.
  4. 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 didn’t see the demonstration.
  5. 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?
  6. 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 directions:
    • 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.

  1. 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?
  2. Imagine what would happen if oil got into the clutch. What problems do you think would occur if this happened when the saw was operating?
  3. 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?
  4. Get some commercial bar oil. Put a little on your fingers. How is it different from regular 30w oil? Why do you think this is so?
  5. 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 oil.
  6. Get a chainsaw bar with a roller nose. Does it have a hole to grease, or is it permanently lubricated?
  7. 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?
  8. Draw or trace a side view of a chainsaw chain.
  9. Ask people in your village what is the best saw they ever owned and why. What is the favorite bar length?
  10. Ask people in the village how they cut wood before chainsaws. If you can, fall and buck a tree using that method.
  11. 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.)
  12. Ask in the village if someone can demonstrate how to splice a broken chainsaw chain.
  13. 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?
  14. 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.
  15. Get an owner’s manual. Draw a picture of two dangerous activities that should be avoided.
Student Response

Student Response

  1. What might happen if the chain and engine of a chainsaw were always connected?
  2. What are the two main parts of the clutch?
  3. How does inertia work in a chainsaw clutch?
  4. What is around the shoes that keeps them from flying outward at low rpm? What would result if it were too loose? Too tight?
  5. Draw a chainsaw clutch where the engine is turning at low rpm and the chain isn’t turning.
  6. Draw a chainsaw clutch where the engine is turning at high rpm and the chain is turning.
  7. If there were low friction between the shoes and drum, what would happen when there was a load on the chain?
  8. What happens to the shoes when the engine is slowed down after running at high speed?
  9. What is happening when the chain is stuck in the tree and the saw is being run at high rpm?
  10. What happens when chain oil is too thin? Explain or draw it.
  11. What happens if the chain oil is too thick?
  12. How is professional bar oil different from 30w oil?
  13. Why is the use of old crankcase oil discouraged?
  14. What is a roller nose on a chainsaw bar? What is the purpose?
  15. 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.


  1. 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?
  2. Would it be cheaper if he ordered an extra bar from a discount place for $22 or commercial chain oil for $2.25?
  3. 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.
  4. 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?

Questions or comments?
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