All engines attempt to match the engine speed to the load on the engine.

• Trucks, cars, and most four-wheel ATVs have to shift gears as they accelerate and decelerate.
• An outboard motor has only forward, neutral, and reverse. The pilot of an outboard motor must change props in order to “change gears” from a work prop to a speed prop.
• Some airplanes can change the pitch of the prop as they fly. Others have a fixed prop.
• A chainsaw clutch is like a one speed transmission. The chain is either stopped or it is turning at the same speed as the engine.

None of these compare to a snowmachine clutch which is constantly sensitive to the load and force generated by the engine, balancing them in a constant flow of mechanical advantage. It is truly a beautiful thing to watch. A few models of four-wheel ATVs use the same type of clutch.

Pulleys of Different or Same Size

1. If two pulleys of the same size are connected by a belt, as one turns, the second one will turn at the same speed with the same power available at the shaft.
2. If a smaller pulley turns a larger one, the larger one will turn slower, but with more power available at the shaft.
3. If a bigger pulley turns a smaller one, the smaller one will turn much faster than the bigger one but with less power available at the shaft.

Example of Shifting a Truck

In low gear, a truck engine turns fast, but the wheels turn slowly. The truck goes slowly, but with power.

In higher gears, the engine turns more slowly, but the wheels are turning fast. There is less power to accelerate in higher gears, but the speed of the truck is much greater.

Effect of Load on an Engine

Overload

If an engine is overloaded, the speed of the engine (rpm1) is reduced, and there is tremendous stress on all of the drive components.

Underload

If an engine doesn’t have enough load, it can turn so fast (high rpm) that it can self destruct. The inertia of the piston going up and down at a speed too great for the engine will stress all of the internal parts.

Conclusion

The load on the engine governs the speed of the engine.

For an engine to run efficiently, it must be balanced with the load. We wouldn’t try to go 60 mph down the highway with a truck in first gear, nor would we try to start the truck in fourth gear.

Parts

A snowmachine clutch is made of two pulleys and a wide belt:

• The drive pulley is on the end of the engine crankshaft.
• The driven pulley drives the chain that connects to the track.
• The belt connecting the two pulleys is strong enough to transmit the energy of the engine to the track. It has enough friction with both pulleys that it won’t slip, except in extraordinary circumstances (as occurs when the track is frozen down.) The belt must be extremely strong yet flexible, even in very cold temperatures.

Description of Low and High Gear

1. When the snowmachine is starting to move, the engine is turning rather fast, but the track is turning slowly. There is ample power to get the machine going.
2. When the machine is accelerating, the engine speed is reducing and the track speed is increasing.
3. When the machine is at a cruising speed, the engine slows down and the track turns quickly.

The size of the drive and driven pulleys change while the machine is running.

Idling

When the machine is stopped, the engine turns, but the belt and track do not. The drive pulley is spread apart.

Starting

When the engine is accelerated, the drive pulley closes together pinching the belt, connecting the engine to the track. The machine moves forward, with the drive pulley small, and the driven pulley large. The machine’s speed is low, but the torque is high. This is like low gear in a truck.

Accelerating

As the machine accelerates, the engine speed increases, and the drive pulley closes together, making the drive pulley bigger. This spreads the driven pulley, making it a smaller pulley. At this point the drive and driven pulleys might be the same size. This is like second gear in a truck.

Cruising

When the machine is at cruise speed, the drive pulley is closed together, making it a large pulley. The driven pulley is spread apart, making it a smaller pulley. This is like high gear in a truck. The track is going fast, but the engine is not. This speed is more fuel efficient with less wear and vibration. The larger drive pulley is driving the smaller driven pulley.

Decelerating

As the machine encounters resistance, whether it is a hill or soft trail, the engine is slowed by the load against it. The drive pulley slows down, and spreads apart. The driven clutch is then driven together by its springs. This is like downshifting to a lower gear in a truck. The engine speed increases and the track speed is reduced, giving more power for the machine to work against the resistance.

We might ask why cars and trucks don’t have this kind of transmission if it is so good. The belt system isn’t strong enough or efficient enough for large engines and large loads.

The Drive Clutch Responds to the Load on the Engine

The drive pulley has weights that move out when the engine speed increases. They force the movable half of the drive clutch towards the other half, making it, in effect, a larger pulley.

When a load is encountered, the engine speed drops, and the springs within the clutch push it apart, in effect making it a smaller pulley.

The force of the weights in the drive clutch balance with the force of the springs in both the drive and driven clutches. Those forces flow against each other, constantly keeping the load and force of the engine in balance. Block up the rear end of a snowmachine, and observe the clutch working as the track spins above the ground.

Maintenance

There is a beautiful balance between the pressure of the springs and the force of the weights in the drive clutch. If any of the parts are not well lubricated, that balance will be hindered.

It is important to use polar grease that will not get too thick in very cold weather. The clutch should be sensitive to load, and not be sensitive to temperature.

Adjusting the clutch

Most machines have a way to tighten or loosen the spring of the driven clutch. Stronger or weaker springs can be installed.

If the spring in the driven clutch is too strong, it resists spreading. In effect, it gives the machine more power, but causes the engine to operate at a higher rpm than is healthy. If the spring is too weak, the driven clutch spreads easily. Top speed is easily reached, but the engine stresses and is weak in acceleration.

Activities

1. Remove the clutch guard from a machine and identify the parts.
2. Block the back of the machine up or suspend it so the track is free from the ground. With students at a safe distance, start the engine. Accelerate and decelerate the engine. Can you see the clutch responding automatically to the throttle changes?
3. Watch the drive clutch. Is it bigger at low or high rpm?
4. Watch the driven clutch. Is it bigger at low or high rpm?
5. Is clutch movement smooth or erratic? If it is not smooth, what does this indicate?
6. At below zero temperatures, put polar grease between your fingers on one hand and regular grease between the fingers of the other hand. Can you feel the difference?
7. What would slip if the track were frozen down and the engine accelerated? Do you see evidence of this around the clutch?
8. Remove and replace a drive belt. Compare a worn and a new belt for width. Find the price of a new belt.
9. Can you see the weights in the drive clutch? If this isn’t possible, try to find an old one that has been taken apart. Describe how centrifugal force causes the drive clutch to close together. Draw what you imagine happening.
10. Compress the spring from both the drive and driven pulleys. Are they stronger or weaker than you thought? Is there a way to tighten the spring on the driven pulley on the machine you are looking at?
11. Improvise some pulleys, even if you have to use a rubber band as a drive belt. Use different size thread spools as pulleys if you can’t find anything else. Predict how many turns the driven spool will turn when the drive spool turns once.
12. Drive a four-wheel ATV, accelerating through the gears. While doing this, try to imagine what would be happening to the drive and driven pulleys if it had a snowmachine clutch. (Actually, Polaris six-wheelers have a clutch like a snowmachine.)
13. Do you think this kind of clutch would work on an outboard motor, giving better performance?

Student Response

1. How do each of the following attempt to balance the engine speed with the load?
   • Trucks, cars, & four wheelers
   • Outboard motors
   • Airplanes
2. What are the three main parts of a snowmachine clutch?
3. Which clutch is on the end of the engine’s driveshaft?
4. Draw a picture of a big pulley driving a smaller one. If the big pulley turns once, will the little one turn more or less than one turn?
5. Draw a picture of a smaller pulley driving a bigger one.
6. Draw a picture of two pulleys of the same size.
7. Which of the above pictures illustrates an engine starting to move?
8. Which one of the above illustrates an engine at cruise speed?
9. What is the main advantage of a snowmachine clutch over a truck transmission?
10. Why couldn’t a snowmachine clutch be used on a truck or car?
11. What is the advantage of a snowmachine clutch over a chainsaw clutch?
12. What is the governor on an engine? What happens if there isn’t enough load on an engine? What happens if there is too much load on an engine?
13. What two things does the snowmachine clutch balance?
14. Why is good lubrication important in a snowmachine clutch?

Math

1. A pulley 3” in diameter turns a pulley 2” in diameter. If the first one turns 50 complete revolutions, how many revolutions does the second one turn?