6.4 - Wheel and Axle
The fulcrum on a lever is the point that the lever rotates about. What would happen if that rotation went completely around? The effort and load forces would trace out two concentric circles. The wheel and the axle is a second class lever in this special circumstance of being able to rotate completely. The wheel is where the effort force is applied, while the axle carries the load force. The radii of each are the effort and load distances.
The fulcrum of a wheel and axle system is the center of the wheel. It is the axis of rotation. Think of a basketball spinning on your finger. Every part of the ball moves around except the spot that your finger is on. Your finger is on the axis of rotation of the ball.
When a second class lever is spun about the axis of rotation, you are going a longer distance around so that the force you apply can be smaller. Looking at the diagram you can see that the effort distance is the radius of the wheel while the load distance is the radius of the axle.
Examples of wheels and axles are screwdrivers, wrenches, doorknobs, pedals to your bike, and steering wheels.
There are times when you need the wheel and axle to work in the opposite way- where the axle drives the wheel. If you took a third class lever and spun it around the fulcrum you would get the wheel and axle pattern again, but this time, the effort is on the axle and the load is the wheel. Fans, propellers, the back tire of your bike, and circular saws are examples of this type of axle and wheel.
When a second class lever is spun about the axis of rotation, you are going a longer distance around so that the force you apply can be smaller. Looking at the diagram you can see that the effort distance is the radius of the wheel while the load distance is the radius of the axle.
Examples of wheels and axles are screwdrivers, wrenches, doorknobs, pedals to your bike, and steering wheels.
There are times when you need the wheel and axle to work in the opposite way- where the axle drives the wheel. If you took a third class lever and spun it around the fulcrum you would get the wheel and axle pattern again, but this time, the effort is on the axle and the load is the wheel. Fans, propellers, the back tire of your bike, and circular saws are examples of this type of axle and wheel.
Gear systems can also be considered wheel and axle systems. The difference is that the gears are not concentric. One gear (the sprocket) drives the other gear (the cog). A good example of this is a bicycle. The pedals are a wheel to the front sprocket. The front sprocket drives the rear cog by the chain that attaches them. The rear cog is the axle that drives the rear wheel. By changing the sizes of the sprocket and the cog, you can change the mechanical advantage of the bike.