the physics in changing gears
Gears are able to transfer energy in a machine. Think about pedaling a bike. You are transferring your own energy through the pedals and over gears that move the wheel. Now think about a car. Whenever your car hits a certain RPM (revolutions per minute) it needs to switch gears. Whether you do this manually or automatically, you're changing the gear ratio which is altering the torque of the wheels. Let's talk a little bit about a gasoline engine. A car's engine extracts the energy from gasoline. It does this by moving pistons ("cylinders") at very high speeds. The engine has a minimum speed it works best at which is usually around 1000 RPM but, since the engine is connected to the wheels, we wouldn't want the wheels to be rotating at 1000 RPM (75 mi/hr) to run efficiently. On your dashboard you'll notice that the maximum RPM an engine can go is outlined in red. "Redlining" is the process of reaching these values of RPM's. Usually when you start to hit the redline for a long period of time, your engine cuts off, but if it didn't, it would explode. The engine has a narrow range of revolutions per minute where horsepower and toque are at their maximum. So, by using gear ratios between the engine and the drive wheels you can shift the gear before the redline is hit and stay in the band where the engine works the most efficiently.
Since cars require a large amount of force to get moving and very little speed, the gear box can increase the torque to get the car moving while not starting the car at 75 mi/hr. As you need to go faster, a different gear ratio is established since you're already moving you wouldn't need as much torque. Now, the energy from the engine can be transferred into speed.
Let's explore the actual gear some more. As you know, gears are wheels with teeth. Gears can come in many different sizes and have a lot of different teeth counts. Gears are used to increase speed, increase force and reverse the direction of the machine. The benefit to using gears with various teeth size is that you could move a gear with 25 teeth (driving) that will move a gear with 75 teeth (driven). So every three turns you do on your gear, the second gear will do one. This is done using the formula driven/driving=75/25=3. When the driving gear is larger than the driven gear, say the driving gear has 60 teeth and the driven gear has 30, then we multiply the velocity ratio by the RPM of the first gear. So 60/30=2, and if the driving gear was moving at a rate of 120 RPM, then the driven gear would be going at 240 RPM. The opposite is true for a smaller driving gear to a larger driven gear. If we were driving a gear with 25 teeth to a gear with 75 teeth, then the ratio would be 3, and if the smaller gear was being rotated at 60 RPM then the second gear would be rotated at 20 RPM.
These different ratios and moving of energy make your car go. If you're driving down the highway and you need to pass a truck, what would you do? Let's say you're already in your top gear of fifth gear. To pass the truck you'd actually downshift to fourth or third gear. This is because you are already going fast (fast enough for fifth gear, so at least 40 mph), but by changing the gears down, you'll get more torque and will be able to give your wheels more power to pass the truck. The downshifting will increase your RPM and make accelerating faster. If you were to downshift but not increase the accelerator, you would start to slow down.
Since cars require a large amount of force to get moving and very little speed, the gear box can increase the torque to get the car moving while not starting the car at 75 mi/hr. As you need to go faster, a different gear ratio is established since you're already moving you wouldn't need as much torque. Now, the energy from the engine can be transferred into speed.
Let's explore the actual gear some more. As you know, gears are wheels with teeth. Gears can come in many different sizes and have a lot of different teeth counts. Gears are used to increase speed, increase force and reverse the direction of the machine. The benefit to using gears with various teeth size is that you could move a gear with 25 teeth (driving) that will move a gear with 75 teeth (driven). So every three turns you do on your gear, the second gear will do one. This is done using the formula driven/driving=75/25=3. When the driving gear is larger than the driven gear, say the driving gear has 60 teeth and the driven gear has 30, then we multiply the velocity ratio by the RPM of the first gear. So 60/30=2, and if the driving gear was moving at a rate of 120 RPM, then the driven gear would be going at 240 RPM. The opposite is true for a smaller driving gear to a larger driven gear. If we were driving a gear with 25 teeth to a gear with 75 teeth, then the ratio would be 3, and if the smaller gear was being rotated at 60 RPM then the second gear would be rotated at 20 RPM.
These different ratios and moving of energy make your car go. If you're driving down the highway and you need to pass a truck, what would you do? Let's say you're already in your top gear of fifth gear. To pass the truck you'd actually downshift to fourth or third gear. This is because you are already going fast (fast enough for fifth gear, so at least 40 mph), but by changing the gears down, you'll get more torque and will be able to give your wheels more power to pass the truck. The downshifting will increase your RPM and make accelerating faster. If you were to downshift but not increase the accelerator, you would start to slow down.
Now that we know how the gears work, let's talk about a sample transmission. The image to the left is of a two-speed gearbox in neutral. When a car is in neutral, it won't stall when you let off of the clutch pedal, but it also won't accelerate when you press the gas pedal. If you drive a manual car, you are well aware that there are three pedals and that you use both your left and right feet. The pedal all the way to the left, the clutch, connects and disconnects the engine from the transmission while you change gears. In our diagram to the left, the clutch is indicated by the green gear. In the red is the layshaft. The layshaft spins as one unit. When the clutch is connected and spinning, so will the layshaft. The layshaft gets it's power directly from the engine when the clutch is engaged. (When you are pushing down on the clutch pedal) In the yellow we have a splined shaft that connects directly to the drive shaft through the differential. This drives the wheels of the car. The blue gears will be spinning if the yellow shaft is spinning (and the wheels). The collar, or flywheel, (in purple) connects one of the two blue gears to the yellow driveshaft. The collar can slide left or right along the yellow shaft to connect to either of the blue gears. The collar has teeth that are called "dog teeth". You can tell that this car is in neutral because the collar isn't connected to either of the blue gears (1st and 2nd gear). The larger the gear number, the smaller it is and the harder it is to get the collar into when starting from a dead stop. You wouldn't want to start you car at a red light at the highest gear, it usually will end up in a stall if not done slow enough.
Lets say you get the car into first gear, now what happens? In first gear the collar is connected to the larger gear (the one on the right). In first gear the green shaft is still turning the layshaft and its energy is being transferred through the collar and into the yellow drive shaft. The smaller gear on the left is turning, but won't have any effect on the yellow shaft. Both of the blue gears move the yellow drive shaft at different rates because of the ratio of their teeth.
If you drive an automatic car your car still changes gears like this, just at the most optimal time. While driving an automatic car is convenient, it isn't as fun! I do know that driving manual can be frustrating when you stall in traffic or make a mistake shifting. When you accidentally make a shift mistake and you hear that grinding sound, it is the dog teeth from the collar trying to engage the holes of the blue gear but it is unsuccessful, resulting in that grinding. So what happens when you stall? When you stall your car you aren't engaging the clutch at the right point and the engine can't communicate with the wheels, so the car's computer thinks something is wrong, so it turns off the car.
Lets say you get the car into first gear, now what happens? In first gear the collar is connected to the larger gear (the one on the right). In first gear the green shaft is still turning the layshaft and its energy is being transferred through the collar and into the yellow drive shaft. The smaller gear on the left is turning, but won't have any effect on the yellow shaft. Both of the blue gears move the yellow drive shaft at different rates because of the ratio of their teeth.
If you drive an automatic car your car still changes gears like this, just at the most optimal time. While driving an automatic car is convenient, it isn't as fun! I do know that driving manual can be frustrating when you stall in traffic or make a mistake shifting. When you accidentally make a shift mistake and you hear that grinding sound, it is the dog teeth from the collar trying to engage the holes of the blue gear but it is unsuccessful, resulting in that grinding. So what happens when you stall? When you stall your car you aren't engaging the clutch at the right point and the engine can't communicate with the wheels, so the car's computer thinks something is wrong, so it turns off the car.
The above picture is that of a five speed transmission. The two speed example is good to use to first understand how the gears work, but a five speed transmission is more practical. As you can see, the largest gear is the first gear which makes it easiest to engage when starting the car from a stop. The gears get progressively smaller till you get to fifth gear. You also notice that there are three different collars. When the collar for a gear is connect, all the others stay in that "neutral" position. The diagram above is a car in neutral. You'll also notice that there is a gear for reverse. This gear spins the yellow drive shaft the opposite way, thus spinning the wheels the other way.
If you've never driven a manual car, I encourage you to try it! It helps to understand what is going on with your car. I've set up a blog about opinions about driving standard. Let me know what you think! Also, if you want to see a cool animation here is a link.
If you've never driven a manual car, I encourage you to try it! It helps to understand what is going on with your car. I've set up a blog about opinions about driving standard. Let me know what you think! Also, if you want to see a cool animation here is a link.
Work Cited
Brian, M. (2000). HowStuffWorks "How Manual Transmissions Work".HowStuffWorks. Retrieved July 15, 2014, from http://auto.howstuffworks.com/transmission.htm
Gears-What are they?. (n.d.). . Retrieved July 15, 2014, from http://bowlesphysics.com/images/Robotics_-_Gears_and_Gear_Ratios.pdf
Woodford, C. (2014, May 15). Gears. - How do they work?. Retrieved July 15, 2014, from http://www.explainthatstuff.com/gears.html
Brian, M. (2000). HowStuffWorks "How Manual Transmissions Work".HowStuffWorks. Retrieved July 15, 2014, from http://auto.howstuffworks.com/transmission.htm
Gears-What are they?. (n.d.). . Retrieved July 15, 2014, from http://bowlesphysics.com/images/Robotics_-_Gears_and_Gear_Ratios.pdf
Woodford, C. (2014, May 15). Gears. - How do they work?. Retrieved July 15, 2014, from http://www.explainthatstuff.com/gears.html