Why do trucks have so many transmission gears?
In: 354
It takes a certain amount of force to move an object at a certain speed. Through various means, you can move more weight with the same force, just the same amount more slowly. One of those methods is gear reduction, which is the “gears” in a transmission. Gears take the input force from one gear to spin another gear more slowly, so the output gear spins more slowly but with more force. The force produced by an engine is a twisting force called torque. A semi-tractor engine produces around 2000 lb-ft of torque. Fully loaded, a semi truck and trailer can weigh up to 80,000 lbs. On flat ground, that means it can take around 30,000 lbs of force to move that much weight. If the engine were directly acting on the wheel with no gear reduction, it could produce about 2000 lbs of force. No where nearly enough to move the truck right? This is where gear reduction comes in. First gear in a transmission is the most speed reduction producing the most force. Typical first gear ratios in a class 8 semi will be something like 15 to 1. That means that the force from the engine has been multiplied by 15 times but is spinning 15 times more slowly. This is further reduced by the axles, which have their own reduction, called the final drive ratio. This is typically around 3 to 1. And these ratios multiply. So in our typical case, in first gear the 2000 lbs of force is now multiplied by 45, but is moving 45 times more slowly. So we’re now making 90,000 lbs of force, more than enough to move the truck! But. A typical class 8 engine spins at a maximum of about 2000 rpm. Reduce that by 45 times, multiply that by the circumference of the tire and first gear gets you a maximum speed of… about 3.5 mph. But, now that it’s in motion, it takes much less force to keep it in motion, because inertia says an object in motion wants to stay in motion, and moving friction is less than static friction. So you go to second gear, which doesn’t increase the force by as much but also doesn’t reduce the speed by as much, letting you speed up. The heavier the load to move, the more reduction it takes to get it moving, the more gears you need to get the load up to speed. So a typical car makes around 200 lb-ft and weighs about 4000 lbs. A typical class 8 truck makes around 2000 lb-ft, 10 times as much, right? But it weighs typically around 60,000 lbs, 30 times as much. And while a typical car engine can spin around a minimum of 6000 rpm, a big diesel engine can only spin around 2000 rpm easily. So, it requires many more gears to get up to a usable speed.
Latest Answers