# why can’t we have gear ratios like 1:300.000

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I’ve recently seen videos on YouTube showing gear ratios like 30 million to 1 when you spin the first gear super fast at 10.000rpm and the last gear spins so slow it would take 300 hours to spin once for example. Searching for the opposite doesn’t give any result, the highest I found was 1:625.

Why is that we can have millions:1 gear ratios but not 1:millions?

In: Engineering

Probably because there’s only so much force the gear teeth would be able to take before breaking under the force of the torque. But that’s just my guess, I don’t really know.

It would be really hard to push the first gear because the friction of the last gear spinning is multiplied back into the first gear. The reason we use gear ratios is because we trade speed for torque, so if we want the last gear to go fast we need a lot of torque.

Two reasons:

* If you’re increasing the speed something spins, you’re decreasing torque (how hard is it turning, or how hard is it to stop). At some point you’d need an insanely powerful motor, just to overcome the friction at the last gear and make it turn.
* Even if you had a motor that powerful, you’d need gears that can survive that torque, and gears that can rotate fast enough without breaking from centrifugal force.

They are essentially the same thing 1:milion and million:1 just means you drive the other end. It would really work in practice, though. There is a simple rule in play, if you multiply the speed, then you divide the torque. This means trying to do the reverse would require so much torque, in practice, that it would simply snap the shaft even if it were made of steel.

When you reduce the speed, you multiply the torque, so very little stress is placed on the driving shaft. The other way wouldn’t work because even a little friction or resistance on the end would multiply itself to a huge torque required on the driving side.

Each gear tooth is like a lever with the fulcrum at the center of the gear. Just like with simple levers, there are practical limits to the forces gears can sustain depending on the materials they are made of. As the gear ratio decreases, it is akin to a simple lever getting longer, and farther from the fulcrum, and therefore needs to be made of stronger materials. The stronger materials are more expensive, and therefore less practical than other ways available to obtain the requisite speeds.

We don’t have an ultra high torque source that spins slow enough to really justify a gear ratio like that, nor do we have materials that can withstand the speeds you’d end up with from the sources we do have.

If you start with one of those big cargo ship engines that spins at 90 RPM (1.5x per second) and run it through a 1:625 gear ratio then your output gear is going 56,000 RPM. If your output gear is even 5 cm in diameter then its edges are being accelerated outward at over 88,000x the force of gravity (867 km/s^2) and the teeth are moving at about half the speed of sound, and that’s just for 1:625

At around 1:1500 the teeth would be moving faster than the speed of sound in air at sea level. At 1:13,000 you’d start bumping into the speed of sound of steel and your gear would undergo some really interesting failure modes as it is unable to pass the force through itself fast enough. And this is just for a gear 5 cm in diameter, if you pick one that’s 25 cm then you hit the speed of sound in steel at just 1:2600 (1/5th the ratio)

There is no useful application for a low torque ludicrously fast gearset (there isn’t one for a high torque stupid slow one either) and you start running into issues with material properties because you just can’t spin the source slow enough. You’d need something spinning once every 12.5 days to be able to match the same gear ratio but in reverse, and you’d need an insane amount of torque on the input and we just don’t have something for that

Have you ever ridden a mountain bike, one of those ones with a bunch of gears? Notice how easy it is to pedal when you’re on a low number, and how hard it is on a high number? And this is with ratios like 1:3 or 1:4. You would need some absolutely *insane* power to get that gearset even started turning. If we need something to spin fast it generally seems easier to make a power source that spins fast in the first place than to gear it up like that.

Torque and power. You need apply very little torque on the reduction gear since you’re trading a lot of initial velocity to get a lot of torque at the end. But going the other way around you’d have to apply a huge amount of torque on the first gear just to move the last one.