Car out of legos using the LEGO Mindstorms and gears

[alex568]

Hi, I'm trying to build a car out of legos using the LEGO Mindstorms set that will go really fast. I'm familiar with the concept of gear ratios (i.e. a 40t gear driving an 8t gear) and using that to increase speed, at the cost of torque. Another method is to attach an axle to the following 8t gear and attach another 40t gear to the same axle and have that turn another 8t gear etc. Somewhere else on this forum, another question relating to gears was asked here:

https://www.physicsforums.com/showthread.php?t=70333

and it was stated that "so a 8t driving 40t then followed by 8t then 40t, 8t, 40t

produces 1:5 * 1:5 * 1:5, which produces 1:125, which produces a lot of torque at the cost of speed?

on the other hand, a

40t driving, 8t, 40t, 8t, 40t produces

5:1 * 5:1 * 5:1, which is 125:1, produces a lot of speed at the cost of torque?"

I don't quite understand this statement, because in the second example, if the first 40t gear was rotating at 1rpm, the 8t gear would be rotating at 5rpm, rotating the other 40t gear at 1rpm, rotating the 8t gear at 5 rpm, followed by a 40t gear rotating at 1rpm. So wouldn't it be the same 5:1 ratio?

 

[russ_watters]

Gears are paired and fixed on the same axle. The second 1 & 5 aren't meshed, they are attached face-to-face. Otherwise, you'd have 1:5*5:1*1:5*5:1*1:5

 

[ravenprp]

Hello! Building a car out of legos using the LEGO Mindstorms set sounds like a fun project. You are correct in your understanding of gear ratios and how they affect speed and torque. The first example you mentioned, with the 8t gear driving a 40t gear, is called a "reduction" gear. This means that the output speed is slower than the input speed, but the output torque is higher. This is useful for tasks that require a lot of force, such as lifting heavy objects.

The second example you mentioned, with the 40t gear driving an 8t gear, is called a "multiplication" gear. This means that the output speed is faster than the input speed, but the output torque is lower. This is useful for tasks that require high speed, such as a car.

In the second example, you are correct that the overall gear ratio is 5:1, but this ratio is achieved by multiplying the individual gear ratios. So, the first 40t gear rotates 5 times for every 1 rotation of the 8t gear. Then, the 8t gear rotates 5 times for every 1 rotation of the second 40t gear. And so on. This is why the overall ratio is 125:1, which is a lot of speed at the cost of torque.

I hope this helps clarify the statement you were confused about. Keep experimenting and have fun building your lego car!