Hello! Welcome back to IntroVex, your robotics guide to Vexcellence.
In our new series: Build Your Chassis!!, we will talk about key ideas that might help you build a sturdy and smooth chassis for your robot. A robot needs a chassis with wheels that spin smoothly and as frictionless as possible to run fast at its intended speed. We'll cover some simple yet handy tips for you to improve.
In this episode, we will first talk about the common gear ratios of competition robots. Gear ratio is a key factor you have to decide based on the characteristic you want your robot to have, commonly between high speed and high torque.
How to calculate Gear Ratio?
At first, when you built your first robot using the VEX IQ super kit, competition kit, and education kit, you will most likely connect the wheel's shaft directly to the motor's shaft. This either makes one of the wheels not connected to motor power, or it connects two wheels with gears to make the power transfer to both wheels.
In the two cases, the latter (connecting wheels with gears) performs better because there wouldn't be one wheel that only provides friction resistance and does not contribute to forward motion. Taking the first-generation starter kit clawbot for example, even though the shaft of the wheel is still connected directly to the motor, the 36-tooth gears bring rotational motion to the second wheel as well.
However, in both cases, they barely covered the gear ratio idea used in competition robots. In competition chassis, we often make the chassis longer. Additionally, we alter the size of gears connected to generate faster or slower speeds; this is called the Gear Ratio.
Gear Ratio is a "ratio", obviously, but between what two values? We have to take into account the "input gear" and the "output gear". The input gear is the one that is directly spun by the motor, and the output gear is the one being driven. Regarding the clawbot, the 36-tooth gear connected to the motor serves as the input gear, and the final gear at the end is the output gear.
What does the middle one do? You may ask. The middle gear in this case is an idle gear that doesn't affect the final gear ratio, meaning we can change the middle gear into 84T, yet the two wheels still spin at the same rate.
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Next, the Gear Ratio is calculated by dividing the output gear tooth by the input gear tooth. Please make sure you don't get it the opposite way because they can mean very different things. In the clawbot case, the gear ratio will be 36:36 = 1:1.
Looking at another case, why not try to identify the input, idle, and output gear, then calculate the gear ratio by yourself? The answer will be at the end of this blog.
Common Gear Ratio Used?
The common Gear Ratios in VEX IQ are normally used for increasing speed, in other words, a lower gear ratio. For example, the most common ones are 1:2, 1:1.5, and 1:1, and two of them are for increasing speed, while the other doesn't change speed.
In super kits, there are only three sizes of gears included: 60T, 36T, and 12T. If you do a little calculation, it is impossible to create the above 1:2 and 1:1.5 gear ratios using only the three sizes. Additionally, 60T cannot be used in a chassis as it has a greater diameter than most commonly used 200mm travel wheels (Note: 200mm is the perimeter, so the radius is about 3.183cm).
If you want to connect wheels and motors through gears, you will have to purchase the gear add-on pack containing 24T and 48T gears. You'll have to use sprockets or pullies, which we will talk about in our next episode.
With 24:48 or 12:24, we can generate a 1:2 gear ratio, making the wheel spin twice as fast as the motor's rotation. However, 12:24 is not recommended in this case because both gears are too small, and you'll have to use additional gears to separate the wheels, like the picture indicates.
Moving on to 1:1.5 gear ratio, it can only be accomplished with 24:36. This might require some more complicated connection method since the wheel covers the center of the 36T gear when aligning both gears side by side with 24T attached to the wheel (shown above).
An example of a 1:1.5 gear ratio is my chassis during slapshot season. As you can see above.
A 1:1 gear ratio can be achieved by any type of gears if the output and input gears have the same number of teeth.
An additional gear ratio possibility is 1:1.3333, using 36:48 gears. This provides a little more speed while not altering the motor torque that much.
What's the difference between these Gear Ratios?
Now, I am here to answer the question of "What's the difference between the gear ratios?". Aside from changing the output speed, it also changes the output torque, which is inversely proportional to the change in the output speed.
Let's use the gear ratios we talked about as an example to calculate the speed and the torque of the chassis. Before calculating, we have to acknowledge that the VEX IQ smart motor spins at 120 RPM and has a stall-torque of 0.414Nm (Nm=Newton Meter), meaning the motor can resist 0.414 Newton of force at a distance of 1 meter.
When we use a gear ratio less than one, the speed is multiplied by 1/gear ratio, which increases, but the output torque is multiplied by gear ratio, which decreases. An easier way to think about the relationship between speed and torque without calculation, the larger the motor gear is than the wheel gear, the higher the speed and the lower the torque.
Next, we can list out all the gear ratios and their input and output gears like above, then proceed to speed and torque calculation.
After filling in the calculated speed and torque, we can observe the fact that all speed*torque equals the original speed*torque, which is evident from the fact that both have an inversely proportional relationship.
"Speed difference can be observed, but what does torque do?" you may ask. Torque determines the maximum weight the chassis can handle before the motors stall. If your robot is too heavy or faces too much resistance, and your motor torque isn’t enough to handle it, the robot will slow down or may not move at all. We'll dive into the detailed calculation in the future, but for now, just remember: higher speed isn't always the best; you have to consider how much structure you'll put on the chassis.
Conclusion
If you know that you need to run around the field fast and don't have to carry a lot of game elements, you could try a lower gear ratio, like 1:2 first. However, if you need a complex structure like the full volume season robots, you might want to consider higher gear ratios than 1:2, or else you might carry weight heavier than intended and actually make your robot go slower than 1:1.5 gear ratio robots.
Feel free to experiment with different gear ratio combos and connection methods on your chassis!! Also, the answer to the gear ratio above is 4:1, with the 48T gear being the output gear and the 12T gear being the input!! Others are all idle gears.
In our next episode, we will talk more about sprockets and pulleys and how they differ from gears. Just a note, the two can also be used in chassis sometimes!! Also, we'll look at how they are used to connect chassis.
This is IntroVex, your robotics guide to VEXcellence. See you next time!!
