Table of Contents
This document goes into detail about the steps you should take when CADing a gearbox for any subsystem or feature on the robot. Make sure you have added all MKCad parts before starting. See MKCad to add the parts.
A gearbox is a compact system of gears that allow for the transfer of energy through rotation of the motors, gears, and output shaft. A gearbox has three main functions: to increase torque (rotary force) from the driving equipment (motor) to the driven equipment, to reduce the speed generated by the motor, and/or to change the direction of the rotating shafts. This all relies on the gears and gear ratio within a gearbox, which help you adjust your speed and torque to your desired values.
Our team uses gearboxes all throughout our robot in various subsystems. For instance, we have gearboxes to rotate our wheels on the drivetrain, spin belts on our feeder, and rotate rollers on our intake.
The first step you should take when CADing a gearbox is decide on the number of gears and gear ratio you want for your gearbox. Our team uses the Design Tool to help us do calculations for us! Here is a copy of our 2022 Design Tool for your reference, but there should be a new Design Tool we use every year in our shared drive. You can essentially think of this as one large spreadsheet, where we can mess around with numbers until we get our desired values.
When using the Design Tool, you should first decide whether the mechanism you’re using the gearbox for has a linear (inches) or rotary (degrees) motion. Think of things like an elevator as linear and a rotating arm as rotary. Make a copy of your desired template as a new tab and name the sheet.
All the boxes highlighted in yellow, you should fill out. The first two tables should be pretty self-explanatory. Keep in mind that Gearbox Efficiency is typically 80% and Applied Load is the total amount of force that will be acting on your mechanism.
<aside> 📌 Although it may ask for pulley diameter, you can just put the pitch diameter of whatever thing you are using to transmit power (pulleys, gears, sprockets)
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When you get to the table involving gears, play around with the values of the tooth count for your driving gear (the gear being powered) and driven gear (the gear being spun by the driven gear) on the table. If your gearbox has multiple stages or gear interactions, make sure you put them all in the columns of the table. Generally we want to go from a low teeth count driving gear to a higher teeth count driven gear to increase the torque. Gear tooth numbers are almost always an even number, and it can be helpful to check the ‣ to see what gears we have.
<aside> 📌 Note that all of the gears we use have a DP (diametrical pitch) of 20. DP is the number of teeth in a gear for each inch of its pitch diameter, or the ratio of the number of teeth in a gear to its diameter.
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As you’re manipulating your gear values, the values in the dark blue tables on the right should change. Continue playing around with gear ratios until you get your desired values for speed and time to move. Keep in mind that while we are generally aiming for a higher speed and shorter time, we also want our values to be reasonable. For instance, having an arm that rotates 90 degrees in 0.1 second is probably not a good idea. For your Current Draw value, refer to the Motor Systems tab of the sheets and find your mechanism. Then, look at the Breaker Rating and that’s the value you should try to stay under so that the breaker doesn't trip.
<aside> 📌 The Breaker Rating is based on the load/power the mechanism is expected to take
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In some cases it’s okay if your value goes a little over because some motors can handle current draw over the breaker rating for a short amount of time. And voila! Now that you have a clear gear ratio in mind, you can start CADing the actual gearbox.
The CAD of the gearbox starts with a plate sketch. The front and back plates help define the mounting holes for the motors, and where our shafts and gears are. First, start by mapping out the locations of gears and motors you decided on in your Design Tool. This helps us determine the overall shape of the plate sketch.
<aside> 📌 The pitch diameter is # of teeth/DP. For example, a 60T gear with a 20DP would have a diameter of 60/20 or 3”. In Onshape, you can type in the expression for the dimension instead of the actual calculation. Instead of typing 2.5", you can type 50/20 instead.
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