Why Spend Time Choosing the Right Motor and Gearbox?
Choosing the correct combination of a motor and a gearbox for a given application is very important, both in the FIRST Robotics Competition (FRC) and in actual engineering projects.  Without appropriate motor-gearbox combos, your team will find that your robot does not function as quickly and effectively as intended, and may have a tendency to burn out motors.

This tutorial will teach you the fundamentals of gearbox design and implementation.  First, I will teach you about motor characteristics.  Next, I will discuss how to choose a motor and gear ratio given application requirements.  I will then provide information about choosing a gearbox, followed by an overview of the motors and gearboxes available in FRC.  Finally, I will demonstrate how to use what you learn in this tutorial in an example problem and point out extra tools and resources if you want to learn more.

This tutorial was made through the Autodesk FIRST High School Intern program.

A basic understanding of physics – e.g. force, torque, power, and gear systems
A willingness to learn

Step 1: Motor Characteristics

There are several important characteristics of motors that provide information about a motor and its capabilities.  They are the motor’s output torque, its current draw, its output speed, its power, and its efficiency, each of which I will discuss in turn.  These characteristics are interdependent and can all be derived from four values: the motor’s stall torque, stall current, free current, and free speed.
A motor’s output torque is the amount of force with which its output shaft can rotate.  If too much torque is applied to a motor, its output shaft will stall, or stop turning.  Other motor characteristics are commonly written as a function of torque.  It is usually measured in N-m when metric units are required and oz-in when English units are required.
Current Draw
The motor’s current draw is the amount of electrical current the motor draws at any given load.  As the load on the motor (the torque) increases, the amount of current that the motor draws increases linearly.  This relationship can be written as
Symbol Name Units Description
I Current Amps (A) The amount of current drawn by the motor
Istall Stall current Amps (A) The amount of current drawn when the motor is stalled
Ifree Free Current Amps (A) The amount of current drawn when the motor has no load placed upon it
τstall Stall Torque Newton Meters (N-m) The amount of torque required to stall the motor
τ Torque Newton Meters (N-m) The amount of torque applied to the motor output shaft
The motor’s output speed is the rotational velocity at which the output shaft spins.  As the load on the motor increases, the output speed decreases linearly.  This relationship can be written as
Symbol Name Units Description
ω Speed Rounds per Minute (rpm) The rotational velocity of the motor’s output shaft
ωfree Free Speed Rounds per Minute (rpm) The speed at which the motor spins when it has no load place upon it
τstall Stall Torque Newton Meters (N-m) The amount of torque required to stall the motor, or prevent its output shaft from rotating
τ Torque Newton Meters (N-m) The amount of torque applied to the motor output shaft

Photo Credit: http://www.engin.umich.edu/group/ctm/examples/motor/motor.html
<p>I am working on project of making small hydro generation unit. In which I have made hydro turbine of 'Pelton Wheel' type. It rotates at 300 rpm but as rated speed of synchronous generator which is to be connected is 1500 rpm, so to generate electricity I have to step up the speed from 300 rpm to 1500 rpm. For that purpose is it feasible to use gear type arrangement?? Which type of gear to be used?? How will it work?? Explain me in detail......</p>
<p>It depends on what type of shaft you're putting it on. Let's be practical. </p><p>You're probably going to use spur gears. We need a ratio of 5:1 (1500/300=5)</p><p>You can do 2x 2.236:1 or 1 5:1. What is the shaft size of your pelton wheel and what material is it? If it's metal or a shaped (like a square or hexagon beam) you can buy a spur gear (or make one if your pelton wheel is large) and as the main shaft spins you will spin your motor (what sized shaft?) 5 times as fast. </p><p>Things to take into consideration:<br>Shaft size helps determine what gear PITCH you use. (The size of the teeth). If you have a really tiny shaft you can't use a large pitch, for power transfer you want a large shaft and large teeth so your gears don't strip themselves. </p><p>Gear material is also determined, wood has the potential to expand and contract, plastic probably won't hold up to how much stress you are putting on your wheel. I would go with metal for you. The gears will be really expensive so honestly, I would go old school and use pressure treated wood to make huge gear teeth (like an old fashion water wheel) which is what I think you're using. </p><p>So all in all, you need a gear ratio of 5:1 (or close to it, will your wheel always be moving at 300 rpm? if the stream wanes you still want 1500 rpm! If the motor can handle more, think about changing the ratio!)</p><p>When I'm saying &quot;huge gear&quot; I'm imagining your wheel to be a foot or two (or more) diameter and you can cut the teeth using a pattern, then another gear. The reason larger is better is because there is less wear, but more importantly the larger the gear the less your human error impacts the gear. (No way you're going to machine a super tiny 188 tooth gear!, but make a 24 tooth gear that's big, you got that!)</p><p>If you pelton wheel is rinky dinky and the shaft is only 1 inch diameter or so you can just buy a gear or two from servocity or another gear provider. Make sure the gear pitches match (mod and pitch are the same thing so match 1 mod with 1 mod or 24dp with 24dp, etc.) Then make sure you can attach the pinion gear to your motor, (press fit, glue, welding). </p><p>Use search words like &quot;spur gear&quot; &quot;bore diameter xx&quot; &quot;pitch xx, or xxP&quot; &quot;pinion gear&quot; </p>
<p>i have a 44 ft long wooden boat that i made it by myself . the boat beam is 15 ft and hull depth 9 ft .Boat displacement is around 22000 lb . the boat is powered by two volvo penta AQAD40 marine diesel engines that produces each 165 hp and a max of 3000 rpm .. i need to install 2 gearboxes could you pls advise about the best ratio suitable for this boat</p>
<p>Hello, I'm having trouble viewing the images (formulas) when downloading this as a PDF. Is anyone else having trouble and if so, found a workaround? Thanks! </p>
This is good. Would love to see a follow-up about actually building geartrains - best practices for making a &quot;breadboard&quot; with parallel plates, sources for gears, axles, bearings, etc.
I posted an Instructable about how I made a gearbox&nbsp;<a href="http://www.instructables.com/id/Making-a-2-Speed-Custom-Gearbox/" rel="nofollow">here</a>&nbsp;- it's kind of the same topic, but doesn't go into quite the detail it sounds like you want.<br> It's definitely a good idea though - I'll put it on my list of ideas for future tutorials.
Great tutorial! Oh yes, I definitely would like to learn more about how to connect wheels to a motor and bearings as well :-) Good luck with your study!
What is the difference between torque and stall torque when dealing with an electric motor? As I recall, the highest torque rating of an electric motor is it's stall torque. I thought separating them out was more a function of usable power in an IC engine rather than an electric motor. Or is that formula generic, it's been a dozen plus years since I've even had to look at that stuff. <br> <br>Also, are you going to get into the differences between DC and AC motors? I seem to recall there being a couple of differences when you start talking about what you want the motor to actually do. BTW, not to sound like a jerk, but I do mean differences beyond what source of electricity is handy.
To answer your second question, the reason why I did not discuss AC motors is twofold. First, this tutorial was meant for students who are part of the FIRST Robotics Competition, which only allows brushed DC motors, though the theory is definitely applicable to any project that uses DC motors. Second, I would have no idea what I was talking about if I tried to cover AC motors. I have never used them, so I might be giving unreliable information if I tried to teach about them. <br> <br>If I understand your first question correctly, a brushed DC motor's output torque is not equal to its stall torque. Instead, it applies as much torque as is necessary to rotate the motor's output shaft. In other words, stall torque is a constant that is a characteristic of the motor, while torque is the amount of torque the motor is outputting at a given speed/current.
I'd been waiting for an instructable like this... For a long time I've wanted to attempt building a solar powered plant stand/turntable which would slowly rotate (about 180&Acirc;&deg; in 24 hours) but I never knew where to start. Though it is still daunting, at least now I know where to begin!
I'm no expert but I think a servo might be better for something like that.
Most excellent work!
Thanks for the feedback! I'm glad you found the information helpful!
This is fantastic, I've wanted to learn this stuff but never knew where to start. Thanks, this will be off great use to me!
Very thorough. Nice job.

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Bio: This summer I am working with Autodesk to make Instructables about the FIRST Robotics Competition. I will be studying mechanical engineering at Oregon State University ... More »
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