Motors that can drive a ~10-15lb robot

VEX V5 General Discussion General Forum
1

I’m new to picking out motors, but I’m currently in the process of building a robot for a school competition, and we’ve estimated our robot will weigh 10 or 15 lbs. Because of this, we’re probably going to go with four motors for the driving.

I was looking at robotmarketplace for motors, and am pretty much overwhelmed: http://www.robotmarketplace.com/products/motors_main.html

Any tips on what to look for, or suggestions of motors to buy? I’ll also need mounts to attach them to the body of the robot. Thank you.

(we’ve already got a motor controller)

2

A couple of things to look for:

First, check the rules of your competition (you’ve probably already done this) but make sure you know full details about any restrictions.

Second, do some speed/torque/power calculations to work out your requirements. These will vary depending on the type of competition. If you’re planning to use a motor controller you already have, then make sure you check the ratings of that as well. Then you can start narrowing down some choices.

This brings me to gearing. If you choose motors that come with gearboxes, then (assuming they have a suitable ratio) you will save yourself much time and effort. Sometimes they are easier to mount as well.

Out of interest, why 4 motors, rather than 2 larger ones (or 6 smaller ones for that matter, though I can think of many reasons not to do that)?

3

Any tips on what to look for, or suggestions of motors to buy? I’ll also need mounts to attach them to the body of the robot. Thank you.

(we’ve already got a motor controller)

If you are using the Vex system for structure and gearing, then have a look at [this motor. It has 1/8" square output shafts just like official Vex motors (pull out the flex-shafts and clean off any residual grease).

I was able to mount it directly to Vex metal using a slotted angle-bracket to attach to the two screw holes at one end of the motor. Runs on 12VDC, which most motor controllers can handle without too much trouble.

You will have to gear it down since it free-wheels at 4400 RPM. I used this to make a really fast & light bot. Lots of scuffed furniture legs and some bent metal until I got the hang of driving it :wink:

Since it doesn’t have any rated torque specs, you can’t do the math to figure out if this is the right size for your bot or not. From experience, it has enough power that I suspect you would be fine with one of these for each side (unless you need to go crazy fast).

Cheers,

4

Thanks for the reply.

There aren’t any regulations on the drive motors for the competition, so I’m safe on that front.

I know little-nothing about motors, so I don’t really know how to do speed/torque/power calculations. What I know is that I’ll need to be able to move ~15 lbs max with motors.

I don’t really know anything about gearboxes or gearing, so guidance on that point would be awesome :slight_smile:

Two would probably work, you’re right, might be simpler, lol. What would you recommend to hold the robot stable as a third wheel/stand in that case?

Thank you

5

Hmm, I’m not really sure how that motor you linked works. Its a single motor with 2 shafts on it? Does it spin them both at the same speed? How does that work for turning?

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It has square axle holes at both ends of the armature, so they spin together. But don’t let the dual-shaft thing throw you - it is just a standard DC motor without an internal gearbox. You don’t have to use both shafts if you don’t need to.

You could use a pair of these motors for a standard skid-steer arrangement.

The bot I made was car-style drive with a single motor driving the rear wheels via a differential, with a pair of crab-drive wheels in front providing the steering.

Cheers,

  • Dean
7

I’ll blindly suggest the Banebots RS-550 motor
You can get gearboxes from the same site and even have wheels that fit right on the output shaft of the stock gearbox
The wheels they offer are pretty awesome, too

8

Gotcha, so one of the shafts could be removed and it would act like a “normal” motor.

I’m liking your idea of using two of these for skid steer. that’d be able to handle carrying a 10-15lb robot around at decent speeds? Would I need to “gear” the motors? I’m not too sure what that means.

Thanks

9

Hmm. I think I need someone to explain a gearbox to me. I googled around and didn’t find a nice guide on gearboxes, does anyone know of one?

10

OK, so I can’t tell you everything, but I’ll try to give a starting point.

The equation relating speed, torque and power is:
Power [W] = Speed [radians/second] x Torque [Nm]

A given motor can supply a certain amount of power*. The purpose of a gearbox is to get the right amounts of speed and torque from that power.
In general, a DC motor will spin way too fast to bolt the wheels straight on. You would have very little torque (therefore you wouldn’t even start to move). So a gearbox is needed. The lower the gear ratio, the slower the speed, but the higher the torque (and therefore driving force).

The trick is getting it right. In vex, you have a kit of parts, so you can play around with ratios and see what happens. But obviously it would be expensive to buy a whole heap of custom gearboxes. So what I would do is:

  1. Work out or estimate the speed and torque requirements depending on what you want the robot to do;
  2. Work out the power you need;
  3. Pick motors that will give you this power (plus a “safety factor” - overdesign slightly in case your estimates were wrong);
  4. Check out the motor specs. Hopefully there is enough information to find the torque and power output at different motor speeds.
  5. Choose a gearbox to match the requirements from 1. E.g. a 10:1 gear reduction will give you 1/10 of the motor speed, but 10 times the motor torque.

Of course you also have to think about how to physically build it; if you don’t have much experience with this, motors that come with appropriate gearboxes already are great.

Also, its not necessarily stupid to have 4 smaller motors rather than 2 larger ones; its kind of a tradeoff between complexity and reliability. With 4 motors, even if one dies, hopefully you can still move (although much slower).You don’t necessarily have to have all the wheels driven - again depends on what you’re trying to do.

*The power output of a motor depends on what speed it is running at. Check out this page for a good overview.

Hint: don’t design based on the stall torque, or the free speed of the motor. You want some kind of balance.

11

The way I had my students go about gearing this motor was pretty simple. (The details below are from memory, so they may be a bit off.)

This group wanted to make a robot fast enough to pop a wheelie. They identified acceleration, mass, and the location of center-of-mass as the key factors to achieve this.

I knew that a skid-steer robot would be hard to control at those speeds, and we thought having 1 motor near the back would help meet the weight & C.M. goals. That is why we chose the car-style steering and drive layout.

Since we didn’t have a spec sheet for the motor, we don’t know how much torque it has, so we started with the no-load rating of 4400RPM.

We then assumed that the motor would be most efficient at around half of this speed, 2200RPM.

We then figured out how far the 4" drive wheels would travel in 1 revolution (about 1 foot).

We then guestimated a top speed goal of around 4 ft per second. (This was a young team that was not quite up to the math & physics needed to calculate this properly, though we did white-board the forces at work so they could visualize it.)

That means the wheels needed to spin at a top speed of about 4RPS, or 240RPM.

So, we needed gearing to get us from 2200RPM down to 240RPM, which is a reduction of about 9:1.

I don’t recall the exact gear configuration, or the ratio we ended up with, but I believe we had a 2-stage reducer.

After some experimentation, we decided to switch out the 4" drive wheels for dual 2.75" wheels to slow it down a bit and provide better grip. It was nearly impossible to drive indoors… Oh, and they did meet their goal of popping a wheelie :wink:

I could probably post a close-up picture of the motor shaft output this evening if that would be helpful for you.

Cheers,

  • Dean
12

Awesome, thanks for all that.

I followed through that math, and estimated 1 foot per second max speed on 8.5inch (perimeter) wheels to get 85rpm on the wheels for 1 foot per second.

Looks like if I use the motors you recommended (and assume 2200 rpm for the freespin), I’d need to gear them down to 25:1.

Is there any way to figure out which gears to buy to make this happen?

Thank you

13

Fortunately, that is pretty straightforward. Since you are reducing speed, you’ll need a smaller gear on the driving side (motor), and a larger gear on the driven side (wheels).

You count the teeth on a gear to see how it figures into the gearing ratio. Vex offers spur gears with 12t, 36t, 60t, and 84t. If you put a 12t gear on the motor, then the gear ratio options would be 1:1 when driving another 12t gear; 3:1 for a 36t gear, 5:1 for a 60t gear, and 7:1 for the 60t gear.

Since none of those get you all the way to 25:1, you’ll need a 2-stage reduction. A pair of 5:1 reductions would get you exactly 25:1, so that is probably the best place to start. That would require a 12t gear on the motor, meshed with a 60t gear. The 60t gear needs to be on the same axle as a 2nd 12t gear which is then meshed with a final 60t gear.

That final 60t gear should spin at 1/25th the speed of the motor, so you can put your wheels directly on that axle, or use additional gears or chain to couple your wheels.

Since you need to do this for both sides of the robot, you need four 12t gears, and four 60t gears. Either the regular gear kit, or the high-strength gear kit offers you all those items in a single package (plus some other gears, in case you want to experiment with other reduction options). Personally, I’d go for the high-strength gears for a heavier robot.

Cheers,

  • Dean