Watch this page for notes and advice as they come up. Feel free to add your own notes as well. Latest updates: 3Feb2016

Motor shafts -- adaptor coming loose?

If the little 3D printed shaft adaptor comes loose, you can repair it with a bit of superglue and some sandpaper or a file. Roughen up the shaft, glue it on. Wait a bit.

There's a ziploc with stuff for motor repair. One of the TAs can help if they are around. (Hot melt glue will not work very well...)

Alternatively: See Aravind! He has some new 3D printed parts which snap on more tightly than the originals. He also has tips for ensuring concentricity.

Crawler bogging down?

This could be due to various things:

1. Do you have enough gear ratio? Even under ideal circumstances, the torque on your drive wheel will have to be at least m*g*sin(60deg)*r_wheel. Now look at the torque from your motor (k*i - Tf). OK, so your gear ratio has to be at least.... And it's gotta be bigger because your transmission is not super efficient, plus you have rolling resistance, etc.

2. Are you getting misaligned and scraping the side of your crawler on the back of the shaft? Add some guides. Maybe extra rollers. Maybe something clever with wire and whatnot.

3. Gears OK? The center distance between gears is somewhat critical. Make sure your motor and its gear are not pressing too hard into the gear they are mating with. In fact, make sure all your gears are just a bit on the loose side. Apart from the worm, they should spin easily. Tip: if your gear train makes a lot of noise, that is probably a bad sign.

Veering up or down inside the shaft?

Some common reasons why this happens:

1. Weight distribution: If your crawler tends to pitch in a certain direction consistently, try shifting some weight (e.g. the bolt) to the other end.

2. Drive wheels on one side of the crawler: Try to put the drive wheel in-line with the CG, as far as possible, to avoid generating additional moments around the CG that will torque the crawler up or down. Alternatively, try putting two wheels on the drive axle -- one on each side of the CG -- to even out the moments.

Worms and wheels...

If you are using the worm gear, you've probably discovered that it does not work conveniently with the 40tooth gear in terms of Lego dimensions (see Preferred Lego meshing dimensions below). It works perfectly with the 24 tooth gears. If you want to use with 40, you might need to "shim" the worm axle and its supporting pieces (perhaps the motor?) with a piece of cardboard or something to move it ~1mm away so that the worm can be centered on the gear and not just contacting one corner.

Worm/worm-gear alignment is critical for good efficiency. Make sure the worm shaft is aligned and not bending a bit.

A number of teams are using the 40tooth gear on the same axle as their wheels. This works OK at light loads but under higher loads the tires deform and the gear contacts the side walls. A solution is to put fat rubber bands around the wheels. Latex rubber as good traction (probably at least as good as the black rubber wheels) but of course it doesn't last as long. However.. you don't care about that.

Lubricants

Lubricants can be helpful, especially for the worm gear. But they are usually the last thing you should play with. They will not fix a design that is otherwise inefficient. Also, they tend to get on the wheels -- which then have to be cleaned with detergent. Also, as Aravind noted:

As you're all testing and improving the efficiency of your designs, be especially careful about using lubricants on Lego gears. In general, any kind of vegetable oil is OK. However, certain petroleum- or silicone-based lubricants (e.g. Vaseline) will degrade ABS plastic and cause your Lego pieces to not fit together properly.

Things coming unsnapped or gears slipping?

If your gears work fine "in air" but start slipping in the track, it's a sign you need more bracing. Remember that in addition to the tangential force, Ftan, there is a significant radial force and, in the case of the worm, an axial force too. Brace the Legos. Add tape and/or rubber bands.

Motor Plots?

What voltage should we do our motor plot? -- Ultimately, you want the motor power and efficiency plots at the voltage that you actually run your crawler. But for Monday design reviews you could pick an estimated voltage (perhaps 6 volts?) and look at the power and efficiency for that case. The point is to get a rough idea what speed your motor wants to be running at. For anybody who wants to check their Matlab script, a file CrawlerPlots.m has been posted which you can use to compare your results.

Preferred Lego meshing dimensions?

Other Interesting links

DIY Lego-compatible gears

https://docs.google.com/a/stanford.edu/document/d/194K2LttM1R28KkHRkAzQaF9A6WYTC7_Ws-NpJPZAmek/edit

Early RISE developments with Legos

Here's a picture from the early days of the RiSE project.

Lego Guy (at left) was a very
early experiment with spiny
wheels and a propellor to try to climb vertical walls.
More on Lego Guy:
Early Leg Platform Development