Fixing Stuck mBot Ranger Wheels
I bought my daughter an mBot Ranger recently because she enjoys building and they've begun Scratch programming in elementary school. The kit supports a Scratch interface and allows her to meld engineering, programming, and seeing the results in meatspace. But, the mBot Ranger has a quality control problem - on several kits, after you run the unit for awhile, the idler wheels can actually seize up. Needless to say, kids don't enjoy a robot that won't move. How do you fix it?
It's actually quite simple. In the following picture, you can see the components that mount the idler wheels. The wheel is held away from the aluminum frame by a nylon spacer and is mounted on two metal bushings. When the system's tolerances are correct, no matter how tight the axle is affixed to the frame, the wheel freely spins on the bushings. But when the tolerance is off, the wheel sticks to the bushing - which itself does not spin as it should be affixed by the nylon spacer and axle head.
Fortunately, the kit comes with more tiny hexagonal nuts that are necessary to build the mBot Ranger. Simply remove the idler wheels and utilize six hexagonal nuts per axle in place of the nylon spacer. Two nuts can tightly hold the axle to the frame. Without the nylon spacer, you'll have to adjust these two frame nuts to make sure the axle's end, and thusly the wheel itself, aligns with the drive wheel to keep the tread straight. The third nut can loosely be placed near the wheel bushings - close enough to hold its position, but not close enough to lock prevent the bushing from spinning. Basically, you're allowing the wheel to be stuck on the bushings and spin as a whole unit by introducing a little slack on the axle. There are enough hexagonal nuts to even use eight per axle - two for affixing the axle on the frame and two to lock the wheel position but maintain "looseness."
Congratulations - your mBot Ranger is functional again.
Alternatively, another solution is to lightly file the inner wheel's hub. Just taking enough material away to allow the wheel to spin freely on the metal hub inserts. Take care not to file too much away or the wheel won't be held onto the hubs anymore. This approach allows you to continue using the nylon spacer for a build that matches the OEM intent.