Friday, July 10, 2009

Waterproof and Water Damage

Well, I haven't RC'd for more than a week, but I will post some things that happened the last time.

Upon my latest run of the KNEX NEW BRIGHT HYBRID, I encountered a motor problem. It just died. This was unusual because it was the stock Losi Mini-T motor, I hadn't opened it up or modified it at all. I was expecting reliability! It turns out either the plastic washer, or armature lamination cover had melted and covered the commutator! I wonder if this ever happened to any Mini-T's.
The melted washer.
The armature. Note the melting at the edges of the white plastic rotor-caps at the upper left and right, and the entirely missing cap at the bottom. The comm has been partially cleaned in this picture.
The comm. This is after I began to peel away the plastic.

I can't help but wonder if this was catalyzed by the one incident from a previous run, in which my roommate was driving. He attempted to climb a steep grade, where he was mostly successful but got stuck and kept holding the throttle. If you continue to apply a current to a locked DC motor, it essentially becomes a short circuit. The current going through the coils is no longer AC (it is usually AC from the comm constantly repositioning itself with the brushes, go read something better explaining how DC motors work if you don't understand), and therefore the AC impedance of a coil is gone and the resistance basically drops to 0. This is almost as if the battery was short circuited.

After the incident, I did smell melted plastic, but I had inspected the gearbox and there was no signs of melting. I hadn't realized it then, but it was obviously something inside the motor.

However, on the run that the motor failure occurred, the truck was running on relatively flat ground (leaves, twigs, etc) and the failure was quite a surprise.


In order to run the truck through the local creek, I had previously put the receiver and speed control into little baggies, and then zip-tied them shut. This worked okay, but then after one run I discovered water inside one of the baggies (the receiver one!). The servo held up well with no waterproofing whatsoever.

This time, however, I wanted to run the truck on the beach, and maybe try to tease the water a bit (it ended up almost floating out to sea!), so I needed something a little better.

Here was the idea:

And the execution:

The servo I opened up, and applied liberal amounts of grease to the seams. I neglected to take photos.

The moist beachy-area I had intended to run on was just water when I went, alas, it was high tide. I continued on to a rocky area, and it was either dry driving, or floating out to sea. Needless to say, it worked pretty well! I hadn't brought my camera, so sadly there's no good photos/footage of this event.

I think it would have made a decent raft if I could have sealed the air inside the tires somehow.

While the run was a success, I cannot say the same about the aftermath. Laziness has set in (I'm in class now though) and I neglected it for nearly a week before performing maintenance. This was made even worse because the water was not fresh, creek water, it was salt water which corrodes much faster.

I don't know if this happened during or after the run (probably mostly during), but some real general chem stuff happened.

This is the battery plug (and the receiving end on the speed control, and a part of the wire where stripping occurred). Note the blue deposit over only the positive contacts. I believe this is cuprous (cupric?) chloride: the contacts are probably made of copper, perhaps nickel-plated, and they were exposed to an aqueous solution of sodium chloride (the bay). The positively-charged electrode of the battery attracted the negatively-charged chloride anions, and formed the precipitate. I don't know if I'm right though, it's been years since I've taken general chemistry and I forget most things a month after classes anyway.

Also, the motor, which turned freely after the run, is now locked tight. I'm guessing some internal rusting caused it, but I don't really want to find out. The truck has been shelved since.

It's been fun, and completely worth it. I just wish somewhere in this city there was some MUD I could plow through.