Scarab Status Report
Part 1:

"Your first robot WILL suck!"
Conventional Bot-Builder Wisdom

Sometime in February or March of this year, I got confirmation from the BattleBots people that the next competition was going to be June 10-12th. Unfortunately, my honeymoon was June 4-15th... My thoughts immediately turn to BotBash in May. A month sooner, and lower weight limits. No way in hell Dozer was going to be done in May, and no way it was going to be less than 50lbs. So, I dust off a few ideas and in April, Scarab was born.

I've always wanted to build a tank-tracked robot, but since I don't have access to the more fancy build machinery, I'd tried to think of ways to implement treads in a simpler fashion. At some point I hit on the idea of fan belts. With a small enough bot and enough belt tension, I figured I could get something going!

The basic design was simple (I didn't have time for anything else): A basic Al box with pulleys larger than the height of the box (to allow running upside down) and a set of fan belts running over the pulleys. One pulley on each side is driven, the other two freewheel and are driven by the belts. Simple, right? Heh...


Here's the start of Scarab's base. Unlike the welded tube frame of Dozer, Scarab is all based on an aluminum base plate with all of the goodies bolted to the "floorpan" and the body/armour built up around it. All of the aluminum used on Scarab is 6061 rems stock ($1.99/lb) from Industrial Metal Supply. In this picture I'm laying out and checking spacing for the motors and bearing blocks.
A crucial component of the fan-belt tread system is proper tension on the belts. Here is the start of the mechanism I designed to tension the belts. I later found out there were some basic metallurgical problems with my implementation...
Here is the completed belt tensioner sub-assembly. The angle piece fastens to the baseplate and the cross piece (which the rear bearing blocks bolt to) floats. The two 1/4-20 bolts are used to draw the cross piece rearward to tension the belts. It worked great at low tensions. However, I found later than when I really torqued it down to get the belts tight that the Aluminum angle stock I used to form the junction between the cross piece and the steel tube bases for the bearing blocks just wasn't up to the challenge. More on that later...
A basic mock-up of the belt drive system. Here I've got a couple of c-clamps mocking up the position for the belt tensioner mount. On the front you can see 2 pulleys attached, I wanted as much belt on the ground as possible, so I spec'd out 2 pulleys on each side. By the way, I got these pulleys from American Science & Surplus
Here is the motor mount and transmission assembly. We used a 3:1 ratio, but probably should have had 4:1. The gears are 16pitch Boston Gear spurs from Grainger. The drive spurs were notched with a Dremel to mate with an existing roll pin on the motor output shafts, and the interior bores were "polished" to make up for tolerance issues with the output shaft on the motors. For the driven gears, a friend bored them out to 5/8" and I hand broached a keyway into them with a Dremel and a file. Let me just tell you how much that sucked! Suddenly, a couple of hundred bucks for a proper broach tool doesn't look so bad...
These are the same 12v Bosch pancake motors I originally bought for Dozer and that were much talked about on the BattleBots Forum. They're surplus from the old EV Warrior electric bike company (got through local surplus stores). They have a slight timing bias to one direction of rotation, so care was made to match a CW and CCW motor for approximately equal power output. You can also see the core of the weapon system. More on that later...
What's hard to see above is that due to the cap at the back of the motor, I've had to cut slots in the baseplate so that the motors can sit flat on the plate. Here you can see those slots. I could have elevated the motors rather than cut the notches, but that would have increased the vertical dimension of the chassis by 1/2", (1/4" above and below) and that would have cut down on the over/under ground clearance. The top armour was slotted in the same way. The other holes are for mounting the cylinder
The bike motors are designed to run two per bike at 12v. For this application, we ran them at 21.6v (3 x 6-cell RC NiCd packs - Thanks Dan!) Here is one of the battery mounts/side armour plates. The clips are modified 9v battery holders (the crown connectors have been dremel'd off) and are fastened into nearly 50 hand-tapped holes. There was probably a better way to do that, but with my small tool supply I just got into a rhythm and tapped away. Broke my first tap halfway through the job, but I blame the cheap spindly little tap...
And here's a completed battery pack. The anderson powerpole connectors on the batteries were very helpful, since I ran two of these packs in parallel for operation, but charged them in series. The genderless connectors allowed easy reconfiguration of the packs for charging without having to remove them from the mounts or the 'bot. The straps are just plain velcro, just to provide a little extra hold, although the 9v battery clips held the batteries on end pretty well.

On to Part 2

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