2007/02/20 - RE: Gene is building a tank! [TANKS] (Gene Burbeck)

After some experimenting I've pretty much decided on running just three batteries rather than all four. This gives 36V total voltage, maximum current of a few hundred amps. 48V is just scary fast, and I'm worried about breaking something (the frame, differential, or track axels), so I'm sticking with 36V for now. Less power, but also 15 fewer pounds to move.

In the above spec sheets (see links) KV is given - 150 rpm/V. KT can be calculated - about .5 inch-lbs per amp. The gearing from the motor to the differential is about 2.5:1, so for every amp we get about 1.25 inch-pounds of torque at the differential. The ratio from the differential the the tracks is about 3:1, so that's 3.75 inch pounds for each amp at the track drive sprocket (7.5:1 total ratio). By running the drivetrain on the bench and pulling on the brake cables with my hands (don't worry, I took many presaftey precautions), I was able to measure current and convert that to torque. When I pulled with 7 pounds of force (measured with a fish scale) on BOTH brakes at once, the motor pulled 200 amps for long periods of time (and didn't slow down very much). That equates to 60 foot-pounds of BRAKING TORQUE at the track drive sprockets. With a 4" track drive sprocket that's 375 pounds of "braking force". In other words if I apply the brake and stop one track, the other side will be able to push with up to 375 lbs of force (minus friction, so call it 250 lbs or greater) before the brake starts to slip. The reason for all this is to confirm I'll have enough braking force to lock one track and turn as sharply as possible. So far so good.

At 36V, that translates to 8.5 MPH. Not terribly fast, but I don't want to start too fast and risk breaking something. Later I can always gear higher and/or add the 4th battery back in. With the 7.5:1 gearing, I expect the tank will easily peel out (or perhaps wheelie!) before the 300 amp current in the esc kicks in. Gearing higher shouldn't hurt anything electrically since I measured 150 amps for one minute and things only got slightly warmer than room temperature. The down-side to more speed is run time, safety, and risk of breaking something mechanical or the frame itself. I'd bet speeds over 30 mph are doable...with a run time of a few minutes. There's a certain robot with a very similar drive setup that peels out as if the ground weren't even there to its top speed of 40 mph. For tank battles I'll most likely keep the speed under ~10 mph, but it would be great fun to have a *fast* setup for fun driving and demos.

The motor and differential is all mounted and running. I added a rienforcing piece over the motor mount and bearing holders, seems real strong now. The brakes are in, just not connected to anything yet. The motor is scary powerful at 48V, 7000 rpm is a lot for a 5.5" diameter motor! It spins smoothly and makes lots of wind.

From:	"Gene Burbeck" <gburb-at-advantagecs.com>
Subject:	RE: Gene is building a tank! [TANKS]
Date:	Tue, 20 Feb 2007 11:27:28 -0500
Reply-To:	tanks-at-rctankcombat.com