Re: motors/torque

[Steve Edwards <sedwards-at-awger.net>]

Mike Butts wrote:

>Then what do you recommend?? What kind of torque is needed to turn? I know torque 
>is based on motor design and voltage adds rpms. Though bumping up the voltage 
>usually increases torque due to the extra power available. However 24v requires 
>very large batteries and you lose ALOT of capacity. For example a 24v SLA and a 12v 
>SLA can be the exact same weight but the 12v will have almost double the capacity. 
>SO I should try to find a VERY strong pair of drill motors...right? I DON"T 
>KNOW...argggg.
>  
>

I haven't actually built one of these contraptions yet (just done a lot 
of research), so take this with a grain of salt.

For "satisfactory" performance you need about .003 hp/lb. For a 100lb 
model, that would be .3hp (two .15hp motors).

How people define "satisfactory performance" differs widely, and you've 
already gotten a definitive response stating that the 600 motors (21 
oz/in, or about .21hp each) are insufficient for 1/6 tank combat in grass.

Hp/lb ratios for real tanks are greater than the recommendation:

    Sherman .007 hp/lb.
    Tiger .005 hp/lb
    T-34 .009 hp/lb
    M1 Abrams .011 hp/lb.

EV Warriors are rated for about .378hp -at- 12V or 1.55hp -at- 24V. Assuming a 
model weight of 175lbs, running the motors at 12V will give you a ratio 
of .0043, whereas running them at 24V will give you a ratio of .0177. 
Compare these ratios to the ratios for the real tanks above; do you want 
your model to move more like a Tiger, or an Abrams? Sticking to the 
"real" ratios will get you scaled performance, but that's not what you 
want when you're going up against these guys.  :-)

Something else to consider: the real tanks have transmissions that give 
them multiple gear ratios, ie they can adjust the torque output to the 
drive sprockets to accomodate varying terrain. Unless you're going to 
build a transmission, you'll have to fudge the drivetrain design to 
accomodate high torque / low rpm situations like standing starts and 
obstacle traversal. The performance numbers you'll find for motors are 
generally no-load RPM and max torque at stall; good for the bottom-end, 
but the top-end is a little more difficult, because you'll have to 
estimate max loaded RPM (considering the drag of the rest of the 
drivetrain) in order to figure out what the final drive ratio should be 
in order to achieve your desired top speed.

Longevity of the batteries is also a challenge to figure, but running 
whatever motors you wind up using at their peak efficiency will make 
them last longer. Peak efficiency (RPM) for a motor is generally in the 
75-90% range of max; if your gearing is such that most of your tank's 
movements are significantly below this range, you'll be burning more 
battery power than you really should and shortening your runtime.  I 
guess this has a lot to do with individual style of fighting; high-speed 
dashes followed by long stationary periods could benefit from higher 
gear ratios (slower acceleration and less climbing ability), but lower 
more constant speeds would call for lower gear ratios (better 
acceleration and climbing, but at the expense of top speed).

I'm personally considering motors in the 2hp range (-at- .015 hp/lb good 
for a 266lb model), simply because I don't want to put a transmission in 
my first model... but the transmission (most likely a two-speed) is 
definitely coming.

There was an excellent article on this subject in "On the Battlefront" 
magazine, written by Howard J Thompson. You can order a copy from 
http://www.OnTheBattlefront.com

I have a spreadsheet that I've been using that has a number of 
rule-of-thumb formulae that I've cribbed from various sources, but it's 
not exactly in a presentable form (you have to plug data into a number 
of different places, none of which are marked). If there's any interest, 
I'll try to clean it up and post it.

    - Steve "well, it looked good on paper" Edwards