Re: motors/torque

["SUSAN GUTBRODT" <showa1-at-msn.com>]

Another question now is how available are the EV's and for how long??? 
Remember, these motors were surplus and are out of production...The key now 
is to ascertain the next motor...
Kurt (What's next???) Gutbrodt

>From: Steve Edwards <sedwards-at-awger.net>
>Reply-To: tanks-at-rctankcombat.com
>To: tanks-at-rctankcombat.com
>Subject: Re: motors/torque
>Date: Thu, 07 Oct 2004 18:19:08 -0400
>
>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
>