Re: Motor to weight question? [TANKS]

[Rick Dulas <rick.dulas-at-oracle.com>]

Title:

Just to throw out something only marginally connected to the topic... I saw a motorized turntable for cars. The specs say the thing weighs 2000 pounds and can support up to 6000 pounds of car. This assembly is powered by a 1/4 HP electric motor. At 746 watts to 1 horsepower, that's about 187 watts to turn more than 6000 pounds. Here is the marginal connection, the turntable rotates at about 3 RPM. Given all this, you should be able to use just about any motor in an R/C Tank and get it to move. The variable is how fast.

Most of the discussions about motors and such talk about current and voltage, fading into watts. If the question were simply, "Will XXXX power my tank?", the answer is YES because the solution is unconstrained. If sufficiently geared down, any motor will move any tank, and there exists some combination of motor and gearing which will yield a velocity. It might be glacial, along the lines of 1 foot/hour, or even slower. In order to have an interesting question, we need to have a constrained problem. For instance, "Will XXX give my tank a velocity of 5 miles per hour?" is a constrained problem. We will assume that you can create a frictionless drivetrain and that there are no losses in the system due to mechanical inefficiencies.

Let's consider the following problem: The Tank weighs 30 pounds and needs to have a max velocity of 5 mph.

One measure of power is watts, which is typically taken as current * voltage (P=I*E). Another measure of power is foot/pounds force/minute. It looks like we have all the information needed to determine the foot/pounds force/minute power required for this tank at this speed.

First get 5 mph into feet per minute. 5280/60 = 88 feet/minute

We need to move 30 pounds 88 feet in a minute. This becomes (30 pounds)*(88 feet)/minute, or 30*88 = 2640 foot/pounds force/minute. And so...

How do we get from foot/pounds force/minute to watts? Google of course. A search turned up a site that said to go from foot/pounds force/minute to watts, multiply (foot/pounds force/minute)*0.022597. Given our tank and speed constraints we have (2640 foot/pounds force/minute)*0.022597 = 59~60 watts.

This suggests that to move a 30 pound tank at 5 mph requires about 60 watts of motor. All this assumes that you have built a frictionless drivetrain, there is no slippage between track and ground, and the playing field is dead flat and level.  Hardly real world. So double the power required and we have about 120 watts.

How fast will a pair of EV Warrior motors move a 30 pound tank? If the EVs are 1 hp each, then we have 746*2 = 1492 watts. To go from watts to foot/pounds force/minute, multiply watts*44.253662. (1492*44.253662 = 66026). And to get the speed in feet/minute, we remove the weight of the tank from 66026 foot/pounds force/minute. (66026/30 = 2200). So we are left with about 2200 feet per minute or 132000 feet/hour or 25 miles per hour. If we reduce the speed by half to allow for the real world, we still have a respectable 12 mph. Remember that Roger Bannister's 4-minute mile is only 15 mph.

burn995-at-aol.com wrote:

I got mine off of e-bay last year. I suppose no one really knows the answer to my question of how much weight the EV worriers can push, probably depends on how much you gear it down. Am I right on the assumption? I weighed everything I got going on my Tiger so far and I'm up to 115 lbs. That's not to include the batteries and controller, and the whole inside of the turret. Considering that I have metal track. I'm predicting I will end up around 150 lbs. How's that compare to other Tigers built for paint ball? OR am I looking at some real problems driving this thing?

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Rick Dulas, Ph.D. | Senior Service Delivery Engineer | 979.846.7713
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