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rjw
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 12:51 pm

have you tried minimum or no pressure on wheels?

How stable or unstable is the shell without the wheels touching at all? Does it rock?

Is the center shaft long enough to install a light coil spring for testing purposes ?

Just to se if the shell finds its home?
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 12:56 pm

rjw wrote:
have you tried minimum or no pressure on wheels?

How stable or unstable is the shell without the wheels touching at all? Does it rock?

I think it would actually end up worse because the two motors would be spinning at different speeds. Whichever one touches more would have a greater load on it than the other, making it overheat.

Maybe I am wrong though.
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rjw
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 12:58 pm

i think that you may be close to getting it resolved, but may be missing something fairly simple

Do you not have a way to check the amperage draw?

i strongly suggest a test with an amp probe

first with no shell installed and then with shell just sitting loosely on the wheels.

If during the 2nd test, you can get up to speed (even if it takes a while), then ??
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 1:03 pm

rjw wrote:
i think that you may be close to getting it resolved, but may be missing something fairly simple

Do you not have a way to check the amperage draw?
I don't know. And honestly, I don't know that it is resolvable outside of a redesign that we can't afford.

Amperage draw is probably really high, but we are pretty much stuck with it like it is.
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rjw
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 1:08 pm

you might be surprised at how easily it might be able to be fixed, but without executing the tests proposed in my previous post, you are shooting in the dark.

As an example, if problem is in overloading the wheels and drives, a possible solution could be to simply remove all motors and just let the wheels support the shell. Then if possible, drive the side wall of the shell.

Just some thoughts, but ampprobe is needed to reveal some unkowns.

Again, if i was closer I would gladly help out in person.
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Cody
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 1:24 pm

They've already done a few redisgns on the bot,Im pretty sure they dont want to completly redo there weapon drive system. Plus the way they are driving the shell has been used, tested, and proven before i have to doubt thats there problem.
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 1:36 pm

Cody Harris wrote:
They've already done a few redisgns on the bot,Im pretty sure they dont want to completly redo there weapon drive system. Plus the way they are driving the shell has been used, tested, and proven before i have to doubt thats there problem.

Any thoughts on what the problem is then?
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 6:17 pm

rjw wrote:
Cody Harris wrote:
They've already done a few redisgns on the bot,Im pretty sure they dont want to completly redo there weapon drive system. Plus the way they are driving the shell has been used, tested, and proven before i have to doubt thats there problem.

Any thoughts on what the problem is then?
We are already fairly certain it is just that the motors are crappy. But we are sort of stuck with them. And a redesign with brushless motors would be too expensive.

So, there isn't really anything we can do.
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rjw
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 6:45 pm

just curious, but with new motors , you say that it only spins 1600 rpm's? full throttle?

Your final ratio is around 6.5...I believe

It should spin around 2200 or better rpm's, once up to speed , wot and no load.

you can only excpect maybe 5 in. lbs of torque with that setup and your outer housing weighs almost that.

So , the good and the bad MAY be...

Bad = it burns up motors

Good = you may just not have enough service factor for your setup.

Were you not running 3 motors before? (at youth fair)?
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 7:16 pm

rjw wrote:
just curious, but with new motors , you say that it only spins 1600 rpm's? full throttle?

Your final ratio is around 6.5...I believe

It should spin around 2200 or better rpm's, once up to speed , wot and no load.

you can only excpect maybe 5 in. lbs of torque with that setup and your outer housing weighs almost that.

So , the good and the bad MAY be...

Bad = it burns up motors

Good = you may just not have enough service factor for your setup.

Were you not running 3 motors before? (at youth fair)?
2 at youth fair

And we don't have weight to add another motor. Whatever we do it has to be with 2 motors.
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 8:04 pm

Hopefully I am not coming across as confrontational in the way that I am replying, because it isn't meant that way. We are just sort of pinned in a corner with the design. Weight-wise, Time-wise, money-wise... It is just a matter of trying to figure out anything we can do to improve what we have right now. And we are pretty much at the end of that list...
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rjw
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 9:11 pm

Not a problem...sometimes it just takes a bit more time than you have to get things sorted.
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 9:43 pm

rjw wrote:
Not a problem...sometimes it just takes a bit more time than you have to get things sorted.
Yeah. We will see how things go with it the way it is though... We should be somewhat competitive at the very least.
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 10:23 pm

I've got some answers for you!

haha well, sort of.

Our multimeter can measure AC current, but not DC ... ? not sure why but that's what it is. I would like to see though, because our batteries can handle 60A max continuous draw, and I really hope we're not too far over that, because we only have three packs.

At full throttle, with relays, we did break 2000 RPM (I wasn't there, so I'm not sure what the exact number was - just that it was screaming over 2000 RPM).

We've always only had 2 motors, just three wheels for stability. Initially, with our FIRST design with 6 direct drive Speed-400 motors, we did have wave-washer springs underneath them (and our current mounting design would easily allow for them) so that could be worth a try.

To be more descriptive about our "method for adjusting the pressure" we have a center shaft through two bearings that have a 1/2" thick UHMW spacer between them. The spacer does not touch the shaft or the inner race of the bearings. The shaft has a disc bolted to the underside, that has a needle-roller thrust washer between it and the underside of the central bearing housing. The top of the shaft is tapped. There is no rocking of the shaft in the housing - it's impressively rigid. The shell has a custom-made shaft collar that is attached to the top. We use a thumb screw with a hollow tube on it to push down on the shaft collar while threading into and pulling up on the shaft. This is how we apply pressure - more turns = more pressure. This was especially helpful when we had all six wheels on wave washers in our first design.

Our shell does weigh about 5 pounds (4.65)....

The duratrax motors supposedly have a stall torque of 91.65 oz-in at 148 A. If they ever stalled, our battery wouldn't like it.

SO. we have 2 duratrax's, so that is (theoretically) 183.3 oz-in of torque, with a 1.34 to 1 ratio (decreasing speed and increasing torque) which (again, theoretically) should give us a grand total of 245.622 oz-in of torque on the shell! That's over 15 lbs! And our drive wheels are 2" so have a 1" radius, so we should really be able to spin a 15 lb shell, right? I can't imagine that there is enough drag on the shell to effectively TRIPLE it's weight... if we pull the belts off, and just free spin the shell, it spins pretty easily. I think we're going to do away with those four runners we had around the edge of our baseplate - they were oil-infused nylon, and were set back but allowed to float a little, so when the bot turned, the moved outwards, and touched the shell. They just spun on a shoulder screw, so it wasn't too much drag, but the clicking noise they make makes me uncomfortable -- it can't be good. So we'll take off the runners (we used the screw heads as support front/back to prevent rocking the shell into the floor, so we'll keep the blocks with the shoulder screws...).

My dad thinks that most of our friction is coming from the thrust washer set up but I think it spins really nicely...

and with our new wiring switch, we weigh in at 15.00 on our scale which is a little too close for comfort. Razz
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rjw
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 11:14 pm

You can't look at the stall torque of the motors, because they will burn up (sound familiar) if you run them there constantly.

As a simple example, look at the banebots 36mm 5:1 planetary gearbox.

It is rated at 31 oz. in. which it will run at all day long. It has a 540 motor.

Compare that to your setup.

2 bb 36mm 5: 1 = 62 oz.in total...intermittent loads can be much higher.

You are running around 6.5:1 ratio, but using pullies, belts, friction of wheels, etc.

Also, you are not lifting the shell, just trying to spin it, so you have that gping for you.

In any case, if properly setup, you should be able to get your 2k+ rpm's.

While it should draw current to get up to speed, once it's there, things should drop off considerably.

In you case it seems that it does not.

So the questions you need to ask is....Do I have a mechaincal problem, or am I just borderline or undersized on the drive motor selection.?

As far as a potential future brushless solution, you would be in good shape weight wise.

A pair of bb 550's weigh around 227 grams each.

I have an align outrunner with performance similar to an axi 4120 and it weighs 320 grams.

You would also save weight in only needing 1 drive pulley setup (possibly)

brushed motors can die a quick death when running constantly over the ideal amperage draw.

Brushless motors can suck the juice constantly..like 50 amps with peaks to 60 in the case of the align that i have.

Just bear in mind thetime and money that you have in that bot at the moment.

If the motors start frying while fighting a destructive bot, you could be back to square 1 or 2.

Especially it that nice alu can gets crunched.

Mind you, that's what this sport is all about.

Good luck with it
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Cody
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PostSubject: Re: hot motors?   Sun Apr 05, 2009 11:17 pm

rjw wrote:


I have an align outrunner with performance similar to an axi 4120 and it weighs 320 grams.


And we would still woop you guys Razz
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PostSubject: Re: hot motors?   Tue Apr 07, 2009 12:02 am

teamjag wrote:
Here is a web site that might help.

http://www.rcgroups.com/forums/showthread.php?t=393591

btw, we're gonna try this tomorrow... thanks for the advice!! Very Happy
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PostSubject: Re: hot motors?   Tue Apr 07, 2009 12:03 am

Dan Curhan wrote:


btw, we're gonna try this tomorrow... thanks for the advice!! Very Happy
What time is the build?

And was there a build today yesterday(12:03 AM...)?
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PostSubject: Re: hot motors?   Tue Apr 07, 2009 12:14 am

yesterday (monday lol) there was no build. Tomorrow no one but me is allowed to come until like 4. My dad got in trouble for having chaos in the workshop during business hours.
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PostSubject: Re: hot motors?   Tue Apr 07, 2009 12:18 am

Dan Curhan wrote:
yesterday (monday lol) there was no build. Tomorrow no one but me is allowed to come until like 4. My dad got in trouble for having chaos in the workshop during business hours.
Ok... I probably won't be able to make it then. Wednesday, Thursday, and Friday I will be able to make it though.. Just so long as the Wednesday build doesn't make me miss Lost Razz
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PostSubject: Re: hot motors?   Tue Apr 07, 2009 12:28 am

Cody Harris wrote:
rjw wrote:


I have an align outrunner with performance similar to an axi 4120 and it weighs 320 grams.


And we would still woop you guys Razz

The new titanium 'ANTISHOCK' setup will neutralize your drums, eggbeaters and whatever else you bring...lol

Resistance is futile.....we will have our revenge....in this life or the next....

How well did you guys do at nat's first time out?
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PostSubject: Re: hot motors?   Tue Apr 07, 2009 12:32 am

5th place our loses came to max impact and vertex, which 1st and 4th that year and in the previous year they took 1st and 2nd
We also realized how crappy our bot really was
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PostSubject: Re: hot motors?   Tue Apr 07, 2009 12:36 am

Cody Harris wrote:
We also realized how crappy our bot really was

We haven't realized that yet.....we're a bit slow down here....sssshhh
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PostSubject: Re: hot motors?   Wed Apr 08, 2009 9:56 pm

okay. I made a shunt from solid 10ga wire... we didn't calibrate it but it should be okay.

with our shell on, the inrush of current was about 140A, then when it was up to speed, it was around 65-70A. Is a drop off of half the amount of current reasonable? Inrush without shell, and with belts and pulleys on got up to about 80A and dropped down to 30-40A.

I need to balance the shell though... there was one piece of fairly thin wall 3/8" chromoly tube that we cut off cause it got squished, so I need to add about that much back in.....
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PostSubject: Re: hot motors?   Wed Apr 08, 2009 10:23 pm

Dan Curhan wrote:
okay. I made a shunt from solid 10ga wire... we didn't calibrate it but it should be okay.

with our shell on, the inrush of current was about 140A, then when it was up to speed, it was around 65-70A. Is a drop off of half the amount of current reasonable? Inrush without shell, and with belts and pulleys on got up to about 80A and dropped down to 30-40A.

I need to balance the shell though... there was one piece of fairly thin wall 3/8" chromoly tube that we cut off cause it got squished, so I need to add about that much back in.....

Maybe you need to find someone who has a similar setup and what they power it with??

If that amount of current is normal, looks like brushless and esc would get you there

I could rent you a mamba max and 1 or 2 motors that I have here sitting in boxes...lol

Actually the motors might be to high rpm for what you're doing
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