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rdubard
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PostSubject: High Torque Belts?   Mon Sep 19, 2011 8:08 pm

We've been working on a belt-driven robot drive for either a heavy robot or one with increased downforce (vac or magnet), and we have shredded some admittedly small belts (1/4 in wide XL series). Have any of you used belts to drive wheels on 120lbers or on 15lb. 'bots with extreme downforce? What did you use?

We have found a link about timing belts that I have been trying to decipher, and if I'm doing it correctly we need at least a 1-inch wide L series belt to withstand the maximum output power of a 555 motor! This just doesn't seem right to me, but if the test with the 1/4 inch XL is any indication, I'm afraid it might be true.

Also, I am intrigued by the mention of "HTD" belts and pulleys, but am not sure just how much better they might be or where best to buy the belts and pulleys.

So, any ideas/experience?

Thanks in advance!
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PostSubject: Re: High Torque Belts?   Mon Sep 19, 2011 10:11 pm

why not use #25 chain?
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Jeff L
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PostSubject: Re: High Torque Belts?   Tue Sep 20, 2011 11:58 pm

Evan Steeves wrote:
why not use #25 chain?
Too heavy.

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rdubard
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PostSubject: Re: High Torque Belts?   Wed Sep 21, 2011 6:34 am

For the 120, chain might be an option, but I agree that chain in a 15 is too heavy.

So, has anyone done HTD belts?
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rjw
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PostSubject: Re: High Torque Belts?   Tue Oct 11, 2011 7:42 pm

rdubard wrote:
For the 120, chain might be an option, but I agree that chain in a 15 is too heavy.

So, has anyone done HTD belts?

We have....but you also need a wide belt.

I would suggest looking at stock drives for the GT2 5mm belts/pulleys. they can come in widths up to 25mm or custom widths with timing pulley stock.....you need the width to not overstress on a 120 or bigger.

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PostSubject: Re: High Torque Belts?   Tue Oct 11, 2011 10:32 pm

look into v-belts, if applied correctly they have more surface area then timing belts with half the width

heres the horse power formulas straight from my hand book, the character definitions are below the chart on page 2414

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PostSubject: Re: High Torque Belts?   Mon Oct 17, 2011 5:08 am

On the downside, V belts require tension to not slip and that will rob power and possibly stress components.

Timing belts are much higher in efficiency than v belts and chains...

Another factor could be pulley diameter. Anything small (< 2") can be tough to tension, although perhaps slipping belts could be a plus...

If width is a problem, coarser pitch T belts could be used...

If cost is a factor, timing belts/pulleys are probably the MOST expensive

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rdubard
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PostSubject: Re: High Torque Belts?   Mon Oct 17, 2011 5:14 pm

Thanks for the replies! I've looked (pretty seriously, actually) at the GT2 belts. They seem like a go. With only the standard width (15mm), the 5mm pitch series can handle stall torque of a 775 motor with a service factor of about 1.8 (if your pulley is about 2.6 or so in diameter--from memory, it's written down at school), and the same belts can do a 555 at stall on a pulley less than 2 inches diameter (lesser service factor, but still).

And they aren't really that pricey--stock drive products has them at under $20 per pulley and around the same cost for a belt for 3.6 in center distance.

I highly recommend downloading the technical part of their manual if you are looking to properly engineer a belt drive system!
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PostSubject: Re: High Torque Belts?   Mon Oct 17, 2011 5:28 pm

I'm on it.....Razz

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rdubard
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PostSubject: Re: High Torque Belts?   Mon Oct 17, 2011 8:37 pm

Actually, my last post requires a bit more detail to explain my results--It's a little involved,
but I'm pretty confident it's solid. (Also, no reason to post TL;DR--it's here only if you want it)

First, I am assuming a 16-to-1 gearbox on both 775 and 555 systems. (Also a 1:1 pulley ratio after that)
Second, I assumed I was running the 18V rated 775 motor from Banebots at 14.8 volts.
Third, I didn't really use the exact method prescribed by the book, because those methods are based on machinery
in which the running speed of the equipment is essentially a known constant, and we know that the drive of a robot
is anything but constant.

So, I used some knowledge of DC motors to reach my conclusions:
I converted the stall torque to lb-in (because that 's what the charts use--for a 775 it's 10.4 lb-in)
Then I derated that figure by 14.8/18 to adjust for the applied voltage (giving me 8.6 lb-in)
Next, I multiplied the adjusted stall torque by the gear ratio (got a whopping 137 inch pounds)
Also, I adjusted the no load speed, by derating it 14.8/18 and then dividing by 16 (got 1002 rpm)
Next, I had to recall that the torque of the motor is a LINEAR function of the speed. So calculate the torque for
each speed listed in the pulley tables by using the equation
torque = stall torque - (stall torque/no load speed)*listed speed
Then I multiplied the values by a service factor of 2 (which didn't work that well, cuz the pulleys would be bigger
than my wheels!) so I recalculated by a service factor of 1.8 and just started looking up the points on the pulley/speed
chart.

It's interesting to me that the most demanding conditions are when the gearbox is turning around 40 to 80 rpm, not
at full stall as I had assumed. I guess that's because an unmoving belt doesn't get flexed.

Oh, I also had to do a little figuring for belt length correction, but it didn't change anything much.

Whaddya think, rjw?
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rdubard
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PostSubject: Re: High Torque Belts?   Mon Oct 17, 2011 9:01 pm

One last detail (assuming anyone cares anymore Very Happy )-With a 3 inch diameter wheel and a coefficient of friction of 1.0, right around stall
this would give 90lbs of pushy per wheel.

This is ridiculous for a 120lb bot, unless you've got some way of increasing the downforce of the bot, the wheels will
just slip against the arena floor well before reaching stall. However, the next lowest ratio gearbox provided by
Banebots is only 4 to 1, and that would make the motors be in 'near stall' far too frequently to be efficient, and a
top speed of the robot that could be uncontrollable. Alternatively, you could use larger wheels and/or a different
pulley ratio to optimize your drivetrain, but the robot gets progressively taller and thus a bigger target.

Just food for thought.
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rdubard
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PostSubject: Re: High Torque Belts?   Tue Oct 18, 2011 9:07 pm

Started to consider the ESC today and realized two things:

1) I was looking at specs for a 550, NOT a 555. The 550 is the one with the impressive stall torque (and 85 amp stall current, which leads me to. . .)

2) For that system the plan was to use BB12-45's, which won't handle the current at anything below about 800 rpm for any reasonable amount of time.

Sigh.

So now we're stuck with the choices of either upping the ESC (and thus the weight/space/price), or dropping the torque or speed. Neither option is especially appealing.

Now I'm wondering--has anyone tried stacking the chips on a BB12-45? I have several that don't work anymore for one reason or another, and it may be worth a try to get a decent power upgrade with minimal cost or weight, but I don't know if it will even work for the chips in the 12-45.

In a similar way, could the chips on a 12-45 be fancooled or heat sinked, or will this add more wieght than just going to a higher rated ESC?
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PostSubject: Re: High Torque Belts?   Wed Oct 19, 2011 8:34 am

rdubard wrote:
Started to consider the ESC today and realized two things:

Now I'm wondering--has anyone tried stacking the chips on a BB12-45? I have several that don't work anymore for one reason or another, and it may be worth a try to get a decent power upgrade with minimal cost or weight, but I don't know if it will even work for the chips in the 12-45.

In a similar way, could the chips on a 12-45 be fancooled or heat sinked, or will this add more wieght than just going to a higher rated ESC?


It is definetly worth trying dubard, I had a crazy electronics whiz freshman last year that used that technique on the old banebots blue 18 amp controller, to match one of the 45's. since we had burned one a few nights before motorama.

It looked ugly but performed great, it actually lasted longer in the competition than the actual 45amp. hooked up to a RS 540 @ 18v in a 16:1 ratio
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rdubard
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PostSubject: Re: High Torque Belts?   Wed Oct 19, 2011 6:23 pm

electronics whiz freshman last year that used that technique

OMG! How do I find out what he did? I cut the heatshrink off a few today and realized I wasn't sure which chip(s) to stack. This is made especially challenging because Banebots SANDS OFF THE NUMBERS from the chips. (No good corporate intellectual property yadda yadda yadda I am the 99 percent whatever)

I made a guess that the ones that look like transistors (square package with three terminals, one of which is also the metal heatsink tab) were the output MOSFETS and stacked those. I also chose to stack these first because there are only 3 terminals on each and there is a lot of space between them.

The thing still works, but now I'm not sure how to know if it made any difference.
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PostSubject: Re: High Torque Belts?   Wed Oct 19, 2011 9:22 pm

I have no idea, thats all over my head. but I just shot him an email asking.

***
on a side note, LS decided to drop the battlebots program this year.
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rdubard
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PostSubject: Re: High Torque Belts?   Wed Oct 19, 2011 9:39 pm

jeeves_m_d wrote:
LS decided to drop the battlebots program this year.

Sad, pointless, and short-sighted. Any idea why?
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PostSubject: Re: High Torque Belts?   Wed Oct 19, 2011 9:55 pm

they want to try first out, and they don't think they could run both.

I mean first is great, but battlebots is what drew kids in. Fighting robots are awesome

There is a new teacher replacing the old advisor, and he wants to change everything so that he wont be compared to his predecessors.


The kids returning from last year were all pretty dissapointed,

My little brother is replacing me at the Highschool this year as a freshman and decided to join.
While it won't be as much fun, i do believe first will look better on a college app.

hopefully they will let me play with the bot parts when i come visit in the summer
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Camden W
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PostSubject: Re: High Torque Belts?   Thu Oct 20, 2011 1:12 am

I think doing FIRST and Battlebots in tandem would not only get the most attention, but would totally be possible. Ideas of ruggedization and functionality would go into both projects more astutely, resulting in total epicness.
As for High torque belts go, any suggestions on a 3mm dia. o-ring?
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PostSubject: Re: High Torque Belts?   Thu Oct 20, 2011 12:52 pm

I'm continuing with the "chip stacking" efforts (should this be a different thread?)

Anyway, here's what I see when I strip the heatshrink off a BB12-45:



(The opposite side has what I think is a BEC output transistor in the place of the control circuit-otherwise it's the same).

So, I took one of the 'broken' 12-45's, desoldered the MOSFETS (both sides) and then soldered them in parallal to the existing ones on a good 12-45 ESC.

Then I set up a bunch of rheostats (high current adjustable resistors) in parallel to create a load of around 20 amps at 13.8 volts and used these in place of a motor. I supplied the ESC with a plug-in power supply (like the kind we use for the battery chargers), and used a servo tester for signal. I powered it up and measured how long until the ESC shut down due to thermal overload.

The stacked one took 2 minutes 30 seconds to shut down.

Tested an unaltered 12-45 the same way and it shut down after 1 minute and 30 seconds.

So, I think this works! It produced an increase in weight of under 4 grams, but nearly doubled the power handling capability. The stacked one looks like this:

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PostSubject: Re: High Torque Belts?   Thu Oct 20, 2011 2:12 pm

I think the holmes hobby esc might be a good replacement if this doesnt work
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PostSubject: Re: High Torque Belts?   Thu Oct 20, 2011 10:33 pm

MODDING bb18s


From what I can remember I simply stacked some N channel High amp mosfets on the two smaller fets.
The two small fets were on the same side of the pcb, i think right next to each other.
With this mod, it outperformed the other BB18-45, so it really makes those small things handle a lot!
Ill try and post a picture tomorrow.

I BELIVE the fets were IFR10 or something like that, really can't remember because it was so long ago.
Yes, chip stacking does work fine. Took me like 15 mins to solder it.

Ill try and post a picture tomorrow.

I really want to do battlebots next year.
and yes sajeev, you are 100% correct:

"The kids returning from last year were all pretty dissapointed."

first sorta sucks right now, we need soooo much moo-la to even register.

BATTLEBOTS = AWESOME!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!




Last edited by abrown on Thu Oct 20, 2011 10:40 pm; edited 1 time in total (Reason for editing : incorrect esc, originally thought I started w/ a bb-9)
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rdubard
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PostSubject: Re: High Torque Belts?   Sat Oct 22, 2011 8:28 am

Update: I was digging around in the physics lab equipment, looking for a legitimate stopwatch to double-check the stacked BB12-45 run times, and I found a handheld infrared thermometer, which I promptly applied to the chips while testing.

I discovered that the "NOT SURE" chip I circled in the prior post was getting hotter than the stacked MOSFETS.

So I stacked those too (Which is a LOT harder, because there are five tiny parallel pins that have to be soldered individually, without shorting between them).

AAAAANNNNDDDD

It continuously ran for 10 minutes at 20 amps and 13.8 volts and then I got bored and quit testing. The hottest chip evened out just under 300 F, which I think is the shutoff temp (from other testing).

It's less than 10 grams increase in weight. Of course, it requires two ESCs to make one, which would not be financially feasible if I didn't already have 8 or 10 nonfunctioning ones in a box to draw parts from, but it's still just about the same size as the original. If someone can find out which chips they are online, it could probably be a lot cheaper than buying new. Also, there are probably ESCs out there that do this too, but I don't know which ones.

Anyway, I would call this a success.
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PostSubject: Re: High Torque Belts?   Sat Oct 22, 2011 8:30 am

Cody wrote:
I think the holmes hobby esc might be a good replacement if this doesnt work

Went to the website--not sure which ESC you mean. The Tekin look pretty good, but I wanna run 3s on the Banebots RS550, and it's not clear that anything but the most expensive one will do the trick.
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PostSubject: Re: High Torque Belts?   Sat Oct 22, 2011 10:06 am

The torqueMaster br xl. Advertised for 80A continuous and weighs 1.6oz. For what you said about having to buy two 12-45s for what you did, $100 for it isn't a bad price
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PostSubject: Re: High Torque Belts?   Sat Oct 22, 2011 11:22 pm

Here is how I did it, seems to work well...



mosfets are NTE 66 N channel, basically just your average power mosfet.

I just piggybacked the chips on top of each other, to get something like this:



This modded esc seems to perform just as well (or better than) a bb45, but that may be because we only used it for short periods of time. We did use this in our robot and it totally handled the current, didn't get hot. I never actually put a load on it and checked how long it would go before the thing shut down. But, given that I made this at 11:00 the night before a competition, its pretty darn good! Smile

That thing marked "not sure" may be two mosfets, plus the temperature thing that gives the signal for thermal shutdown. not sure though.
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