Belt Pitch

hey guys we were thinking about trying timing belts for our drums, so my question is what pitch do you use? I know you have to design around the belt because it isnt one size fits all, also any other suggestions? We use a 1:2 ratio

Typically not the way to go....

Alot of teams use inverted timing belt because its just a flat belt

Timing belts used normally will get you up to speed quicker even as the belt get a little loose. but they put an unnecesarry shock through to the motor which will result in greater current spikes during hits.

What we have used for timing belts in weapons have been XL series belts 0.2'' pitch, but we inverted them

what do you mean inverted?

inverted as in like in side out? so the teeth aren't touching the gears?

That is what it means

do you still use timing pulleys? Is it bad to use them as you normally would, I know jeeves explained why not to, but will it wreck the motor? Do they slip alot when inverted?

We've used the both ways, never had any motor problems and we've snapped them both ways from REALLY big hits, usually a drive belt would snap before or along with it

inverted timing belts are used with smooth pulleys.

You do need a tensioning system of some sort.

We usually set them with a bit of looseness, so that things can slip a little on hits.

i'm thinking of trying xl pitch normally, do you think that would be okay? or go with L?

It will be fine, but you may find that when the belt doesn't slip on big hits, and the motor stalls or slow siginifigantly that it will draw ALOT more current then your used to, and depending on how the rest of the bot is wired you may heat stuff up and take away power from your drive. Granted this is only for a short amount of time but it still is a risk, after a big hit you want to show that you didn't take yourself out.

XL series belts are commonly used, both inverted and not. 3/8 wide or 1/2 wide are also the most common. I had thought (before reading this thread) that weapons were almost always run inverted, and drive was almost always with 'teeth in." Clearly, this is not always the case. The advantage of 'teeth in' is that the center distance calculators will give accurate values, and you never have to make your own pulley. Inverted has the advantage of slip (and really, you can use the toothed pulleys, but you don't have to). Inverted does NOT work with the center distance calculators, so making your own pulleys is one way to 'guess and check' your way to the proper fit.

Will Bales' Fluffy uses THREE 3/8 wide inverted belts at a 3:1 ratio. The big problem with his implementation is 'teeth in mesh'--that is, his small smooth drive pulley is very close to his large output pulley, so there isn't much contact on the small pulley, so it slips easily. He used three side-by-side to offset this effect, and for redundancy because he has a 5 horsepower motor driving the darn thing.

Yes, you read that right, FIVE horsepower in a 15lb weapon. I am gonna miss that crazy guy when he graduates this spring. . .

IIRC the mamba monsters claim 5hp output as well????

here is quote from the castle wesbsite


"Mamba Monster systems typically pull 4,000-5,000 watts (5 to 6 horsepower) under acceleration"

I also noticed that they now have a 1/5 scale setup as well....where will it end?? can you say 10hp?

From what I remember wills motor is pushing around 5 hp at the voltage he is running while the monster is certainly capable of 5 to 6 hp at 24+ volts
could be wrong idk

from experience, I think it's fine to properly use timing belts (teeth side). I would suggest XL size for 12/15lb bots. But several things have to be taken into account: as people mentioned, shock to the motor is bad. For this I have developed several things to fortify the weapon motor (Axi 2826 in my case, also most of these apply to outrunners)

1. Solid connection between weapon motor and pulley (putting a flat on the shaft and putting a set screw through the pulley)
2. Gluing magnets - had problems with magnets coming loose, so as soon as I get a new motor I go ahead and make sure all the gaps between magnets are filled with epoxy
3. Tapping larger motor-mount holes. The Axi 2826 comes with 4 mounting holes with m3 threads which strip quite easily in combat, so we drill it out with a 6-32 tap drill-bit and tap it. be careful when tapping so that the small metal bits don't go in the motor (it's jus non-magnetic aluminum, but u never know what might go wrong..)
4. (untested) If i ever were to build another 12 and get a new motor for it, I was considering swapping out the bearings of the motor entirely and just increasing the shaft size to 6 mm or 1/4"
5. (used by paul in 'land shark' aka 'ntertainment') when mounting the weapon motor, mount against a wall and use a ball bearing to support where the shaft exits the motor since that's a common snapping point of motor shafts on outrunners.

Aside from these factors that I've used when using outrunners, I usually use the SDP-SI distance calculator to find what size belt I need and then order 1 or 2 teeth larger so there's more 'slip' or 'slack'. I don't think I have had any large issues with spin-up with a slightly larger belt, you just have to account for the belt slipping off the pulleys by using flanges or some other method.

donsun wrote:

5. (used by paul in 'land shark' aka 'ntertainment') when mounting the weapon motor, mount against a wall and use a ball bearing to support where the shaft exits the motor since that's a common snapping point of motor shafts on outrunners.

If i would consider supporting the end bell with a bearing also. The small set screws in the back of axi like to come loose and seem to be a common point of failure also

5mm shaft inrunner with inverted timing belts has been working pretty well for us.....but not the cheapest way to go.

My 3pounder,Sting used to have an outboard bearing on a 3mm (not oem) inrunner shaft, but that got removed prior to moto last year and I haven't had any problems...again, inverted belts

just some food for thought....

Cody wrote:

donsun wrote:

5. (used by paul in 'land shark' aka 'ntertainment') when mounting the weapon motor, mount against a wall and use a ball bearing to support where the shaft exits the motor since that's a common snapping point of motor shafts on outrunners.

If i would consider supporting the end bell with a bearing also. The small set screws in the back of axi like to come loose and seem to be a common point of failure also

oh yeah, forgot about those.. we usually just unscrew them and put them back in with red loctite