Hey guys,
This might be a really simple question, so forgive me if it’s common sense to some.
Our team is looking into using timing belts to drive our full court shooter. Looking at the available motors, we want to use 2 AM-9015 motors and 1 RS 550 to achieve the desired speeds (about 4000, 7000, and 10000 RPM).
The issue that I have is our team was planning to use half inch axle to spin the wheels on. The motors have an eighth inch shaft. However, I could not find appropriate timing belts to fit both of these. On McMasters, the MXL series fits the eight inch, and the L fits half inch, but I couldn’t find anything that fits both.
Any ideas on how to get around this? I honestly have no idea of where to go from here…
Why not use similar shafts? One great way to achieve the speeds you’re interested in is to use the VEXPro VersaPlanetaries with BB550’s, BB775’s, AM9015, or BAG motors in them all with the same 1/2" hex output shaft. By standardizing the output shafts, you should no longer have the problem of mixing pulleys.
I assume that the reason for no gearbox is efficiency.
We used 2 mini cims, and had no trouble hitting 1 shot per second full court with a belt setup. We had chains before, and it took quite a bit more current on the mini cims to maintain the same wheel rpm. Our shot time was also significantly slower. As good as the versa planetaries are, they’re not quite 95%+ efficient.
Our Full Court Shooter was setup almost exactly as you desire.
For speeds we are running a AM-9015 on the first wheel with a pulley setup at 42T:12T ratio (~4500RPM). The next wheel is a Banebot 550, setup on a 30T:12T ratio (~7700RPM). The third wheel is also a 550 with a 22T:12T ratio (~10500RPM).
We used the 12T MXL timing pulleys pressed on to the motor shafts and secured with the set screws. For the wheel side, we used 1/2" hex shafts drilled and tapped for a 1/4"-20 bolt to attach the pulley to the top of the wheel shafts.
You just need to make sure you place the wheels on the right side of the shooter, so when you’re shooting, the bolts in the wheel shaft want to tighten and not loosen. This positioning also helps with arcing yout shots into the 3pt goal.
If you want more information, send me a PM and I can email you more detais.
Our team modified a 4:1 banebots transmission to be a 1:1 transmission by grinding down the ring gear and gluing the planet gears together. After continued use (>10 minutes) it would get really hot (we think the gearbox’s output shaft was slipping in its bearing), but we must have made thousands of full court shots with that setup.
I would strongly encourage you to use the GT2 tooth profile rather than the english series. There has been a lot of design efforts that have gone into improving the tooth profiles to give higher load capacity in a smaller belt, and these advancements are only present in the metric profiles with GT2 being the best. For this reason, we use GT2 or HTD if we can’t get GT2 in the size we want. There is a great document on the gates website that outlines the power rating differences, unfortunately it is buried down in there. It is well worth finding and doing the math on the proper sizing, or you run the risk of stripping the teeth off of your belts.
Since McMaster only carries the english profiles, you need to get them from someplace like SDP-SI.
I’ve been looking at some of the different pulleys, and here’s the issue I see.
We can’t tap the axle because we don’t have the machining capabilities to do so. I’ve checked on several websites and found that there are no belts that have half inch bores and eight inch bores with the same pitch and numbers of teeth that we need.
With that in mind, are there any other potential options?
If you do not have access to a lathe, shoot me a PM. It is quite easy to find machining sponsors or local teams who can help you. Shoot me your location/team number/school name and I can help get you set up.
One option is to build a set of 0.1255" x 0.4996" bushings. The change in diameter is enough that you can put some tiny set screws in the bushing. Barring that, you can machine a pin to go through it.
Another option is to make a shaft saver similar to what RC hobbyists use to attach propellers directly onto motor shafts, such as http://hobbyking.com/hobbyking/store/_5678__Prop_adapter_to_suit_2_3mm_motor_shaft_collet.html. Some designs hold the shaft using collets, others use set screws. Both can be built with a lathe and a drill press, or you might be able to find COTS ones that fit your specs.
There have been some very good suggestions here already. The best ideas unfortunately require a lathe.
To add one more suggestion, (again if you had access to a lathe) you could turn down the 0.5 inch shaft to a more reasonable size for the pulley sizes you are looking at. You wouldn’t want to step down the diameter TOO much as there will be a stress concentration and a potential point of failure at the stepdown. At the torque ranges here it may not matter.
As mentioned before SDP-SI.com has a much bigger pulley selection than McMaster.
