This is my 2nd year teaching robotics and this is the first year we will be competing in Vex Robotics… Gateway. All we have are 269 two-wire motors and external controllers. We are going for the tread system. The tread on two sides and hoping to get the spheres and cylinders that way. I have been watching TONS of video but still confused on how to get the “arm” up to get over the goals. Any suggestions, tips, ETC? I have a zero budget (Thank the Lord for donations) and do not have tons of parts. The students and I need some help.
Thanks in advanced,
I am waiting on high strength chain…I have tons of sprockets and gears. I have seen several videos and have some sort of idea. Do I put the little sprocket to the motor and the larger one below for more torque? I haven’t messed with stuff like this much.
Well, here’s an example from last years roundup robot which used a 4 bar. Not necessarily the best design (in fact there were problems with shafts twisting) but it shows the principle of increasing torque from the motor at the expense of speed. This just shows one side from behind, the other side was the same.
Here is a simple robot from the Elevation season with an arm a lot like you are considering. This robot only had one motor to lift the arm, most teams now would use two. Take a careful look – there is a lot of basic VEX construction in this picture.
This helps a lot… Do you have a picture of it from the side? I just thought you might put a gear on the top, one on the bottom and run a heavy strength chain, but this good b/c I should have all the parts.
You don’t need a chain to lift an arm, as long as you have room for the motors to hang out the sides of the towers. You should note that the robot in the picture is only 12" wide. With a wider robot you could mount the towers farther inboard and make room for the motors. If you’d like to see more pictures of this robot, go to the gallery and search on “zippy.”
That helps even more. So all that is lifting that arm are the 2 huge 82 tooth gears and then those little ones that are connected to the motor? I REALLY APPRECIATE this information.We built one side of the “arm”/intake today… Just trying to get down on how to tilt/lift arm… Makes me feel better about how to do it.
The new turntable bearing kit is very effective at raising an arm. The arm can be directly attached to the turntable and the turntable attached to the robot tower. No drive shaft to twist, no chains to let go, not gear teeth to strip, just smooth action. http://www.vexrobotics.com/products/accessories/motion/276-1810.html
In my opinion a regular gear system is better (for gateway anyway).
The turntable is nice because its the whole mechanism right there. But, what do you do if one of the gears on the turntable break? You would have to buy a whole new turntable (I think so at least). An example of a broken turntable can be seen here.
I haven’t had any problems with axles twisting this year, yet. Hopefully I won’t have to deal with them this year.
In conclusion, I think it is partially a personal opinion on the turntable and it is not as great for gateway. It would have been nice for Round Up though because we twisted shafts when we try to de-score and our claw would get caught on the post.
Thanks for the tips guys. Love the turn table BUT I have zero budget now… Hahaha… I started out w/ $0 got enough donations to buy some booster kits and stuff, but no where close to what I would like to have. Am I correct w/ the two big gears only connected to the 2 -12 teeth gears to raise arm as shown?
Yes, that arm was bolted directly to a pair of 84-tooth gears which were also bolted together. The 12-tooth gears are on the motor shaft. The most fragile part of that system is the 12-tooth gears, which can break teeth under a load. The gears are not doubled up to protect the big gears, but to add redundancy to the little ones.
been doing vex for 4 years now and every time i go to build an arm i just revisualize that a big gear power a small gear would make the smaller one spin faster and you want strength not speed so flip it
motor on smallest gear
arm on biggest gear
for highest torque then you get into 2 and 3 stage gear reduction like that one is a 2 stage
In that compound gearing of 1:5 then 3:7, you would want to flip the order, so it goes 3:7 then 1:5. That way there is about half (7/15) of the torque on that middle axle, so you will be less likely to twist axles and slip the gears.
Tradeoffs to consider when using a turntable for arm shoulder joint:
turntable is huge
turntable may be heavier than alternative solutions, adding to high center of gravity
turntable is pivot joint is much bigger/stronger than a normal axle & bearing joint
turntable is still cool and unusual at this point
turntable allows only 5:1 torque-up range, not 7:1 like 84:12
harder to add multi-stages(maybe?)
= same type of gear teeth as High Strength 60 tooth gear
= Provisions for two motors
= Forces direct mount of arm to internal gear, which prevents even the possibility of poor design practice of passing final arm torque directly through axle torsion.
Too bad the Vex hinges don’t line up (in either X or Y) to gear axles.
Hinges make great (unpowered) arm joints: tight fit and 4 narrow shear joints.
We’re also a low-budget team, but one tightwad addition you’ll not regret is rubber bands (#32 from an office supply store are equivalent to the Vex product, with no shipping charges). If you use 2-stage reduction for the lift (like jpearman’s example), the arm will probably be able to hold its position, but an arm with one-stage reduction (like Rick Tyler’s) will probably flop down under its own weight when undriven. You can see that he uses elastics (black rubber tubing in the back) to counter this. Rubber bands are almost as good if you use a multiple thickness and are quite a bit cheaper. Countering the weight prolongs motor life as well – we’re using ancient 5-year-old motors that are still hobbling along.
