What’s the general opinion as to a good drivetrain speed? Right now my team is running off of direct drive and it is working fine but we feel like we need more speed. We think we can score more points in programming skills if our bot is faster because the drive time takes up most of our skills run. Anyone have a good suggested gearing ratio? The bot is heavy but not a complete tank and we have 2 269’s and 2 393’s available for the drive. Also, high-strength gears or high-strength chain and sprocket?
You might be able to sustain a .67 drive ratio depending on your weight, wheelbase, and wheel type. That would be a 60-tooth gear on the motor driving a 36-tooth gear on the wheel.
I prefer spur gears to chains because it is easier to build a low-friction drive with gears. If you decide to use chains make sure they aren’t too tight, and remember that it is easier to build a good drivetrain with larger diameter sprockets.
You didn’t mention your wheel size.
An old Rule of thumb is that near 3 feet per second, per vex gear ratio speed chart, is about right, but that was for games with less congestion.
It takes more practice to get good at high speeds.
It takes more coding or gentle driving to prevent PTC brownout at speedy gear ratios.
Do you find that gear sizes forcing the placement of wheels & motors at fixed intervals to be limiting? We’ve always used gears and find this to be a major frustration, so we recently purchased our first chain & sprocket kit with the hope of getting around this problem.
As an aside, do “spur gears” refer only to the thin, “low strength” gears? I noticed the term “spur” in the product description of the non-HS gears, but not in HS ones.
Gears are limiting, but I prefer them when possible (although I do almost no robot building any more - I spend a lot more time talking to teachers than I do on robots these days).
All the “regular” and high-strength VEX gears are spur gears. It’s a type of gear shape: Gear - Wikipedia
I believe that some ratio around 1.6:1 or 1.4:1 (my favorite) is best for both speed and torque. I also enjoy spur gears since I have found that they are more reliable. 3 out of the 4 robots my team has built this year have been 7:5 which is 1.4:1 which is extremely reliable using spur gears. It only browned out 2 times because one of the wires got unplugged from the cortex, but we fixed it. One of our robots we built this year was 7:3 and it was crazy fast but nearly impossible to control so we decreased it back to 7:5. We are going to try and build a good chained base for our next robot using 1.6:1 because it is a little faster and I also want to do well on programming skills and driver skills. Our last chained base was around 2:1 at worlds last year and it was a disaster with a 19 pound robot. We even had 6 motors on it but it kept browning out probably because we had too much tension on the chain. So as Rick said before, try to use spur gears, you can do a 6 wheel drive alternating the gear ratio 5:7:5:7:5 with 7’s attached to the motor and it shouldn’t fail you.
If you want to kinda compare speeds: here is a video of me failing at driving a 7:3 robot which is 2.333:1 12/10/11 Ceres Vex Robotics Tournament - YouTube
The 2.3333:1 robot is really fast at scoring the doubler around 1:45
And this shows a 1.4:1 which is fast enough and controllable http://www.youtube.com/watch?v=neRRcijsPUs
hmm, someone should have a modular 2:1 (speed) drive for skills runs only!
and are you running 1:1 with internal speed setting on your motors?
that would make it 1.6:1 which is the ideal setting for speed and torque
Something I’ve been messing around with in code is mapping joysticks to max ~75% of motor speed, then using a shoulder button as “turbo” for when you can trade maneuverability for speed. Also, if you can remap the joystick on a log scale instead of a linear one, you could get greater accuracy at higher speed.
Hey thanks for all the the replies. We arm using the 4" omnis and we are planning on using gears. So here’s another question, if we are going to go with a 1.6:1 ratio, should we internally gear the 2 393s and externally gear the 2 269s or should we just externally gear all of them? I would assume the first, but I just want to check.
I would externally gear them all because the hs is 1.6:1 but the 269’s are 1.67 :1
I’m not sure myself if there is a real world difference in that or not. But i’d do external all just to be safe
I think external 1.6:1 would be better like murdomeek said cause the motors tend to overheat if you gear it internally and if your robot is around 20 pounds. We are going to use a 24 cog with the 15 tread cogs to create 24:15 = 8:5 = 1.6:1
Wait, 20 lbs?? O.o
It’ll probably be safer for u to do 1.4:1 in this case…
Haha I didn’t say it was 20 pounds! It’s low to mid teens in terms of weight. I can easily picks it up with one hand.
Oh lol my bad.
oh haha
our robot is around 13 lbs and its geared 1.6:1
at worlds last year, Titan had a 19lbs robot going at 1.6:1 but that was pushing it to the max
(he had motor tripping issues if the fields have anti-static spray on them)
what does motor tripping mean? and why did the anti-static spray affect it?
This is on the 393 motor wiki page: https://vexforum.com/wiki/index.php/2-Wire_Motor_393
Vex foam field floors are a very good insulator, and are subject to triboelectric charging: when you walk across them, and touch a ground, you’ll usually get a shock.
To avoid or reduce this, competition venues spray the foam with Anti-static spray.
However, Anti-static spray seems to increase the ‘stickiness’ of the field, and increases traction with the robot wheels. This doesn’t affect straight-line driving, but any wheel-scrub from turning has much more drag than before, even with omniwheels.
This extra drag-load on the motors increases the likelyhood of tripping a PTC in the cortex, or the thermal overload breaker in the motor.
Swerve steer drive would theoretically be less affected, since it has less wheel-scrub.
X Holonomic omni-wheel drive is observed to be more affected.
- Holonomic omni-wheel drive, may be less affected if it is lightly loaded, and driven primarily orthogonally.
I have not seen any data on the new mechanum drive.
A longer answer than you were looking for, I’m sure…
tl;dr version:
anti-static spray increases drag on drive motors, which makes them more likely to trip PTC or overheat.
This is a theory which no one (to my knowledge) has tested objectively. The vast majority of robots at WC seem to work just fine – static spray or not. Personally, I would want to see some data as I’m more than a little skeptical. I think can think of a couple of credible alternative hypotheses:
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New tiles are clean and soft – old ones tend to get dusty and slightly compressed. It might just be that the resistance-to-turning of clean tiles is greater than that of dirty ones.
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Teams that test on hard surfaces are nearly always shocked when they play on foam.
It wouldn’t be hard for an experienced roboteer to test this.
What kind of anti-static spray are we talking about here (I.E. What stuff is used at worlds), and where can we get it from?