Gear Ratio help....plZ..someone???

[Dick Linn]

Planning on jumping up on the platform from the 6" side, 4 WD otherwise, eh? My guess anyway.

Maybe a 12:35 on the 12.5 inch wheel and then come off that axle with a 15:12 to the 10" wheels to step them up 1.25 x. This should get you in the ballpark of 6-7 FPS (I seem to recall drills in low was around 360 rpm, maybe lower when accounting for friction & etc.) If you can find a dual-chain 12 tooth sprocket it wouuld be lighter. If you use separate sprockets, the 12 tooth sprocket on the drill tranny can be changed out to most any number of teeth from 10 on up to balance speed and pulling power. Make sure you get exact wheel diameters under the loaded conditions before finalizing the gear ratios. Tire pressure will be important.

Lots of sprockets = lots of weight.

[Instigator]

Matt Adams >>>> Only 4 wheels will have contact with the surface at one time....and you guessed it right "(the 12" wheels centered and "rock" between the 10")"
- The 12" wheels are the ones they provided us with and the 10"s are hard rubbered.

Dick Linn >>>> "(Planning on jumping up on the platform from the 6" side, 4 WD otherwise, eh? My guess anyway)".......how did you our game plan?.....yeah you are right, we will "try" to do that....don't know if we will be successful at it.
"(If you can find a dual-chain 12 tooth sprocket it wouuld be lighter. If you use separate sprockets, the 12 tooth sprocket on the drill tranny can be changed out to most any number of teeth from 10 on up to balance speed and pulling power. Make sure you get exact wheel diameters under the loaded conditions before finalizing the gear ratios. Tire pressure will be important. Lots of sprockets = lots of weight.)"<<<<<< Thanks, we will keep these things in mind....

[Dick Linn]

Instigator,

We're trying something similar, but only on one end, assuming that 2 wheel drive is good enough (time will tell). 4WD is heavy and 6 is even heavier. Plus 4WD can be less maneuverable if it is a simple scrub-steer 'bot. We're using the drill motors in low, so we aren't faced with the problem of getting a 40:1 reduction in speed at the end.

Try to make the wheels drive at the same ft/sec, so carefully measure the diameter ratios of the two types of wheels, under typical load conditions, and select the sprockets or gears accordingly. If you drive the smaller wheels off the axle shaft that the large wheel is mounted on, you can add a small sprocket there and a slightly larger one on the 10 inch wheel to get things evened up. That may reduce the number of large gears (read weight) that you need. If the wheels are exactly 12.5 and 10 inches, a 15-tooth on the 12.5 driving wheel axle connected to a 12-tooth on the 10-inch driven wheel axle should do it. A 12.0 inch tire would require either a 12 & 10 or 18 & 15 tooth sprockets. If the wheel is about 12 1/4, you are in between a 16:13 and a 17:14. These are assuming you're using commonly available sprockets for 3/8 (#35) chain.

[Matt Adams]

Originally Posted by Dick Linn

assuming that 2 wheel drive is good enough (time will tell).

There's no need to let time tell you- many many teams have gone down this path before.

If you're using two wheel drive, you should loose almost every pushing match against and four wheel driven robot, simply because half of your weight (approximately) will be dragging instead of being pushed. I can not stress enough that you need to have all wheels in contact with the ground be powered if you plan on using a simple tank style drive and wish to be competitive.

The pneumatic wheels are very light, and drivetrain is by far the most important part of your robot. Don't cut power to two wheels because of weight unless you plan on doing something so specacular with the other 5 or 8 lbs that if offsets the immense disadvantage you'll be at from having half of the pushing/pulling force.

I plead that you'll reconsider having unpowered wheels if you haven't finallized your design already.

Matt

[Dick Linn]

Originally Posted by Matt Adams

There's no need to let time tell you- many many teams have gone down this path before.

If you're using two wheel drive, you should loose almost every pushing match against and four wheel driven robot, simply because half of your weight (approximately) will be dragging instead of being pushed. I can not stress enough that you need to have all wheels in contact with the ground be powered if you plan on using a simple tank style drive and wish to be competitive.

The pneumatic wheels are very light, and drivetrain is by far the most important part of your robot. Don't cut power to two wheels because of weight unless you plan on doing something so specacular with the other 5 or 8 lbs that if offsets the immense disadvantage you'll be at from having half of the pushing/pulling force.

I plead that you'll reconsider having unpowered wheels if you haven't finallized your design already.

Matt

Who said pushing was part of our strategy? We've built both 2 and 4 wheel bots in the past, and this years can be converted to 4 WD in about a day if need be, so no worries there. Weight distribution, tire patch on the ground and gear ratios are equally important. We'll try maneuverability for a change. Thanks for the concern.

[Matt Adams]

Originally Posted by Dick Linn

Who said pushing was part of our strategy? We've built both 2 and 4 wheel bots in the past, and this years can be converted to 4 WD in about a day if need be, so no worries there. Weight distribution, tire patch on the ground and gear ratios are equally important. We'll try maneuverability for a change. Thanks for the concern.

Dick, I wish you guys the best of luck. I guess my fear is just that pushing isn't as much a stratetgy as a fact of life- you can get in pushing matches and pinned by other teams even if you don't want to be.

I think it's also fair to assume that unless you have a crab or swerve style drive system, the manuverability increase isn't too significant between a tank style 2 and 4 wheel drive system. This is what my experience has shown, hopefully yours has been different.

Again, good luck!

Matt

[Matt Adams]

Given:
- 2 Drill Motors
- 2 12” and 4 10” wheels will be powered
- The final robot weight is roughly about (or will be) 130 lbs
- Speed we would like to achieve is around 6-7 ft/sec.
-Only 4 wheels will have contact with the surface at one time, the 12" wheels centered and "rock" between the two 10"
- The 12" wheels are pneumatic and the 10"s are hard rubbered

Find: optimal Gear ratio.

Assume:
Drill Motor Characteristics, Low Gear:
RPM: 450
Stall Torque: 319.95 in lbs
Stall Current: 127.0 A
Breaker Limit: 40 A
Torque at Breaker Limit: 103.35 in-lbs

Solution:
Okay, so you have 130 lbs of robot that needs to be pushed having 4 wheels of different diameters in contact with the ground at the same time. It's assumed that you don't want to get into a pushing match and trip the circuit breakers that are on the drill motors.

What makes this a tricky situation is that you have different wheel diamters and presumably an uneven weight distribution since the robot will be rocking. For the sake of assumptions, I'll have to assume that there is more weight on the center 12" wheels, say 60% of the robot's weight, and the remaining percentage will be rocked back and forth.
Assuming the robot has even weight side to side, we'll break down the overall robot to each motor.

Now, we're dealing with 65 lbs, and 60% of it is on the center wheel, that's 39 lbs. We'll assume that you get a descent coefficient of friction of about 1.2 since you have those nice pneumatic wheels, which means that you need to be able to exibit a torque equal to:

12 in / 2 * 39 * 1.2 = 280 in-lbs.

On the back wheel

10 in / 2 * 65 lbs * 40% * 1.2 = 156 in-lbs.

What is very unfortunate is that as you can see, you'll need to have more torque on your middle wheel than on your outtermost wheel, but you need to hold a gear ratio of 10:12 between those wheels, or you'll have a situation where one wheel will be forced to slip on the carpet. That's bad!

So what you're forced to do is actually design based on the conservative wheel, the larger middle one.

The total torque required for the drive train is 436 in-lbs, which means you'll need to have a 4.2:1 ratio between your drill motor and your primary drive shaft, which could presumabley be the center drive wheel. From here, you can place any sprocket ratio of 5:6 from the 12 inch to the 10 inch wheel.

This is the required ratio to make sure you don't trip the 40 A breakers on your drills. In my opinion, this is how you must design a drive train.

Your overall speed would be ((450 RPM / 60 s )/ 4.2) * 1 ft dia * 3.1416 = 5.61 ft / s

The short story is that you really can't ever do much better than this with just the drills if you don't want to run the risk of tripping your breakers. If you wanted to adjust the ratios a little bit, you could edge yourself up to 6 or 7 feet per second, but you'd have to understand that you can't get in pushing matches.

Also note that I entirely nelglected mechanical efficientcy losses beetween sprockets, which could amount to perhaps as high as 15% overall.
I hope that his has been useful for everyone, if you have any questions comments or corrections, please let me know!

Good luck!

Matt