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

We are trying to figure out the gear ratio for 10" and 12" wheel that is run by a motor of 5000 RPM.

We desperately need some…Any sort of advise or support would be greatly appreciated!!!

Thanks.

If you search around the forums and papers, you’ll find much help on this.

Last year I made up a spread sheet to do the calculations with an atwood only drive and different sized wheels.
http://www.chiefdelphi.com/forums/papers.php?s=&action=single&paperid=225

I think that this would be a great problem to work out for all teams to see. Please answer the following questions for me, and I’ll go through all of the information you need.

Which motor are you using?
How many wheels will be powered?
What wheels are you planning on using? (Both size and material)
What will the weight of your final robot be?
What speed would you like to achieve?

When you answer those questions, I’ll run through the math for you (and everyone) to see.

I’ll also make a shameless plug for John’s whitepaper that can truly take a lot of the math out of the equation.

Matt

Look for the Gear Ration Calculator Spreadsheet in the PAPERS section - under TECHNICAL. It’s about the second page down. This should be the one originally provided by Frank Saladino and it is very useful for both single and multi-stage gear reduction.

You’ll have to decide how fast you want to go or what you want to pull. The drill motors on low with this year’s 10:45 sprockets would be about 4.4 Ft/sec with the 12.5 inch wheels. Last year, using the 2:1 gearbox and the stock 8.75 wheels gave somewhere around 7 ft/sec with the drill motors in low. 5 to 8 ft/sec is in the ballpark unless you have more motors or a gear shifting device.

The Chiaphua (Atwood) motor runs around 5000 RPM at no load. The drill motors in low are probably less than 400 RPM after you adjust for the forward/reverse speed difference, so you’ll need quite a bit of reduction.

even easyer…here is the one i did also

drive train calculator

• Thanks Rob Colatutto for the speed calculation equation. And yeah…how exactly can you find the Wheel RPM??

• We are using Drill Motors
• 2 12” and 4 10” wheels will be powered
• Skyway rubber wheelchair wheels
• The final robot weight is roughly about (or will be) 130 lbs
• Speed we would like to achieve is around 6-7 ft/sec.

• Greg Needel>> the link to your drive train calculator doesn’t work for some reason, but thanks anyways.

See John-V-Neun’s totally awesome white paper! It will help you figure out anything and everything drive train oriented you ever wanted to know.

Cory

In the spread sheet you can look on the left in A19, that is the wheel rpm.

This makes for a rather unique situation. Since you’re using 6 wheels total, with different diameters, I’ll need slightly more information.

Are all 6 wheels going to be in contact with the ground at the same time
or
will you be having the 12" wheels centered and “rock” between the 10" having only 4 in contact with the surface?
or
Another situation?

Are these wheels the pneumatic or hard rubber?

With this known, I’ll be more than willing to chug numbers.

Matt

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.

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…

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.

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.

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.

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

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