Panzerpaul

If any of you out there are experts on this subject or just highly knowledgeable, perhaps you can guide me thru this. I'm attempting to determine what gear ratio I need to design to move a model vehicle at scale speeds.

The parameters are:
Max motor RPMs are 2000
Powered by a 7.2 v DC battery
The prototype vehicle ran at a max speed of 56 MPH
This is 3.5 MPH if in 1/16th sxale.....=18,480 ft/hr....=308 ft/min...=3,696 inches/min

There are 6 driving wheels on the model…..each with a circumference of 7.59”….meaning the model vehicle covers 7.59” for every one revolution turned.
Thus in 1/16[SUP]th[/SUP] scale, it would take 486 revolutions/min to travel the 3,696 inches.... to be at a scale 56 MPH speed.

However, all of this is at “no load” on the motor. The model vehicle weighs 2.44 pounds……=39 ounces.

The pinion gear has 8 teeth. Thus the questions are:

How many gears and what teeth per each gear is needed?

At “no load” , using an 8 tooth pinion (#1 gear), a 14 tooth #2 gear, with a second 8 tooth gear attached to it, driving a 19 tooth #3 gear, works out to 481.2 revs/mins…..which is close enough !!!!! (I used Impact’s Gear Ratio Simulator for this calculation)…. http://jbwid.com/art74j.htm

But how do I factor in the 39 ozs of load that the motor will be pulling?….and what about effectts of friction from the moving parts....(drive shafts, U-joints, gear teeth, axels, etc....maybe these can be ignored on a model)
Certainly this extra load requires more revolutions/min to move everything !!
Is there a formula for this calculation?

And lastly…….does anyone know where to order plastic gears with the required number of teeth……like those used in Tamiya gear boxes?

Thanks for any assist. PP

Panzerpaul

Gentlemen: I see many of you have viewed this thread.......isn't there someone out there who can give the help needed? Thanks, PP

lposter

I tuned off once I saw Imperial units......but.... I notice that in the post you say that once the 39 oz load (the weight of the thing) is applied, "Certainly this extra load requires more revolutions/min to move everything ".

I dont think that may be completely correct. The extra load will require more torque to move everything but the revolutions per unit time should be the same (ie. moving the mass at the desired speed).

Its also a little unclear whats going on....have you six drive wheels (ie. six wheels connected to motors) or six wheels with just two connected to motors?

In theory...you have forgotten that you wont always be running on flat surfaces. A motor/gearbox might get you running on the flat and then fail miserably on a slight incline. So you have to factor in that.

Related to the torque is also the problem of acceleration. If you are trying to get completely scale speeds (And I think you are), then scale acceleration should be something to think about. There is no point achieveing scale speed (to my mind) if the vehicle attains that speed in a completely unrealistic period of time (ie. from 0 to the desired speed in a half second or so).

IF I was you (and you dont have to heed this advice), I would find your motors torque (can be done by just hauling weightys with it) and start from there.

3.5 mph is 1.45 meters/s.

If you have two drive motors and the maximum incline you want to drive up is 30 degrees and lets say you want an acceleration of 0.5 m/s/s and your vehicle weighs 1.10 kg then you need two motors with torque of ...

and IM assuming you rdrive wheel has a radius of over 9 cm (is this a tank?)...... then you will need a torque for the motor(s) of about 0.5 Nm.

Then if you know your original ungeared torque...the reduction is calculated based on the torque.

How you achieve that reduction (ie. one gear, two gears x gears) is probably up to yourself.

bear in mind (and you may not have thought of this) that the efficiency of the gearing is important. If you achieve the reduction using for example, two spur gears then the efficiency will be higher than if you used 10 spur gears or 1 worm gear. And your battery will last longer and so on and so forth.

BUt if I was you, and I didnt know the original torque of the motor , I would try and find that out. If you are fairly certain that the motor will deliver the torque you need, then the rpms are probably fine to work from (seeing as they are easier to figure out).

I dont know where you can buy gears loose but robot shops are your best bet.

For all the equations, I use "Building Robot Drive Trains" by Dennis Clark and Michael Owings and published by TAB Robotics.

Sorry I cant help more and I know its probably not the answer you were looking for but it might help and at least its an answer of sorts.!

Its annoying when you feel no one is answering a valid question.....I dont suppose you know th thickness of a 1945 Soviet towing cable do you?

p

lposter

Sorry....instead of buying that book, all the equations you need can be found here: http://www.instructables.com/id/Unde...sign/?ALLSTEPS

but its all metric and I dont know if they are the same as for imperial. Possibly not....


p

Strato50

I was confused about the max motor RPM.. most 'can' motors in this hobby are 10-20,000 RPM for say a 380-400

A 2000 RPM motor is usually.. really big.

DeadTom

I agree with lposter that your motor's rpm will fall off as your machine approaches any incline so therefore you will need torque to mount any incline and torque will not found be in the faster motors.
You can get by with 380-400 can motors as long as you have the voltage and amperage to torque the motors up any incline.
Direct drive will not work as you will never acheive any torque at lower speeds but on flat out ground it will be like a rocket ship, so you must gear it down. You will need to know what torque you proposed motors will be able to produce before you start adding gearing to it to match your final speed.
One question though, if you are producing a 1/16th scale model why would you not just buy a nice metal gear set and then control the speeds with some of the nice new boards out in the market?
The cost to produce a one off gear set is much more than it would be to just purchase them; please do not let me discourage you at all just wondering if you have a new wizbang idea for gear sets?

Perry S.

This can be worked out but you have not given enough information. To start out you will need the torque-speed curve for the suspected motor. No load rpm values are of almost no use as any drag will affect the speed, drag, current consumption, etc.

So you can pst the T-S curve and work it out from there then build some custom gearboxes for a lot of money but chances are you can just use commercial units that are already available for way less money.

Perry

Panzerpaul

Gentlemen: Thanks for all the advice. I'm working my way thru the info on this page...http://www.instructables.com/id/Unde...sign/?ALLSTEPS Going to take some time.

To answer the question why not use a readily available gearbox......well because there is no room, and I would like everthing to be as protypical as possible. This is an armored car and as you can see in the first pic of the underside of the model, there are three drives that need connected, the motor is connected inside the metal hull with the transfer case (gearbox) running from above to below the floor. The second pic shows what the prototype looks like.....you are looking at it with the floor to the bottom of the frame. Its very unique looking and more rectangular in size..............Yes, this will be lots of work to design it, but worth it in the end.

ausf

That is one sweet project.

I would ask the question with the rock crawler or 4WD guys. They may have a better feel for what you're trying to do than us that push tracks around.

Panther G

Paul you simply need to use a RC truck transmission and transfer case to drive that vehicle. they are easy to find and build and there are hundreds of ways to do it. you are on the wrong track trying to use a gear box system like we use in tanks for this project.
And for the middle axle you need a driven through differential like on a semi truck. They are easy to find also like a Tamiya TLT axle. They would be perfect. 6x6 vehicles are as easy to build as 4x4, you just need the axle that is driven through with an input and out put shaft.
I also think it is possible to get an RC truck transfer case with 2 same side out put shafts. Look at RC4WD.com. The may have one.
I also think you are over thinking it. You need to remember that your axles are going to determine your final ratio. No matter what you put into them their ratio is the the final. You need to determine what the axles ratio's are first then you can determine what transmission and trans case ratio you need to get the desired out put.

Panzerpaul

PG: Thanks for pointing out the axel ratio also counts. Mine is 2.75 turns on the drive shaft to move the wheels one full rotation. I will factor this in. I checked RC4WD.com but neither transfer case seems to work. So I'm still looking at building one from scratch. PP

Panther G

Well maybe as I said you could find a similar axle to what you have with the through put. input shaft and out put shaft. that way you can run a conventional transfer case and be done with it. Ihave a similar set up for the 8x8 I am building. i used the TLT axles. 2 steering axles and 2 straight.
I'm not sure what brand axle you are using but they look common and maybe the TLT's would have a close enough end to end dimension that you could use them. If you want i can measure mine an give the dimension so you can compare yours. i still think this will be the most practical and easiest way to go.

Panzerpaul

Hi: Could you provide the link to TLT axels. Thanks

Panther G

Sure, you can very easily find them on Ebay also. that is where i got mine and still get notifications when they show up on the bAY FOR SALE.

Panther G

http://www.ebay.com/sch/i.html?_trks...at=0&_from=R40
These are just some. they do have the center drive axle with shafts in and out you just need to look for them. It will make thins so much simpler for your project and you may even be able to just substitute your center axle for an TLT with out needing to change all 3 axles.
What I have done too is just use 2 of the shaft side axles housings to make a center axle. It works in either direction just need and extra input shaft which are easy to get too.

Panzerpaul

Thanks PG. RC4WD is where I got the axels i used .... They are 1/18 scale. PP