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Help me choose the right gearbox

by 9988776655
Tags: choose, gearbox
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Mech_Engineer
#19
Feb27-08, 05:38 PM
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Quote Quote by 9988776655 View Post
Im thinking of using the gearbox that I was originally looking at but replacing the motor with of the ones you suggested. The gearbox is meant for a 2mm shaft but the 18-turn motor is 3mm. I do not know the diameter of the 17-turn motor. Do these motors also run on 3V?
I don't think that gearbox will be a very good application for what you're trying to do. As said before, it looks to be more of an automation/robotics application for low speeds and forces.

While it's theoretically possible to run on 3V, why not just buy a single 6.4-7.2V battery pack? They're cheap, and specifically designed to work with the high current draw of this kind of motor.

Commonly the gear reduction used with these specific motors is a spur and pinion gear combination that drives a transaxle differential (combined transmission/differential), which in turn drives the wheels. Common pinion/spur ratios are around 1/3 to 1/4 (for example, a 78 tooth spur gear and a 24 tooth pinion gear, you can select different options to suit your driving style) and the transmission might be about 1/2.6, making the output speed to the wheels about 1/7 to 1/10 that of the motor's output speed. That setup (primarily off-road) would normally be used with 8.89 cm diameter drive tires, that are 6.1cm wide. On-road applications would be faster (less total gear reduction) with slightly smaller diameter tires.

Quote Quote by 9988776655 View Post
I thought to make the output shaft of the gearbox run the wheels.
But how? You're going to need some sort of transaxle at very least to make the single output into dual axle shafts...

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I dont really need a speed controller. Im happy with max speed all time.
Basic speed contrtollers are cheap, and these motors cannot be run without some sort of speed control. It is the throttle for the motor, so you can slow down in turns, and accelerate on straight aways. You have to remember it's going to have a pretty fast top speed and running with an on/off switch to the battery isn't really an option.

Quote Quote by 9988776655 View Post
I worked out that the small components+wood+batteries+gearbox is 527g. If we change the motor it could be a bit heavier.
My guess without any calculation at all is you should aim for about double that, at least.
9988776655
#20
Feb27-08, 06:54 PM
P: 22
I thought about this differential:
http://www3.towerhobbies.com/cgi-bin...P=7#otheritems

I also found these preassembled versions:
http://www3.towerhobbies.com/cgi-bin...?&I=LXSXA9&P=7
http://www3.towerhobbies.com/cgi-bin...?&I=LXLXX4&P=7

But I dont know what pinion gear to get to fit between the motor and the differention and I dont have housing for the differential. Why dont they include a pinion gear and housing set?
Mech_Engineer
#21
Feb27-08, 11:28 PM
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You're on the right track. Some R/C cars or trucks will have a transaxle that essentially houses the transmission and differential together, while other ones have separate differentials and transmissions.

It might be worth looking into parts for a Traxxas E-Maxx or T-Maxx, they have separate transmissions and differentials, and transmissions are available in single or dual speeds for the electric version of the truck.
9988776655
#22
Feb28-08, 02:29 AM
P: 22
The traxxas differentials have many parts to put together.
What do you think of this combination instead:

http://www3.towerhobbies.com/cgi-bin...?&I=LXSXA9&P=7
http://www3.towerhobbies.com/cgi-bin...?&I=LXSWW1&P=Z
http://www3.towerhobbies.com/cgi-bin...?&I=LXAMH5&P=Z

Do I need any other components? I can buy steel rods and a battery from another place cheaper and I have wheels already as well.
Mech_Engineer
#23
Feb28-08, 09:38 AM
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I hate to tell you now, but if you're looking to build a functioning R/C car, being afraid of putting a lot of parts together is probably not a good thing...

The motor you linked might be a little on the big side, but would still work (it's made for a monster truck it looks like). The differential "assembly" is the gear, bearings, and output shaft, but you will still need the entire transmission assembly and plastic housing that all of that goes in. you can't just drive the 'differential" directly with the motor, you need an interface between the two.

I would suggest looking up the exploded drawings of some stadium trucks or monster trucks such from companies such as Team Associated, Losi, or Traxxas (Traxxas has very easy to read drawings for our purposes). They are available free online, and will show you how the parts go together. For the Traxxas E-Maxx for example:

http://www.traxxas.com/products/elec...ews.htm#manual

You can see that it has a motor mount/transmission assembly that then has an output driveshaft that goes to the front and rear differentials (4WD). While it has the option for 4wd and two motors, it would be pretty easy to use just one motor with the transfer case, and then a single differential in the rear to put power to the ground. The nice thing about the Traxxas differential is it has it's own case, and can then be mounted where as the differentials you are finding go inside the case of the transmission/transaxle.

For an integrated transmission transaxle, you can see how it goes together on something like the Traxxas Rustler:

http://www.traxxas.com/products/elec...ews.htm#manual

Looking at the Rustler's Transmission assembly, you can see that the differential, motor mount, and "trasmission" are all combined into a single unit. So, if you bought a used (or new) complete Rustler transmission assembly, you would have everything you needed to put power to the ground on a rear wheel drive vehicle. All you need in addition to that would be a motor (and spur gear) and speed controller, and your radio receiver equipment. Then you would have to just try and mimic the truck's body mounts on the back that the transmission assembly mounts to.
Mech_Engineer
#24
Feb28-08, 09:53 AM
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In addition to the off-road trucks I linked, which are designed with long axle shafts and big suspension travel in mind, Traxxas also makes some street vehicles, such as:

http://www.traxxas.com/products/elec...ews.htm#manual

You can see that the Street Sport is a sort of a NASCAR model that has a transmission very similar to the Statium truck, just smaller suspension travel. The street truck they make:

http://www.traxxas.com/products/elec...ews.htm#manual

The Spirit actually has NO suspension travel in the back because the entire transmission assembly pivots up and down, the wheels mount to the transmission directly. That might be one of the easiest integrations just because you don't necessarily need any shocks or suspension components.


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