

The robots are just vehicles taking kids on a ride to STEMville.  FTC5110 [more] 




Thread Tools  Rate Thread  Display Modes 

#1




Help with gear ratios
Hi, guys, I'm sorry to seem like a rookie but my friends and I are having problems understanding gearboxes. From our understanding, the first stage of most drive gearboxes (the tough box) includes two motors each with their own
gear attached to it. Both gears act on the same middle gear. We tried treating it as a planetary gear system without a ring gear but we just got more confused. How do we calculate the ratio of a gearbox with two 10 teeth "planets" that are spinning at the same speed in the same direction as a 40 tooth "sun" gear? Is it a 4:1 ratio (40 for driven/10 for the drive) or 2:1 (40 for driven/adding both 10 tooth gears up.) I also read that no matter how many motors are added, the speed of the output won't change but the torque will increase. For example, 2 motors mean twice the torque 3 means thrice and so on. I don't really understand how this works, shouldn't adding more motors increase the speed of the output as well? Thank you so much 
#2




Re: Help with gear ratios
To calculate this ratio you take the # of teeth on the gears on the motor and then divide that by the # of teeth on the gear that you a driving with the motor and small gear so in your case this 40 tooth gear. This should give you a number of .25 which means you have a gear ration of 4:1. This means that for every time the gear on the motor turns once the large gear will make a 1/4 of a turn. This means that the large gear is spinning 4 times slower than the motor speed.
Edit: This video will probably do a better job explaining it than I can. https://www.youtube.com/watch?v=49DxlXs8tyk Last edited by FrisbeeFunTime : 06182017 at 06:36 AM. 
#3




Re: Help with gear ratios
Theoretically adding motors will do nothing to increase the output speed of the system. The only things that will do this is changing the type of motor and changing the gearing.

#4




Re: Help with gear ratios
Think about it this way:
You have the two motors going at, say 400 RPM. That means each of those little gears is going to turn at 400 RPM. If we have 1:1 ratio, that means the middle gear is going to go at 400 RPM as well. There's no way for the middle gear to "outturn" the outer gears as they are the ones pushing it. However, with two motors, you have two things pushing the middle gear instead of one  hence double the torque. 
#5




Re: Help with gear ratios
First off, most gearboxes used for drivetrain applications that you'll encounter in FRC won't be planetary gear systems. Please don't treat them this way. The only common ones that are would be the VersaPlanetary from VexPro and the Banebots Planetaries from Andymark. As you can see, their names literally note the fact that they're planetary gearboxes.
To better understand motors and gear ratios, you can look at this resource here: [1] It may be from VEX EDR, but the same principles and concepts transfer over to FRC. Take some time to explore beyond that unit to learn some other fundamental topics commonly used in robotics as well. Once you understand the underlying concepts behind gear ratios and gearing, you can use the JVN Design Calculator to make better educated decisions on how you gear your robot. For example, the Toughbox Mini used in the Kit of Parts drivetrain is a two stage, single speed gearbox. Stage means the number of reductions in the gearbox to get the desired output. The way these work is much more straightforward than a planetary gearbox. Assuming that you're using a standard setup for the KoP drivetrain (6" HiGrip wheels, 2 CIM motors per gearbox) and a max weight robot (154 lbs which is a 120lb robot + 10lbs of bumpers + 14lb battery), you get something that looks like this: [2] Note that all the salmon colored cells are spots that are customizable. When designing, you can experiment and plug in different gear ratios, motors, wheel sizes, and more to see the resulting speed and current draw. On the left there's 2 stages of reduction: 14:50 and then 16:48 which has an overall gear ratio of 10.71:1. A ratio of 10.71:1 using 6" wheels results a free speed of 13.02ft/s and an adjusted speed (assuming a speed loss of 81%) of 10.55ft/s, which will be approximately the speed your drivetrain will go in the real world. Note that you have a pushing current draw per motor of [EDIT: 71.14 amps], which is pretty safe. [EDIT: It's best to make sure that your PDP breakers can tolerate your current draw, see post below for details] Current draw is distributed among your motors, so having more than one for your gearboxes is usually pretty safe. Stay with two CIMs per gearbox (total of 4 for your drivetrain) since having three CIMs (with a total of 6 for your drivetrain) can cause your roboRIO to brownout at higher speeds. A common rookie mistake that I want to point out with gearing is that they'll select very large diameter wheels in order to go "fast" (usually 8" or more). However, its harder to turn larger wheels and it significantly increases the current draw on your motors when your drivetrain is stalled (sometimes they don't even move). Stick to smaller wheels and you'll experience much more success on the field. Hope this helps! Last edited by SPang : 06182017 at 12:32 PM. Reason: Cropping bad advice 
#6




Re: Help with gear ratios
Quote:
If you're gearing conservatively, you're going to want to stick below 40A per motor. If you're willing to push it, you can go up to ~50A since the breaker can technically hold 135% of rated current indefinitely (according to the datasheet). You'll still want to watch your total current draw for your main breaker and avoiding brownouts. 
#7




Re: Help with gear ratios
Actually if you were to accurately measure the output speed of a transmission with one motor and again with two of the same model motor you will see an increase in output shaft speed.
If you had a motor that had a true free speed of say 5000 rpm and then connected it to a 5 to 1 transmission you will not see the calculated output of 1000 rpm. Why? Friction. That friction is due to both the bearings and the gear to gear interaction. When you add a second motor you do add another friction point with it's gear to gear interface but you don't add any more friction to the bearings. (in fact proper placement of the second motor can reduce the friction on the driven gear shaft) That means that while the total friction in the gear box increases with the additional motor it does not double. So back to our 5000 rpm motor on a 5 to 1 transmission. Put one motor on it and the actual output rpm may be say 950 rpm instead of the calculated 1000 rpm. Put an additional motor on that transmission and the actual output speed will rise. No it still won't rise to that theoretical 1000 rpm but it could rise to say 970 rpm. 
#8




Re: Help with gear ratios
Quote:
* Ratchet wrenches sometimes have unintended, but useful consequences. Play the breaks when you get them. This is at best a second order effect, where the fraction of energy dissipated by friction is reduced a bit by greatly increasing the amount of energy on the drive shaft. Adding a second motor to a gearbox essentially doubles its torque (and acceleration, if the wheels don't slip on the carpet), but increases the top speed by a few percent at best. Last edited by GeeTwo : 06182017 at 10:53 PM. 
#9




Re: Help with gear ratios
Quote:
I just want to make sure I understand completely thank you so much. 
#10




Re: Help with gear ratios
Quote:
Basically, the equations are: ratio = driven/drive speed = input_speed/ratio torque = input_torque*ratio In this case, the input torque is really the input torque per motor times the number of motors (which is where you get the *2 from). 
#11




Re: Help with gear ratios
Quote:
I think I was getting confused by mixing torque and speed together but this really explained everything. If only there was a Chief Delphi equivalent of Reddit gold I could give to you. Thank you so much 
#12




Re: Help with gear ratios
Quote:

Thread Tools  
Display Modes  Rate This Thread 

