Newcomer In Need of Help: Gears


#1

Hi everyone. I am a part of a team tasked to build a drivetrain with a required amount of two 393 motors. The task is to build a ‘dragster’ at max speed. For our gear ratio we initially tried a compound gear ratio of 84:36. However, the gears were not meshing properly. With limited time, we changed our gear ratio to 84:36 (regular gear ratio). However we are still facing some problems. The gears aren’t meshing properly and I believe it’s due to too much weight on the axles. I have a few questions regarding this matter:

  1. Where do you put the shaft collars on a motorized and non motorized axle? Also how many shaft collars?
  2. How do you adjust the weight on an axle?
  3. How do you ensure that the gears are meshing properly? Also, how do you make sure that the gears are not wobbly?
  4. What is the best gear ratio for speed? Are any other elements such as chain advised?
  5. If the wheels are leaning when its on the axle, how do you fix that?
  6. What are some tips in making the robot faster in regards of RobotC? (Programming)

#2

A picture of your 7:3 gearbox would be nice for suggestions. Make sure you’re placing your 84 gear right above your 3 gear - the spacing should be perfect if you have 4 spaces in between axles.

For one, weight on axles isn’t the problem with meshing. It sound like you need to use bearings more effectively - put one on both sides of every axle , and make sure to tighten the extra screws.

  1. on a nonmotorized and motorized gearbox, it’s always a safe bet to put the collar lock right next to the gear or wheel - just make sure it’s not touching any metal on the other side, that increases friction. Make sure to use nylon spacing. 1 collar lock per axle, 2 tops.
  2. you shouldn’t have to. Gears meshing problems are usually a problem of placement of the gears or build quality issues
  3. add spacing to both sides (washers and nylons spacers, make sure to not pack them too tightly, but use enough to constrain the motion of the gear), and use bearings.
  4. gears are better than chain usually due to less slop - chain is only used when the motions must be connected far apart. For a speedy bot with no torque applications, a 2 torque motor 7:3 sounds perfectly fine.
  5. Either the inserts are broken, you aren’t using spacers, you aren’t using bearings, or your bearings need to be tightened.
  6. Run your motors at the max speed of 127.

#3

@henryl, thank you so much for the pointers! Tomorrow I will definitely take a picture and post it. Regarding your answer to number 4, why is it better for the motors to be modded at torque when the task is to go at max speed? Also, is 84:12 ratio advisable? We have not tested it yet as some people have advised that may take a long start up time. Finally, do you think a compound gear ratio is better for speed? And will traction wheels help?


#4

84:12 is not advisable. 7x160 = 1120 rpm and you’re at roller speed there. the absolute highest I would recommend you to go is 300 rpm (torque ~120 rpm, 120/3*7 = 280), especially with only 2 393motors. The reason it takes long to start up is you are trading extremely high cap speed for extremely long acceleration, which in your case if you are racing is not a good idea.

More gears cause more friction. Compound gear ratios are only to be used in extremely high torque (lifting the entire bot) or high speed( flywheel) scenarios, neither of which you are doing.

Traction wheels are useful if you are doing high-speed turns, but in that case you want to make your drive base all traction and as square as possible(wheels placed equal distances from each other).

If you are only powering two wheels, making only the powered ones traction is likely the way to go.


#5

Thank you very much. What is rpm? How do you calculate it? What rpm is recommended for both torque and speed?


#6

RPM means revolutions per minute- the number of full rotations a shaft makes in a minute.


#7

Thank you! For VEX V5 Programming Studio, is RPM suggested rather than percent? What is the max amount of RPM?


#8
  1. Where do you put the shaft collars on a motorized and non motorized axle? Also how many shaft collars?

Shaft collars are used to pin a shaft, that is to stop it from sliding back and forth. You want to lock down 5 of the 6 degrees of freedom (XYZ translation and XYZ rotation). We want a shaft to rotate in one and only one direction. Use shaft collars to accomplish this. Using spacers this can be one collar but teams usually use two, one at either end.

  1. How do you adjust the weight on an axle?

Don’t worry about the weight of an axle.

  1. How do you ensure that the gears are meshing properly? Also, how do you make sure that the gears are not wobbly?

The VEX EDR system is bsed on a 0.5" (half inch) spacing. If gears are used the appropriate way they will mesh correctly with an engineered amount of play. (the term “play” is a technical term meaning an amount of give).

  1. What is the best gear ratio for speed? Are any other elements such as chain advised?

This will depend on wheel size, motor type, motor internal gearing, etc. Experiment then start using gear ratio calculators.

  1. If the wheels are leaning when its on the axle, how do you fix that?

You must have a cantilevered wheel, try to brace the axle on both sides of the wheel or in two places on one side of the wheel. Post a picture of your bot here so we can comment.

  1. What are some tips in making the robot faster in regards of RobotC? (Programming)

Faster? If you’re commanding full speed from the motor then that’s all you’re going to get.


#9

image


#10

For some reason, we are now facing the problem of it going slanted. FYI: Since we don’t have enough shaft collars, our teacher told us to use tape.


#11

Wow that’s bad.

Those rails are really old VEX parts and kind of suck. Furthermore you should really have the shafts going through two structural points on either side of the wheel or two on one side. Look at the chassis kits.

10%20AM


#12

This is the best answer. Those rails are awful, and you need two points of support on every axle ALWAYS. Plus, you don’t have to worry about so many collars if you just use one and then have the axle go through two chassis bars.


#13

Reminds me of this bad boy
IMG_20181121_212420 JPEG_20181118_172119


#14

@AperatureLabs @anon35188833 Thank you very much. We will take this into account. Are there any other general building tips that we can use?


#15

Dont cantilever your axles and try to only have plastic on plastic. Also make sure that you dont have motor side bearings unless it is a very high stress joint.


#16

What does cantilever mean?


#17

have your shaft going through 2 points of support rather than just one. basically I advise following the images @anon35188833 posted as closely as possible.


#18

Google the word…


#19

Got it. Thank you everyone for the helpful pointers!