# what is the best gear ratio?

what is the best gear ratio to lift an arm? like the robotic version of the human arm. its got a shoulder, elbow and hands. ive seen this done on youtube but i dont know which gears to put on. basically i want the most strength.

If you want strength you are going to have to give up speed. There is no such thing as “the best gear ratio” it all depends on what you are doing with it, how much weight you are trying to lift and at what distance from point of rotation, how many motors you want to drive it with and what type of motors you want to use.

If you want strength you are going to put a small gear on the motor shaft and a larger on attached to the arm. You may even want to do a compound gear ratio…

~DK

to sum it up,
faster = less strength
more strength = slower
more strength AND faster = more motors

typical robot arms that lifts their manipulator and game objects mostly use the 1:5 or 1:7 ratio depending on their design

We used a 1 : 3 ratio on a high strength motor for the lifting motion on the shoulder, and a 1 : 1 for all of the other motions with 2-wire motors. It wasn’t strong at all. We used all aluminium pieces, and it lifted it’s own weight, but not much more than that.

I’m not sure what the standard is for gear ratios, but for torque, you want the smaller gear on the motor geared to a larger gear on the axle of the arm’s rotation.

1:1 is definitely not an ideal ratio for a lift. Your exact ideal reduction will depend on your application, but experimentation is very easy in VEX.

• Sunny G.

Just remember, don’t exceed a 1:3 gear ratio on any one set. If you want to get more power than a 1:3 can provide, you need to have a compound gear ratio. For example: two 1:3 together make a 1:9, and three make 1:27. So your never really limited on your power you just have to sacrifice speed.

Why not? I’ve rarely had a problem with 1:5 and 1:7 reductions.

Currently my teams base is a 5:7 (7 on motor 5 on wheel). We feel that its fast, and has lots of torque.

What would happen if we added another gear to this ratio and this gear is attached to our wheel? So the gears would be 84 tooth on motor, then 60 tooth ,and after that a 36 tooth attached to the wheel.

It would be the same as if you simply removed the middle 60-tooth gear. The ratio would be the same. In a long chain of gears, no matter what is in between, the gear ratio will always be the difference in teeth/circumference of the gear on the motor and the gear on the wheel. So, in your case, the ratio would simply be geared up 3:7.

~Jordan

## It increases the chances of the gears slipping. You can do it but I do not recommend it. Last year Sheila had a 1:9 ratio on the lift (two 1:3s). I’m not exactly sure why it’s just our experience.

[Delta]

Gear slip is a function of load, not reduction.

High strength gears and metal pinions are both great ways to prevent gear failure.

## Yes I agree and we’ve done that, but based on our experience, it is better to go 1:3:9 then to go straight to 1:9. It will slip less.

[Delta]

We have used direct gearing to achieve a 2-second high hang in Round-Up, so I would say it’s more matter of how you support the gears and axles than the gearing strategy.

Looking at the 1:9 for torque example with 2 393s and a total stall torque of about 25 in-lbs:
Going directly puts 25in-lbs/(radius of pinion) of force on the teeth.
Going compound puts 3 times that force, so it would seem to be more inclined to slip.
There are naturally other factors involved, but just looking at the forces on the teeth, directly gearing will slip less that gearing “compoundly”.

254D: Looking at the 1:9 for torque example …
How do you ever get 1:9 direct with Vex? (12:84 is only 1:7) Did you cut your own gears?

My usual old problem at high torque was breaking gear teeth off the 12t original strength pinion driving the output gear.
The old solution to pinion tooth breakage was to use 12t only on the motor shaft, and 36t as output pinion.
New solution is 12t metal pinions anywhere.

We used to use a motor drive shaft with several 12T gears held in place by lock collars. Two would be driving and the other 2-3 (depending on how many would fit) would be in-place spares. When one would break, the students would loosen the collars to slide the broken one out of the gear train and slide an unbroken one in. Lunch time was for removing the shaft, slipping off the broken gears, and putting new ones on. O for the good old days!

Well I made a nice long post explaining everything more in-depth, but naturally the page timed out
The “9” gear was just a hypothetical example based on Delta’s post.

Cutting the sides of the plastic 12-tooth gears off and sandwiching two together (just like how teams effectively make 84-tooth high-strength gears) will increase the strength significantly. Even if you’re only powering one gear off of your doubled gear, centering it will still make it stronger.

Aside from there being no way to have a 1:9 with a single reduction in vex (to my knowledge) when you construct a compound gear ratio as you described you are obviously incorporating more gears into the system which means more friction…
Can you explain what exactly you mean by “slip”?

~DK

nice trick!
i wish i had known that when i built my very first robot
it almost shredded our whole 12t gear stash!

Actually, it would be 1/3 the force applied laterally on the gear teeth, not 3 times. Plus, plus this would increase your perpendicular force by 16.6666%. Those two facts alone would decrease the chance of slipping (the gears losing contact with each other due to the excessive torque load). And yes, a 1:9 gearing was hypothetical.

Yeah, what he said.

That said, I just have never broken a pinion. Maybe I’m lucky or something.