Arm gear help

Aloha VEX forum users!
Our team 2443 is asking help with are arm for Round Up.
Currently we have 1 HS motor on each side of our arm, which makes 2 HS motors total. We never changed the gearing of the HS motors yet so they are not on speed setting.

Currently we have our arm geared at 36-60, but find that it is to fast at full power so we changed it since it would hit the ground with alot of power. After changing the power of the arm to half power it is way slower, but it doesen’t lift all the way.
What is the problem with that?

Our team was thinking of adding a regular motor on each side of the arm. While talking about it we are not sure how to gear it.
First idea was justing adding another 36 tooth gear. 36:36:60
Second idea was adding a 12 tooth. which would make the gearing 12:36:60

Could you please help us understand gearing arms, and what the results may be. WE sre so sorry about asking,but we need help and we are running out of time, we need to be ready by friday morning.

So what may the possible results from the first idea?
I am thinking that it would move the same speed, but have more strength because of the additional regular motor on each side of the arm .

Also what would be the possible results from the second idea?
I am not sure how it would turn out, but I am guessing that it will also have more strength since of the additional motor on each side also.

Lastly we are using alot of rubberbands. Our goal is to have the arm be able to not slam into the ground, and be able to go strong/fast enough to stay up with its own weight.
Thank you so much.
From team 2443 Blue Thunder. Maui High

Really quickly - Before I head to work.

Use a Vex Potentiometer (or limit switches) to keep track (in your software) of the arm’s location and slow down the motors (change the commands you send to them) when the arm is near the limits of its travel (all the way down or up, or any spot you want to it to stop in between).

To have more than one motor drive an axle, consider using chain.

To have more than one motor drive an axle, consider putting up to 4 motors around the axle: 1 above, 1 below, 1 on each side - This allows all the motors to turn in the same direction which helps them respond identically to your commands (it helps a little mechanically, and it simplifies the software). Do this instead of putting them on opposite ends of the axle.

To have more than one motor drive an axle, definitely don’t use a trio (or more) of different size gears unless some of the gears are just idler’s that connect identical motor output gears to the axle. If the motor output gears are different you will probably dislike the result. Trying to get them all to turn at the correct (different) speeds simultaneously will almost certainly not turn out well.

To have more than one motor drive an axle, consider using rack and pinion gears to push on the arm instead of using spur gears to twist the arm.

To counterbalance an arm consider using a weight on a lever arm that is connect to the pivot point on the “working” arm. a small weight on a long arm, connected to the working arm by latex, by chain, or by spur gears can counterbalance a lot of the torque from the “working” arm. The point of putting the counterweight on an arm of its own is getting more torque than if you attached the same amount of weight to the little stub that usually sticks out of the back of a “working” arm.

OR… reduce the torque on that axle and avoid needing more motors. In the world of Vex, two HS motors are pretty powerful. There has to be some way to reduce the torque and dead weight loads you are putting on them… Use fewer or smaller screws, use lighter pieces, move parts of mechanisms from the end of the arm to a spot closer to the pivot point and only use lightweight linkages to transfer their motions out to the tip of the arm. Maybe switch to using a 4-bar linkage if you aren’t already. Etc.

Scour through the Photo Gallery to find pictures of how other folks have done it in the past… Searching is your friend.


Hm…When you say you changed the “power” of the arm, did you guys change the gear ratio or actually changed the value for the motor speed in the programming? If you changed the ratio, then the next two paragraphs pertain to your issue. If you changed it in the code, then keeping fiddling with the power setting until you get one that you like.

Um…from what I read, it looks like you have a 36 tooth gear on the motor, and then a 60 tooth gear on the arm? Is this correct? If so, then the 36:60 gearing is for torque. To increase torque, and consequently slow down your arm, you are going to want to increase that ratio you have (3:5). Compound gear it again, another 36-60 reduction, and you’ll get a slower arm speed and have more torque on the arm.

If you have your gear ratios backwards, then what you might be noticing as “slower” is actually the motors straining to raise the arm, and when they reach their max, they don’t lift anymore.

And to answer your question about your ideas. Do you want to add the two regular motors with conjunction to the HS motors, or just two regular strength motors?

  • Sunny

I’m not sure what you mean by “just adding” a gear. I’m assuming you mean that you have 2 gears in a row and will add a 3rd gear to the end of that row. If this is the case, only the 2 gears on the ends will have any effect on the speed.

A gear ratio is determined by making a fraction with drive/driven. So if your drive gear (connected to the motor) has 36 teeth and your driven gear has 60 teeth, your ratio is 36:60, or 3/5. That means you have 3/5 the speed but 5/3 the torque (strength)

If all 3 gears are in the same row, only the 2 end gears will affect your ratio. For example, 3 gears in a row 36-36-60 will create 2 fractions, which are multiplied:
(36/36)(36/60) = (1)(3/5) = 3/5, the same you had before.

Even if the 3 gears are different sizes, only the 2 on the ends matter. For example 12-36-60 (small, med, large) creates the ratio:
(12/36)(36/60) = (1/3)(3/5) = 1/5 (for the purposes of the gear ratio, it’s as if the middle gear didn’t exist)

To slow it down, you need a smaller gear (say a 12 tooth gear) on the drive end and/or a bigger gear on the driven end (say a 84 tooth gear).
A 12-84 connection makes the ratio (12/84) = 1/7

The other way to further reduce the gear ratio (slower & stronger) is to “ladder” the gearing using 2 stages. In the attached photo, the 12-tooth gear drives the 36-tooth gear. The axle with the 36-tooth gear also turns a 12-tooth gear (on the same axle), which drives the 60-tooth gear (the second stage).

The ratio here is:
(12/36)(12/60)= (1/3)(1/5) = 1/15 (15 times slower and stronger)

There are more detailed explanations on gearing and other basic concepts in

Vex for the Technically Challenged (a theory-based guide)
Vex Machinations (a collection of building projects)

Hope it works out in time!

P.S. These links will only be good until mid-December, when the college where I work migrates all of its websites. I’ll provide new & improved links when I have them.
2-stage gearing.JPG

I am so sorry for my bad explanation! :frowning: I will try again.
Currently we have a four bar linkage.
We are using 2 HS motors one for each of the arms (2 arms 1 HS for each 2 bar linkage) The first 36 gear teeth gear has the HS motor then it connects to the 60:36:60.
We have our arm geared at 36:60:36:60. Each bar of the four bar linkage is connected to a 60 gear teeth gear.

Thanks for telling us to use a pontentionmeter! But the ones we have make the shaft not move smoothly like in the shaft encoder. Is that normal, or is that bad?

I was planning to y connect the HS with the regular motor so yes they will be on the same side.

Lastly we are thinking of adding 2 regular motors. So over all 2 HS motors and 2 regular motors for the arm.
So hopefully what would you think the results to the ideas I had in the other post?
I hope this clears up any questions.

Ah, much better.

I would recommend going another stage of reduction. ManicMechanic posted a picture of compound gearing. That might really help. You might also want to try to add rubber bands in key places so that that will help.

Since HS motors are considered to have 2x torque at the same speed as a regular motor, technically you’re using 4 regular motors on that arm, and adding another pair of regular motors just isn’t worth it.

The potentiometer does have some added friction, but this is perfectly normal, and is by design, as that is how it senses where the shaft is turned to, using friction.

I am not sure weather you are using the Cortex Microcontroller or not, but I am assuming you are. If so, using a Y-cable to connect two motors really has no advantages this year, as the Cortex has 10 motor ports (Maximum amount of motors allowed in VRC.), 8 3-wire ports, and 2 2-wire ports.

2 high strength 393 motors is a lot of power, if you think about it. 2 high strength 393’s are equivalent to 4 3-wire “regular” motors, which is probably the most you would ever need on an arm, realistically, so I don’t think you would need to add 2 3-wire “regular” motors as well. (Equivalent to 6 3-wire “regular” motors.) Instead, if you need more power, try gearing down the arm a little more, a 5:3 (or 3:5, depending on who you are) geared down for torque is still pretty fast for an arm, and you might think about gearing it down a little more than that just for greater control, (rather than slowing the arm down in code) so that you also get some “free” added torque.


You should be able to lift a relatively light load (basically, an arm that’s not picking up your entire robot) with one, maybe two regular motors. If you don’t want them in your drivetrain, a single high strength motor should definitely do it.

If the arm moves too fast, increasing the reduction (my gut says 12:60 36:60 should do it) not only slows it down, but increases the amount of torque in the arm so it lifts more easily. It’s a win-win.

You shouldn’t ever need to slow your arm’s motor down in software for normal use (other than ramp up / down at the beginning and end of travel); if you find it’s running too fast there’s nothing to lose from increasing your gear reduction.

A potentiometer that isn’t broken should resist turning somewhat, but that resistance should not be erratic/jerky and it should be small enough for the Vex motors (old or new) to very easily overcome it.

Good luck - Maybe post some pictures?


A simple rule of thumb is that a 1:5 (12:60) arm is fast, and a 1:7 (12:84) arm is pretty fast. A 3:5 (36:60) is really, really fast, and usually not controllable. In the experience of our club, you end up choosing between 1:5 and 1:7, depending on the load. Our robot that lifted itself using an arm in Hangin’ Around was more like 1:twentysomething.

Thank you VEX users for all your help. We probably fixed our problem with our four bar linkage.
Although it may seem a waste as I said earliar we decided to add 2 regular motors to the 2 HS motors we have on the linkage. Learning from the gear ratio we had earliar we didnt want to pay the price. So today while at school (during class we had a sub :P) So we went with this gear ratio.
As you can see we used 2 small gears the reason is because if we only had one the 2 motors would be touching each other which is bad.
Although we never tested it yet :frowning: it seems prettly fast, and strong. The reason why I am saying this because the arm can hold up the load its carrying.

a future change we will probably do is the gearing on our arm so it only takes up 2 hs motors.

But this probably will be for the World championships since we qaulified at mckinley, and our builders dont want to change it yet.
The main reason is because we are having a scrimmage with other maui teams tommorow.

Sure I will post pictures as soon as possible. Possibly right before the maui competition or right after it. :smiley:

wait so which gears are your motors on?
i supposed the 36’s but what would be the point of the 12’s?

and ask rick tyler said, 1:5-1:7 is probably the “controllable” ratios
my team generally goes with the 1:5 and add more motors to it if there is more load
and we practice driving a lot so the speed isnt an issue