VEX ARM: weight issue

So a while ago I said that was going to start building an arm out of VEX parts. Well its halfway done and there are very few issues.

My arm is too heavy

So I tried to cut metal and I’ve cut off all of the excess so far. Right now I have one motor for the Y-axis of the shoulder. I’ve also added a small counter weight in the back. However it is still to heavy. Any suggestions?


all aluminium except for mounting plates and bicep
has 3 turntables
5-6 motors
only one is geared for high torque (shoulder)
wires are threaded through the middle of turntables

A picture could help get us all on the right reference frame. Them we could giving ideas and pointers.

  1. Rubber bands to act as springs to help the arm motor not have to work as hard. Limits some range of motion. Kind of like a counter weight but a spring force over a (nearly) static force of weight.
  2. Lower gear ratios to make the motors carry more torque to the arm segment. If you direct drive it, you have a poor little motor carrying a lot of torque needs of the rest of the arm. A motor only has about 7 in-lb of force before it starts straining and getting in to cautionary areas. But put in a 5:1 ratio and now you may be in business! The turntable I think is a 5:1 ratio already. Put rubber bands on it and you could be alleviating the load more.
  3. Multiple motors per joint working together. On that first joint you may want 2 or 4 motors helping you.
  4. Lots of weight on the base to prevent tipping. But that is a different problem…

Like @tabor473 said, pictures are a big help…

Here are some pictures of it

I would put some motors in the second turntable hole on the first two joints at a minimum.

Make sure they are geared for strength on the inside for maximum torque.

For counterweights, steel plate stacked together with screws in as many holes really adds up.

Maybe some rubber bands like biceps and triceps can help strengthen the arm too.

How exactly do I internally gear my motors for high torque? I know I have to replace some gears but which one and what does it look like? Also I don’t think I can use rubber bands.

If you haven’t done anything to your motors, they’re already geared internally for torque. 393 motors come with “high speed” internal gears on the side, and Vex sells “turbo” internal gears as an accessory, but the “torque” configuration is installed by default. If you’ve replaced the motors’ internal gears with the vex “high speed” or “turbo” replacement internal gears, you should swap these out for the default “torque” internal gears.

I noticed your arm did not have any sensors on it. You will need some eventually. IME on the back of the motor or a potentiometers for segments going less than 270 degrees would be needed.

You will be having relative position as of that joint which you then need to do some math to get back to the base’s frame of reference.

Path planning for the arm from point a to point b is another bit of complexity too but doing things simply can help reduce the complexity. One thing at a time. Get the arm working first.

Or they could control it via controller so it reduces the program but expands what you can do with it


Yeah thats what I’m going for. I don’t think i want sensors unless I have it do a complex autonomous demo.

Having that many degrees of freedom makes it a tough mechanism to control. You’ll see once you get the torque/weight issues settled down.