# 4-bar Linkage problem - Wont lift

I have a problem with my 4-bar linkage. It wont lift up. It lifts up once, then after it acts as if its tired and wont go up. What can I fix?

First of all, collar that 1 unit gear, its never good to have loose gears as they cause friction. Second of all you might need more torque, as that is often a symptom of lack of torque or friction problems. From what I could see you only have a 1:3 gear ratio running off of either 1 or 2 369 motors. Considering you have a pretty long 4 bar link, I would suggest increasing the torque to maybe 1:6 drive to driven if you can. BTW in case you don’t know, to calculate torque you take the gear ratios. For example you have a 1 gear that is connected to a 3 gear. That is equal to a 1:3 gear ratio. I would also suggest just going over all the mechanisms and making sure there is no friction, excess metal, or any pieces that are inhibiting movement. Also make sure your batteries are fully charged

Thanks for your help, I will try to raise the torque. It’s running on 1 393 motor. But my batteries are fully charged. It’s like it needs more torque for sure. I’m pretty new to robotics how do I know the gear is a “6” gear or a “3” gear?

Count the number of teeth on the gears, and simplify. So for example, you could have 72 teeth on one gear and 12 teeth on the other, and if you meshed them together would have a 1:6 gear ratio as described by Thorondor above.

It looks like 12 driving 60, or a ratio of 5:1 torque increasing ratio.

“Works once, then acts tired”; sounds like PTC current limiting.
You can get a 10Amp DVM from Harbor Freight for \$4 on sale sometimes; hook up a few wires to put it in parallel with power to motor, or power from battery and see how much current is flowing.
Assuming thats the problem, you need to reduce the load.

Add some rubber bands between the top back bar to the bottom front bar,
and that can help a lot to neutralize the deadweight of the arm.
There are lots of other threads on this topic you can find through the search box.

If you can afford longer pieces to make a simpler arm, it will also be lighter.
The arm looks overbuilt: using one piece from 35 length metal will help, especially if its aluminum.

The tower looks overly complex also: two pieces of 5 wide C channel often work well.
The flat plates you have at the top of the tower could flex side to side and bind on the axles. Hinges can be strong and precise fitting pivots for the 3 unpowered arm joints.

The pictures show nuts next to the gears. Its easy for the gear to slip sideways and catch on the bolts. Make sure you have spacers to prevent the moving parts from contacting the sides.

Good work on non-cantilevered wheels, although I usually like to put my wheel frame rails smooth side out to make it less likely to catch on field edges, game pieces, etc.

seems there is too much stress on the motor
you can either
a) use elastics to relieve some of the stress
c) gear it down so you have more torque

I’m trying rubberbands, can I put another 393 motor on the other side or even a 269? I’m trying not to gear it down because that means take it all apart again.

I would definitely add an additional motor for starters, either 393 or 269 will help your arm (obviously the 393 will help more, 13.5 in-lbs vs. 8.6 in-lbs of output torque when compared to the 269 motor.)

It also looks like you have no elastic assistance helping the arm raise up. You can quickly add some by adding standoffs and by running rubber bands or surgical tubing from the top left corner of the 4-bar linkage down to the bottom right corner (example). Do this on each side of the arm, I’d suggest on the inside of your arm due to flat faces of the metal being on the inside - it will make running the rubber bands & tubing easier & will look cleaner.

You should ideally have the driving shaft not engaged with the motor when you do this, so you find the true ‘sweet spot’ without the motor being involved. Once you do this, plug the shaft back into the motor and give it a test. Let us know how it goes!

You also have to make sure those bars arpleyely parallel or it will go up and bind.

I realised that gearing up was the way to go, adding a second motor would have been harder because there wouldnt be a place to put it without interference. I took the idea of “compound gearing” with the help of my robotics coach and we got it up. It has so much torque that it twisted a drive shaft…

Bending drive shafts indicates too much load, try adding elastic to add mechanical assist as mentioned above.

You need a lower gear ratio, add a second stage of reduction.

You can also avoid twisting VEX shafts by bolting your arm directly to the larger gears and sprockets. This will allow the torque to be transferred through the bolts and/or standoffs instead of just the shaft.

For example in the photo below, the arm on the left will have a much more durable connection than the arm on the right.