Need Double Reverse 4-bar / 6-bar help

VEX V5 General Discussion Technical Discussion
#1

We have a double reverse 6-bar with 6 motors at 1:7 and we’re having issues with stalling, and we were wondering if anyone has any ideas on what could be the issue?

Photos
Photo (old photo without intakes, motors have been moved, etc.)

Render of back

Motors
Right now we have 4 motors in the back area with all the gears, and 2 motors at the uprights (all of them are 1:7).
The 4 motors at the back are on a power expander.

In this image the 4 axles pointed in red are powered by axles.

Axles
Every single axle in the system is a high-strength axle except for the ones connected to motors. Every axle is supported by bearings.

Weight
We have a skyrise claw and arm at the top (not sure how heavy it is… maybe 1-3 lbs?). We also have several crossbars but everything metal on the lift is aluminum.

Elastics
We have rubberbands spread out over 6 areas (one group for each paralellogram), about enough that it balances the weight when the lift is about 1/3rds to 1/2 way up. They become ineffective when it’s high up though and we think this might be part of the problem

The problem
When all the motors are at full power, the lift works fine the first time, then at the second and third times it starts stalling at about halfway up, then after that it doesn’t go up very far.

Things we’ve tried
We have

  • checked that all motors are working
  • checked that all motors are turning in the right direction
  • we have encoders and we also tried slaving them together and enabling pid control but I think this ended up making it slightly worse

(edit: when PID and slaving was enabled, one of the motors kept jerking back and forth (as if it was desperately trying to correct itself but changing it’s mind on which direction it should go))

Any advice or ideas would help a lot!
Thank you!

#2

When my team ran into this issue, we solved it by nearly doubling the number of rubber bands on our lift. This was enough so that when we pulled out our drive shafts the lift would shoot up on its own.

I’m not clear on the number of cubes you are trying to lift, but if you are trying to lift anything more than one cube (or skyrise), you’re going to struggle with a 1:7 gear ratio. I’ve seen most teams run a gear ratio of 1:15 or 1:9 (1:15 with motors internally geared for speed.) My team runs a gear ratio of 1:13.

#3

I disagree. A 1:7 reverse double 6 bar is very doable. Even with two cubes.

I think either your problem may be that there are simply not enough rubber bands, or that there is a friction problem somewhere in there.

Have you tried pulling the axles out of the motors and moving the lift by hand to check if it is smooth?

#4

Most likely you’ll need more elastics. Pull the axles out of the motors so you can move the lift freely. As Jarred suggested, check if it moves smoothly or if there is friction. Then add more elastics until the lift (possibly with a cube on top) can balance with just the force of the elastics. This way the motors strain the same on the way up as on the way down.

#5