Hi forum people.
we have nationals this friday and are doing our usual thing of still building the robot at this stage. We have a double reverse four bar, but unfortunately when we tell it to go up and down the the gears on the the middle section start skipping. This only lets us go to about 1/3 of our potential height. Most of the lift is steel due to budget limits. I put all our aluminium on the upper bar (the upper bar is actually a 6 bar). Despite the weight problems we have counteracted that with rubber bands, so i don’t think it’s that.
Another problem is that the lift prefers going up on the left hand side for some reason which we can’t ascertain. The lift is completely laterally balanced in terms of build. I have heard that people can put lift stabilisation programs in, but i am only proficient with easyc and we don’t have enough 3-wire extenders to put any sensors on the middle section of our lift.
I’ll be able to post pictures and videos tomorrow.
We’d appreciate any tips anyone can give us 
Sorry i should clarify. we have a 1:15 ratio on our lift. Due to this compound system we weren’t able to mesh the two arms together, so they move independently. However we would like to be able to get round this problem as well maybe via programming. thanks again
Make sure that your gears are well supported on both sides and have bearing blocks. Also, you can try using the high strength shafts, if you have them, as they will flex less under load. In terms of it not going up straight, it could be due to one side having more friction than the other, so check all the joints. You can counteract this by adding or removing elastics on each side, but you should also add braces connecting the two sides.
It will be much easier to help with some pictures.
We have some high strength shafts but no bearing blocks that fit them. How do you mean well supported?
I will upload some pictures tomorrow
You don’t want to have an axle that is really long with the gears on it, as it will flex and lead to skipping. You want to have the metal with bearings on either side of the gears as close together as possible, so that there is as little space left on the shaft between the gears and metal.
Essentially, here is a diagram of what I mean:
G = Gear
---- = Shaft
| = Metal
Good:
| |
|-G-G-|
| |
Not Good:
| |
|----G-G----|
| |
| |
|-G-G-|
| |
Not Good:
| |
|----G-G----|
| |
[/CODE]
Ok i get it. all the axles on our middle section are 3 inches and supported at both ends
3 inches is a bit far. A piece of metal in middle can help too. Zip ties on spacers holding the distance firm is another solution.
To get the bottom and top synced, you can make what looks like a 6 bar mechanism from one of the lower gears to the upper arms.
Stick a bar on to the 60 or 84 tooth gear that is moving at the same speed as the bottom gears (but the opposite direction so the arm moves the direction you want). Have this bar make it’s own parallelogram linkage connected to the top bars - kind of like you would have done in a 6 bar.
Now they are in sync.
having both layers of the dr4b geared together is much better than separate.
there are a lot of forums giving great support to teams building dr4b. including many we have posted on. (we are also doing a dr4b lift) … and won state:D
https://vexforum.com/t/double-reverse-four-bar-help/28509/1&highlight=dr4b&page=4
and…
https://vexforum.com/t/dr4b-help/27643/1&highlight=dr4b
and…
https://vexforum.com/t/my-reverse-double-4-bar-is-tilting/28526/1&highlight=dr4b
The biggest things we have found is to put C - channel across both stages and to connect the middle stages with a shaft to sync up the rotation. After that, just keep adding elastics to reduce the weight the motors have to lift and cut motor power. We have 2 of our motors run at 90 and 2 at 127 to go up, and all at -15 to bring the lift down.
Well on the programming side of things, a quick solution to help keep the lift at certain point would be to have them running at ~25% - 30% all the time. It may drain your battery though but its a good option.
the lift seemed to sort itself out today. I added tonnes of elastics and tied some of the axles together with strong which is keeping the gears meshed. Now it is just leaning back. I saw a thread on that so i will check that out. It isn’t letting me post pictures.