So the mid section of my reverse double 4 bar seems to be tilting back 
Reinforcing it and rubber bands do not see to help with this. Any suggestions?
With scissor lifts or bar lifts that are not double reverse 4 bars this problem does not occur nearly as often.
Why?
In RD4B’s it’s a common practice to use axles in the middle section because of the gears there. The problem is that the axles have a lot more slack than screws do.
http://puu.sh/fQM8F/32b91bb766.jpg
As you can see in this image, this RD4B uses 2 inch screws, nylock nuts, washers/nylon spacers, and standoffs insead of axles. In the axles that have gears you can use circular high strength gear inserts or for low strength 84 tooth gears drill out a 8-32 screw diameter hole so that you can have a screw run through those as well. This will greatly reduce slack and increase performance.
Lastly another factor that may cause your rd4b to tilt back is the spacing between the bars of your RD4B. I recommend a space of 5 holes between the holes that your screw or axle runs through. Note that I am not talking about the space between the pair of 4 bars but the bars of one 4 bar linkage itself.
It’s really hard to fix the backwards tilting of an rdnb. There’s no good way to do it, except maybe with weight distribution. Try to avoid having anything that sticks out from the top of the rd4b to the back of the robot. The methods Stanley mentioned are definitely useful but will not fully solve the problem. It’s just something you might have to get used to. If I were you I would probably do what Stanley suggested if it’s not too inconvenient.
Try advancing the top 84-tooth gears one tooth higher. It worked for me
can you give some pictures?
we are using a dr4b since the beginning of the season and solved this problem by increasing the distance between the two c channels and adding high torque gears and axles.
these images are of our robot about 2 months ago.
we have never actualy grabbed 3 cubes in a match. but can lift them.
My robot is on tilt
Do you think maybe it’s also because I have gears between my largest gears (with arm attached to them) ?
Just wondering, when building, did you double check that you had the correct teeth placement on the gears? If possible, some closeup pictures would help a lot more.
How Do I create a New Post So I can help people?
find out where the excess motion is coming from. my guess would be from the 6 bar layer with the rubber band placement you have. the trick is to eliminate slop in the lift. I would suggest widening the distance between the bars on the 6 bar layer (which would also help it lift higher). or using a 4 bar. make sure you are also using bearings and the right tension of screw (not axle) on all joints.
just widening the 6 bar a few holes would significantly help your problem. even though you would have to re-do your gearing in that mid layer it will significantly help in the performance of your robot.
hope this helps
974x Cyberbrains
are you using the new high torque gears and axles on the center layer of your lift?
Common issue of double reverse class. Every class of lift has its issue:
Scissor: generally wobbles, requires tremendous power at lower position
Reverse double: extremely vulnerable at highest position/tilts
8 bar/12 bar/bar lift: long arm requires more torque; less height
Elevator: slow, hard to have 2+ cube capacity, chain breaks
Every class also has its advantages as well:
Scissor: extremely fast and stable if built correctly; easily supports stationary skyrise builder
Reverse double: having similar speed as scissor while a lot more strength; easy to build and maintain (scissor can never handle 1:7 three cubes)
8 bar/12 bar/bar lift: huge horizontal expansion, can score and get skyrise across cubes and blocking robots
Elevator: extremely stable, programming-skills and autonomous oriented, easy to program
Every class has its own advantages and disadvantages, and is important to know your class’s limit and play style. The easiest solution to the problem of reverse double is to build your lift higher than objective and never raise that high. Actually, most lift classes are vulnerable when they reach their maximum mechanical limit of height.
Remember, try to incorporate your drive’s factor into building. If your driver is not adroit enough to pick up 3 cubes in competition, don’t worry about giving the robot a three cube capacity. That is for the future. If your driver is more than capable of capping all middle and low goals, then you need to seek height from the lift and try to make the driver cap high goal as well. If not, settle down and focus on capping all middle goals and low goals first. A robot that cannot reach high goal but is capable of dominating middle goals and low goals is still a very formidable opponent, remember that.
A good robot is your potential. Good driving, programming and teamwork are your actual strengths that can win games. Potential does not equal to strength. An important thing to remember in VEX is to balance the time you spend on structure, programming and driving. Knowing your limit is very important.
We are using the new high torque gears but not the axles. I don’t really get how the high torque axles since you can’t put them through the metal’s holes.
you do end up having to drill into the square holes that they have to go through.
the large axles WILL make a huge difference in the quality of your build and with your original problem. I am actually really surprised that you have a working dr4b without large axles.
these are some pictures of my teams dr4b arm drive assembly. if you want to stay with your gear set up, I would put high torque axles and gears on all 4 gears connecting the two lifts together because there is a lot of torque between them and you don’t want them to come out of alignment during a heavy match. otherwise, you can do what we did and gear 2 directly and then bar in the gear to the top 4 bar. less moving parts and less slop.
-Cyberbrains
foregot the the pictures.
here
you just did. 
I can’t be sure from the picture but I think that your joints at the top of the six bar portion are not in a straight line or the spacing between the joints are different from the bottom.
Nah they’re the same. It’s the slop in action.
My 8 bar also tilting to the left and sometimes to the right.Can someone really help me with this?i tried to change it but it seems doesnt change.pleassee help me…:(
It would help forum members help you if you can provide some photos, pictures of the whole robot and perhaps pictures closer up of the various gears and joints.
pictures would definitely help but I think I can guess what the problem is. assuming the lift is the same on both sides (rubber bands, holes, etc.) it is probably that you don’t have enough cross braces attaching the two sides together. you should preferably have at least one for each lift layer you have. if it is leaning more to one side than the other, I would look at joint tension and replace rubber bands so they are even.
if you are still having a problem, double check that you have no bent axles or bad motors on the lift. as it can be hard to spot sometimes.
do you have the same type of metal and same type of bar on both sides?
hope this helps
pics would be great.