2" screws are long enough for all joints if built with using them in mind. We have two shafts in our entire lift, into the two motors. That being said, it takes a little bit of doing to design a gearbox that has less than 1.5" of gap for the gears and bars mounted to them.
In general though, friction is your enemy. You can counter-act a little extra weight with more rubber bands, but friction is never good.
Symmetry is very important, not just weight left-right, but also the length of the bars. If you have a longer bars it can cause the geometry of the lift to change as it moves, making the lift “non-linear.” An ideal dr4b has all 4 bars identical lengths. Changing the top 2 to be shorter or longer than the bottom 2 will cause the lift to “drift” forward or backward as it goes up. Longer top means forward, shorter pulls backward.
Another issue to watch is wobble. Now this seems like tightening the joints would make it wobble less, and it does, but it also make the lift get “stuck” leaning, which then get worse the more it tries to fix it. So by making the joints looser, the lift can fix itself easier, which overall decreases wobble. And cross bracing is important, of course, but not always easy to do if the middle of the lift has to be open.
Those are the 3 biggest things to watch IMO. Hope it helps.
one thing that threw us off for a while is to make sure that the teeth of each arm are meshed exactly the same we always thought it was rubber bands or cross bracing but if one arm is a tooth off it will make the lift go up uneven no matter how many rubber bands you put on it
We did a rebuild and built a DR4B. We used long bars and had trouble getting our lift to rise straight. We couldn’t figure out why, but it turns out it was just a motor controller issue. As far as tips go, make sure everything is exactly the same for both sides and secure the bar that is connection the lift to the base well. We had some problems with it wobbling which we eventually fixed.
If you’re planning on using a sensor on your lift (which if you aren’t, you should), bear in mind that at least two of your gears will actually need to be on axles, as potentiometers don’t exactly fit screws. Even if there is no sensor on the other side of the lift, you should still use an axle as it keeps things even on both sides. For the rest of the lift (even the other gears), go ahead and use screws, just keep in mind that one set that will need axles.
We just use screws for the connection to the towers. The rest, we use axles. That means we have space to make the gearing work well. Using a 2 inch standoff (I think) connected to a .5 inch standoff makes the spacing super nice (like not very many washers needed).
What I would recommend is using single rotating joints so that this problem would never happen in the first place. Basically you would firmly attach a screw to one of the bars with a fully tightened Keps nut. This will remove a lot of the slop and keep the screw perpendicular to the bar. Then you would run it through a bearing on the other bar so it can rotate, and then hold it all together with a Nylon Lock nut, just tight enough to keep it from coming apart. This joint only rotates in one place and you don’t have to compromise between a tight lock nut to remove slop and a loose lock nut to prevent friction.
Other than this I would recommend:
Eliminating friction entirely from the system
Use rubber bands in a triangle shape
Only use aluminum.
Don’t use excess amounts of gears
Have the top and bottom bars directly connected to the gears transferring power from the bottom to the top.
Bracing is your best friend. Use X braces on your actual lift bars (straight across bracing does nothing and bars in between a 4 bar do nothing)
However, the biggest tip I can recommend are looking at really good dr4b’s from the past. The most impressive ones in my opinion are team 118’s from skyrise. There are a lot of little tips and tricks on that robot which make it work perfectly you can watch their reveal here
Another robot with a really great dr4b is AURA’s 24" skyrise robot which you can watch here%between%
But besides that practice makes perfect (it took me 5 dr4b builds to finally get one that worked really really well). If you keep improving and finding the little ways to make this type of lift, I think it is generally the most useful lift in VEX.
One thing that can throw you off is the difference in friction per side. I can not stress enough how important it is that both sides have very little difference in friction compared to each other. I would also recommend to reduce friction as much as possible with Teflon washers. Another thing to stress on is bracing the lift to keep it from swaying. I personally had an X brace on the top section of mine and simply a C-channel running across on the bottom.
By sensor, I presume you are referring to either quad encoder or potentiometer?
The quad encoder or potentiometer is to help you with your programming, so that you can control how much you want the lift to raise.
Of course, you can always use them to ensure both sides are going up evenly as well. But I always prefer mechanical balancing rather than using programming to compensate for the construction faults.
It depends on whether or not you’ll need it for programming. When my team had an autostack function, it was definitely necessary to have a potentiometer on the lift. Now, even with manual stacking, I use the potentiometer to make the secondary lift (chainbar) adjust to the changing arc of my lift (dr4b) because the bars on top and bottom are not equal lengths. If you’re not planning on going as advanced as this, then don’t even worry about a sensor unless you raise your lift during autonomous and want to make sure it stays up.