What is the longest bar that is used in a 6 bar lift, is it the combination of two bars or is it a 1x2x1x35? My team is throwing around a couple ideas as the only thing that we have finished is the drive train.
Also, how do you keep the joints from moving around where the bars connect? In my head I thought of using the Delrin pillow bearings but my worry is that the screws would either be in the metal and not allow the arm to move or that they would rub against each other and again inhibit movement.
You will have to play around with the length of the longest bar. Assuming you want to get to the 30" goal you will most likely need to connect two bars to extend past the 17.5" maximum length of any VEX metal. As for preventing wiggle room between joints, you should look into using long screws and lock nuts. You will just need to screw them as tight as possible while still allowing full rotation of the metal.
With a 6-bar, you can get 25 inches (the diagonal being ~25.5 inches) by joining two bars together. One 17.5" bar will not be long enough to reach all the way to the opposite corner unless you attach another bar to it.
We always use the black axle holders whenever there is an axle going through metal so it doesn’t move or rub. Put a spacer on the axle between the two bars you’re connecting to keep the screws from hitting, and you should be good to go. They should not inhibit movement.
As dontworryaboutit says, using the black sleeves when using axles is ALWAYS recommended. This gives the axle more width to go through, stabilizing it more than if the axle was only going through two thin pieces of metal. We find the constant loosening of axles is not worth the hassle and just use long screws.
Even with the black sleeves I can’t seem to comprehend this. In my mind I see screws falling off from any little vibration and then the arm falling apart. I thought about using really small axles with collars on each side but then it seemed like I would never be able to get them close enough together to stop the arm from moving around a lot.
Also, does anyone having any tips for cutting the C Channel 1x5x1x35? We’ve been using a bandsaw and trying to angle the pieces in but it seems to be extremely unsafe as the metal suddenly slips.
Vex hinges make great joints for 4 bar. Very low wiggle, very strong, etc.
Using a 1x5x1x35 for the top bar, and for the bottom bar, with hinges at each end makes a very solid arm, without need for doubling (left/right).
Since the hinge pin does not easily line up with a mounting hole, it is hard to put a gear on it. You can use a stacked 3rd bar mounted low (as proposed elsewhere) to provide the actual power.
Other ideas for joints:
Vex shoulder bolts with castle nuts, maybe a teflon washer between metal.
Non-Vex shoulder bolts (< 2" long) with castle nuts, might work great with or without bearing blocks on 1x2x1 crossways C channel.
vex flat bearing blocks inside the metal, with spacers to escape the screwheads.
vex flat bearing blocks outside the metal, with scraped down profile or counter sunk to escape the screw heads.
Right, this is the problem I was elaborating on. Using axles and collar locks comes loose quite easily. Instead, using long screws and nylock nuts (we just say lock nuts) is tight and doesn’t unscrew. Usually, using a keps nut in this mechanism would do as you say, falling off quickly to vibration. Nylock nuts, however, have a nylon insert that increases friction between nut and screw. The nylocks require a wrench to put on as they are very tight, however once put into place they are EXTREMELY difficult to take off. Thus if you utilize long nuts and nylocks in your arm joints, then tighten them as close as possible without limiting joint rotation, you will have a very sturdy arm.
wait, i lost this thread somewhere in the third post XD
what part of the robot are you guys talking about?
(that part of the arm uses hinges, “black sleeves”, and small axles with collars on each end)
and whats wrong with 17.5"?
we currently have a 17.5" c channel for the “long” bar and it reaches 30" just fine
the MAGIC OF VEX STRIKES AGAIN!! =D
What are screws that are falling out connected to? They should be screwed through the axle holders, through the metal and onto a nut without interfering with anything else.
I believe it was just a misunderstanding of the use of nylock nuts. It is absolutely necessary to use nylocks instead of normal keps nuts. If you used keps nuts your joints would either come loose or be totally immobile depending on how you tightened them.
Murdomeek, we are discussing the best way to attach bars at the joints to minimize wiggle room and instability in the arm.
“black sleeves” and “axle holders” are informal names for delrin bearing blocks right? We’ve found that having a longish screw through bearing blocks on each piece, with a 0.125" high 8 mm spacer (white) between, and a reasonably tightened nylock nut to be very effective at minimizing wiggle.
Well, actually, the extra length on the “long bar” can be unnecessary, depending on the design of the robot. For us, using a 17.5in stock piece is actually long enough to reach 30" with an inch to spare (Had to double check that; it does in fact work).
Our team never uses lock nuts, ever, because they are always a pain to screw in on longer screws, and don’t work on shorter screws. Keps nuts do not just fall off whenever; tightening the keps nuts works quite fine, where tightening involves using pliers on your allen key. Seriously, Murdomeek stripped quite a few screws and keys before we found some made of better quality material.
For support on an end-to-end joint, bearing blocks are good to reduce wiggle, and I would recommend bar locks on the other side of those for structure.
If I misinterpreted you guys and you are talking about the rotating joints, then consider using bearing blocks on both sides of the metal (It acts as double support in some applications, and I think it might help here too). Also, use shaft collars with 8/32 screws instead of set screws and press them inwards as tight as you can.
The only times we use lock nuts are when we need a pivot that can be done with a screw or when there are axle holders that have a lot of force on them that absolutely cannot move. Other than that, Kep’s nuts work for all scenarios and should not loosen quickly if tightened well with wrenches.
Set screws are 8/32 screws, just without a button head and with a depressed end on the other end. If the axle is stationary, regular screws work well and tend to be harder to strip, but if the axle is rotating I prefer to use set screws, especially the silver ones, not the black oxide ones (that strip more easily). They may be easier to strip, but the set screws significantly reduce the size of the collar on a rotating axle.
By “axle holder”, I do mean the black delrin bearing blocks. Doubling up does help to reduce wiggle, but bar locks (those flat metal things with 2 round holes and 3 square ones) will bring it to practically 0.
I’m sure you know this, and I’m probably just arguing semantics, but for those new to the VEX system:
Don’t ever use a wrench to tighten a keps nut. Use the wrench to hold the keps nut in place, and use the allen driver to tighten the bolt into the nut. The nut should be stationary; the bolt should be turning.
If you do tighten the nut onto the bolt, the metal ring will dig a ring into your metal and severely reduce the efficacy of the keps nut.
Also remember - black goes with black, silver with silver.
I’m not sure what you mean by “black goes with black, silver with silver”. You mean you use the black oxide screws for screwing in plastics, and the silver screws for screwing metal together? Or the screw heads go with the plastic, and the nuts with the metal?
I can see where you would want to avoid tightening the Keps nut against plastic like the black delrin bearing blocks to prevent damaging the plastic, but sometimes you have to do that to make things fit.
The metal ring the the Keps nut spins independently of the nut itself, so it hasn’t gouged any holes in our parts yet.
We’ve found that to be a recipe for disaster. The keps will quickly gouge the plastic a large ring, so not only do you have a nut-bolt assembly prone to coming apart from vibrations, but a thoroughly demolished bearing block as well.
For first-time use of the nuts, yes. We’ve found that with repurposed nuts, however, the ring mates with the nut.
Another useful thing to remember is that Keps nuts are disposable items. After one or a few uses, the lock washer get squashed flat and no longer works. I recommend tossing any that look flat and replacing them with new ones.
I’ve never experienced “squashed lock washers” that have not allowed the nuts to tighten well. Maybe I’m not just noticing it, but in any case we almost never throw old nuts away.
On all of our Keps nuts, the rings spin freely of the nut itself and do not gouge holes, except for an imprint that holds the nut more securely. Maybe we just have super-Keps nuts?
