I think that in a scenario like that, using standoffs would be better. By tightening a standoff very tight, you can eliminate that slop between a screw and the hole more than you can with a spacer. The standoff will weigh ever so slightly more, but the rigidity is worth that trade off in my opinion. Also since standoffs don’t compress like spacers do, you can really tighten those screws a lot more than you could the spacers. If you do both types right, the difference will be pretty small, but I believe the standoffs should be better. (I have not objectively tested or proven this, so this is just from my experience)
Of course, when mounting a piece like in this example, the biggest source of flex in your structure will come from that bottom c channel itself, to vastly reduce that you would definitely want to have box bolts going through the whole c channel. That would be a much more important part of this particular attachment’s robustness than the spacer vs standoff decision.
Forgot to mention, but we do this a couple of times too with using aluminum nuts to lock down the screw and spacers (or standoffs depending on the distance) for mounting.
We specifically use this for the bottom channel braces on our base to enable us to easily take off the outer drive channels for maintenance or fixes as the actual hardware can remain on the outer drive channels with only screws holding the bottom channel in. Pic for reference below:
I mean the c channel on top probably doesn’t need to be boxed unless it’s an extremely high load bearing piece of structure, or you don’t really care about weight and you just want the strongest build, but the box bolts on that bottom channel are pretty crucial.
Standoffs have some special uses in structure. For example, if you want to place a c-channel inside a u-channel for extra sturdiness in a drivetrain (what I did this year), it is impossible to box the c-channel in the corner that is created between the u-channel “bottom” and c-channel long side with spacers. In this case, I simply boxed it using a .750" standoff and four steel spacers, two on either side of the standoff.
I’ve noticed this seaon that teams have some super heavy duty bases (6 motor, 6 wheel, double channel supports with extra frame supports) and then a superstructure of mostly standoffs.
Roboteers are telling me the super low center of gravity makes high speed motion without tipping. With either just skills uns or LRT without the defensive contact that makes sense. Wonder if ext season with physical contacts if that will still be the case.
A question for @vexreally: wouldn’t those aluminum nuts for boxing be heavier, more likely to vibrate out, and weaker than using spacers?
For everyone else: where are places when spacers would be better to use?
Its fine to do what vexreally did, though I would personally use keps nuts or threadlocker if I had to use hex nuts. in some cases, like exactly that, you cant use spacers. Also, whule aluminum is denser, it is also a much smaller volume and may even be lighter than nylon.
spacers are best for box bolts, but they’re also good for applications that need low weight, aren’t under heavy load, and obviously for their intended use on shafts and screw joints. they’re also useful for when a standoff isn’t the perfect length you need on a connection.
If you’re really into box bolts (which you should be, they’re the best) you can even buy spacers and standoffs that are the right size for a box bolt.
For your standard spacer box bolts, you can get a ton of 7/8" long, 3/8" diameter nylon spacers from McMaster for real cheap.
I haven’t gotten any yet but I plan to. Traditionally you stack a 1/2" and a 3/8" spacer to make a box bolt, but being able to use one spacer for this job seems like a nice convenience thing. Also the smaller outer diameter means these spacers can fit almost any box bolt, where the fat 1/2" diameter spacers might not fit in all situations.
Also on McMaster are some interesting 7/8" length standoffs and aluminum spacers.
I also plan on getting some of these in the future for applications where you want the most robust box bolts. Nylon can compress, which means that it’s possible to overtighten a box bolt and deform the c channel. You really have to tighten it a ton for this to happen, but there are some applications where this could be an issue.
I would also recommend the 3/8" diameter robosource spacers just for general use. They’re lighter, and lower profile than the fat 1/2" ones.
it is worth keeping in mind that 3/8" diameter spacers will compress more easily than 1/2" diameter ones, so its not like the 1/2" ones are bad, but only use them if you need that extra strength.
And while we’re on the topic of build quality, I would also recommend using shoulder screws whenever possible. The thread-less part of these screws keeps the screws fitted tightly inside the vex holes, keeping them perfectly centered. This improves the squareness of a build, and prevents slop from the screws from un-squaring a connection. https://www.robosource.net/71-shoulder-screws
spacers and standoffs have some really good use as structural components, and you can really improve your build quality by using them well, so I think having a variety of spacer and standoff dimensions for a variety of applications is important.
In this specific scenario I would use all spacers; inside the horizontal C-channel and between the horizontaland the vertical C-channels, then run a screw through the whole thing and tighten a locknut at the end. The entire thing is kept together through compressive force and it’s much easier to maintain if needed.
A solution to the spacers compressing is that you can always use metal spacers (allowed under R12).
As for spacers vs standoffs, generally I like to use the screw-running-through-everything approach unless I need to start stacking spacers.
Aluminum nuts are negligible in weight savings compared to using spacers, but the main reason we used aluminum nuts there is due to bearings being in the path of the box. Because of that, we drill out the bearing horizontally to make room for the screw to pass through, with the nuts being tightened against the metal to prevent it from bending in. Keps nuts can very much be used there, but there is no real difference as long as it is tight.
In regards to spacers, 0.5" works fine a majority of the time unless you want a narrower OD due to space constraints. Personally, I wouldn’t go lower than 0.375" OD (the robosource link that @Xenon27 has in his post).
Alright, that makes sense. I had looked at the images you had provided, but I thought that they weren’t only around the bearings. Wouldn’t just space boxing just around the outsides of the bearings work too, though?
Correct, using spacers in areas where bearings (or any obstructions really) are fine. We personally use either custom length 7/8" 0.313" OD spacers or a combination of 0.5" OD spacers depending on if there are screwheads that may cause issues.
By the way, I see that these come with some washers. Are those to space the screw head a little out away from the metal, so that the shoulder doesn’t poke out the other side, getting in the way of the nut or whatever else is on the other side?
standoffs are fine in that situation, as long as the bottom of those towers is braced properly.
the main purpose of the standoff here is to maintain a fixed distance between the front and back towers, and since standoffs have no problem resisting compression or tension, they work well. Just keep in mind a standoff there will not do much to resist the towers from moving side to side, but typically that’s an issue you solve towards the bottom/middle of tower mounts.
you are correct that the higher the brace, the more stability it would add, but you really don’t need to have a brace at the top for a good amount of rigidity. And keeping the braces lighter and nearer to the bottom improves the robots acceleration and center of mass.
Its good to keep in mind that very little force will be applied to these towers, so prioritizing lightness over rigidity to some degree is a good idea. If these towers were for something like a lift, I would agree completely that high on the robot c channel braces are in order.