How did this Michigan team attach their metal intake arm to the vertical metal bar? In the image below, I circled in pink the pivot point of the intake arm. We’re not sure what the object is called.
Here’s the video of how we want our intake arms to deploy: https://www.youtube.com/watch?v=smczGyIbQR0
Thanks!
Looks like a bolt (also known as a machine screw), with a nut on the end. Probably has some washers between the two metal tubes, to space them apart.
There might be some bushings in there, but I kind of doubt it…
Yeah, this is the inside
edit: From their doc: https://docs.google.com/document/d/1689t3vj-MkVeXKqDyHuAS06EUKrcGfDtaPSUG-uIbPY/edit#
Hopefully at least a washer. Doesn’t look like any bushing or shoulder bolt.
Let’s say we put a machine screw through a vertical support and an intake arm, added washers between the vertical support and the intake arm, and added a nut at the end of the machine screw. The intake arm would easily pivot, correct?
Yes it looks like they used a nut in a bolt.
What I would have done is drill it out with a step drill to one and 1/8 in put in a bearing to both pieces. A lot less friction that way smoother operation
I’d probably at least go a bit bigger than did with the bolt.
If you wanted to do a shoulder bolts, you’d likely need to drill the inside vertical and outside rotating tube larger for access, just because longer shoulder bolts aren’t as usual. Then the rotation would be on the hardened should instead of the bolt threads.
It can likely pivot, yes. You would want to look at how tightly you bolt it all together (a nylock nut would really help, as you could secure it with it being loose still), as too tight would add a lot of friction, making it hard to turn.
All that said, a better way is to use a bearing and a shaft, in order to reduce friction as much as possible. You also want to consider how you plan to move the arm. For example, in the picture below, we have bearings in the polycarb framework, with a hex shaft going all the way through. The intake arm is attached to the shaft using a hub. This way we can drive the shaft with a normal hex bore sprocket in order to drive both sides of the intake at the same time.
Also, from our CAD so you can see the assembly a little better:
If you check out the Everybot CAD, they have an example of how to attach an arm via bushings.
I guess I shouldn’t tell these guys, but we’re going to have our arms pivot on a 3/8" bolt, with no bushings…it’s crude, but it usually works OK. Kind of depends on how quickly you need to get it done, and your machining capabilities, and what materials you have or can get easily, etc.
Sometimes we forget that the machine we’re building has to operate for less than half an hour, it doesn’t need to be built as perfectly as possible. Although it’s also a good idea to make sure its’ going to work well enough that you won’t have problems with it during competitions.
Well, less than half an hour on the field during competition. That doesn’t include all the testing and practice time you’ll need to have the team actually be able to drive that robot effectively during that critical half hour, which will amount to quite a few more hours of use.
Honestly, I’d never allow my team to put in a pivot joint like that without at least using bushings to ensure the aluminum didn’t wear or tear out around the hole (especially since the OP is saying they’re only using 1/16" tube.)
We used Shoulder screws and Oilite bearings from McMaster-Carr to connect a 4-bar linkage to vertical bar in the past. That combination worked well for us.
https://www.mcmaster.com/shoulder-screws/alloy-steel-shoulder-screws-9/
https://www.mcmaster.com/thrust-bearings/lubrication-method~embedded/oil-embedded-thrust-bearings-7/
Choose the shoulder diameter and length according to the number and width of the aluminum tubings you are connecting.
That will be true, if we actually get it done on time…otherwise…not a big deal.
But even if we get to use it for several hours, it’s not likely to wear out, nor cause excessive friction, nor have any alignment issues. The way build is going this year, we have very little time to add any unnecessary complexity to the robot, at all.
Remember that bolt threads can be very abrasive on aluminum. If the bolt threads rest on/contact the hole in your aluminum arm or support, the hole edges will be machined away by any movement (i.e. every time the arm moves). Thus the call for a bushing.
If we opt to ignore all practice time, we’re seeing match lengths of 2.5min. For a regional robot, let’s look at an average of 10 matches. You’re at 25min. If you play a single round of playoffs, you’re at 30min (losing both matches). If you’re a district bot, you play 12 matches. You’re at 30min before considering playoffs. Double that to get to an hour.
If you advance at all, you’re going well beyond that 30 min. You’re adding a point of failure where there doesn’t need to be one. Given the ease in changing this is pretty easy, why would we look at fixing this poorly?
I guess I’ve seen too many unfinished robots show up at competitions, and do really poorly.
Note the qualifiers in my posts, if you can do it well, then do it well. But if you can’t do it well, just do it so it will work, and don’t sweat it. We’re not building spaceships here, we’re building students, eh? And learning expedience is just another lesson.
This isn’t a place where we’re talking about something complex that’s going to lead to those problems. We’re not talking about expedience.
The time to place the designs shown here are roughly the same amount of time as just the bolt. Though, they’ll save time in maintenance later.
If you can do one, you can do the other. This is the wrong place to be providing that lesson.
I wish we had the hours available to do everything right…but we don’t. Getting something done is our goal this year. I’m glad your in a better situation.
You’re misrepresenting what I’m saying intentionally. Shame on you.
It’s worth noting that every team has different resources. While one team may find drilling matching bearing holes to be simple and easy (due to the presence of CNC machines, mills, etc), another team may find it more difficult and time consuming. It may me simpler to clamp things together and match-drill them, stick a bolt in and call it good. When using bearings and shaft, you tend to need things to line up more precisely.
The intake my team has took a total of a couple of hours - some quick CAD once we knew what we wanted, 10 minutes cutting on the CNC, and then some time assembling everything together. We’ve done similar intakes in the past, without the benefit of CNC, and they’ve taken much longer. Instead of being able to go from stock to assembled in a single meeting, we’ve taken a week before.
I think it’s always worth pointing out the pro’s and con’s of different methods, but it’s also worth remembering that what works for you may not work for others!
I guess I’m not understanding what you’re saying. I’m surely not trying to misrepresent it.
