These things are neat and I haven’t heard much of their use on FIRST robots. Do any teams have experience using these on their machines? What do you perceive as the good and bad of them?
Rivet nuts and rivet studs require a special tool to install – McMaster-Carr sells a hand-operated tool for about $100. They install like regular pop rivets, but are threaded on the inside or have a protruding stud.
I’ve been using them for some projects at work where we have blind holes and we can’t cut access holes to reach hardware inside of tubes and the like. They’re a bit expensive, but like PEM products, they eliminate the need for nuts or bolts in some locations.
We used them on this years robot. We had bad experiences with them. I know they fell out and had to be replaced. I think we eventually ended up coming up with a different solution (to whatever the problem was, I forgot).
We’ve used nutserts in the past… are they the same thing? We used them to mount our AM transmisions and didn’t have any problems whatsoever. You can spot them in this picture. (that’s our 2007 robot, I don’t recall any being on the 2008 robot)
I started using them many years ago. I needed to add hardware to a leaky (RF) transmitter to get the second harmonic down. (A Hot Water 101 for those of you who remember back that far.) The nurling that is formed when the hand tool is used proved to be perfect for making contact between the covers and the chassis. The real trick, just like PEMs is to use the right size drill bit. As I remember, McMaster has both aluminum and steel.
We have used them in the past with fairly good results; especially for light duty. They work well for blind locations as you mentioned and in thin material where you can’t tap a hole. If your application is light, harbor freight has an assortment pack for ~$10 which is much more expensive than rivets. But if you are planning for a connection that you can remove without having to drill it out each time, they can be quite useful.
I really like them and think they are a fantastic idea. They can be especially useful when mounting something to thin-walled tubing. The tubing wont get crushed if you tighten the bolt too hard. I feel like any situation where a riv-nut is possible, it should be used because why worry about a nut and a bolt when you could just worry about the bolt?
Of course, because of the flange, you wont want to use them when you want the two parts to be flush.
1293 used them in 2006 to allow access into the ball elevator if need be. It was one of the few parts of the shooting mechanism that worked well. There wasn’t much load on the rivet nuts, but they were just what we needed for the application. 1618 has the equipment to do them, but we have yet to have the right application.
We’ve never used them on 237’s robots but we do use them on some of our plastic injection molding robots where I work. They are used on our smaller robots on the lighter duty axis to attach linear bearing rails and brackets.
The ones we have are ribbed on the outside so they “bite” in the hole as you tap them in place with a hammer. Hole size is critical as Al said, otherwise they won’t grab. We don’t use/have a tool to insert them. Once they are tapped in place the bearing rail is installed. As the screws are tightened the rivet nuts expand and hold in place for good. We use rib washers* and Loctite too, either 222 or 242, depending on the size of the screw. Vibration and loosening fastners is something we’re very worried about with our equipment due to the constant movement of the robot and the press and even vibratory bowl feeders when used.
*Rib washers are these things. I’ve never seen them used anywhere before other than all of our equipment. Might be a European thing . Rib washers aren’t anything you’ll find at Home Depot or Lowes either. We never use regular split lock washers.
There are any number of “rivet” type inserts and studs and fasteners that have very good application possibilities. This is especially true for high speed assembly of mass-produced commercial products where labor costs are high. If you don’t mind going out and fund-raising to get the $$$ to buy some of the (sometimes) required insertion tools, great. To my mind, they are great for fastening thin panels where you have no access to the back, and not enough thickness to tap a hole. They do prevent a loose nut from going missing in electronic components as well.
We had good success this year using 10-32 aluminum rivet nuts mounted into 1/8" and 1/16" box aluminum. We put a fairly large load on them when we attached our upper body to the lower body through just 4 of these. Its loose now after a season of wear, but still holds the 'bot together.
We did have problems when using locktite to hold the threads though. The rivet nut would slip in its hole before the locktite would break when trying to remove a bolt, causing the rivet nut to just spin freely. The only way we found to remove it then was to clamp the two surfaces together, putting more pressure on the flange of the rivet nut and thus holding it in place enough to remove the bolt.
In a non-FIRST robot, I have had problems with rivet nuts rattling loose in serious off-road conditions, but this shouldn’t be a problem when rolling on carpet as long as your initial hole is the correct size (just like others have said).
I used about a dozen 1/4-20 nut rivets on this year’s robot for mounting chain tensioners and mechanical stops to our arm. It was my first experience using them, and it worked quite well. Both applications were places where we had a large tube with wires inside, which I could not (or did not want to attempt) to drill a through-hole in. They were very light-duty applications, as any forces would be applied parallel the bolt, in an inward direction. I would not use them in any situation where the bolt could conceivably be pulled out, or where a significant moment could be applied. They’re just not that durable, and the aluminum ones in particular can strip very easily.