About nut inserts (which kind is best)?

[graham-xrf]

My thanks to all of you. Clearly @WobblyHand has used many. It's completely OK to use more than one fixing type on the same project if the aim is quality functionality. I admit I was also somewhat distracted by the sight of the seriously businesslike motor drive electronics with some thought given to the cooling

I will keep in mind the 2-lug type mentioned by @Advil for a different project. That sort is clearly uncompromising.

Good picture from @Mill Lee farm Thank you.
The second rivet nut tool in my picture (Rivet Tool1a) looks to be a near clone of the Astro Pneumatic brand in your link. At $82.32 bucks, I guess that one is the premium quality version. In my case, the initial project does not require high strength, not more than a few nuts, so I suppose I could get up to something truly cheapskate, and skip the tool altogether, but I can see that these things are useful, hence why I consider investing in something decent for future use.

The actual task
Please forgive the almost off-topic aside..
The brake splash-plate issue is from a truly bonkers bit of Mercedes rear hub design that finally afflicts the brand. The car is quality, and is long-lived enough that the brake cover plate will finally rust first. The rear hub has disc brakes, but combined in the same disc hub is a cable operated conventional drum brake that is used for the parking brake. The plate locates the drum shoes, and springs ends only. This cheap part is behind the bearing hub which is bigger diameter than the hole in the plate. You can't get it off without cutting it, or pulling off the wheel bearing hub.

To change the plate requires a seriously high force risky hub pulling operation that is unlikely to leave the axle bearings intact. They might survive OK, but are normally replaced as part of the procedure. The bearings are expensive, as is the labour cost, and all that goes with heat-fit bearing installation on a Mercedes. All this x2 (to do both sides), just to replace a cheap pressed splash plate! Anyone looking at what it costs to have this done to a older Merc will feel ill just by glancing at the price!

Of course, the folk on YouTube have a variety of work around ploys. One plan simply removes a big chunk of the plate, avoiding the critical spring anchor points. What is left does hold the springs, and locate the brake shoes, but is hardly anymore a "splash plate".

Most others require cutting through enough of the plate to allow it to be somewhat "twisted" to go over the hub, then rely on the bolts to set all straight (with a cut slot left in there). It is functional, but is not the best. The best solutions split the plate in two places, and install rivet nuts to allow them to be held together with one (or maybe two) stainless patch 1.5mm thick sheet add-ons.

Some thoughts still arising are ..
1. Should the rivet nuts be fixed into the backplate, or the patch?
2. Should the repair plate be on the outside, or the inside of the backplate?
3. I have to check carefully the space clearances, and that the bolt heads will be accessible, and will not foul any other feature.
4. Either M4 or M5 seems about right. It's overkill, but M5 (0.2 inch) feels about right.



OK - so before all you folk get to musing on how pathetic is such an endeavour, know that I think so too, and I end up wondering why is it always me having to get into such desperate stuff?

 

[Mill Lee farm]

I'm never one to talk anyone out of buying a tool...
but if you're looking at installing 8 rivnuts you would be fine without a tool.

Quick search found this guy who has an interesting take on the nut & bolt method. Basically a handle instead of a washer to help keep the rivnut itself from spinning while tightening.

Are the white circles showing the bolt holes for bolting the backing plate to the hub? If so that seems reasonably sturdy.

Install one rivnut on a scrap piece of similar gauge sheet metal and double check that the remaining stickout of the threaded portion wouldn't interfere with any of the brake parts etc.
I would think it would be hard to bolt from the inside (the opposite side shown in the above picture). Better to bolt from the outside.

Of course another option would be to install the rivnuts 'inside out' and use a thicker 'repair plate' with clearance holes for the threaded portion of the rivnut. It would need to be thicker than the remaining stickout of the threaded portion. A washer would hold the repair plate on. You probably have more room on the outside than in there with the brake system?

The rivnut should have as much strength from either side once installed.

 

[Mill Lee farm]

quick ugly sketch might help explain my thoughts?

 

[WobblyHand]

Rivnuts can be very handy. Don't forget there are other types, like PEM nuts. Here's a link to them in McMaster. Pretty sure there's an equivalent supplier in the UK. https://www.mcmaster.com/products/pem/metric-steel-press-fit-nuts-for-sheet-metal/
To install them, you use either the fancy tool (too expensive for casual use!) or simply press them in place. I used an arbor press, which worked fine. The PEM nut sits on the outside, and allows a flush mount. I used PEM nuts to hold the stepper motor driver flush to the inside of the case for better thermal contact to the enclosure.

 

[graham-xrf]

Are the white circles showing the bolt holes for bolting the backing plate to the hub? If so that seems reasonably sturdy.

Yes indeed! The white circles highlight the 4 bolt holes that secure the plate. This is how one chooses where to cut. For some folk, they seem OK with simply cutting the plate into two, and relying on the two bolts each half to hold everything in place, as if the cuts were not there.

Maybe it's just me, but I would be unhappy with that. A repair plate goes some way to restoring the radial stiffness to a part that does, after all, anchor the shoe return springs, and mounts the end of the actuator cable outer taking the force of ones foot pressure on the parking brake.

 

[graham-xrf]

quick ugly sketch might help explain my thoughts?

View attachment 448089

Thank you so much for taking the time to draw all it all. Your sketch alerts me to remember the repair plate hole must go over the rivet nut OD, and not just clear the bolt threads. It should still locate just fine. I plan to make up the whole repair plate mounted by rivet nuts, and then finally make the cut afterward. The cut edge will need a dob of protective paint or sealant, but that's OK. I expect the repair will outlast the rest of the time I remain alive, let alone up to when I replace the car.

That inside-out approach with a thick plate is interesting for an ultra strong, truly uncompromising fix, and I think there is plenty of space on the outside to allow it. Mechanically, it is well over-the-top . I am checking the "stick-in" clearance for rivet nuts mounted the other "more usual" way around. I think there is enough space between the plate and the disc. 1.5mm repair plate is thicker than the brake plate anyway. I have even considered mounting the repair plate on the inside.