Seat belt reinforcement plate attachment

[chevynut]

I am working on installing seat belts and I have made some steel reinforcement plates for the mounting points. I was going to plug weld them onto the sheetmetal, but I got to thinking that maybe I should epoxy them instead. The strength of the seatbelt mount comes from the fact that it's behind the sheetmetal and would have to tear through it to fail. The plate interface with the sheetmetal would be in shear. The plates are 4.25X2.75" or about 11.7 square inches. Epoxies typically have a shear strength of 3500-5000 psi, so this should support 41K-58K pounds. Four 1/4" plug welds would only support about 10K pounds. So it seems like the epoxy is actually a lot stronger.

Is there any reason you would recommend NOT doing this? I have some high strength aircraft-grade epoxy that I'm thinking of using. The nice thing about it is that if I ever wanted to remove it, I think I could do it by heating it.

[markm]

It soumds like a good plan to me, but I dont wear seatbelts.

[Rick_L]

The plate's attachment to the sheetmetal needs only minimal strength. Its primary force in a crash, relative to the body, is compression, for which it needs zero strength. I suppose there might be some shear also, but the bolt takes care of most of that.

Plug welds spaced away from the bolt hole will not degrade the overall strength with a heat affected zone or a stress concentration.

I think you're overthinking this one.

[chevynut]

Rick, this is for my 3-point front seat belt. I have the upper reinforcement plate welded into the b-pillar and I need to add two more plates.....one for the reel, and the other for the end of the belt. They will both be on the inner quarter sheetmetal. The plate for the reel will be mostly in shear since the belt goes from the reel to the b-pillar anchor point. That's the one I was most concerned about. If it was the rear belts, the force would be mostly perpendicular to the sheetmetal so I don't think the attachment is very critical. I actually have different plans for the rear belts so I don't have to put the plates under the car.

I was wondering if there was any other reason besides the strength of the plate to not use the epoxy. Let's say the epoxy degraded and failed for some reason. Would the bolt alone be strong enough? In other words, let's assume I bolted the belt to the plate but didn't even attach the plate to the sheetmetal. The bolt would have to tear the sheetmetal in an accident as it pulls the plate toward the b-pillar anchor. Would that be strong enough? I'm thinking it wouldn't since the sheetmetal is so thin.

Markm, you've got to be kidding. I thought virtually all states required seat belts. I personally don't drive without one.

[Maddog]

It soumds like a good plan to me, but I dont wear seatbelts.

Do you smoke cigarettes also?

[Maddog]

Either will work, I do think you're over thinking this too. But consider since the welds do nothing harmful but you know they will never "degrade" and have unknown concerns with the epoxy, weld it.

[chevynut]

Nope, probably gonna epoxy it. ;) My calculations show that the epoxy is stronger than the plug welds, and I'd rather not weld it if I don't have to. The 1/8" plate is actually going inside the car and the reel outside the inner quarter. According to my plans, the only place you'll see the belt is from the b-pillar attach point at the top and down to the floor. If I can make it work, it's going behind the headliner.

[Maddog]

Nope, probably gonna epoxy it. ;) My calculations show that the epoxy is stronger than the plug welds, and I'd rather not weld it if I don't have to. The 1/8" plate is actually going inside the car and the reel outside the inner quarter. According to my plans, the only place you'll see the belt is from the b-pillar attach point at the top and down to the floor. If I can make it work, it's going behind the headliner.

So why did you ask?

Your real concern is the shear of the attachment bolt, if anything is going to fail that will be it.

[chevynut]

The reason I asked is to see if there's any other reason I shouldn't epoxy the plate that I was over-looking besides the strength. The bolt is grade 8 and 7/16" diameter so it should support around 13,500 pounds of shear load. If that's good enough for a seat belt mount then the epoxy should be good enough too.

[Maddog]

I haven't looked at every car, and mostly older ones at that but all OEM have multiple spot welds. I think your inability to reproduce the same quality of spot welds at the plate is the major concern with welding other wise I would weld it. Hopefully the epoxy holds for years to come and the plate doesn't become movable.

[Rick_L]

My main argument against the epoxy is how do you know you achieved the bond strength advertised? I work with epoxies, urethanes, and potting materials at work often. What we find is that they are very sensitive to what you're applying them to and they are also sensitive to process variables and surface prep. Also to shelf like, storage conditions, etc. I think you'll know whether you have good plug welds. If you don't think they are strong enough, increase the quantity of them, or increase the diameter of the plug.

[chevynut]

Rick, did you change your position on this?

"The plate's attachment to the sheetmetal needs only minimal strength."


If it only needs minimal strength, then what's the big deal? I assume it's because the plate is in shear? The way I look at it is that the epoxy bond strength is about 4 times the shear strength of the bolt. Even if I only got 25% coverage, It would be just as strong. I think I can do way better than that. Epoxies are used to put airplanes together and to hold panels on cars. I think today's epoxies are very good, and this area has no moisture, heat, or anything like that.

I have never done a plug weld through a 1/8" plate onto 20 gauge sheetmetal. Seems like it might be trickier than two pieces of sheetmetal together. The 1/8" plate is on the side that I need to weld from.

[Rick_L]

In a way I changed my position. In my original reply, I was thinking of a seat belt mount on the floor - on a horizontal surface. Then you came back and said you were doing the 3 point mount - on a vertical surface. There is a difference in the loading between the two.

As for the epoxy vs. the welding - well I know which one I'll count on if I weld it. Also, I did my plug welds from the thin side - the easier side.

[chevynut]

If I plug welded it from the thin side it would be from inside the inner quarter. I don't think that's feasible. I should have done it when the quarters were off the car, but who was thinking about seat belts then? ;)

I'm a little concerned about being able to plug weld reliably from the thick side, although I could try it on some scrap pieces.

I suppose another option would be to use grade 8 bolts to mount the plates.

[Rick_L]

Why don't you put the 1/8" plate behind the sheet metal? Then you could plug through the sheet metal rather than the 1/8" plate. And the retractor mount bolt would also hold some of the load. In fact I would not do it any other way. If you epoxied it on the outside, you would be relying 100% on the epoxy.

[chevynut]

I thought the whole point of the mounting plate was for it to be on the opposite side of the sheetmetal as the retractor. If I put it inside the quarter, it will be on the same side as the retractor. I welded a nut onto the plate, so I would have to cut a hole in the sheetmetal for clearance, so I don't see how the bolt would support anything. I would have to take the nut off the plate and use a loose nut, which I could do.

If I epoxy the plate to the "outside" which is inside the car, both the epoxy and the mount bolt are holding the retractor.

Maybe I didn't make it clear. The retractor is going between the inner quarter and the outer quarter. The plate would be on the inside of the car, just behind the side panel. So the sheetmetal would be between the retractor and the mounting plate. Maybe I need a picture. :)

[Rick_L]

Ok I get it now.

[markm]

Do you smoke cigarettes also?

Hell no , but I am not a pussy either. The goverment and insurance companies want met to so. hell no.

[Maddog]

OK, but you lost me after "want"

[Geoff]

Lazlo,

Please post pictures of how you're doing your seat belts. I always like the creative solutions you come up with.

[chevynut]

After doing more work on my seatbelts I think I've changed my mind on the attachment of the retractor reinforcement plates. I don't want to plug weld the plates in so I made two more 1/8" steel plates that go on the retractor side of the sheetmetal the same size as the anchor plate. I'm going to use five or six 1/4" grade 8 flathead screws through both plates, sandwiching the sheetmetal between them. I think the screws will be stronger than plug welds anyhow, and I won't have to worry about making a good plug weld through a 1/8" plate onto the sheetmetal. The 7/16" bolts will also go through both plates and the sheetmetal. ;)