Technical materials question. Structural engineering...

[Stoo]

I would like opinions/views from engineers or like minded please.

Say for instance you had an engine mount.

Made from steel bracketry with a solid polyurethane bush of 75A durometer.

Bolted to the chassis and gearbox to the manufacturers required torque settings.

Now take the same mount but make it from aluminium bracketry. Same bush, same torque settings.

Would one or the other material be more likely to cause the threads in the gearbox to strip?

I'm an engineer myself, and I don't believe it would.
Regardless of material, the mount is still torqued in to the same torqulevel and has the same hardness of bush. The forces applied are going to be equal are they not??

Anyone care to share their opinion??

[Lee]

The only way the threads are going to strip is if the mount is yanked out under an extreme amount of force. So the question is, would the aluminium fail before the threads in the gearbox strip? If it would, then I suppose the answer is, the steel mount could possibly strip the threads, but the ally mount would fail before this happened?

Ally is also mildly malleable and may act as a mild shock absorber against any shock to the bolt, which could prevent it from stripping, or at least make the tolerance higher.

I'm only speculating though as I never got that far into my mechanical engineering course back at college lol

[mark69]

it all depends on the quality of the ally, we ally weld at my work and some ally can be strong as and some can just bend when you turn the welder on,
if you braced it enough and use quality metal then it sound be alright i recon.

[MattBrown]

As Lee said.

It's softer, so will have a microns more give in it.

It will also transfer heat slightly better, conduct better etc.

It obviously won't rust, but will oxidise, so this could force the mount away from the gearbox, but that's a 1/100000 chance.

I would say no, it won't ruin the threads.


What inspires the question? Sounds like mechanical hnc?

[Stoo]

The aluminium type would be billet.

There's a trader on the DC5 forum selling the alloy variety and claiming the steel variant are likely to crack and cause the strippig of thread type issues.

I said I had tr steel type, very happy etc and then he comes at me with all sorts of pish that he has seen them crack an gbox threads pulled.

Can't and won't provide evidence however.

In my research and yrs of DC5 experience, I have never ever once heard of this problem.

[Stoo]

Whoops

Double entry...

[Andy]

Sounds like hes just trying to scaremonger folk into buying ie
"ZOMFG must buy his product NOWW!!!"
Ive never heard of that happening to any car.
I dare say the shell would be splitting before that king of carry on happened

[dhdev (Oli)]

Is the design of the mount the same?
Billet is merely a description of the initial form of the material, not of the material itself. There are countless different compositions of aluminium alloys and an equal number of treatments to improve mechanical properties.
I'd imagine that the deflection in the bush will be several orders of magnitude greater than either the steel or aluminium alloy mount. So a change of material should have no bearing on the stripping of gearbox threads. I'd imagine that the only benefits may come through the increased corrosion resistance and potential weight saving.

[Saloony]

Ive only ever seen this happen on a 1.4 Civic, 5years ago. And to be fair, on sight of the box it was the actual ally casting itself. Certain amount of corrosion if you can call it that, basicly the thread had turned to poweder.

[Sturge]

From your follow up post the cause of the thread stripping is caused by the mount cracking which will allow movement which could result in the stripping of the thread.

Theoretically (assuming both designs are sound) yes, there is far more chance of a welded steel mount cracking than there is of a billet ally one due to the stresses in the heat affected zones (or a cast mount) not liking vibration. If it's billet steel also though, I can't see it

[dhdev (Oli)]

Good point about the HAZ and susceptibilty to fatigue. However I'd like to think that I'm safe in assuming that honda tested their engine mount design sufficiently to be able to deal with the extremes of the vehicles life. Therefore a new standard mount shouldn't shear, if however we're now comparing an old welded steel mount to a new billet alloy mount, the playing field has changed somewhat :thumb:

[Stoo]

It's a comparison between two aftermarket mounts. Same company.

One is billet alloy. One is steel.

Same durometer rated bush.

Basically a trader on the DC5 forum is discounting the steel variant because he says he has seen/heard of issues but cannot or will not produce evidence of this. Incidentally, he has just become a dealer of the alloy type. :s

[dhdev (Oli)]

If the stripping of threads is related to the collapse of the mount, then I can't possibly conclude whether the Steel mount is better than the Al Alloy mount. The way the original question was worded seemed to imply that the mount's material would cause a bolt to strip the threads in the box somehow.
I just suggest doing some digging and finding out if the failure of the steel mounts is substantiated, by the sounds of it you already have and the Al mount guy is full of it.

[Sturge]

But if the same company makes both, I'd imagine if there were issues they'd now be redesigned or discontinued

[Stoo]

This is the point i am trying to make.

He is implying that the steel mount is more likely to cause the thread to strip than the alloy ones!

No evidence, no proof, just his hearsay

Ive done plenty of digging, and i cannot find any issues relating to his claims

Even guys running 500WHP+ dont suffer this fate!

[Stoo]

This was his PM to me when i asked him what the issues regarding Steel Vs Alloy mounts



Stoo,
We have had issues with bolts working loose and snapping on solid front mounts and also gearbox threads eventually stripping under normal torque settings, we think it is due to direct shock loading across the thread pitch which might fatigue the alloy thread eventually.
We dont have scientific evidence of this but many racing assumptions are based on our 25+ years racing car building experience and not direct science. We did helicoil the DC5 alloy threads (which we first started doing on alloy gearboxes in 1984)and this cured the problem but it is more of a "corking a leaky dam"than building a proper dam.... http://www.itr-dc5.com/forum/images/smilies/icon_idea.gif
Also we consulted some of our dealers who have fitted both Avid and Inovative steel mounts and the story is the same irrespective of make.Thats why we opted for Alloy Billet.
Hope this helps you in your decision.
Absolute pish in my opinion.
Regardless of mount material, the forces acting upon the threads are going to be similar or the same.

Here is a picture of the mount in question (Alloy type)
http://i889.photobucket.com/albums/ac99/j0shmills/Innovative%20Motor%20Mounts%20Install/IMG_0515.jpg

http://i889.photobucket.com/albums/ac99/j0shmills/Innovative%20Motor%20Mounts%20Install/IMG_0518.jpg

Steel variant (middle of the 3)
http://www.innovativemounts.com/v/vspfiles/photos/90650-2.jpg