Tap & thread directly into 7075 or use Time-Sert's insert?

[littodevil]

Hi! This is my first post and wanted to get some opinions from you guys.

I'm retrofitting a brake caliper from another vehicle onto my vehicle and I designed a bracket to bolt a brake caliper onto the car but due to space constraint I'm limited by the thickness of material where the caliper bolt will be screwed into.

The material is 7075-T651, the bolt is M12 x 1.5 pitch

The MAX material thickness I have (I know it's not ideal but it's as much as I have) where the screw goes into is about 10.5-11mm

The torque specs for the bolt going into the original vehicle (not mines) is 63 ft lb but I can only use about half the thread that's exposed on the bolt from the original application vs mines.

I was originally going to use a time-sert or even the big-sert but I learned the material used in those is 12L14 Carbon Steel and the specs is Tensile PSI is 78,000, Yield PSI is 60,000

Yet.. for 7075 T651 Aluminum Alloy, the Tensile Yield Strength I found on matweb says 73,000 PSI

This bolt wont have to come off and on often at all since I don't need to remove the caliper to change out the pads and the rotors are carbon ceramic so unless something happens, I wont have to be removing it much at all.

Should I just thread directly into the aluminum or use an insert?.. if I use the insert should I use the normal Time-Sert one or the thick wall Big-Sert?

Thanks!
Larry

 

[BrianPetersen]

Hah, I recognize your username from another website (pertaining to a certain small car).

I think for this safety critical application, your thread engagement length is marginal. Can you make it out of steel? I'd be less concerned. Can you flip the bolt around and bolt it together from the back, potentially with a steel nut? If you have clearance limitations on the back, can you use a flat-head screw in a countersunk hole bolted together from the back (used essentially as a stud)?

 

[gruntguru]

Does the caliper mount normally include dowels or shoulders to isolate the bolts from shear?

je suis charlie

 

[littodevil]

Brian, haha yea I've used this same username for over a decade.. my first business was designing titanium alloy parts for r/c cars back in high school, then I did high end headlight design and retrofits and now I own an auto body and mechanic shop and been working on performance vehicles for about 10 years lol.. time flies..

I could make it out of steel but isn't 7075 aluminum stronger than.. some steel? The mount is actually very small so I'm not concern with flex... I can plate it to prevent corrosion but I assume your talking about the threads. There isn't any room behind the bracket to mount the bolt as a stud.. I thought about that.. The flat head screw countersunk sounds like a good idea and I thought of it but wasn't sure if it was stronger.. or better.. there is enough room all around the bolt to use a counter sunk screw.

Gruntguru, no the caliper originally doesn't have dowels or shoulders to isolate the bolts from shear. This is a radial mount caliper so the bolts don't bolt in from the side.

Any more ideas from anyone else? Still deciding if I should tap or use an insert...

Thanks!
Larry

 

[littodevil]

Actually that countersunk with nut might not work unless I also use a standoff on the screw before the nut goes on.. the nut might not be able to go on because where the nut would eventually secure and sit.. is a counter bore area wide enough just for the diameter of the socket head style bolt that is originally used. Using a standoff would push the distance away from the height of the counter bore but.. it just seems like a lot of pieces involved at this point.. finding and machining a stand off that can take the compression of the nut and stud.. I don't want to have to harden the stand off.. hmm..

Larry

 

[Tmoose]

Hi littodevil,

You said "This is a radial mount caliper so the bolts don't bolt in from the side."

Got pics of the caliper and bracket?
Is this an automotive application using motorcycle brakes?
I'm a little surprised to hear a non-motorcycle OEM uses radial mount calipers.
But then I don't get out as much these days.

 

[littodevil]

Tmoose, no this is a Porsche caliper made by Brembo that's used on Porsche's PCCB system.. the Porsche Carbon Ceramic Braking system, it's a very high end caliper with ceramic pistons and all that. It's also forged and very light, I'm retrofitting it onto my Tesla Model S P85D so I can knock off 20 pounds of rotational mass PER corner and hopefully have a 0-60 mph time faster than 2.6 seconds as tested by motor trend hehe..

I don't have easy access to get the pictures onto here but for now this is the exact caliper I'm using for the front, you can see the radial mount design.

http://www.6speedonline.com/forums/...ale-wanted/366490-fs-pccb-6-pot-calipers.html

Larry

 

[tbuelna]

Given the high cost of the components you are using for this retrofit, I would recommend using the best quality thread insert you can find. However this would not be a Timesert. A lightweight self-locking Keensert would be a good choice, but the standard insert length is more than the mating part thickness it will be installed in.

 

[gruntguru]

I have experienced a number of race car applications where hardware is threaded direct into 7075 T6 with no failures of the threads. I suggest you thread a scrap piece of 10mm 7075 plate and torque a bolt to failure. You might be pleasantly surprised. You could repeat the test with a variety of inserts if readily available.

je suis charlie

 

[mikeneal]

Not exactly same application but I have a 316SS brake caliper bracket on a boat trailer and it is .375-16 thread and has about .25" engagement. I hate it and always use some thread locker (typically blue) when I assemble it. If it was me, I would put a keensert in your 7075 bracket.

 

[littodevil]

Thanks you for the feedback guys!

gruntguru I most likely will thread into a spare bracket and see what it will torque up too.. but..

Wanted some more opinion...

If I can torque the bolt comfortably to it's torque spec on my bracket... what would the benefit be to using an insert then?

Does using an insert make my part "stronger" in terms of not having the bolt come out.. or does it just ensure the threads dont wear out if I remove and reinstall it a few times? Or a little of both? Sorry if I was an expert at this I wouldn't be posting it here so thank you for your time, patience and understanding guys!

Oh yea.. the keensert I found goes down into the hole 18mm but I believe the locking pins go down 10.7mm.. if I could make the part 11mm.. can I just install it and machine off the back of the keensert?.. I know it's not ideal but.. that's better than nothing right? haha..

Is there anything better than a keensert? I like how it has locking pins that keeps it from turning. Thanks for the idea tbuelna and mikeneal!

Larry

 

[MikeHalloran]

Like gruntguru said, take an exemplar of the bolt you plan to use, and thread it into an aluminum plate that's as thick as the plate you intend to use, and torque it >until the bolt breaks<.

If the bolt breaks, you are probably ok.

If the threads in either bolt or bracket strip, then you don't have enough thread engagement to use that material combination.

The whole idea of only 11-ish mm of thread engagement on an M12 bolt in a safety- critical system makes me ill; I just wouldn't do it.

Why do you have so little bracket thickness there? The calipers are normally used with much bigger rims? Something like that?



Mike Halloran
Pembroke Pines, FL, USA

 

[Screwman1]

In aluminum I would generally want to see at least 1.5 fastener diameters of engagement into a nut material for a class 10.9 bolt. The use of the insert will increase the stripping and tensile strength of the threads due to the larger diameter involved.

Like Mike said; given how critical brake systems are and the forces involved, I would not due something like this without being able to actually test parts to failure. If the threads pull out; what will be the effect on your stopping distance? You know that the time when maximum load is on those bolts will be when you need to have maximum braking capability.

Scares the daylights out of me.....

 

[littodevil]

Guys I understand your concern being this is a safety critical application but that's why I'm on here asking for ideas that maybe it's something I haven't thought of yet.

MikeHalloran, I only have so little thickness because the calipers were designed for a Porsche but I'm retrofitting the entire carbon ceramic braking system onto my Tesla and the steering knuckle design limits where I can place the caliper around the rotor and the only place I could place this caliper, is also where the stock mounting ears are for the Tesla caliper. Unless I use a larger rotor to push the caliper further out, I don't have any other solution. I spent $20,000 dollars on these brand new carbon ceramic rotors and $2500 dollars on the calipers and they were suppose to go on another project vehicle which I scrapped and now I want to use them on this vehicle so I don't have the luxury of just buying new parts and starting the R&D process all over again.

I could make the bracket out of steel or something stronger but I was just thinking how the material specs for 7075 are actually similar to some steel out there.

If the purpose is to keep the threads from pulling out of the aluminum then I will definitely use a threaded insert either way. In all reality I don't think I can torque the bolt until it breaks with the amount of thread I have, but I also know in all reality I'm probably just marginal on how strong it actually needs to be in real life. There are people out there taking much less in consideration and retrofitting brake systems and making brackets that I feel would fail way before mines would but.. I still want to get the opinion of people here that's why I posted this.

So from what I've read the limitation isn't so much on the material or bracket itself breaking but the threads pulling out, in that case I'll use a Keensert insert and also test the bolt to failure and see how much I can torque it down.

I can probably get away with 12mm of thread engagement if I start shaving material off of the steering knuckle on the car...

Thanks for all the feedback and I'm still all ears!

Larry

 

[tbuelna]

littodevil-

Using a steel key-locked thread insert (like a keensert) in an aluminum part serves several functions. First, the larger thread diameter at the interface between the insert OD and aluminum tapped hole provides a better match in tensile strength capability with a steel bolt. Second, the keensert's self-broaching keys are very effective at preventing the insert from rotating after installation. Third, thread inserts like a keensert are used to provide an internal thread locking feature for the mating bolt.

It would help if you can provide a picture of the part the caliper attaches to. Have you checked to see if there is room to install a press fit D-head stud or flanged nut from the back side of the caliper mounting holes?

 

[berkshire]

If you put a threaded insert in , do you still have enough edge distance from the edge of the hole to the edge of the part?
B.E,

You are judged not by what you know, but by what you can do.

 

[littodevil]

tbuelna, thanks so much for explaining!

The problem with putting a stud or anything that self locks from the back side of the bracket.. it takes away from the thickness of my bracket which i don't have a lot to begin with. And the second problem is.. unless it's a jet nut or something small, I wont be able to secure the nut on the end of the stud where it comes out of the caliper since I wont be able to get a socket or anything unless I use a stand off since it's like in a "counter bore" like area.. sorry for the bad explanation.

Anyways the stock bolt is a M12 x 1.5, I can get the inserts from McMaster but they come in M12 x 1.75 or M12 x 1.25, if I wanted to switch type of bolt used.. which one should I go with? I looked up the difference between using fine and course thread and got the general idea that fine threads I can exert more torque but they are easier to cross thread.. The stock bolt with the stock caliper uses M12 x 1.75.

berkshire, yep plenty of material! That's probably the only good thing! haha..

Larry

 

[Screwman1]

What is the property class of the current bolt? Make sure to check things like thread length and shoulder diameters against the appropriate standards- many caliper attaching bolts are specials dimensionally.

 

[BobR101]

Read a bunch of this but i didn't see any mention of testing the threaded connection at temperature. 7075 ultimate strength quickly diminishes with temperature. Steel has a far less fall off.