Torque Specs for Aluminum

Can't they go through the base and have nuts on the back?

Del

No, the base is machined to another engineers specifications. I cant modify it in any way. Plus its 4 inches thick...

Well ask the other engineer!

Del

Are the holes blind? What's the threaded depth?

In a lot of applications where there is to be high stress on aluminum threads they use steel inserts.

Here is some info calculate thread engagement depending upon tensile strength of the material.

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm

my exact thought

Use a high tensile strength bolt tightened until the aluminum threads strip out. Note the amount of torque required to strip threads. Then drill and tap the aluminum for a steel insert that can hold the load you will actually need.

I did ask the other engineer, hes taking forever to get back to me. They are not highly stressed, they're holding a frictionless torque transducer on, so no need for steel inserts. I just dont want to ruin a few hundred dollar piece of aluminum by over torquing, that's all.

I expect that the torque sheet you have is for the bolts only and has nothing to do with any base material as normally, the sheets are based on a bolt/nut combination. If you know the grade of the bolt you can find the yield strength of it and compare it to the yield strength of the particular grade of aluminum you are using and do a calculation to see how much less the torque should be for your application. Personally, I would do the calculation and then deduct for a factor of safety, just in case not everything is perfectly machined in the aluminum base plate.

And, check the depth of the tapped hole in the aluminum. The more thread engagement you have, the better.

As Lyn's test show the quality of the treads is important. Those test where done on steel. Also as you change the tensile strength of the material you have to make thread length changes. Generally on steel the length of thread engagement is one thread diameter. On aluminum the length of thread engagement is generally two thread diameters. To do it right use the calculations in my previous post to calculate the length of thread engagement need for the tensile strength of the aluminum.

If your material is 4" thick how deep are the threads. Again Lyn's conversion to pound feet is correct. That is not a lot of torque speck for aluminum that is 4 inches thick we are constantly torquing front suspension parts that are 1" thick to 105 Ft pounds and not had any problems, I think someone is trying to over think this one. Just my humble opinionDuke

The female thread being in the softer material the shear resistance at the bolt diameter should be higher than the maximal force resulting from tightening.

You have to know several factors in order to do a safe job:

- mechanical values of the aluminum as yield limit (Re) and friction coefficients for the bolt head against the surfaces it clamps.

according to the thread profile you should compute the height(h)of the thread in the aluminum part at the outer bolt diameter.

For une turn the shear area will be A1=d*pi*h, the maximal shear force per turn will be

F1= A1*0.57*Re

The maximal force comes from the torque equation:

F= M/(0.16*P+µ*(0.5*d2/cos(a)+0.2.5*(D+Di))) where

M= tightening torque

P= thread pitch

a= flank angle

d2= mean diameter of thread

D= outer diameter of contact bolt head

Di= hole diameter considering as well the hole chamfer

The number of turns will be N=F/F1 the depth will be L=N*pitch + the parasite length on bolt and thread end.

It will be good if N is increased with at least 2 threads since end threads are not complete on the bolt.

In many books you could find for aluminum a recommended engaged thread length of at least 2...2,5 d but I had some surprises in soft alloys so better compute according to material portance.

I would also recommend after the computing to make a test since many times Re values promised are not delivered.

I agree with Lyn 500 inch pounds is not a lot of torque for a 4" plate or a 1/2-13 bolt

I think that would be 42ft/lbs. 500in/lbs / 12

The bolt goes 3/4" into the aluminum. The engineer finally got back to me and gave me that formula...and then a link to a site that would calculate it for me . And yes the spec sheet is a generic one that the shop uses. I guess its only for a nut and bolt or a nut through tapped sheet metal.

"holding a frictionless torque transducer" should not require a 1/2 inch bolt.

Torque the thing to 15 foot/pounds with some thread locker on it and move on.

Haha I agree 100%! If it were my call thats what I would do. I think the 1/2 bolt is overkill itself.

A Suzuki Swift lug nut torque spec. is 50 foot pounds, and that's steel to steel.

!5 still gets my vote.