Aluminum manifolds clamped with steel tie rods

[kcj]

I am looking for design or experience info on clamping a stack of aluminum manifolds with steel tie rods.

Long past, I had an application with about 3 or 4 D03 sandwich blocks, aluminum totalling about 5 inches thick, clamped under a D03 valve with steel #10 Sun stud kits. Due to differences in thermal expansion, the joints never worked right. If tightened when hot, they got loose enough to weep at low temps (-25F or so). If tightened when cold, they overstressed at higher temps (160F maybe) and occasionally broke the tie studs. The answer was a rule of thumb of no more than 2 or 3 aluminum blocks under a D03 in our applications, no further problems.

On the other hand, I inherited a design with about 18 inches of various blocks 4 x 6 inches in cross section, clamped with (5) 7/16 threaded rod studs. Despite my worries, about 20 of these joints have worked fine for over 10 years now.

The project of interest involves a single aluminum block about 6 inches thick, 2024 or 6061, clamped to a D03 pattern on another much larger aluminum manifold base by (4) #10 Sun stud rods .

In theory, it’s a classic bolted joint with elasticity of the studs and of the manifold determining the stresses, and the joint could be calculated. In reality, how much cross section of the aluminum block deforms is unknown, and I don’t think any FEA that we have is sophisticated enough to get meaningful information.

I don’t know it would be a problem, but don’t have a warm fuzzy sense over this. My thoughts so far:
I can counterbore the nuts down into the block and shorten the effective length of aluminum/steel in the clamped joint. This reduces the thermal expansion difference, but also the flexibility.

I can counterbore the aluminum and use a long stud with long steel hollow tube over it. This is similar to the clamping method used on high temperature piping flanges, or engine exhaust manifold joints.

So, any manifold or bolted joint experts out there with rules of thumb or ideas?

Tks,
kcj

 

[jbthiel]

One thing to consider besides the tie rods - the O-rings. The material type and durometer have a significant influence on the amount of movement that you can tolerate. I had fretting problems with 70 durometer Buna-N that went away with 90 durometer. The 70 durometer was the valve manufacturers standard solution, as their experiece was that harder material could not seal the occasional imperfect seats found in a production environment.

As for the current design, maybe stainless steel bolts? I think they have a higher rate of expansion than steel, but still less than AL.