Need advice with PHH (1 Viewer)

[AJP]

Basic physics of welds makes the "weaken/crack the actual pipe" pretty unlikely. Enough so that I wouldn't worry about it. The break should follow the weld hardened zone and stress riser of the tab long before it affects the relatively stress riser free pipe.

Could be ... and I will ask my son (who managed to teach himself some mandarin) in EnSci at UofT to gather up his brilliant Chinese buddies to write a paper on this (which I wont understand!) in the interim I am betting on two things I DO KNOW:
1) if the metal used to form the tab is stronger/thicker etc than the tube wall - the tube wall will fail where the heat of the weld has annealed (softened) the tube wall -- ie the tube wall will fail before the tab, and 2) Murphy's law -- a well accepted principle of physics will prevail.

Now I understand that this would not be catastrophic as you can always bypass the tube with a length of hose -- not as elegant but just as functional. And if Pinhead is correct you can always purchase a new tube cheaply (?) and reinstall it without the lower bolt. But as I live in Canada and US dealers are not supposed to ship parts to me in Canada and Lexus Canada have decided to place a usurious profit margin on every part I purchase from them ... I would not go that direction if I can avoid it.

 

[120mm]

in the interim I am betting on two things I DO KNOW:
1) if the metal used to form the tab is stronger/thicker etc than the tube wall - the tube wall will fail where the heat of the weld has annealed (softened) the tube wall -- ie the tube wall will fail before the tab, and 2) Murphy's law -- a well accepted principle of physics will prevail.

I'm not going to address Murphy, but no way is the tab stronger than the tube. The tab, being flat, is a stress riser rich zone. The tube is many times stronger than the flat surface, such as a tab, because the force vectors prevent stress risers from forming.

Welds do not fail in the annealed area, but in the hardened zone. That's why the best welders reheat electric welds by running over them with "soft" heat from a torch and annealing the entire welded area.

I've repaired enough tube fuselages and am pretty confident this.

 

[120mm]

Actually, it's probably more likely the tab will break where the face of the bolt head contacts the tab, near the hole. That is where the biggest concentration of stress risers should be.

 

[gsdog2]

Doing the PHH removal and bypass was pretty simple. Hardest part was rocking the tube for 8 minutes. What I've never seen mentioned is the smaller coolant hose in front of the PHH which goes up underneath the manifold. This little guy busted on me in the middle of the desert. Way more of a pain in the ass than the PHH. So if your PHH is going bad, I'd assume this smaller hose isn't far behind. I'd check it also.

 

[AJP]

I'm not going to address Murphy, but no way is the tab stronger than the tube. The tab, being flat, is a stress riser rich zone. The tube is many times stronger than the flat surface, such as a tab, because the force vectors prevent stress risers from forming.

Welds do not fail in the annealed area, but in the hardened zone. That's why the best welders reheat electric welds by running over them with "soft" heat from a torch and annealing the entire welded area.

I've repaired enough tube fuselages and am pretty confident this.

I didnt say that the "weld" would fail in the annealed zone ... I said that "the tube wall will fail where the heat of the weld has annealed (softened) the tube wall" ... I'm very glad it worked for you. I wont (and would not) depend on that for me. Have you by any chance been working on any aircraft I might be flying in?

 

[120mm]

I didnt say that the "weld" would fail in the annealed zone ... I said that "the tube wall will fail where the heat of the weld has annealed (softened) the tube wall" ... I'm very glad it worked for you. I wont (and would not) depend on that for me. Have you by any chance been working on any aircraft I might be flying in?

Again, annealing, or softening does not cause metal to fail. Hardening along welds, cause metal to fail, where it is hardened. That and stress risers, which are prevalent among materials with sharp edges. (Like those found on flat pieces, like tabs) Tube by definition do not have sharp edges, as they are tubular.

I'm sorry you find basic metallurgy so difficult. Maybe you don't know as much as you think you know.

FAQ: What is the relationship between hardness, microstructure and toughness in steel heat affected zones?

"Low toughness may be experienced in Hardened Area Zones due to the presence of inherently brittle microstructures, including unusually coarse microstructures."

Note that "toughness" is a function of strength plus malleability. Toughness is caused by ANNEALING. Hardness does not equal toughness.

 

[Pin_Head]

I would venture to guess that the tube will never fail by the type of metal fatigue imparted by rocking it back and forth because it is stiff and won't bend, compared to the thin and flat tab. It is a specious suggestion.

 

[Texasknowhow]

If you want to get the lower bolt out, you can try the gear wrench method I used:
PHH OEM style close up pics

Chris, Awesome pictures and additional write up to what others have posted. Nice work. Clay