[DML] Re: Anatomy of A Failed TAB

["Toby Peterson" <Toby@xxxxxxxxxxxxxxxxxx>]

In the interest of accuracy I thought that I should chime in here on a couple of points.  I will comment on each of the statements that have not been cut from the original post.

5)  Corrosion pits are indeed the primary initiation sites for the start of cracking.  However, the original bolts do not have an adequate margin of safety above the stresses induced by preload due to torqueing.  They are not "more than strong enough".  They are just barely good enough to survive for a while.  You can find my analysis back in the archives someplace.

6)  Stretching is caused by the tension stresses of torqueing (see above) as well as some fatigue effects.  The primary cause of loosening in the joint is likely wear on the flat washers that are also critical to the health of the joint.  Tighteneing the bolts will allow for some life extension, but (as you indicated) is probably not a long-term solution.  

7)  Again, stronger bolts are imperative in this joint, along with either corrosion-resistant materials or adequate plating.  Plating is sacrificial, meaning that it actually corrodes away instead of the bolt that it is applied to.  The plating will eventually dissipate, leaving the bolt unprotected.

8)  Inconel 718 can be found in many heat treat ranges, from "wimpy" (same as the original bolts) up to the upper range of about 270 KSI.  TOBY-TAB's were heat treated to 248 KSI, so they are about 50% stronger than stock originals while still having adequate ductility so that they won't rupture at high loads.  The advantages to the TOBY-TAB's are in the extremely high strength and also the corrosion proof materials.  I do agree with the very last statement with one important caveat - the joints should always be inspected on an annual basis because of all the other elements in the joint that can go bad.  The TOBY-TAB's will last forever if the rest of the parts involved in this load path are kept healthy.  However, a joint that becomes sloppy for other reasons, such as worn washers, bad bushing, or loss of bushing attachment bolts can kill even a TOBY-TAB.

Toby Peterson  VIN 2248 "Winged1"
DMC Northwest
DeLorean Parts Northwest


--- In dmcnews@xxxxxxxxxxxxxxx, "Steel Wings" <steelwings@...> wrote:
> 5. The stock bolts were more than strong enough for their designed and intended utility. Unfortunately, they simply outlived their useful life expectancy. The primare cause of failure is surface corrosion compounded by stress.
> 6. Apparently "loose" bolts may indeed be the result of "stretching" due to fatigue. If you observe this symptom, don't tighten the bolt..That's probably not the solution. It's time to replace it before it breaks. Tightening it will only serve to hasten the failure.
> 7. Bolts having a greater hardness over the originals are probably not necessary. However, they should have some cadmium or other corrosion resistant coating on them if you decide to use them. Regardless of what they are coated with, they will one day succumb to corrosion and will have to be replaced again. Be vigilant with this.8.
> 
> 8. Although I do not have "Toby Tabs" , I have been told that if they are indeed made of the 718 Inconel, they have approximately the same tensile strength as the stock OEM bolts that came with the car when new. The primary advantage to having this bolt is not the strength but the corrosion resisitance and they should last the life of the car.Toby Tabs will allow you to replace the originals and virtually forget about this issue.
> 
> Steelwings
> #1559




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