Hi-lok cracking in threads 1

[Deltaengr]

We have had a small number (4 out of 2000)of hi-lok fasteners fail during collar torqueing. The collar hex did not shear and the fastener failed. The failure mode was determined to be a bending induced crack which propogated to failure during collar torqueing. The installation is a 1/4" hi-lok, spherical collar and washer in about a .003 interference hole. the collars were tested and were shearing at the appropriate torque. We have UT inspected the other installed fasteners and found no cracks on installed pins (.031" UT standard)
Has anyone had similar failures?

 

[Deltaengr]

I forgot to note that the crack started in the root of first or second thread (shoulder end of threads).

 

[WKTaylor]

Deltaengr...

A few questions to refine details of the problem.

1. Exactly what HL Pin/collar combo is in use? If pin has exceptionally long grip, are all the pins "straight" [try turning them in a machinist's "V" block? NO wobble is acceptable].

2. Did the pin-thread fracture occur before or after the point where the collar would contact the mating surface [IE: did it bind-up during instl or run-smoothly down to surface contact]?

3. Is fastener perpendicularity to the collar-mount surface [opp HL pin head] within HL limits, at holes where failures occured? Was the problem repeatable at the same hole (IE: multiple tries, multiple failures)?

4. Are HL collars "riding" next-to, or into, a fillet radius? Look for fine scoring or rubbing along an arc [part wurface and collar flange-face) to indicate if collars are "riding" an inclined surface or into a fillet.

5. Check HL collar squareness in the pin threads. Look for collar wobble when threaded on (re-try collars that caused the Pin-threads to fail, if available).

6. Have You tried self-aligning collars or Radius fillers [flat or tapered A/R]?


Regards, Wil Taylor

 

[Deltaengr]

1) The fasteners that failed were HL12V-8-10 and -13. They are being used with a HL75G-8AGW collar/spherical washer. We did not specifically check for straightness but we did inspect the hole and it was still within specification after removing the failed pin. I will take a look at others in inventory.

2) No significant preload had been applied prior to failure based on conversations with the techs and verified by inspection of the silver plating on the collar and washer. No issues were noted while running the collar down.

3) Angle of the surface did not exceed 5 degrees and the collar is designed for up to 7 degrees misalignment.

4) There were no riding conditions in a radius.

5) Threads in collar were perpendicular to hex portion. New fasteners installed into the same hole had no installation issues. We did find a few collars in inventory with non-perpendicular threads but we tested them and noted that they sheared at a lower, not a higher torque than required. These defective collars were not found anywhere on the vehicle.

6) As noted we are already using the spherical collar/washer combination.

Thanks for your help!

 

[WKTaylor]

Delatengr..

Only a few things I can speculate-on before departing for the Holidays..

a. The SA collar-to-washer interface lacked adequate acetyl alcohol lubricant to allow parts to freely "slip" into alignment, retaining a bending load.

b. The HL Pin fasteners are BOGUS [forgeries, imitations of certified parts]... or were improperly manufactured [many critical issues with the metallurgy of titanium parts].

c. Bare Ti Pins were damaged by some chemical or mechanical means.

NOTE: I recommend STRONGLY that You contact the manufacturer, or manufacturer's local area rep who services You [who's ID(s) is(are) stamped on the Pins and the collar assy]. Describe the failures in detail and offer to express ship the pieces [100% of fractured pieces] to them for lab failure analysis. A reputable company should accomplish this in an expedient and forthright manner. If NOT [or maybe in addition to the mfgr's lab], then suggest You contact an independent metallurgical lab and have them perform the failure analysis.

Note: I recently dealt with some 1.375 Dia Inconel nuts that were improperly [not] deburred on the threads and had poorly adhering SFL coatings. We discovered this problem when they seized on the mating Inconel bolts [very expensive parts, trashed]. What a pain... and by a "reputable" manufacturer.

Good luck in Your investigation. If I can think of any other avenues to pursue


Regards, Wil Taylor

 

[Stache]

Over the years I have installed to many Hi-lock to count, but we would find the same cracking problem. We had the metals lab test some and we found when the treads were rolled the dies would wear and toward the end of a MIL run the dies were out of tolerance and caused stress cracks. Needless to say the source of the Hi-locks was the problem coming from out of the country (quality control problem).

Stache

 

[DaveWW]

We had this problem about 10 years ago in my last company.
Memory is very vague, but I'm sure the problem had something to do with the speed / torque of the installation tools.