Installing a hi-lok with a nut instead of a collar. 2

[sferrin]

thread2-197752

This question is directed to "WKTaylor" but anybody who could help would be appreciated. He stated the following in an old, closed, thread:

"3.4 WARNING. Shear-rated HL collars 1/2"-and-larger [0.500"D+] and tension-rated HL collars 7/16"D-and-larger [0.437"D+] SHOULD NOT be installed by "hand". The collar-break torques are so high that most technicians will strain really hard to attain the torque; then loose control of the wrench/socket head when it suddenly breaks... which will fly around and damage structure [or hurt someone]. In these cases consider installation of a shear or tension rated nut at similar torque values [HL collar-break ranges]."

The application in question has 1/2" hi-loks in extremely tight quarters (inside the tailcone of a helicopter - tails inside of course). We haven't had any serious injury but we'd like to avoid it. What type of nut would you recommend be substituted for the collar (HST1572)?

I would have DM'd poster directly but I can't seem to locate any functionality of that sort on this board.

Thank you,
SF

 

[tbuelna]

sferrin-

Your HST1572 titanium collar is for a tension type pin. The 1/2" pin has a thread that mates with a .5000-20 UNJF-3B nut. Any aircraft quality tension type titanium self-locking nut with the appropriate thread should work. Here is a good example.

However, there are a couple things you need to pay attention to.

- First, you need to determine the correct thickness of washer(s) required for each pin installation to prevent the nut threads from bottoming out on the pin threads. Below is a cross section drawing of your hi-lite pin/collar installation. You'll note there is a .105" deep counterbore at the contact face of the collar, and the unthreaded portion of the pin body protrudes slightly beyond the material face. The counterbore is there to ensure the collar threads don't bottom out on the pin threads.

- Seond, you need to check that there will be at least one full thread pitch visible protruding beyond the nut locking element after installation. This is standard QA practice in aerospace. In the locknut example linked above, the deformed thread locking element is at the upper end of the nut.

- Third, the torque-off value listed in the spec for your collar is 720 in-lbs. This controls the installed preload. You'll need to develop a torquing procedure for whatever substitute locknut you select so that the correct preload is achieved. Also remember that while the locknut is torqued, the pin must be restrained with a 7/32" hex key inserted in its end.

Hope that helps.
Terry

 

[RoarkS]

STR-10 "Starlite" nuts made specifically for Hilok

 

[sferrin]

Thanks guys. Now to get the customer onboard. They are very picky. Pray for me.