Hi-lok collar substitution with MS nut.

[jonny76]

Is there any document allowing the substitution of a Hi-lock collar with a nut. This has been a common practice, where interferance with surrounding structure does'nt leave room for the collar or socket wrench.

 

[MNLiaison]

Seen it. Douglas DC-9 SRM allows the use of MS nut with washer in lieu of Hi-Lok collar.

Regards

 

[RPstress]

Also seen it done. We did this quite a bit on a nacelle a few years ago. However, as far as I'm aware we didn't have a document okaying it.

 

[FieldTeam]

I have a 707 SRM that allows alternate to hi-lok collar (HL79) using NAS679 tension nut (we used to call these "mexican hat" nuts), torqued to standard torque table in SRM 51.

Saying that, there will be plenty of people who will cry foul with a standard torque table being used as seen in this forum - post- MS21250 bolt torque(and forum - mechanical engineering other topics - bolt stretch versus clamping force)

 

[bf109g]

It's in most of the Boeing SRM's I've seen.

 

[FieldTeam]

We've all seen it done and can reference SRM manuals stating so, but hi-shear considers the hi-lok a fastener system and stresses the proper selection of collar to fastener being used. I don't see where hi-shear allows sub of collar for standard nut.

Ref section C of link:

http://www.hi-shear.com/brochures/H..._scan_filename=Hilok_Hitigue_Installation.PDF

 

[bf109g]

You also need to consider the context of the SRM. Are fastener subsitutions per the SRM acceptable when reinstalling or replacing heavy structure? Probably not when you consider hole/fastener fit and clamp-up.

However, if you have a repair which calls for the use of Hi-Loks, such as a skin patch or stringer splice, and you can't get the collars on, by all means, use the nuts.

Hi Shear may be the manufacturer of the fastener "system," but the Type Certificate Holder determines what is acceptable to maintain their product in an airworthy condition. On the other hand, I don't think you'll get Boeing or Airbus to sign off on replacing an engine mount with nuts instead of collars.

 

[FieldTeam]

Just looking at all the angles.
Bottom line....as the poster stated, he has no reference he can show to allow the practice. As per the reference I stated earlier in the post, I would only be able to show using the nut where a HL79 (1/64 over) collar is called out.
If you can't reference/prove it, it has to be approved through your engineering dept.

 

[WKTaylor]

Jonny76...

I always specify a nut ILO of a HL collar for Large dia HLs used in repair, as follows

Shear rated nuts ILO shear rated collars for 0.4735-dia [-14] and above. Typical HL collar break-off torque-range for 0.4735-dia shear collars is 270-330-in#

Tension rated nuts ILO tension rated collars for 0.3125 or [-10] and above. Typical HL collar break-of torque-range for 0.3125-dia tension collars is 360-420-in#.

Note: a quick check of the torques listed will indicate that the collars break ~10%--15% above the "typical" torque-range of a nut. This excessive break torque is deliberate: it allows for rapid installation and clamp-up with faying and fastener sealant... which inevitably leads to a slight joint (tension) relaxation.

Reason for using nuts on large Hi-Lok pins: collar break occurs almost without warning and varies more than the average mechanic can estimate and "absorb".

If the collar is installed by a robot in-production the resultant force-vector at snap-off can be attenuated by the robot effector and do no damage.

On-the-other-hand, the sudden break when manually installing HL collars (with a hand-wrench or gun) will cause the wrench/socket to inetivitably fly-off. A great mechanic will catch most and protect the surrounding structure... and himself. A less experienced mechanic will reglarly damage adjacent structure/fasteners...and whack their knuckles until bleeding... and cuss every engineer and their kin, in-sight.

A nut, on-the-other-hand, can be torqued in a typical cross-pattern sequence to a precisely established final torque-tension requirement slightly lower than the HL collar break. Structure settling and sealant squeeze-out occur slower and the result is the same as the HL collar install.


Regards, Wil Taylor

 

[A380istoobig]

As per the Boeing 737 classic or next gen SRM 51-40-02 paragraph 7.D.(3) it says, "Where access is not available for the installation of the special collar, it is permissible to use locknuts."

I typcially use MS21042 nuts (MS21042L really, the L indicated a dry film lubrication) in lieu of a hi-lok collar where access does not allow it.

 

[WKTaylor]

A380istoobig..

CAUTION:

Shear rated HL Pins and tension rated HL Pins have MUCH different torque requirements. I have also see shear rated HL Pins crack or break-off heads when the preload is too high... immediately or after a short period in-service.

A mechanic MUST be given specific instructions regarding torque limits for EACH type HL pin and Nut installation.

Regards, Wil Taylor

 

[Rob130]

Will, the C-130 SRM allows a MS or KFN542 nut as a substitute for HL70 and HL86 nuts. The SRM does not specify a torque for the nuts so we use the break off torque for the specified collar. Just our assumption of best practice.

An additional comment on the use of a MS nut for those that use them. The MS nut does not have the 1/16in counter bore that allows for the shank portion protruding above the surface of the structure. An inexpierenced technician might not use a washer and torque the nut so it shanks out and does not clamp up the structure. Useing the KFN style nuts prevent this problem.

Just some general info for those who might not be familiar.

 

[A380istoobig]

Mr. Taylor,

Thank you for your caution, but the SRM covers me.

As per the Boeing 737 classic or next gen SRM 51-40-04 Figure 2, it provides the torque values for MS nuts for both shear and tension head hi-loks.