Nylon insert nuts can't be "spun down" with your fingers.
An assembly tech can't forget to put an nylon insert on a nut.
Contaminates on the bolt don't significantly change the performance of a nylon insert.
Nylon inserts don't require extra steps during assembly.
I've seen techs apply loctite, spin the nuts down finger tight, then go to lunch. Come back from lunch and torque the nuts, breaking the loctite bond in the process.
A nylon insert will be effective for a few install and remove cycles. Loctite is one-shot. So if it's a part the needs to be removed for maintenance, the maintainer will probably put the nylon nut back on. He probably won't clean and reapply loctite.
Loctite is theoretically better then nylon inserts, but there are more opportunities for assembly techs to mess it up. Nylon inserts are more real-world proof.
MintJulep did a good job of summarizing the trade-offs. If high strength/threadlocking is needed for vibration resistance, then an adhesive such as Loctite will perform better, provided assembly is clean, correctly done, etc. Nylon inserts are temperature limited to ~ 120 C, while some adhesives can be used up to ~ 230 C.
It really depends on the application. If you want to prevent loosening, thread adhesive (especially the pre-applied style) is much better. If you want to have a fastener that won't allow the joint members to separate if there are loose fasteners (and the joints are taken apart frequently) then the nylon insert fasteners could be better.
In an assembly line application they are both frowned upon. Obviously liquid thread lockers are messy and can be forgotten. (You can get fasteners with thread lockers preapplied as a special order) Nylocks are better, but you do waste a little time compared to a standard nut because you can only install it one way.
And I know you didn't mention the deformed style of locknut. They are cheap, but they are not reusable. And bad nuts can damage the thread of the fastener.
Both nylocks and loctite are generally avoided in aircraft fasteners. However, given the choice between the two, a nylock nut would probably be better.
The reason for this is that the prevailing torque of the nylock nut can be verified at installation. So it is more reliable from a QA standpoint.
There is no way to verify the threadlock function of loctite without loosening the nut. So the use of loctite requires a carefully controlled process performed by trained personnel to guarantee quality assurance.
Wicking loctite is pretty good on previously assembled fasteners with out much lube on them. Very effective at securing carb linkage adjustments on screws that go thru thin threaded levers, NOT long screws going deep into carb bodies. Seems to set up pretty quick even without primer.
Patch bolts are ok, not as good as a full nylock nut.
For a blind tapped hole application they will work if you can't use loctite for one of the reasons noted above.
Castle nuts aren't very good.
It's common that the specified torque doesn't happen when the holes are lined up. Then you don't know what the assembly tech will do. More tighter might yield the fasteners. More looser will not have the necessary preload.
There will be clearance between the hole through the bolt, the slots in the nut and whatever pin you use. Therefor there can be relative rotation and loss of preload.
Castle nuts are ok for applications where the bolt is used as a pin and all you need to do is keep the nut from falling off. They are no good for maintaining preload.
My apologies... I always say castle nut when I mean flex-top expanding nuts.
from mcmaster...
Flex-Top Expanding Hex Locknuts
Stronger than nylon-insert locknuts of the same size, these nuts are made from a single piece of metal. Also known as Flexloc nuts, they have a slotted top that expands when the nuts are tightened—locking them in place and providing superior vibration resistance. Nuts are not rated for hardness. Inch sizes have a Class 2B thread fit; metric have a Class 6H thread fit. Cadmium-plated steel nuts are made from low strength steel.
It will also depend on where it is being used or at least what kind of material it will be oozing onto if it is the liquid choice. I always double check before specing but I recall there being issues with combinations like the blue LocksNuts and ABS where eventually the inserts would loosen in the plastic.
Harold
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