Fastener Substitution NAS 1008

[boo1]

We are looking to replace fastener NAS1008 1/2"-20, CRES, A-286 (UNS S66286) per AMS 5731, 140,000 PSI Min. These are used in wetted military marine environment, where galling, cross threading and fouling is a problem. We would consider a material substitution in the similar strength, and corrosion resistance (Inconel 686, ect). We are looking for a specification or recommendation for 1/2"-13 coarse thread fasteners.

 

[dinjin]

Any loosening or error is greater for course threads.
Why are you changing these if it is a military application and the screws are specified?

 

[boo1]

galling, cross threading, fouling from salt

 

[TVP]

boo1,

A couple of questions for you...

1) Do you absolutely have to use a standard military/aerospace fastener like NAS, etc. or can it be a custom manufactured part?

2) Quantity needed?

3) When you say "fouling from salt", does this include actual corrosion (red rust, etc.) during use? Are the fasteners disassembled and reassembled often?

4) What are the mating internal thread details (material, coating, etc.)?

In general, changing from fine to coarse thread will improve the galling and cross-threading situation. Are you using any type of lubricant during assembly? This is an absolute must for stainless steels and nickel alloys, especially if you are trying to generate high clamp forces by using high assembly torque.

 

[boo1]

Since it is a military application, traceability and drawing requirements spec items make it easier. McMaster-Carr did carry a commercial product in A286, 1/2-13.

Each assembly uses 1000 of this fastener. Fasteners are typically removed every few years. They go through an aluminum extrusion, providing a clamp up. I have seen salt and aluminum byproducts on the threads. They are installed dry torque to 600 in-lb.

 

[dinjin]

Why would you dry torqwe these. Check with loctite and see what they say about a coating for this application.

 

[mighoser]

Aluminum + Stainless = Galvanic Corrosion. I'm guessing the aluminum is oxidizing heavily which makes removal/installation a nightmare. I'd try to isolate this galvanic couple with a coating. Solid film lubricant? Alodine?

 

[boo1]

We changed to wet assembling with Pro-Seal trying to eliminate the galvanic cell.

Ended up with specifications:
ASME Specification ASME B18.2.1
ASTM Specification ASTM A453 Grade 660