E-Rivet Mystery Question 6

[Chiefmarty]

After years of frustration, research and practical experience I have failed to find a rational explanation for why the USAF, and industry, has replaced the D and DD rivets with the E-RIVET for retrofit maintenance.

I have experienced more damage to the aircraft (C-130 and A-10) than could possibly ever be saved in heat treat and refrigeration expense.

The E-rivets are very difficult to shoot: they must be shot in a single burst because they work harden rapidly and will not take a second hit to complete the buck tail, resulting in drilled out rivets or damaged surrounding structures. Apparently the E-Rivet is adaptable to squeeze equipment, that can rarely be adapted to maintenance repairs and modifications.

On the other hand, an annealed D or DD rivet will shoot much easier and is a very forgiving rivet to shoot.

Does anyone out there in the repair and retrofit business have any information with reference to the E-rivet (MS20426E)? This seems to be the best kept secret in aircraft structural history.

Thank you for your time:

Frustrated at Hill AFB, Utah

Martin Tobert, Contr.
Lear Siegler Services
309 AMXG/MXAYS
Engineering Tech
801-586-6225

 

[Kwan]

I agree with you about the icebox rivets. They are quite strong and are driven with very little effort. Here is my opinion on the E versus DD.

I believe that bean counters showed that the process that is required (dry ice, heat treat) is expensive as compared to just ordering "E" rivets. They did not account for the amount of work required to successfully buck the E rivets.

I worked for a certain company and had the oppurtunity to talk with a bean counter about this. He was very proud to have saved the company x dollars with his idea of replacing icebox with E's.

 

[rb1957]

DN (or is it ND ?) rivets wrok well as a replacement for DD. they are a specially controlled AD rivet (so easy to drive) with DD properties (without the icebox). there were issues with the early production lots, but i understand that these have been sorted now.

 

[WKTaylor]

Chiefmarty, Kwan and rb1957... in addition to your astute comments...

Ahh… the “simple” solid rivet…

1. My current acft was assembled primarily with Ice-Box DDs… +45-yrs ago… for all reasons cited above. In recent years we have switched to D rivets, driven cold. The small differences in strength were off-set by the fact that holes in old structure are generally oversized [1/32] for mate-drilling repair purposes… and that the reduced strength is factored in for nominal diameters when repairs or structural modifications are developed. These rivets are substantially harder to drive than Ice-box or AD types. Above ¼-inch diameter, I hesitate to allow cold-driven Ds because driving forces start approaching the limit for hand-driving. Also possible damage to thin members and inducement of Stress Corrosion Cracking in “7XXX-T6” members due to the higher interference forces. Instead tend to I specify anodized Ds and have them heat treated and installed in the “ice-box” condition. We have been approached to use E/KEs… but driving forces are slightly higher than DDs and there appears to be some “spring-back” [relaxation] after driving… which some testing suggests reduces fatigue life.

2. There are many subtle issues about ice-box rivets [D and DD] that tend to cause LOTS of problems for non-assembly-line work.

2.1 Heat treatment MANDATES a “natural color” anodized finish: only anodize will survive heat-treatment. DDs universally come with an anodize finish: HOWEVER, Ds and ADs [and E/KEs] have MOSTLY chem-film finishes that work well when driven cold. If heat treated Chem film finishes are destroyed leaving the rivet vulnerable to corrosion. It is hard to find ADs and Ds [and E/KEs] with anodized finishes [yes… ADs could be ice-boxed if anodized… but why bother since they drive nicely “cold” up to 3/8-inch diameter].

2.2 Ice-box rivets require significant process controls. For a few rivets, the process can be a nightmare. To name a few: Incoming receiving to insure alloy/anodize; heat-treatment/quench process controls; quick-freeze controls; cold-storage controls; cold-transportation and shop-delivery controls; and of course, final installation controls [out-time, moisture removal, “spoilage” of unused rivets, etc].

3. Here is a unique problem we face. Customer repair data does NOT allow any cracked or chipped bucked-tails. Our riveting data, NASM47196, MIL-STD-403 and MIL-STD-40007allow fine cracks under specific circumstances. Unfortunately cold driven Ds and E/KE rivets tend to develop fine cracks that are OK within our standards: but guess what… our customers mechanics are in a rock-and-a-hard-spot! What is interesting is that field unit mechanics tend to have more rivet-cracking problems than depot mechanics. I suspect that some of the Ds used in the “field” tend to be “old***”… whereas Depots uses hundreds of thousands a year. I also suspect that the field may have anodized [brittle coating] rivets instead of soft chem-filmed rivets [used to be that either coating system could be supplied under that same basic part numbers]. Note: MIL-STD-40007 had some interesting possibilities for “why” rivets cracked.

4. Rivet Notes:

4.1 One problem recently uncovered [but I has suspected all along, due to mechanics comments] are that AD [depressed dot head-code] and D [raised-dot head-code] rivets have been mixed together in shop containers. The Ds appeared to drive “hard”… while ADs drove “easily”… in the same rivet rows!!! This problem has turned into a nightmare for skin lap-joints…with NO obvious “starting point” [when did this begin???]!

4.2 Also, a little known fact is that –T3/-T4 tempers tend to “migrate” towards the -T8/–T6 temper over long periods of time… depending on environmental exposure. My dad built a small all metal homebuilt acft [T-18] in the early ‘70s… left over 2024-T3 sheet metal was exposed to +130F in the corner of a garage under a south facing window [high desert climate]. In the mid ‘90s, I tried to bend some “2024-T3” sheet 0.016 thick and it turned-out to be amazingly hard… and it cracked on a typical –T3 bend radius. It was OBVIOUSLY no-longer –T3! That something for You DADTA guys to chew-on!


Regards, Wil Taylor

 

[edmeister]

We recently had a situation with E rivets where we experienced 100% failure rate with cracked tails. Although we would accept the occational cracked tail but every one was not acceptable. (Sample size 80+) When we returned the rivets to the supplier they insisted that the batch passed the shear & tensile tests. We applied the same installation technique / tooling to another batch & we had no difficulties with every tail perfectly formed.

 

[WKTaylor]

Guys...

One thing I noticed when working on F-15s was that all solid 7050-T73 rivets had McDonnel-Douglas Part numbers... implying MDC procurement controls. These rivets invariably "shot" as anticipated... with NO mechanics EVER complaining about cracked buck-tails, that i can remember. I used these rivets in many acft types with virtually No problems. They WERE tough to drive... but mechanics made-it-happen. Makes You wonder how MDC got it right thru the vendors. I suspect that Rivet-wire stock [controled metalurgy], cold-heading [forming], quench and bake cycles, and coatings, were exactly controled.

Regards, Wil Taylor

 

[Stache]

Being of the old guys who shot many a D, DD and AD rivets I know for a fact different lot numbers buck differently. Not only that, but it seems recently a lot of rivets are cracking. I recently had the shop remove a patch they install after checking the rivet shop heads most were cracked. One of the problems was the mechanic used a 3X rivet gun to drive 3/16-inch rivets. These rivets should be installed with at least a 4X rivet gun and a heavy bucking bar (kids). One of the guys had a chipping hammer for a rivet gun. Seems lack of experience is part of the problem.

 

[FieldTeam]

I believe part of the answer to your question is that air force metal technicians use T.O. 1-1A-9 for rivet installation guide lines. Here, the technicians are interpreting that the 2017 D rivets are to be annealed prior to installation, then aged for 96 hours after shooting.
This means using the D rivet for repairs where the airframe is required back in service in less than 4 days is out of the question.
AF will not shoot D's in the age hardened condition.
Anyone have any thoughts on this?

 

[WKTaylor]

Field Team...

Are you sure that USAF T.O. 1-1A-9 [metal Tech] is correct? I think Your techs are probably refering to USAF T.O. 1-1A-8 [Aerospace hardware and fasteners].

I strongly advise You to refer to my previous posting [above]. "D" rivets with alodine coatings were ONLY intended to be drive COLD: and a high % of D rivets are only alodined (as opposed to being anodized). The folks at XX-ALC drive thousands-a-day successfully... although large diameters than #7 can be painful.

About 2-years ago I had a long talk with the engineers/techs responsible for USAF T.O.s 1-1A-8 and 1-1A-9... explaining the problems with D rivets in detail... as related to these (2) T.O.s. However, I'm unsure of how much was absorbed... and if any actions were taken to change the T.O.s (to explain the "do's and don'ts" of driving D rivets cold or driving D ice-box rivets... and the heat-treating process) NOTE: 1-1A-8 is the basic for riveting; and 1-1A-9 can be used for rivet heat treatment...

Regards, Wil Taylor

 

[FieldTeam]

Will,
Thanks for your reply. I am sure the 1-1A-9 states this. Please refer to para 3-37 heat treatment of rivets. Table 3-11 addresses the 96 hour cure time. Even though the technician sees that it states D rivets can be driven in the age hardened condition and our SRM states that D rivets are stored and driven at room temp, he will not do so.