 Okay, now we move into one of the sections that I think is a good one for the design engineers on fastener design. These questions are kind of a working list that I made up based on questions that I'd gotten in the past from people. The most frequently asked questions for fastener design. Number one is what torque value should I use? Now this is a difficult one to give an exact answer on, so what you have to do is you look at the material strength, lubrication, the loading that you're going to have on the fasteners, the differential expansion, and then give a torque range for the mechanic to work in because you cannot expect someone to go with exactly 345 inch pounds on a fastener because his wrench won't read that accurately, so you have to look at things and determine what range you should be in, and the range usually on torquing is about plus or minus 10% of the value, really, that you have to give for a tolerance, and you have to determine whether you can live with the lower limit and the upper limit on it. Now should locking torque be added to regular torque? Now the first thing to do is check to see if locking or running torque is a small percentage of the total, and Marshall Standard 486 gives guidelines for this. If you are using an extremely small fastener, and when I say small down to maybe a number of six or below or something like that, then locking torque can be a sizable amount of the total torque value, so small, and it's hard to get something on a small fastener that will give you some locking without giving you too much because there's a limit on how small can you go on a plug or deform thread or something until you get into problems with it. But in general, the running torque is such a small proportion of the total torque that you include it in the total torque, but you have to look to see how much it is and Marshall Standard 486 gives tables in which they give you upper limits and lower limits for locking torque. Well if you are running a fastener down and it's taking a lot more than the maximum amount given to Marshall's table, you've got to go back and check to see what's wrong because chances are there's something wrong in the threads or something that's causing you to have that higher running torque. So yeah, you add it on, but it should be a small add-on. Next question, do I have to use inserts and aluminum if I install steel bolts? Well, you try to avoid tapped holes if you can, but if you have to have the tapped holes, you can delete inserts if galvanic corrosion and disassembly don't need to be considered because chances are if you put a steel bolt directly into aluminum, it's going to corrode up so much that you'll never get it out. So that's one of the problems that you have with it. Otherwise, you need inserts. The other question is why did the heads pop off my socket head screws within a few hours after installation? Well, as I pointed out in the inspection criteria, the depth of the socket in a socket head screw is not measured normally during inspection. So if it goes below the bottom face of the head, the net cross sectional area is so small that it will fail in tension. Of course, the other reason for it failing would be hydrogen embrittlement, which would cause the heads to pop off. Assembly disassembly life cycles for a locking fastener. Now, it's usually good for about 10 to 15 assembly disassembly cycles. Nylon, nylock plug is good for about 5 to 10. Of course, the deformed thread is what I'm talking about. It's good for 10 to 15. Now, if the running torques, once again, if you check and they're below the minimum values given in the Marshall Standard, then you should discard the fastener. In other words, if you have ground up the nylock plug to where it's no longer effective, then you discard the fastener. Now, the assembly disassembly life for a fastener. If the fastener was torqued above yield, it shouldn't be used again at all. The number of cycles for a fastener, it depends on the criticality of it. For instance, in the aircraft business, you're not supposed to use fasteners over again at all. This is one of the places that some of the small charter type airlines have gotten torqued in trouble, not as often as they should, but they have gotten in trouble with taking cannibalized parts off of an airplane and using them as new parts for rebuilding, both fasteners and just structural type pieces. That is a no-no because you are supposed to put new fasteners in when you rebuild an airplane. I know that here at Lewis with the Grumman plane we have or did have, I was told by the mechanics that Rolls-Royce will not allow them to use anything but Rolls-Royce supplied fasteners any time that they work on the engines because they want to make sure that the right bolts are in there and that they use new ones each time. What do I use for a shear area when I have a bolt and shear in a tapped hole? A bolt loaded in shear and installed in a tapped hole, first of all, is not a good design to begin with, but if you can't avoid it, use the root area of the thread for the shear area. If the design is critical, use a stress concentration factor of like about 2.7 to 6.7 depending on how many threads you got exposed and Peterson, most of you I think are familiar with Peterson's book on stress concentration factors. They give the factors in there for all sorts of cases like this with threaded holes. The next question, should I use a washer under the head and nut of a bolted assembly? The answer is yes you should as much as possible to provide a smooth hard surface to minimize the friction forces during torquing and provide in the case of high strength fasteners with a bigger fillet radius under the head to make sure that you have a counter board interwarsher diameter to avoid point contact on the head fillet. Next question, when should I use coarse fine or extra fine threads? Now, whether you're going to use fine threads or not, fasteners below a number 10 usually have coarse threads unless you special order them due to the difficulty of making very fine threads. Corsed threads are easier to install without cross threading, so they're the most popular one in the industrial world, but fine threads are easier adjusted and have a larger root diameter than coarse threads, so they're a little bit better in fatigue. So the aerospace world uses fine threads. You normally do not find coarse threads being used on airplanes. Extra fine threads are used for fine adjustments and for jacking screws on mechanisms, that type of thing, and tapped holes and thin materials. Now this next one is a wide open question that can never be satisfactorily answered. And one of the problems with it, the government sometimes, when they're infinite wisdom comes up with things to get them in all kinds of trouble. And one of the troubles that they're in with customs is that they have a different tax rate for screws and bolts on import tax. Now the problem is, and I've gotten several calls from customs on this, asking my opinion on the definitions of a bolt and a screw, how do you define a bolt versus a screw and differentiate between them? You really can't because here's all the different answers that you have. A screw is installed in a tapped or drilled hole and a bolt is threaded into a nut. But you look at these and you take a bolt or a screw either one, and you can buy them that look exactly the same, but one says screw and the other one says bolt. Because they can have a head on them, the heads are alike, they can be threaded, they can be threaded all the way to the head, and they can be only partially threaded and still called a bolt or a screw. Now here's another definition, a bolt is installed with an external wrench and a screw is installed with an internal driver. Now that's one of the common definitions that we come up with sometimes, that we feel that okay if it's something you use a screwdriver on it's a screw. But then on the other hand, a socket head bolt, you use an allen type internal wrenching driver for it, so you could say that that one was a screw instead of a bolt. And here's another fine definition, a bolt has a shank diameter equal to the minor diameter of the threads and a screw shank diameter equals the major thread diameter. Now the only time that you use the reduced diameter bolt is if it's a fatigue application where you don't want to fail in the threads near the end of the threads, then you machine the shank down to the minor diameter of the threads to prevent stress concentrations. But most 99.9% of the bolts that you get are formed by starting out with a given diameter rod which is formed and you actually cold form it to reduce the threaded area before threading so that when you do the threading, the deformation that you get will only make the OD of the thread equal to the diameter of the shank. So that kind of shoots that one down. The other one is a screw is threaded all the way to the bottom of the head and a bolt has some unthreaded shank. Well, that's not true either because when you get into a longer length fastener, then you only have a partial thread on it because they don't want to bother to spend that much time threading it. So the only ones that are threaded all the way to the bottom of the head are the ones that are say an inch and a half and less in length. So because the normal way of threading a fastener is that you go with a minimum of one and a half to about two and a half times the diameter of the fastener in thread. So if you have a four inch bolt that's a quarter inch in diameter, you will have three-eighths to a half inch of thread on it and that's it. Now, you can say that's a bolt or you can say that's a screw, whichever you want to call it. So that definition. So all of these definitions have overlaps. So what you need to do when you're specifying something is give the geometric terms of how much thread you need and what you have to have rather than depending on a definition of bolt or screw. Then the next question is when should I use threaded studs with nuts instead of fasteners and tapped holes? Well, the threaded stud works better than a fastener if you're not going to remove it because you can just let it rust in place. For instance, the manifold on a automotive engine is usually installed with threaded studs because once the thing rusts into the manifold it's like a bolt sticking out and your chances of getting a nut off and then chasing the threads and putting a new nut on are much better than trying to twist a rusted threaded part out of a hole and replace it. So the threaded stud would work better, particularly here in the rust belt. Now, next I have some comments on some of the thread specifications and do you have that one in there? It's this one. You do not have that one in there. Well anyway, I can just give you from here. As some of you may have heard about the PERI initiative, which was, PERI was the Secretary of Defense and he came up with the idea of doing away with mill and federal specs because that way the government would no longer have to look after him. You just do away with him until people use something else. Well, of course he's a non-technical type so he didn't realize all of the thousands and thousands of specifications that were being done away with no replacements. So two of the ones that were really a concern in the fastener world were mill S7742 for screw threads, the standard threads that covers your grade eights and socket head cap screws and stuff like that and mill S8879 which covers J-threads. Well, the 7742, they made it inactive for new design but it still can be used on old designs. And 8879 for J-threads, they made it inactive for new design and actually as of May 14th of this year and said other consensus standards can be used. Well, of course this creates a big problem because J-threads are not covered very well in anything else except this mill S8879, there's an SAE spec that covers it to a certain extent. The other thing, there's kind of a turf war between the industrial people and the military people on control of these specs because you don't want to be a party to a specification like an ANSI spec if you're working for Boeing and ANSI, the ANSI committee decides to change that spec and makes it unsatisfactory for your usage, you don't like that. So the good old national aerospace standards committee is coming to the rescue and they are a part of AIA which is the Aerospace Industries Association and SAE. So the committee decided to convert all of the MS, AN, mill type specs to NAS specs and what they will do since they could not rewrite them, they're just saying okay we'll make them an NASM and then give the same number they had before. So what this does, this transfers custody of the spec to the NAS committee so that future revisions of the specs will be made by the NAS committee but the spec can still be used and the other thing that they emphasized was that people see this notice saying inactive for new design. Alright, you can still, if you want to specify that that spec be used, you can still put it on a drawing and that's up between you and the contractor that's building the part. If you say I want it to this mill spec that is inactive for new design and he says give me a copy of it, that's fine. So that's the way that we're getting around that with the NAS committee taking over the custody of those specs. The other thing that I had mentioned was of course the Fastener Quality Act has been delayed until May or at least presently delayed until May 1998. So now we go into the summary which you can show as the agenda because the agenda and the summary are the same thing. And here we have it. These are all the things that we have covered and there was heavy emphasis at the beginning on materials. We covered all of these other topics and then we got down to the appendices and the references which you don't have yet in your notebooks but we will be distributing those to the people who took the course and the final edition of the manuals will contain the appendices and references. One of the things that I added to that was the, that I forgot to mention when we covered bearing stresses, Tony Herman and I wrote an article for Fastening Magazine on bearing stress distribution and it was published in the December edition of the magazine and if anybody wants to look at the copy and see me or Tony, both of us have copies of the article and I think I listed that at the end of the references. The other thing too that I would like to acknowledge once again is the references that I used from John Bickford and Dave Peary's book. Peary was, I met him when he was at General Dynamics. He used to be head of their research division at General Dynamics. His book is one of the best there is, the old edition of it. If anybody can get a copy of it, it's out of print. It's a 1950 edition. He was head of aeronautical engineering at Penn State at the time he wrote it and a lot of the stuff that I got out of there for the fasteners in a pattern, how to analyze them and so on came from Peary's book. So any of you that would like to pursue that further could check Bickford's book and Peary's book for more information. Also I'd like to mention again the book which is supposed to end all quests and so on for fasteners will be published around the end of the year. I was a contributing author to that book. It will be a two volume set published by Marcel Decker on fastener design and each volume is to contain somewhere in the neighborhood of 1300 pages. So you can see even though this may have seemed like a long course, this only scratches the surface in the field of fastener design. So with that we will close and thank you very much for your participation.