Structures For EV's

Introduction

Recently on the EV discussion list, there have been several discussions about how to build things like frames, suspensions, and mechanisms. As a mechanical engineer in aircraft structures, and having designed and built pumps and compressors for industrial use, I have had a lot of experience designing various structures and devices. In this page, I have tried to put together discussions on several subjects relevant to EV mechanical structures and systems. The information presented is generally accepted as good design and assembly practice, but is in no way claimed or intended to be the definitive and final word. It is intended as "General Information" only. In all cases, check with a local engineer for structural analysis of your specific design. He or she might even like the idea that you're building an EV so much that they'll help you for free (expect them to want to borrow it a lot, though).

My intention is to update this as I have time with more information and topics as requested. It may grow to include a materials database, but I hope it doesn't come to that. If you would like a topic addressed, please e-mail me.

As a guide to how much time an engineer should spend on your design, a structural analysis of a relatively standard metal battery box for the NEDRA required 8 G's horizontal and 4 G's vertical loads should take no more than 4 hours, and that is if the engineer has to take all the measurements himself while crawling around the car. Most ME's will do this for you just as a favor to a friend (and to get to see a really cool gadget). You may have to buy them lunch, sometimes, though.

"Before we begin, I'd like to take a moment to talk about shop safety" I do address some specific safety concerns, but I expect that the sort of people who are considering building an EV are smart enough to wear proper protection, such as safety glasses, gloves, etc., and to follow all the safety precautions associated with any chemicals or machines they use. The paints, primers, sealant, resins, etc. I discuss here contain the same sorts of things that companies get sued over when they have a safety lapse and release some into the environment. We can't very well complain about their practices if we do the same thing ourselves. If used according to the instructions, these chemicals are safe and useful. Most important, if in doubt, ASK! Local shops that deal with the materials will be very helpful, and dealers are supposed to have the MSDS's for their products.

Contents (links are on this page):

Introduction

References

Design Practices

Metal Structures

Aluminum

Steel

Stainless Steel

Titanium

Composite Structures

Joints and Fasteners

Bolts

Screws

Rivets

Solid Rivets

Table 1: Shear Strengths Of Solid Rivets 

Blind Rivets

Blind Bolts

References:

I use several references. Some of the most useful are described below. You should peruse some of these as they are written in a lot more detail than this page. (Remember, "general info. Only, etc., etc.")

"Manchinery's Handbook" has everything you need to know about materials, machining them, and working with them. Every serious mechanic and engineer should have a copy of this book. It has its own website ( http://www.industrialpress.com/mh.htm) and is available on CD. A search on Amazon.comshould turn it up, too.

AR-MMPDS-01 (formerly known as MIL-HDBK-5), "Metallic Materials And Elements For Aerospace Vehicle Structures." This book contains nothing but material (and fastener) strengths. It has all the grades of titanium and aluminum you need, plus a bunch of materials you'll never need to use, including magnesium, nickel alloys, heat resistant alloys, beryllium, and copper alloys. It is available in .pdf format, and is well indexed. However, the file is over 65MB! It is available for download.  search for it on google.  If you design in aluminum or titanium, you must have this book.

Jeppesen publishes some paperback manuals on how to work with composites and sheet metal. Although geared towards the aircraft mechanic, they are great for general information on assembly and machining practices, as well as the tools you need. An Amazon.com search should turn them up. The first is "advanced composites". The next is "Aircraft sheet metal".

"Building Your Kevlar Canoe : A Foolproof Method and Three Foolproof Designs" by James Moran. Again, this is available from Amazon.com. He gives good information on tooling and lay-up methods.

Design Practices:

A lot of the practices I describe will provoke the question "why do you do this?" I've tried to sum up in this section. Remember these practices are performed on vehicles that your lives depend on (airplanes), and have had very good results. While not all of them are necessary in an EV, using them will virtually guarantee that it will still be in the same condition 50 years from now as it is when you are done.

Mainly, these practices deal with corrosion. With titanium, fretting can be just as bad, as it can lead to embrittlement. I always suggest joining two pieces of metal only after priming them or applying some other coating (like powder coating for instance). Even then, a coat of sealant is applied between them before the fasteners are installed. This is called a 'fay surface' sealant. It keeps moisture from getting in between the parts and causing corrosion you can't see. It also keeps the parts from rubbing against each other during vibration, and fretting. Fretting could make the parts come loose or become weakened later on. This type of wear also invites moisture, which invites corrosion. In aircraft, a two part, chromate cured sealant is usually used. For us mortals, a good RTV silicone works fine. For the same reasons (avoiding corrosion), holes and countersinks are primed or coated before installation, and fasteners are covered with sealant ("installed 'wet'") before installation.

Metal Structures:

In general, the three metals used most often that apply to EV construction are aluminum, steel, and titanium. I'll talk a little about each.

Aluminum:

Aluminum is the lightest of the three, and is available in a wide variety of strengths. For design, use the values in the MMPDS. The info. In the machinery's handbook seems unconservative. Aluminum, like titanium and stainless steel, resists corrosion by forming an oxide layer on its surface. Clad aluminum has a thin layer of pure aluminum rolled onto its surfaces to enhance the corrosion resistance. Since the thickness of the alloy under the clad is less than that of bare alloy for a given sheet thickness, clad aluminum is typically 3-5% weaker than bare aluminum. Aluminum is prone to corrosion in places you can't see it, like under fasteners and between parts that are joined together. For this reason, several steps are used in assembly. First, before assembly, all aluminum surfaces (including holes and countersinks) are at least primed. Some primers have aluminum or chromates in them to enhance corrosion resistance. The primer can be left as is, or paint may be applied over the primer, or the aluminum may be powder coated instead of primed. Surfaces that are assembled in contact with other metal surfaces are coated with sealant over the finish (be it primer, powder coating, or primer and paint) prior to assembly. Fasteners installed "wet". That is, they are coated with sealant and immediately installed. Finally, after final assembly and after application and cure of all primers, paints, and sealants, there are anti-corrosive sprays that can be applied to displace moisture in tight spots and keep it out. These are petroleum based products that are waxy and form a skin over the area. They are similar to the sprays available for steel, and the steel sprays can probably be substituted without any problems. If a structure is subject to road spray or rain, these sprays are a good idea.

The proper sequence to do all this is to make the parts, temporarily assemble them using clamps and the like, and drill and countersink all the necessary holes, them disassemble them for cleaning and priming and sealant application. Then reassemble them and install fasteners.

Certain grades of aluminum can be welded fairly easily, but the grades that are weldable have poor strength. Also, aluminum is more prone to warping from a welding operation then steel is, although steel does it too.

For parts that will bear weight, use 2024 or 7075 aluminum. These are the most common structural alloys, and are readily available. They are available in a variety of tempers. 2024-T3 and 7075-T6 are the standard tempers. -0 (annealed) tempers are available, and allow the user to form tighter bend radii, but they must subsequently be heat treated to be strong. Also, unlike some steels, you can't heat treat 2024 back to 2024-T3 once you've annealed it to the -0 condition. 2024-T3 is only available from the mill, as part of the treatment includes a stretching process. The available tempers via heat treating 2024 are T4, T6, and T81. By the way, the T4 temper of 2024 is virtually worthless (IMHO). If you get something that is heat treated 2024, get the T6 or T81 condition. 7075 doesn't come in a T3 or T4 condition, and does not have these limitations.

Steel:

What can I say? Easy to weld, via many methods, with welding classes available inexpensively at vo-tech schools, cheap, easy to get (but be sure you know what you are buying!), easy to machine, lots of suppliers and shops that can do anything with it you want at reasonable prices. Easy to heat treat, with lots of shops available.

If you are building structure out of steel, you may want to look at various grades. Anybody can make a perfectly acceptable battery box out of A36 angle iron, but if you use a stronger steel, you can cut the size and weight substantially. Homebuilt aircraft frequently have a frame of welded 4130 tubing. 4130 is very strong, and using hollow sections increases strength without increasing weight. Steels can be heat treated to resist wear or can serve as flexible members.

You can get a wealth of info. on different steels from the Machinery's Handbook. Also, local machine shops and steel supply houses will be able to recommend specific steels for your application. Some steels don’t come in certain forms, like structural shapes, but you can usually find something that will work. The trick is to pick a couple of steels that come in the shapes you know you need, then design your structure. And remember that you can weld two or more simple shapes into a complex shape. Here are some I recommend: A36 structural steel is probably the most common and available in the most forms.. Its design strength (from machinery’s handbook) is only 33,000 psi, though. 8620 is a steel I like to use for machined parts, with a yield strength of about 51-55 ksi. It can be effectively carburized to a hardness of Rc 60-65 for great wear resistance, and this is just a surface treatment, so it is still flexible. 4140 is also go for machined parts, and hardens all through its thickness on heat treatment. 4140 has a yield strength of about 60-95 ksi depending on the heat treat.

Obviously steel needs to be protected from corrosion. In storage, a film of light oil works well, and there are several aerosols available that form a slightly harder coating for temporary protection. Finished items should have surface corrosion removed and be degreased or solvent cleaned to ensure the primer or coating will stick. Primer and paint is a typical treatment, and powder coating is also available. Powder coating is very tough, and is available in as just about as many colors as paint is. The powder coating places do all the cleaning for you. They then apply an electrostatic charge to the part so the powder will stick. It is then oven cured into a seamless finish. Galvanized steel offers an extra layer of corrosion protection. As with other metals, surfaces that are to touch should be coated with a primer at minimum, and then a sealant should be applied over that. The pieces should be installed while the sealant is wet, and fasteners should be installed "wet", or covered with sealant. Be sure all the holes and countersinks are primed or coated before assembly.

Stainless steel:

The most common grades (300 series) don't give very high strengths. However, 301 sheet is available in various degrees of cold work that can yield greater strengths. These grades are called ¼ hard, ½ hard, ¾ hard, and full hard. Stainless steels may not be subject to corrosion themselves, but can cause corrosion in less noble metal that they touch. That is another argument for all the sealing between surfaces that touch. Other grades of stainless with the "PH" designation (precipitation hardening) are available, and these can be heat treated. But, as with carbon steels, they are more expensive and available in fewer forms. Most stainless steels are easy to weld. The 400 series of stainless steels can still rust. They have just enough chrome and nickel in them to be called stainless, and not enough to have a really effective corrosion resistance. They can be very strong, however. The most common stainless grade in aircraft use is 301 SS. The most common industrial grade is 304, and the "premium" grade is 316. 304 has "great" corrosion resistance, and 316 has even more.

Different kinds of steel can be welded together, but you need to get a welder who knows how to do it. Where I used to work, we regularly welded together 17-4 PH and 1018, while maintaining the H900 heat treat of the 17-4 PH. One former boss knew a welder who welded 316 SS to Pyrex (he used 5 weld fillers as intermediate materials). Not your everyday welds, though.

Although the more common stainless steels are relatively resistant to corrosion, they will cause corrosion when installed with dissimilar metals such as aluminum. Stainless items installed against aluminum should have two coats of primer applied to both of the substrates before installation, and should be installed with sealant covering the mating surfaces. Powder coating is an alternative to the two coats of primer. The pieces should be installed while the sealant is wet, and fasteners should be installed "wet", or covered with sealant, both to prevent dissimilar metal corrosion from introducing a third material and to prevent forming an electrical circuit for the dissimilar metal corrosion between the two pieces. Corrosion that starts under a fastener is hard to find until the fastener falls out. Be sure all the holes and countersinks are primed or coated before assembly.

Titanium:

Those of you in the Seattle area are blessed with the ability to buy titanium sheet and ingots for $8 a pound from the Boeing surplus store (at least that was the price last time I was there in 4/00). This metal does not require ANY corrosion protection for itself. It forms an oxide like stainless steel and aluminum, but it is much more tenacious. It is in between steel and aluminum for strength and stiffness and weight. Personally, I consider this the metal of choice due to its great ductility, strength, temperature resistance, and invulnerability to most corrosion.

It is kind of finicky to work with, but its great properties can make up for it if you are willing to take the extra care. It has a tendency to weld to the tool during machining. Low speeds and high feeds are recommended. Ample use of cutting fluid is recommended, as titanium isn’t a great heat conductor. Don’t use chlorinated solvents or fluids on titanium. They can lead to hydrogen embrittlement. Fine particles (such as those from grinding) and shavings can ignite and burn. This is because the tiny particles want to combine with oxygen in the air to make their oxide layers. This takes place at a rapid rate, and can generate substantial heat. Also, an accumulation of titanium dust/shavings/etc. is a fire hazard. Remove as soon as possible, and store outdoors in a steel drum. Covering the shavings with water may also help. Locate a metal recycler to take the waste.

When placed next to aluminum or steel, dissimilar metal corrosion can occur in the other metal. Aluminum or steel details should be degreased, primed, and then installed with sealant between the titanium and the other metal. It sounds like a lot of work but it is worth it. Also, fasteners can create an electrical path between the titanium and the other metal, causing corrosion in the presence of moisture in the air. All fasteners must be installed "wet". That is, coat the fastener with sealant, then immediately install it before the sealant cures. Finally, titanium is subject to oxygen and hydrogen embrittlement. Most aircraft fasteners used in titanium have an aluminized primer coating as well as being installed wet, so they do not scratch the hole when they are installed and create a point that could be subject to embrittlement. Aircraft hydraulic fluid causes this embrittlement, and some types of brake or steering fluids can as well. Welding of titanium is conducted using the gas tungsten arc welding method, and requires extensive shielding. It is easier to just avoid it. Spot welding of titanium has been done successfully, though. If you have to weld titanium, take it to a local vo-tech school that trains aircraft mechanics, and get a welding instructor to do it for you. Don't let the local weld shop do it, they won't have the right equipment or filler, and any shop that can do it right would probably charge an arm and a leg. To tell if a weld is good, look at the color. If it looks burned, with a brown and blue haze in the heat affected zone, is has been embrittled by the oxygen. These welds are substantially stronger than the surrounding base metal, but have only 1-2% ductility, and will crack when they are flexed.

Titanium sheet is easy to bend, and is used as fire shields in aircraft because of its ability to remain stable and fairly strong at high temperatures. It is also used where a steel structure would corrode or be too heavy, and aluminum would be too massive (i.e. 777 landing gear). There are several types. The ones to get are CP-1 (commercially pure (but with a few alloying elements), 70 ksi yield strength), and 6al-4V, also known as AB-1 (120-130 ksi yield strength when annealed, 130-145 ksi yield strength when solution heat treated and aged).

Composite structures:

Composites hold the possibility of significant weight reductions for structures. High stiffnesses and strengths are also possible. Special fasteners are used when these items are mechanically fastened, and special methods are needed to work with them.

First of all, anybody with a chopper gun can build a boat hull. Someone with the right training and materials can make one from fiberglass cloth that weighs half what the other one does, and is stronger. If you are familiar with boat hull construction the first thing to do is forget everything you know, or you won’t realize the full potential of the strength and light weight that are possible using these materials.

I am not going to go into the various materials available very deeply because there are so many. Any catalog for people that build their own airplanes will have many listed. Bondo even sells some in Wal-Mart now for boat repairs and patching large holes in cars. Basically, any homebuilt composite structure will likely be hand laid up, and that is the technique I will discuss here. Other techniques are available, but require a lot of equipment. Hand lay-ups or "wet" lay-ups require four things: A tool or mold, Dry cloth (Kevlar, carbon, or glass), Resin, and tools to cure the resin (these are not always required. They can be an autoclave or an oven, as well as anything used to apply vacuum and/or pressure). Note that the exact same procedure I am about to describe can be used to make a composite copy of a hood or trunk lid for weight savings. The kevlar canoe book describes the composite lay-up process in great detail, from tooling preparation to final finishing. I will only discuss generalities here.

First, a very general discussion of materials. There are three dry cloth materials available, and usually these would be purchased through a homebuilt airplane catalog. Kevlar has the highest strength to weight ratio, but is difficult to machine in its cured state, and absorbs water through exposed fiber ends. One note here: a Kevlar composite panel will not make your EV bullet proof. Bulletproof vests are bulletproof because the kevlar is not set up in a resin. It is arranged like batts of fiberglass insulation inside the vest. It stops bullets by deforming and absorbing the impact. The fibers stretch and entangle the bullet. A laminate is not as free to deform and absorb the impact. Also, a laminate usually has less total mass of fibers to stop a projectile. That said, a composite laminate CAN provide protection from road debris. Fiberglass laminates are used on the bottom of some aluminum airplane flaps for just this purpose.

Carbon has high strength also, but fiberglass is much less expensive and can be almost as strong, depending on the type of weave the material has. As thin as the plies are, lay-ups with more layers will not be too noticeable. It may be better for your application and budget to lay up two plies of glass rather than one of carbon. After all, fiberglass is a lot cheaper, and you can better afford to make mistakes as you learn if you use fiberglass. If you are going for that carbon fiber look, just spray paint the glass with Krylon J .

All composites are difficult to drill, though kevlar has the particular problem of "fuzzing" at the cut surface. This is due to the high strength and high flexibility of the fibers. (These are the same qualities that make them good for stopping bullets.) Kevlar lay-ups are usually covered on each side with a fiberglass ply which serves to seal the fibers against moisture and serves as a sanding ply. A fiberglass sanding ply is recommended for carbon and glass parts, too.

There are several resins available. Polyester resins are generally considered inferior in the area of moisture resistance and UV resistance, but if you use the right paints to exclude moisture, it should be OK. Aircraft companies use epoxy resins, and these are excellent, but can be more expensive.

You must have some sort of tooling to lay the composite material into to give it its shape. The kevlar canoe book and R.Q.Riley’s website give good information on making tooling. To reverse engineer an item (an example is making a carbon fiber hood), lay the original part into a bed of gypsum cement mixed with loose glass or flax fibers to leave an impression. The fibers are necessary for large tools to keep the cement from cracking when it dries, and when it is moved about. You could also use a composite lay-up on top of the part as a tool, but it is usually less expensive to use something like gypsum cement to "mold" a tool or to "sculpt" a tool. It is also easier to correct errors. Obviously the original part must first be coated with some sort of mold release and there must be a structure to hold the cement. Once the cement has set up, remove the part. Apply wax to the tool mold surface as detailed in the kevlar canoe book, and your tool is ready. Commercial mold release sprays are available, too.

The lay-up procedure is simple. Lay the cloth in layers on the tool, thoroughly wetting each with resin. A Teflon spatula works well for this. Most of the resins that are practical for the homebuilder to use are room temperature curing, and it is not necessary to add heat, although heat will speed up the cure. If you have been able to obtain the special plastic films that are used, you can use a shop-vac to apply vacuum and compress the layers together. This brings excess resin to the top, and results in a more uniform material. There are many books on what to lay up and in what order. In general, a non-woven fiberglass mat will be placed atop a perforated plastic film to absorb all the excess resin. The vacuum is not to draw out the resin. It is only to compact the lay-up.

Once the resin has cured, remove and discard the excess material. Usually, the surface away from the tool will need to be sanded, and will have a bumpy texture. The mold or tool side will usually be very smooth. You may need to add body filler at this point to get the surface contour correct. Once that is done, apply a primer that blocks UV rays (available from aircraft homebuilder’s catalogs) to keep the UV rays from embrittling the resin over time. Paint as desired.

A thin lay-up by itself may not be very stiff if it is a large panel. Usually the way to make a large panel stiff is to make a sandwich of honeycomb core between two lay-ups. This pretty much goes beyond the realm of homebuilders, as it is hard to get a good bond to the honeycomb with a liquid resin. Maybe you can find someone around your area that can make one for you. A way around it is to use foam as a core. R.Q.Riley does this in their kit cars. Solid foam is easier to bond to than honeycomb. It is heavier, but is still a lot lighter than metal. Seal up the edges of panels with potting compound (a thicker epoxy than the laminating resin – you can add a lightweight filler to the resin or buy a compound especially formulated for this purpose). Remember moisture wicks into the cloth through the exposed ends of fibers, so they need to be covered up. And if you're lucky enough to get a honeycomb panel (check the scrap bins of local aerospace manufacturers), the core needs to be protected from moisture as well.

One way to increase strength, stiffness, and fastener holding ability is to encapsulate a metal stiffener or fastener attach plate in the lay-up. Warning: Don't do it with carbon, unless you use a titanium metal structure! Carbon is a conductor, and will cause dissimilar metal corrosion with other metals, which will pop open the lay-up. Titanium is the only metal that should be attached to carbon composite with NO surface preparation. The titanium is very noble and does not corrode. A fay surface sealant should be used to keep moisture out of such a joint, so it won't seep into the carbon cloth, though. If encapsulating a piece of metal, make sure that you do it in a climate controlled area with low humidity, so moisture from the trapped air won't give you problems later. The moisture in the trapped air is enough to start the corrosion process. The worst combination is carbon and aluminum. Remember carbon plus aluminum equals a battery. (Maybe you could make it so that the body of the EV IS its power source…hmmm.) Also, the metal must be bare and completely free of oils. Special adhesive primers are available to increase the adhesion of the resin to the metal.

Another way is to lay up a stiffener out of the same composite material, then bond and/or fasten the stiffener to the base lay-up. Typical shapes are angles, zees, and hat sections.

Joints and Fasteners:

Fasteners have been mentioned on the EV list a lot recently, and are critical in any structure. There are three general types of fasteners: Bolts, Screws, and Rivets. Any or all may be used in a structure like an EV. All fasteners are designed to be placed in shear. That is, the pieces of metal the fastener is holding together are trying to slide apart, cutting the fastener shaft through it's circular cross-section in the process. This is the type of loading fasteners are best at. It is easy to take steps to ensure that the fasteners are loaded in shear, but it must be done during the design phase. Sometimes, a fastener must be placed in tension. Only use bolts in tension. The other types will fail. Also, when a bolt must be placed in tension, use the biggest one possible.

Bolts are threaded fasteners with threads that do not go all the way up to the head. The proper way to use these is to place the non-threaded portion of the shank at the place the two pieces come together. A slight interference will result in a nice tight joint with a long fatigue life. Then add washers if needed (a proper design should not need many), and attach with a nut. Yes, I know a lot of items don't follow this rule. An example is the idler pulley attach bolt on early 90's Chrysler minivans which fatigues on a regular basis. All I can say is that this is the way bolts are designed to be used.

As far as locking bolts into place goes, in the eyes of the FAA, only a deformed thread on the nut, or a safety wire really locks the nut on. These work well, but for an EV application, you may want to consider other options. First, nuts and bolts with polymer inserts are available, and they work well. The locking action wears off, though, after a couple of disassembly/assembly cycles. There are also bolts with pre-applied threadlockers. The messiest way to go, although just as good, is to add your own threadlocker before installation. Threadlockers provide a more tenacious lock than the polymer inserts, but the bolts and nuts are harder to remove, and the stuff must be reapplied on reassembly. It's kind of hard to get these to work well if installing fasteners "wet", though. For home mechanical work, I prefer the nuts with the polymer inserts.

Screws are threaded fasteners with threads that go all the way up. They are not as strong as bolts, and the threads bite into the edges of the holes when parts are loaded in shear. They should only be used to attach non-structural items like covers, closeouts, cable tie anchor points, etc.

Rivets can be further divided into solid rivets and blind rivets. Blind "Bolts" are also available.

Solid rivets are a solid piece of metal which is driven or "bucked" into a hole. Basically, a hole is prepared, the rivet is inserted (the fit is a bit loose, as the shank expands when driven), and a "bucking bar" is placed against the end of the shank. A rivet gun (pneumatic hammer) is used on the factory head of the rivet. This "bucks" (effectively mashes) the other side down. There are certain parameters to look at to make sure it is done right, such as the diameter and height of the bucked head. It does take practice to learn, but goes pretty quickly thereafter. Solid rivets are preferred for their light weight, ease of installation (although you must have access to both sides), and their good resistance to vibrating loose over time. Rivet strength is covered in Section 8 of MIL-HDBK-5. Rivets come in sizes smaller than most average home mechanics are used to. The reason is that many are used to spread the load out over a larger area. This results in a very durable and fatigue resistant structure.

The rivets packed in dry ice have been mentioned on the list. DO NOT use these. The material code for these is "DD", and they were called "ice box" rivets for obvious reasons. The reason that they are no longer used in new designs is that nobody could effectively control all the process parameters they required to fully develop their strength, and they were a pain to use. They had an allowable out time, you had to install them wearing gloves to keep from getting frostbit, and if they got too warm, you had to send them out to be re-heat treated (those that didn't crack, anyway) and put back in dry ice. They have been superseded by the "E" or "KE" material code rivet, which requires no temperature control. "E" and "KE" rivets are the same driven strength level as the "DD" code rivets.

That said, most people use "AD" rivets, which are common, cheap, and easy to drive.

TABLE 1

RIVET SHEAR STRENGTHS (From MIL-HDBK-5H, Table 8.1.2(b))

Rivet Material Code

Rivet Material

Rivet Shear Strength (psi) (multiply by cross sectional area to obtain strength in pounds.

B

5056-H321

28

AD

2117-T3

30

D

2017-T3

38

DD

2024-T31

41

E (KE per NAS documents)

7050-T7351

43

M

Monel (Nickel-copper alloy)

52

T

Ti-45Cb (an easily driven titanium)

53

N/A

A-286 (a high strength stainless steel)

90

TABLE 1 NOTES: The driven or "bucked" head should be 1.5 times the nominal diameter of the hole. DD rivets are driven in the "W" or "ice-box" condition to minimum 1.4 times the nominal hole diameter.

Blind rivets are installed from one side only, so they are easy to put in. The only problem is that you cannot inspect the driven side, so you don’t know if that rivet "pulled" correctly. In every batch, some don't. The reliability is just not there. The aircraft grades I am primarily addressing are stronger in static strength that solid aluminum rivets, and are available in a variety of materials.

Blind rivets are not used in primary aircraft structure. One reason is the lack of inspectability mentioned above. The other reason is that they come loose under fluctuating (fatigue) loads. There have been fatal accidents due to their use where solid fasteners were required. The standard hardware store "pop" rivets lack the stem locking features available in aircraft grade blind rivets, so have even less reliability. I also don’t know what sorts of strengths they have.

To summarize, blind rivets are great for items that are not subject to fluctuating loads, as long as it is a structure about which you can say "if it does fall off, it won't hurt anything". They are also good for temporary fasteners to tack something together while you install solid fasteners.

Blind Bolts are a special category of blind rivets. Also known as jo-bolts, these are threaded fasteners designed to be installed blind. They have the strength of bolts, and are not susceptible to "pulling" incompletely, as the threading action accomplishes the formation of the bulb on the opposite side, and the protrusion which forms the bulb usually stays on the far side, as opposed to a blind rivet, where the bulb former actually rides up inside the stem. They are smooth on the outside. They are placed in the hole, and a driving nut is then turned, which pulls up the protrusion that forms the bulb. They do not fill the hole because the shank does not swell. Therefore, the parts could be subject to some vibration unless they are installed in a hole with a small amount of interference.


Return to EV technical papers page

Return to main EV Page

Return to Home Page

Return to Top