Blind Rivets Application Considerations Part 1 of 2 Parts

(Reprinted by Permission From ENGINEERING DIGEST (Canada), Volume 16, No. 10)

SELECTION of fastening or assembly methods Ily anNforTHE production lines, industry is confronted with virtualinfinite number of types and sizes of fasteners from which to choose. Over 500,000 fasteners can be identified by type, size and material and hundreds of standards and specifications defining fastener properties confront the user. Under such circumstances a fastener type, whose use clearly has grown and continues to grow at a great rate in a wide variety of industries, is the blind rivet, so named because it can be set from one side of the work unlike many other fasteners which require access to both sides. Furthermore, simplification of product design, higher assembly speeds, lower assembly costs, and the fastening of dissimilar materials, all have contributed to high volume usage of the blind rivet. Blind rivets are mechanical fasteners. When a blind rivet is set, a self-contained mechanical, chemical, or other feature forms an upset on its inaccessible — or blind — end and also expands its shank, thus securing the parts being joined. However, blind rivets are increasingly being used in applications where both sides of the joint are accessible in order to simplify assembly, save metal, improve appearance, or decrease cost. Blind riveting has the added advantage of portability which becomes especially valuable in the case of large assemblies. Blind rivets should be considered when the fasteners will not have to be removed for maintenance purposes, will have to function in a high-vibration environment, will serve only as tack or temporary fasteners, or will be used as repair fasteners by untrained operators in the field.

through leaving a hollow rivet; break-mandrel rivets (open or closed end) where a part of the mandrel remains a plug in the rivet body; the non-break or self-plugging rivets where the mandrel is pulled into but not through the rivet body. The projecting mandrel end is removed in a subsequent operation. Drive-pin: Drive-pin rivets consist of a rivet body and a pin assembled and located at the head side of the rivet body. The pin is hammered into the rivet body and flares out the blind side. Chemically expanded (Explosive): A one-piece rivet (open or closed-end type), it has an explosive in its body. Application of heat or electricity to the rivet head activates the charge to detonate, thus expanding the rivet walls to set the blind end. Threaded: This type consists of an internally threaded rivet and an externally threaded mandrel which is torqued or pulled, usually with a special tool. This action expands the walls of the rivet body to form a blind end. Factors that can influence the choice of one of the three types over the others are cost, structural integrity of the joint, speed of assembly, clinching ability of the rivet, ease with which it may be removed after setting, size range of available rivets, and the range of grips that each will accommodate. Only with extensive knowledge of these factors can a proper choice be made. MATERIALS AND FINISHES

BASIC TYPES

Basically, blind rivets are classified by the method of setting. There are four basic types: pull-mandrel, drive-pin, chemically expanded (explosives), and threaded (See Fig. 1). Pull-mandrel: This type consists of a rivet body and a mandrel. The mandrel is pre-assembled in the body. In setting the mandrel is gripped and pulled axially so that its head upsets the blind end of the rivet body to form a set rivet. Pull-mandrel rivets are subdivided into: pullthrough rivets where the mandrel is pulled completely 26

AUGUST 1971

Blind rivets are made of various materials, including steel, copper, stainless steel, aluminum, and Monel. Regular steel rivets are specified where high strength with minimum corrosion resistance is required. Such rivets can be plated, chemically treated, or painted. Aluminum blind rivets are commonly used for exterior work. They are manufactured with an ordinary mill finish or any one of several special finishes, e.g., anodizing. Monel and stainless-steel rivets provide high shear and tensile strengths and superior corrosion resistance. They can also be used in contact with detergents and most corrosive liquids commonly used in industry.

STANDARDS AND SPECIFICATIONS

Fig. I—Basic blind rivet designs and how they are set. STYLES

Body style: A blind rivet is classified according to its as-manufactured condition. The closed-end rivet has a solid end and its blind side remains closed after it has been set. The open-end rivet has the as-manufactured end open. In a split-end rivet, a portion of the body is split axially into two or more segments. The shank in a slotted-shank rivet has one or more axial slots that extend from the underside of the head and terminate short of the open end; the remaining portion of the shank is a hollow cylinder similar to that of the open-end body style.

Military specifications published to date have been prepared for government applications. As blind rivets continue to be used in increasing volume for application other than aircraft, undoubtedly new specifications will be prepared and published by government and defense agencies. At the present, it is impossible to assign typical joint-strength values to any of these fasteners without knowing the tolerances that are to be allowed on the rivet length versus assembly thickness, hole clearance, joint configuration, and the type of loading. It shall be mentioned here that the Industrial Fasteners Institute — the Technical Committee of which prepared this article — has issued its first IPI Standard "Glossary of Terms Relating to Blind Rivets" (IFI-110 of March 20, 1969). (Continued on Next Page)

Core styles: The core of a blind rivet is the axially located hole in the rivet body. Styles are based on the post-setting condition of the rivet.

A filled rivet contains enough of the mandrel or pin so that the break point of the mandrel or the end of the pin is approximately flush with the top of the rivet head. This style provides high shear strength. A semi-filled rivet contains a short length of the mandrel in its core. A hollow rivet has a completely empty core, as in a pull-through mandrel rivet, and is advantageous when light weight is important. Head styles: The head of a blind rivet is the as manufactured upset portion of the rivet body. After setting, the head is located on the access side of the joint. Styles include round, truss, countersunk, brazier, flat, and domed. These can be either flush or protruding. End styles: The end of a blind rivet is the part of the rivet body at the extremity of the shank and opposite the head. Some rivet ends are open or split, some are completely enclosed, and others are plugged. When the end is set, it becomes the blind head of the rivet. Sizes: Sizes are usually in increments of 1/32 in. Oversized rivets are also available for dimpled-sheet construction and for replacing standard diameter rivets in holes that have been enlarged by the removal of those rivets. Explosive rivets are also produced oversize, i.e., the actual rivet diameter is about 1/64-in. larger than the diameter specified. Nominal sizes of this type are 1/8, 5/32, and 3/16 in. Grip range: The minimum-to-maximum total thickness of component materials that can be joined properly with a blind rivet of a given length is its grip range. Thus, the grip range does not correspond to the length. A wide range of grip means a smaller inventory for many different applications. Most production requirements would dictate a length consistent with the grip range involved.

(a)

(b)

(e)

Most common types of riveted joints: (a) lap joints; (b) single-riveted butt joint; (e) double-riveted butt joint. SPORT AVIATION

27

DESIGN CONSIDERATIONS

The factors determining the type of rivet most suitable for a particular application have been mentioned earlier. The designer can profit by remaining aware of certain assembly tips and techniques called from actual field experience of blind riveting specialists. Such knowledge can provide the designer with a relatively simple means of introducing significant improvements in the performance or appearance of an end product. Seven examples are worthwhile to remember: 1. Fastening relatively thin sections often is a stumbling block for designers faced with reducing the overall weight of an assembly or obliged to use relatively thin sections for functional objectives. Particularly for units that cannot be assembled by adhesive bonding for one reason or another, blind rivets with large flanged heads can serve to minimize the possibility of fracturing the surfaces of the members being fastened. They also provide the extra bearing strength required to ensure a quality fastening. 2. When fastening sections of dissimilar thicknesses, best results are obtained if the rivet is expanded with the thin section directly under the rivet head. This causes the heavy sheet to bear the brunt of the clinching action. To expand the rivet on the thinner member risks damage to its surface. 3. Riveting metal to another type of material, for example, plastic, often is a problem for the designer. If the rivet is expanded against the harder material, this can be easily solved. 4. When fastening a flat section to a curved section, it is possible to set the rivet from the curved side and for the rivet so set to hold. However, it is strongly recommended that the rivet be set from the flat side. 5. When riveting corrugated material such as roofing, rivets should be on the flat slope if possible. Second choice is the peak. The poorest place is the valley because water will accumulate at the rivet. 6. Care should be taken to use a rivet material which is compatible with the material being riveted in order to eliminate or reduce corrosion. 7. It is sometimes most advantageous to design a part for rivet failure since the broken rivet can usually be replaced much easier than fractured or torn sheet metal. In addition to the rather specific tips and techniques just described, the designer can also profit from the following design recommendations.

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Fig. 3

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(To Be Continued Next Month)

SPORT BIPLANE ENGINES The first big change in Sport Biplane Class rules since souped-up engines were outlawed on January 1, 1970 is about to take place. The Class Engine Committee of the Professional Race Pilots Association is about to approve use of a second engine in addition to the popular Lycoming 0-290 — the 235-cu. in. Franklin "Sport 4" series rated at 125 hp. According to the Committee, this engine must be used in strictly stock configuration, with NO exceptions! The Committee will also be extra careful in checking Lycoming engines, especially the induction and ignition systems. £) 28

AUGUST 1971

Helpful Hints WELDING TIPS

By Antoni Bingelis (EAA 2643/Designee 306) 8509 Greenflint Lane Austin, Texas

LTHOUGH 4130 STEEL (chrome molybdenum or A "chrome-moly") is very tough and strong when cold, it is even weaker than mild steel in the heated condition;

therefore, it must not be stressed or shocked while in a white-hot state. The following guidelines should be heeded if problems are to be avoided: 1. Do your welding in a draft-free area, otherwise the metal will chill too fast and thereby be weakened. 2. Never use tightly clamped jigs. Spring-type clamps are just fine for holding parts to be welded, and they come in various sizes. 3. Good welders say that starting welding on an edge is not a good technique. Rather, it is best to start at a point away from the edge and work to it. Caution — heat builds up fast near the easily heated edges and it is very, very easy to inadvertently burn through the edge of the metal. Watch it and draw the flame away slightly as needed when the edge itself is reached. If you do ruin a piece, make it over. That's where the educational part of homebuilding comes in. 4. In all cases where parts have been tack-welded together, it is most important that you melt completely through the tack as you complete the final weld. 5. As the thickness of the metal being welded decreases, the selection of the proper tip and the adjustment of the gases becomes very important. Thin metals are easily buckled when too much heat is used. 6. Especially on thin metal and thin-wall tubing, care needs to be taken to clean any dirt, scale, or oxide from the parts to be welded. Percentagewise, as the parent metal becomes thinner, the chances of having dirty metal in the weld is increased. Take time to clean the weld areas. 7. Get in the habit of preheating the metal in the area to be welded. 8. Don't clamp your work in the vise and then try to weld on the part near the jaws of the vise as the heavy metal of the vise will draw away the heat and you'll have difficulty getting the metal hot enough to do good work. Remember, any large or heavy metal areas near the weld areas of work will draw away the heat from the joint and will require a larger flame, thereby increasing the risk of burning the adjacent metal. 9. Be sure your line of weld in the parent metal is heated to proper melting point. Try to keep the weld pool size as uniform as possible. 10. Heat the filler rod to the same melting point before introducing it into the melted pool of metal. 11. Add filler rod as evenly and steadily as possible. 12. Don't rush! Be sure that the added metal and parent metal are puddled together properly. 13. Keep playing the outer envelope flame over the pool to protect it from the oxidizing effect of the air. 14. Melt a certain portion of the parent metal on both sides for the entire length of the weld. 15. Avoid reheating of weld metal which has cooled.