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.
rff
1d\~
J Fig. 2
Fig. 3
Fig. 4
(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.
Blind Rivets Application Considerations Part Two of Two Parts
(Reprinted by Permission From ENGINEERING DIGEST (Canada), Volume 16, No. 10) Rivet Length: There is a delicate balance between joint thicknesses, mandrel-head dimensions, the coldworking properties of the rivet material and, of course, the
shank diameter of a blind rivet. On the other hand, one length can be used for a wide range of panel thicknesses.
For any use beyond the tabulated data, actual limitations should be determined by test (see Fig. 7). Fig. 2
Fig. 3
Fig. 4 ^-
CLEARANCES AND ALLOWANCES Tool Clearance: Minimum allowable distance from a wall or projection to the rivet-hole centerline is critical and
should be checked against tool specifications. Depth of a channel or cylindrical section likewise limits the application if the rivet is inserted as shown in Fig. 2. However, tool
extensions can often be employed, and the nosepiece of the tool can be extended by as much as an inch. Edge Distance: From the standpoint of joint strength,
Poor
Good Fig. 8
Good
qPPoor
y Good Fig. 9
Good
Angles and Formed Section: Avoid obstructions to the tool, which should set flush on the work. Thus, the "poor" joint in Fig. 8 requires a special offset tool. Reverse the rivet and set from the other side, or reduce the length of the angular section.
dimension can be greatly reduced. Example: The rivet functions only to hold a bracket or component in position.
Channel Sections: For adequate tool clearance, set from the underside of the channel or widen the section (see Fig. 9). A tool extension may be used to reach inside the section, but makes the tool somewhat more awkward to handle.
Head Clearance: Standard head dimensions vary with rivet size. Height and diameter can readily be determined
set the rivet against the thicker material. Rivets of this type can be set against 0.030-in. and thinner aluminum stock
from specification sheets. Frequently, the work can be
(see Fig. 10).
countersunk, or the joint designed to prevent interference
Flush Joints: A truly flush surface can be obtained only by countersinking the outer wall or plate. Dimpling generally is not recommended because of the possibility of the rivet not fully "bottoming" and setting against the hole edge. A smooth, watertight, but not completely flush surface is obtained by plugging the rivet (see Fig. 11).
the recommended distance from the rivet centerline to the edge of a sheet should not be less than twice the rivet diameter (see Fig. 3). If joint strength is secondary, this
with mating sections (see Fig. 4). Hole Clearance: Permissible clearance depends on
whether the fastener grips the wall or clinches underneath. In the latter case, oversized holes are less critical, and one does not depend on wall construction to prevent breakout (see Fig. 5).
! t
or
V^d^Before
Fig. 5
Fig. 6
After
I I I d\ Fig. 7
Back Clearance: Distance from the underside of the rivet head to end of the stem head determines the required back clearance — not dimensions after setting. These values
should take into account the maximum combined material thickness (see Fig. 6). 28
SEPTEMBER 1971
Thick and Thin Sheets: If there is a choice, it is best to
Pivoted Joints: The method shown in Fig. 12 is satisfactory for low-cost consumer products where some binding can be tolerated until the'joint works in. The one shown in Fig. 13 is perhaps best, because there is some
small clearance between the rivet head and the wall. The others shown in Fig. 14 and 15 are satisfactory. Also, a special nosepiece can be used which allows a blind rivet to be set loosely so that the fastened parts can pivot freely. Washers: If a metallic section is to be assembled to a
softer material such as leather, a washer may be necessary to provide the necessary rigidity for clinching action. The
soft material should be outside with the washer under the expanded head. For conventional assemblies either way is practical (see Fig. 16).
Sotisfactory Good Fig, W
Countersunk
Extruded Sections: Blind riveting is common, because of the limited access and clearance in most extruded sections. Rigidity of the joint at the right is far greater than that at the left in Fig. 24. Setting against unsupported internal projections is seldom recommended. Compressible Materials: Compressible materials do not provide a firm base for the setting action, and the rivets may tear through. Use of a formed sleeve or bushing, or a washer, can solve this problem. If the material is to be riveted to a metal section, the washer is under the plastic
Copped
fig. / /
(see Fig. 25).
Large head
Sleeve Back-up
Good
Satistactory
Satisfactory
Fig. 13
Fig. 14
Fig. 15
Poor
Good
\ Wash* %
Fig. 76
Tight Joints: Since the blind rivets exert considerable gripping force when clinched, the joint shown in Fig. 17 is impractical. Conversely, either of examples in which a single rivet is employed may prove as satisfactory as the one with the multiple rivets (see Fig. 18). Multiple Assemblies: One, two or more components or sections can be riveted to a base or panel with a single rivet, provided they can be mounted from the front, or the base section is accessible from the rear or underneath (see Fig.
IL
Unsoh'sfoctory
Good
Unsatisfactory Good
Fig. 17
TH-
Good Good
Fig. IH
Fig. 19
Fig. 20
Straps and Clamps: A blind rivet can be used to assemble straps, clamps, and similar members around a component. Allowance must be made for the squeezing action on the clamp (see Fig. 20). Housings and Panels: Of the many ways to construct a housing or panel, the ones shown in Figs. 21, 22 and 23 are perhaps the most common. Example: A double-channel section forming the joint to which the panels are assembled and riveted; a back-up stringer with a finishing strip between the gap. In selecting the design, consider cost, appearance, strength, and thermal expansion.
Good
Fig. 25
Fig. 2-1
19).
Unsotisfoctory Good
Washer
Unnecessary
Tubing: For maximum rigidity, the rivet should not extend through both walls of tubular members. The rivet will set blind against the curved inner wall section, or the tubing can be flattened to increase the contact area (see Fig. 26). Plastic Members: Preferably, the rivet should be set against a metallic rather than a plastic member. However, the rivets are being set successfully against many molded and extruded plastics, which resist setting pressure without cracking (see Fig. 27). Blind Holes or Slots: Clinching the rivet against the side of a blind hole or into and against a milled slot, intersecting hole, or internal cavity is possible because of the clinching action of the rivet (see Fig. 28). Plastic
Metal
stringer.
\Finishmg
strip
Finishing strip
Depends on Material
Fig. 21
Fig. 22
Weather-proof Joints: It is difficult to produce a pressure-tight riveted joint, but reasonably moistureresistant, weather-tight assemblies have been evolved, with open-end "POP" rivets. For maximum degrees of air and liquid tightness, closed-end "POP" rivets are recommended. Hole-filling characteristics of the closed-end "POP" rivets are provided by the high degree of radial expansion generated in the rivet bodies as they are being set.
Fig. 27
Good
Good
Fig. 28
R I V E T I N G TOOLS The fastest, most efficient installation of rivets is done with power tools — air, hydraulic, or electric. Manual tools, such as special pliers (Fig. 29) can be used efficiently with practically no training. Selection requires consideration of such factors as cost, design, and versatility of the tool in accommodating various rivet designs. Tool prices vary with the size (strength) or the rivet selected, and power tools are more expensive than manual tools. SPORT A V I A T I O N
29
-4P
enough for the tool to enter. Many of these access problems can be solved with tool accessories, such as extensions, special heads, or nosepieces. It is a good idea to select pneumatic, hydraulic, or electric tools (see Fig. 30) that are flexible so that different rivet-head attachments can be used for more than one size or style of rivet.
OTHER CONSIDERATIONS
In-Place Costs: Blind rivets often have lower in-place costs, although they cost more than solid rivets or tapping screws. These lower costs result from a combination of low
Fig. 29—Typical hand riveters.
tooling investment, high installation speed, and single
operator requirement. Also, savings can be realized if one
long rivet is used, because of its large grip range, for several applications of different thicknesses.
Fig. 30—Pneumatic and air-hydraulic riveting tools and pulling heads.
Size or shape of the setting tool may require special attention. For example, the side members of deep channel sections on the assembly being joined may not be big
Loading: A blind-riveted joint is usually in compression or shear, both of which the rivets can support somewhat better than tensile loading. Performance of rivets subjected to vibration can be influenced by minimum rivet-hole clearance and such installation techniques as applying compression to the assembly with clamps before the rivets are set. Material Thickness: Thickness to be joined can be critical for some types of blind rivets and of no significance for others. The setting method can also be a factor in this matter. Some rivets can be set in materials as thin as 0.020 in. if the head is properly formed and shank expansion is carefully controlled during setting. If possible, however, it is best to form the blind head against the thicker sheet of a combination. Also, if one component is of compressible material, rivets with extra large head diameter should be used. Dimensional Tolerances: Tolerances of blind-rivet holes vary according to the type of rivet. Usually it is more economical to select a fastener that clamps against the surfaces of the parts being joined rather than one that depends only on hole filling. Q
Blind Rivet Selection Guide Drive Chemically Expanded Threaded
Pull Mandrel
Blind Rivet Type
Pin Application
(Explosive) Open-End Closed-En d
Pull-Thru Break Mandrel Break Mandrel Nonbreak Open-End Closed -End
Angles and Formed Sections
X
X
X
X
X
X
X
X
Channel Sections
X —
X X
X
X
X
X
X
X
X
X
Flush Joints
X
X
X
— — X
X
Thick & Thin Sheets
X
—
—
X
Pivoted Joints
—
X
X
—
X
X
X
Washers
X
X
X
X
X
Multiple Assemblies
X
X
X
X
X
X X X
Straps & Clamps
X
X
X
X
X X
X
Housings & Panels Weatherproof Joints
X
X
X
—
—
X
—
X —
X X
X X
X
Extruded Sections
Compressible Materials Tubing
X
Plastic Members
Blind Holes or Slots Honeycomb Sections 30
SEPTEMBER 1971
X
X
X
X
X(3)
X(3)
X X
X
X
X
X
X
X
X X
X
X
X
X
X
—
X
—
X
X
X
-U)
X X
X
—
X
X
X(2)
X
X
X X
X
X
X
X
X
X X
X X
(1) Chock manufacturer! (t) U»« only
on
haavy-wall tubing.
(3) Umited UM— Chock manufacturer* recommendation.
