Machinedesign 7322 P53 2009 03 19 Rivets 0

Straight Facts on Blind Rivets

March 16, 2009
Blind rivets, or breakstems, provide vibrationproof fastening for a wide variety of materials. But designers must know at least the basics before specifying them for applications.

Authored by:
Pete Beecherl
Engineering & Business Development Director

Tim Bartlett
Product Manager

Avdel North America
Stanfield, N.C.

Edited by Stephen J. Mraz
[email protected]

Rivets have been used to put things together since the Bronze Age, but it usually took two workers to set them: one with a hammer to pound the rivet through the hole, the other with a bucking bar to set it on the opposite side. Then, in the mid-1930s, engineers developed blind or breakstem rivets. They let workers install fasteners from one side of a workpiece when the opposite side is inaccessible or cannot even be seen.

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Blind rivets are now used to assemble everything from Blind rivets come in a variety of sizes, styles, and materials.small electronic components in cell phones to steel beams in the world’s largest bridges. They join different materials of varying thickness and can handle the most demanding environments. High-tech quality control delivers foolproof installation that lets only the right rivets be installed the right way in the right holes.

How they work
A blind rivet has a smooth, hollow, cylindrical body topped by a flared head and a solid-rod mandrel with a bulbed head that extends from the hollow rivet shaft. It looks somewhat like a nail. To install one, a jaw on a setting tool grips the mandrel, pulls it into the rivet body through a drilled or punched hole in one or more layers of material. The mandrel head expands the walls of the rivet outward, pressing them firmly into the walls of the hole while forming a tightly clinched load-bearing area on the opposite side of the material. Unused parts of the mandrel break off when the tensile force exceeds that needed to fully deform the rivet body.

Rivet manufacturers put grooves on some mandrels to weaken them, in essence predetermining the spot where tension will break them. Some mandrels also have a mechanical lock that snaps into place, plugs the opening in the rivet shell, and captures the remaining portion of mandrel inside the sleeve. Installation, from the time the operator pulls the trigger to the final setting of the rivet, takes about a second and leaves a permanent, vibration-resistant joint.

Controlled expansion of the rivet body is accomplished through proper design and material selection. Together, they ensure uniform compression and hole fill. This, in turn, lets blind rivets join a variety of dissimilar materials, including metals, plastics, and woods.

Designers select blind rivets based on the specific applications, and mandrels for how they perform as a built-in tool. The design challenge is to combine the right rivet size and configuration (tapered, float, sharp edged, or serrated) to the right mandrel for strong joints and rivet integrity. The goal is to have rivet bodies deform as specified, and mandrels to break at precise loads to ensure joint consistency, strength, and durability.

Benefits of blind rivets
Blind rivets are the fastest way to join materials. They can be set at 150 to 500/hr, and each setting is identical. This cuts assembly time and costs. Unlike threaded fasteners, there are no concerns over tool clearance, rotation failures, and secondary parts such as bolts and tapping plates, all of which add assembly time, weight, and cost. (Tapping plates are flat parts fastened atop the workpiece to reinforce the threaded parts installed in thin materials.) Blind rivets, unlike threaded fasteners, avoid problems of over or undertorquing, and stripping. Rivets also won’t loosen once installed because the rivet body is as large or larger than the hole it’s put in. They also won’t shake out or break off, and the tight seal helps prevent leaks and seepage.

Breakstems also work in ductile materials and thin gauge metals. If necessary, rivets with large heads (at least 50% larger than the hole) can be used to spread the load further. And on soft plastics, three or four metal legs unfold diagonally across the blind surface to spread the load even further. As a rule, mandrel heads must be big enough to spread the load but not so large as to waste material and add unnecessary weight.

Blind rivets can compensate for irregular holes, as long as they are within 0.004 in. of the rivets’ expansion range. Compressing the rivet body during installation compensates for such irregularities, as do metal legs that expand the load bearing surface.

Breakstems can fasten painted parts, as well as those with other finishes without causing surface blemishes. Thread and bolt fasteners, on the other hand, can scratch, dent, and dimple surfaces.

Types of blind fasteners
There are two classes of blind fasteners: open end and closed end.

Open-end breakstems are the most common. The mandrel breaks near the blind-side head and has flanges or countersunk heads for use in thin-gauge metals and low-strength materials.

Closed-end breakstems seal holes by closing the tail end of the rivet body and capturing the mandrel inside the rivet bore. They are often used to seal liquid-handling components such as hydraulic pumps. They are not used as extensively as they once were, having been replaced by open-end versions that seal the rivet body with equal effectiveness.

Selecting and installing the right blind rivet in the right hole involves rivet diameters, grip ranges, hole preparation, head style, and corrosion resistance.

Rivet diameter: Blind rivets are available in diameters from 3/32 to ¼-in., with 1/32-in. incremental increases available. Diameter selection is based on space, strength, and material thickness. The larger the diameter, the higher the shear and tensile strength, which is due to the thickness and material of the rivet body.

Grip range: The materials being joined must have a thickness that falls within a fastener’s grip range. (Grip range is the allowable difference between rivet length and material thickness.) Grip ranges increase in 1/16-in. increments, with each addition having better stem retention. A 0.5-in. grip range is usually the maximum length attained by standard breakstems, but custom designs can stretch this to an inch. Standard-size blind rivets join materials as thick as 0.75 in. Custom rivets handle materials as thin as 0.020 in., such as components for cell phones and computer chassis, or as thick as 2 in. to bridge gaps in tubular assemblies such as lawn chairs.

Hole preparation: The rivet must completely fill the hole. Principal factors that ensure this are material thickness, grip range, and rivet diameter. If a hole is too big, it will not be filled and the result is a loose rivet. If the hole is too small, the rivet won’t fit. Otherwise, no special preparation is required as long as holes are free of excessive burrs.

Head styles: There are three basic blind-rivet head styles: protruding, large flange, and countersunk. Protruding heads (also called dome or button heads) are the most common and simplest. Large flange designs have heads four to five times the size of rivet-body diameter to increase the bearing surface. Countersunk heads, after installation, are flush with the material. They are used for a better surface appearance or to reduce wind drag, such as in assembling aircraft.

Corrosion resistance: To slow or prevent corrosion, rivet bodies and mandrels are often made from identical materials, including low-carbon and stainless steels, and nickel-copper and aluminum alloys. When joining dissimilar materials, engineers specify protective coatings and sleeves to prevent corrosion or when rivets do not share the same physical and mechanical properties as the parent materials. Stainless steel breakstems offer the best corrosion protection in environments such as swimming pool ladders and city buses exposed to salt.

Installation tools
There are many types of blind-rivet installation tools, including handheld, lightweight, and semiautomated. All consist of a pulling head that grips the mandrel during rivet-body expansion.

Handheld tools install fasteners with lower break loads, typically 1,000 lb or less. They are portable and let technicians quickly install rivets of different sizes and types. Hydropneumatic hand tools are most commonly used to install blind rivets. Handheld installers can also be pneumatic, electric, or manually powered. Special nose assemblies adapt tools for putting rivets in narrow spaces, channels, insets, or right angles to the rivet body.

Up to the mid-1980s, most hand tools were steel and weighed a hefty 15 lb or more. Modern versions are lighter and more powerful. Hydropneumatic tools made from plastic and aluminum, for example, weigh an ergonomic 4 lb and set one breakstem per second. Older versions installed blind rivets within a couple seconds, but operators were slowed by having to lug heavier tools through an 8-hr shift.

Multiheaded semi and fully automatic workstations can install up to 268 breakstems simultaneously. Some of these tools also carry sophisticated monitors and foolproof sensors and controls to assure proper riveting and placement.

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