rivet

rivet

[riv-it]
rivet, headed metal pin or bolt whose shaft is passed through holes in two or more pieces of metal, wood, plastic, or other material in order to unite them by forming the plain end into a second head. The button-head rivet has a hemispherical head; the countersunk-head rivet has a flat head made to fit a countersunk hole. A large rivet for building construction is first heated so that the pneumatic hammer used to set it can more easily squash the plain end into a head. When the hot rivet cools, it shrinks and pulls the parts tightly together. For critical work, holes are drilled and reamed to exact size; for most other work they are punched. Full tubular and split rivets can be driven through soft materials without the necessity of first making a hole; after passing through the materials, the rivets' plain ends spread out to form heads as they strike a hard substance. More complicated blind rivets are used when only one side of the work is accessible. The mandrel type is a tube in which a rod with an enlarged end is inserted. After the rivet is pushed into the hole, the rod is pulled back through, crushing the end of the rivet into a head and forcing the sides of the tube against the walls of the hole. The drive-pin type is a tube with an opening at the headless end smaller than at the head end. As the pin is driven through from the head end, it spreads the tube out over the edge of the hole. The explosive blind rivet is filled with an explosive; when the head is heated with an iron, the explosive ignites and expands the headless end over the edge of the hole. Rivets are made of steel, aluminum, copper, and many other metals, and of plastics.

A rivet is a mechanical fastener. Before it is installed it consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a pre-drilled hole. Then the tail is "upset" (i.e. deformed) so that it expands to about 1.5 times the original shaft diameter and holds the rivet in place. To distinguish between the two ends of the rivet, the original head is called the factory head and the deformed end is called the shop head or buck-tail.

Because there is effectively a head on each end of an installed rivet it can support tension loads (loads parallel to the axis of the shaft); however, it is much more capable of supporting shear loads (loads perpendicular to the axis of the shaft). Bolts and screws are better suited for tension applications.

Fastenings used in traditional wooden boat building like copper nails and clinch bolts work on the principle of the rivet but they were in use long before the term rivet was invented. So, where they are remembered, they are usually classified among the nails and bolts respectively.

Types

There are a number of types of rivets, designed to meet different cost, accessibility, and strength requirements. These include solid rivets, blind rivets, multi-grip rivets, grooved rivets, peel type blind rivets, self-pierce rivets, plastic rivets, tubular rivets, etc.

Solid rivets

Solid rivets are one of the oldest and most reliable types of fasteners, having been found in archaeological findings dating back to the Bronze Age. Solid rivets consist simply of a shaft and head which are deformed with a hammer or rivet gun. The use of a rivet compression or crimping tool can also be used to deform these type of rivets; this tool is mainly used on rivets closer to the edge since it is limited by its depth of frame. A rivet compression tool does not require two people and is generally the most foolproof way to install solid rivets.

Solid rivets are used today in applications where reliability and safety count. A typical application for solid rivets can be found within the structural parts of aircraft. Hundreds of thousands of solid rivets are used to assemble the frame of a modern aircraft. Such solid rivets come with rounded (universal) or 100° countersunk heads. Typical materials for aircraft rivets are aluminium alloys (2017, 2024, 2117, 7050, 5056, 55000, V-65), titanium, and nickel based alloys (e.g. Monel). Some aluminum alloy rivets are too hard to buck and must be softened by annealing prior to being bucked. "Ice box" aluminum alloy rivets harden with age. These rivets are likewise annealed the then kept under sub-freezing refrigeration (hence the name "ice box") to slow the age hardening process. Steel rivets can be found in static structures such as bridges, cranes, and building frames.

The setting of these fasteners requires access to both sides of a structure. Solid rivets are driven using a hydraulically, pneumatically, or electromagnetically driven squeezing tool or even hand held hammers. Applications in which only one side is available require the use of blind rivets.

Blind rivets

Blind rivets are tubular and are supplied with a mandrel through the center. The rivet assembly is inserted into a hole drilled through the parts to be joined and a specially designed tool used to draw the mandrel into the rivet. This expands the blind end of the rivet and then the mandrel snaps off. (These are also commonly called pop rivets from the sound and feel through the setting tool when the mandrel breaks.) These types of Blind rivets have non-locking mandrels and are avoided for critical structural joints because the mandrels may fall out, due to vibration or other reasons, leaving a hollow rivet that will have a significantly lower load carrying capability than solid rivets. Furthermore, because of the mandrel they are more prone to failure from corrosion and vibration.

Prior to the adoption of blind rivets, installation of a solid rivet typically required two assemblers: one person with a rivet hammer on one side and a second person with a bucking bar on the other side. Seeking an alternative, inventors such as Carl Cherry and Lou Huck experimented with other techniques for expanding solid rivets. Unlike solid rivets, blind rivets can be inserted and fully installed in a joint from only one side of a part or structure, "blind" to the opposite side.

Due to this feature, blind rivets are mainly used when access to the joint is only available from one side. The rivet is placed in a pre-drilled hole and is set by pulling the mandrel head into the rivet body, expanding the rivet body and causing it to flare against the reverse side. As the head of the mandrel reaches the face of the blind side material, the pulling force is resisted, and at a predetermined force, the mandrel will snap at the break point of the mandrel. A tight joint formed by the rivet body remains, the head of the mandrel remains encapsulated at the blind side, although variations of this are available, and the mandrel stem is ejected.

The rivet body is normally manufactured from one of three methods:

  • Wire, the most common method
  • Tube, common in longer lengths, not normally as strong as wire
  • Sheet, least popular and generally the weakest option.

There is a vast array of specialty blind rivets that are suited for high strength or plastic applications. Typical types include:

- TriFold, a rivet that splits into three equal legs like a Molly bolt. Typically used in soft plastics where a wide footprint is needed at the rear surface. Used in automotive interiors and vinyl fences.

- Structural rivet(a), an "external" mechanically locked structural blind rivet that is used where a watertight, vibration resistant connection is of importance. Typically used in manufacture or repair of truck bodies. A special nose piece is required to apply this rivet.

- Structural rivet(b), an "internal" mechanically locked structural blind rivet that is used where a watertight, vibration resistant connection is of importance. Typically used in manufacture or repair of truck bodies.

Internally and externally locked structural blind rivets can be used in aircraft applications because, unlike common "pop-rivets" the locked mandrels can not fall out and are water tight. Since the mandrel is locked into place they have the same or greater load carrying capacity as solid rivets and may be used to replace solid rivets on all but the most critical stressed aircraft structures.

The typical assembly process requires the operator to install the rivet in the nose of the tool by hand then actuate the tool. However, in recent years automated riveting systems have become popular in an effort to reduce assembly costs and repetitive disorders. The cost of such tools range from $1,500 for autofeed pneumatics to $50,000 for fully robotic systems.

Drive rivet

A drive rivet is a form of blind rivet that has a short mandrel protruding from the head that is driven in with a hammer to flare out the end inserted in the hole. This is commonly used to rivet wood panels into place since the hole does not need to be drilled all the way through the panel, producing an aesthetically pleasing appearance. They can also be used with plastic, metal, and other materials and require no special setting tool other than a hammer and possibly a backing block (steel or some other dense material) placed behind the location of the rivet while hammering it into place.

Flush rivet

A flush rivet is used primarily on external metal surfaces where good appearance and the elimination of unnecessary aerodynamic drag are important. A flush rivet takes advantage of a countersink hole, they are also commonly referred to as countersunk rivets. Countersunk or flush rivets are used all over the outside of many military and commercial aircraft, including the AH-64 Apache helicopter.

Sizes

Rivet diameters are commonly measured in 1/32 inch increments and their lengths in 1/16th inch increments which are expressed as "dash" numbers at the end of the rivet identification number. A 'dash 3','dash 4' (XXXXXX-3-4) designation indicates 3/32" diameter and 4/16" or 1/4" long rivet. Some rivets lengths are also available in "half sizes" and will have a dash number such as -3.5 (7/32") to indicate it as a half size rivet. The letters and numbers that precede the dash numbers, in the rivets identification number, indicate the specification under which the rivet was manufactured and the head style. On many rivets the size in /32nds may be stamped on the rivet head. Ohter makings on the rivet head such as small raised or depressed dimples, or small raised bars indicate the rivet's alloy.

To become a proper fastener, a rivet should be placed in hole ideally 4-6 thousandths of an inch larger in diameter. This allows the rivet to be easily and fully inserted, then setting allows the rivet to expand, tightly filling the gap and maximizing strength.

Applications

Before welding techniques and bolted joints were developed, metal framed buildings and structures such as the Eiffel Tower, Shukhov Tower and the Sydney Harbour Bridge were generally held together by riveting. Also automobile chassis were riveted. Riveting is still widely used in applications where light weight and high strength are critical, such as in an aircraft. Many sheet-metal alloys are preferably not welded as deformation and modification of material properties can occur.

Common but more exotic uses of rivets are to reinforce jeans and to produce the distinctive sound of a sizzle cymbal.

Joint analysis

The stress and shear in a rivet is analyzed like a bolted joint. However, it is not wise to combine rivets with bolts and screws in the same joint. Rivets fill the hole where they are installed to establish a very tight fit (often called interference fit). It is difficult or impossible to obtain such a tight fit with other fasteners. The result is that rivets in the same joint with loose fasteners will carry more of the load—they are effectively more stiff. The rivet can then fail before it can redistribute load to the other loose fit fasteners like bolts and screws. This often results in catastrophic failure of the joint when the fasteners "unzip". In general, a joint composed of similar fasteners is the most efficient because all fasteners will reach capacity simultaneously.

Installation

There are several methods for installing rivets. Rivets that are small enough and soft enough are often "bucked. In this process the installer places a rivet gun against the factory head and holds a bucking bar against the tail or a hard working surface. The bucking bar is a specially shaped solid block of metal. The rivet gun provides a series of high-impulse forces that upset the rivet in place. Rivets that are large or hard may be more easily installed by squeezing instead. In this process a tool in contact with each end of the rivet clinches to deform the rivet.

Rivets may also be upset by hand, using a ball-peen hammer. The head is placed in a special hole made to accommodate it, known as a rivet-set. The hammer is applied to the buck-tail of the rivet, rolling an edge so that it is flush against the fastened material.

Testing

A hammer is also used to "ring" an installed rivet to test for tightness and imperfections. The inspector taps the head (usually the factory head) of the rivet with the hammer while touching the rivet and base plate lightly with the other hand and judges the quality of the audibly returned sound and the feel of the sound traveling through the metal to the operator's fingers. A rivet tightly set in its hole will return a clean and clear ring, while a loose rivet will return a recognizably different sound.

High Strength Structural Steel Rivets (ASTM 502A)

Until relatively recently, structural steel connections were either welded or riveted. High-strength bolts have completely replaced structural steel rivets. Indeed, the latest steel construction specifications published by AISC (the 13th Edition) no longer covers their installation. The reason for the change is primarily due to the expense of skilled workers required to install high strength structural steel rivets. Whereas two relatively unskilled workers can install and tighten high strength bolts, it took a minimum of four highly skilled riveters to install rivets in one joint at a time.

At a central location near the areas being riveted, a furnace was set up. Rivets were placed in the furnace and heated to a glowing hot temperature, at which time the furnace operator would use tongs to individually remove and throw them to catchers stationed near the joints to be riveted. The catcher would place the glowing hot rivet into the hole to be riveted, and quickly turn around to await the next rivet. One worker would then hold a heavy rivet set against the round head of the rivet, while the hammerer would apply a pneumatic rivet hammer to the unformed head, causing it to mushroom tightly against the joint in its final domed shape. Upon cooling, the rivet would contract and exert further force tightening the joint. This process was repeated for each rivet.

The last commonly used high strength structural steel rivets were designated ASTM A502 Grade 1 rivets.

Such riveted structures may be insufficient to resist seismic loading from earthquakes if the structure was not engineered for such forces, a common problem of older steel bridges. This is due to the fact that a hot rivet cannot be properly heat treated to add strength and hardness. In the seismic retrofit of such structures it is common practice to remove critical rivets with an oxygen torch, precision ream the hole, and then insert a machined and heat treated bolt.

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