Rivet
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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.
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[edit] 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.
[edit] 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.
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 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). 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.
[edit] 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. Blind rivets are often avoided for critical structural joints because they generally have less load carrying capability than solid rivets. Furthermore, because of the mandrel they are more prone to failure from corrosion and vibration. A blind rivet consists of the rivet body and the setting device or the mandrel (sometimes called the nail or stem).
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 molybolt. 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.
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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] Installation
There are several methods for installing rivets. Rivets that are small enough and soft enough are often "bucked"[1]. 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.
[edit] 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.
[edit] 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. [2]
[edit] Alternatives
- Nails
- Bolted joints
- Welding
- Soldering
- Folded joints
- Self-Pierce Riveting
[edit] See also
[edit] References
- ^ The term "bucked" comes from a name for one of the two parts of a rivet. The "head" of the rivet is one part, and is the part that the air-hammer strikes. The other part is referred to as the "bucktail", and is the part that the bucking bar is held against. Hence the terms "bucking bar", "bucked", and "buck-man"
- ^ Segui, William T. (2007). Steel Design, 4th, Toronto: Thomson. ISBN 0495244716.
