A rivet is a permanent mechanical fastener. Before being installed, a rivet consists of a smooth cylindrical shaft with a head on one end; the end opposite to the head is called the tail. On installation, the rivet is placed in a punched or drilled hole, the tail is upset, or bucked, so that it expands to about 1.5 times the original shaft diameter, holding the rivet in place. In other words, pounding creates a new "head" on the other end by smashing the "tail" material flatter, resulting in a rivet, a dumbbell shape. 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 a head on each end of an installed rivet, it can support tension loads. However, it is much more capable of supporting shear loads. Fastenings used in traditional wooden boat building, such as copper nails and clinch bolts, work on the same principle as the rivet but were in use long before the term rivet was introduced and, where they are remembered, are classified among nails and bolts respectively.
There are a number of types of rivets, designed to meet different cost and strength requirements: 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 of a shaft and head that are deformed with a hammer or rivet gun. A rivet compression or crimping tool can deform this type of rivet; this tool is used on rivets close to the edge of the fastened material, since the tool is limited by the depth of its frame. A rivet compression tool does not require two people, is the most foolproof way to install solid rivets. Solid rivets are used in applications where 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 rivets come with rounded or 100° countersunk heads. Typical materials for aircraft rivets are aluminium alloys and nickel-based alloys.
Some aluminum alloy rivets are too hard to buck and must be softened by solution treating prior to being bucked. "Ice box" aluminum alloy rivets harden with age, must be annealed and kept at sub-freezing temperatures to slow the age-hardening process. Steel rivets can be found in static structures such as bridges 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 actuated squeezing tool or a handheld hammer. Applications where only one side is accessible require "blind" rivets. Solid rivets are used by some artisans in the construction of modern reproduction of medieval armour and metal couture; until recently, structural steel connections were either welded or riveted. High-strength bolts have replaced structural steel rivets. Indeed, the latest steel construction specifications published by AISC no longer covers their installation; the reason for the change is due to the expense of skilled workers required to install high strength structural steel rivets.
Whereas two unskilled workers can install and tighten high strength bolts, it takes a minimum of four skilled riveters to install rivets. At a central location near the areas being riveted, a furnace was set up. Rivets were placed in the furnace and heated to glowing hot so that they were more malleable and deformed; the rivet warmer or cook used tongs to remove individual rivets and throw them to a catcher stationed near the joints to be riveted. The catcher caught the rivet in wooden bucket with an ash-lined bottom; the catcher inserted the rivet into the hole to be riveted quickly turned to catch the next rivet. The holder up or holder on would hold a heavy rivet set or dolly or another pneumatic jack against the round head of the rivet, while the riveter applied a hammer or pneumatic rivet hammer to the unformed head, making it mushroom against the joint in its final domed shape. Alternatively the buck is hammered less flush with the structure in a counter sunk hole. Before the use of pneumatic hammers, e.g. in the construction of RMS Titanic, the man who hammered the rivet was known as the "basher".
Upon cooling, the rivet exerted further force, tightening the joint. The last 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. In the seismic retrofit of such structures it is common practice to remove critical rivets with an oxygen torch, precision ream the hole insert a machined and heat treated bolt. Semi-tubular rivets are similar to solid rivets; the purpose of this hole is to reduce the amount of force needed for application by rolling the tubular portion outward. The force needed to apply a semitubular rivet is about 1/4 of the amount needed to apply a solid rivet. Tubular rivets are sometimes preferred for pivot points since the swelling of the rivet
Manuel María de los Santos Acosta Castillo was a Colombian General and political figure. He served as the president of Colombia from 1867 until 1868. Acosta was born in Miraflores, Boyacá, on November 1, 1828, he died in Bogotá on January 9, 1901. Although Acosta studied and graduated in medicine, he did not practice this profession. Rather, he pursued political careers, he was elected several times to the House of Representatives and the Senate. Santos Acosta was one of the main players during the constitutional reform of 1853. In 1867, Congress elected Acosta as second Vice-President. Congress had elected Santos Gutiérrez as first Vice-President and Joaquín Riascos as third Vice-President. Santos Acosta at virtualology.com
The IEEE 802.21 refers to Media Independent Handoff and is an IEEE standard published in 2008. The standard supports algorithms enabling seamless handover between wired and wireless networks of the same type as well as handover between different wired and wireless network types called Media independent handover or vertical handover. Vertical handover was first introduced by Randy Katz at U C Berkeley; the standard provides information to allow handing over to and from wired 802.3 network to wireless 802.11, 802.15, 802.16, 3GPP and 3GPP2 networks through different handover mechanisms. The IEEE 802.21 working group started work in March 2004. More than 30 companies have joined the working group; the group produced a first draft of the standard including the protocol definition in May 2005. The standard was published January 2009. Cellular networks and 802.11 networks employ handover mechanisms for handover within the same network type. Mobile IP provides handover mechanisms for handover across subnets of different types of networks, but can be slow in the process.
Current 802 standards do not support handover between different types of networks. They do not provide triggers or other services to accelerate mobile IP based handovers. Moreover, existing 802 standards provide mechanisms for detecting and selecting network access points, but do not allow for detection and selection of network access points in a way, independent of the network type. Allow roaming between 802.11 networks and 3G cellular networks. Allow users to engage in ad hoc teleconferencing. Apply to both wired and wireless networks the same list as IEEE P1905 specifies to cooperate in software-defined networking Allow for use by multiple vendors and users. Compatibility and conformance with other IEEE 802 standards 802.11u unknown user authentication and 802.11s ad hoc wireless mesh networking. Include definitions for managed objects that are compatible with management standards like SNMP. Although security algorithms and security protocols will not be defined in the standard, authentication and network detection and selection will be supported by the protocol.
Implementation is still in progress. Current technologies such as 802.11 that accomplish handover use software to accomplish handovers and suggest that software will be the way that handover will be implemented by 802.21. The use of software as a means to implement 802.21 should not cause large increases in the cost of networking devices. An open source software implementation is provided by ODTONE. Crossing different administrative connectivity domains will require agreements among different network operators; such agreements are still not in place. In smart phones today, a user can manually select to use WiFi or cellular LTE, but the connections are not automatically maintained should a disconnection of one network occurs. Hence, seamless handovers across different wire/wireless networks are still not available today. A user should be able to get handed off to an 802.11 network. A cellular phone user in the midst of a call should be able to enter an 802.11 network hotspot and be seamlessly handed off from a GSM network to the 802.11 network and back again when leaving the hotspot.
Unlicensed Mobile Access technology is a mobile-centric version of 802.21. UMA is said to provide roaming and handover between UMTS, Bluetooth and 802.11 networks. Since June 19, 2005, UMA is a part of the ETSI 3GPP standardization process under the GAN Group; the Evolved Packet Core architecture for Next Generation Mobile Networks provides the Access Network Discovery and Selection Function element. Its S14 interface provides the communication path between the Core Network and the User Endpoint device on which to exchange discovery information and inter-system mobility policies, enabling as such a network suggested reselection of access networks. Mobility management Mobile IP Handover Radio resource management Roaming Vertical handoff Handoffs in Wireless ATM / Mobile ATM Asynchronous Transfer Mode#Wireless ATM or mobile ATM IEEE Standards Associations IEEE 802.21 web site Is Cellular Handoff Nigh? Article Internet Protocol Journal Introduction to IEEE 802.21 Open source implementation ODTONE