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Chapter 7 Relays and Switches

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Chapter 7 Relays and Switches Chapter 7 Relays and Switches As the Bell System moved from manual to automatic switching, relay tech­ nology played a key role in logic, memory, and switching functions previously performed by human operators. Research and development efforts focused on reliable and inexpensive relays and switches to handle the demands of the rapidly growing telephone system. The number of contacts per relay was in­ creased to permit more complex functions, while the final assembly was simplified through the use of premolded subassemblies. Sealed enclosures improved the quality of relay contacts. As semiconductor devices started to perform the logic functions, there was a demand for miniature relays compatible with the physical design of semiconductor electronics. In other developments, crossbar switches replaced panel and step-by-step switches, and later the ferreed switch was designed to provide the switching path in electronic switching systems. I. INTRODUCTION In 1925, the Bell System was serving about 12 million telephones. Manual switching was still dominant, although automatic switching was being introduced. Two types of automatic systems had been developed in which the talking path was established by special switches: step-by-step and panel switching systems. In addition to these switches, general-purpose relays were required to perform various peripheral logic functions. The development of these systems is described in the companion volume The Early Years (1875-1925).Another companion volume, Switching Tech­ nology (1925-1975),covers the system developments after 1925, while this chapter covers the relays and talking path switches for these systems. As of 1925, Western Electric was manufacturing about 3 million relays per year. 1 The E- and R-type relays were the general-purpose relays of the period; they used punched parts to permit economical large-scale man­ ufacture. Contact springs were stud activated and single contacts were used. The relay magnet was capable of activating a maximum of 12 contact 2 3 springs-4 transfer (break-make) contact sets, for example. • Principal author: S. J, Elliott 285 286 Enginei?ringand Sciencein the Bell System During the next five decades, the number of Bell System telephones grew to more than 100 million; mechanized switching almost completely displaced manual switching, and the demand for relays increased dra­ matically. By 1970, Western Electric was manufacturing more than 80 million relays per year. In addition to increasing the need for relays, the evolution and expansion of mechanized switching also led to more stringent demands on relay performance. As relays and other electromechanical devices took over logic and memory functions formerly performed by human operators, the av­ erage number of relay operations per call also increased dramatically. For example, a manual line (in which an operator selected the desired lines of both calling and called customer, rang the station, checked for busy signals, and kept track of charges) needed 2.5 relays per line. The mech­ anized step-by-step line of the mid-l 950s, on the other hand, required seven relays. The No. 5 crossbar system of the same period needed 30 times the number of relays performing over 12 times the number of relay operations per call. The increasingly large capital investment required for complex automatic switching machines dictated that the useful service life of such machines be increased also. Consequently, where a relay life of 10 million operations had been adequate in the early manual switching systems, some later relays had to operate 100 million times in step-by-step and panel systems, and as many as 1 billion times in crossbar and electronic switching systems. At the same time, the switching networks used in the talking path were undergoing dramatic changes. The early step-by-step and panel switches were replaced by crossbar switches, which were incorporated into switching systems with common control units. Hermetically sealed ferreed contacts became the basic building blocks in the switching networks for electronic switching systems. II. EXPOSED-CONTACT RELAYS During the first half of the 1930s, development started on crossbar switching systems using a matrix switch in the talking path (see section IV) and a common control unit for establishing connections. Because the control unit is shared by many users, it is heavily used while executing complicated logic. Requirements for a new family of general-purpose rellays to meet these needs led to the development of the U-type relays. (Fig. 7-1] Compared with the earlier E- and R-type relays, the U-type relays provided up to twice as many contacts per relay, faster operation, generally more reliable contacts, and longer life. The U-type relay continued to lean heavily on punched parts, including flat, stud-actuated contact springs. But twin contacts were provided, and one spring of each pair was bifurcated to provide a degree of independence between the two contacts--a form Relays and Switches 287 (a) (b) (c) ---- 5 INCHES---- Fig. 7-1. The three types of general-purpose relays developed during the period from 1925 to 1952. (a) The U type was the first Bell System relay to employ twin contacts and bifurcat ed contact springs. (b) The UB type was the first to us e card-release actuation, and the AF typ e (c) was the first of the wire spring relays. of redundancy to increase contact reliability. 4 Manufacture of U-type relays started in 1936 . A decade later, in a refinement of the U -type relay design, stud actuation was replaced by card-release actuation in the UB-type relay . Card actuation was an important step in improving reliability by substantially decreasing the tendency of wear to reduce contact forces and by reducing the tendency of contacts to lock mechanically when "pip-and-crater" erosion occurred. [Fig. 7-2] The design also permitted a longer slot in the bifurcated contact springs, thereby increasing the independence of the twin contacts and further improving their reliability. 5 [Fig. 7-3] The AF-type wire spring relays , the next generation of general-purpose relays , also used card-release actuation and continued the trend set by the U-type relay of providing completely independent twin contacts, more contacts per relay where required, either low battery drain or faster op­ eration, greater contact reliabili ty, and longer mechanical life . [Fig. 7-4] In addition , the wire spring design permitted substantial reductions in the amount of hand labor required to assemble and adjust a relay, an important factor in holding down the cost of telephone equipment during the rapid rise of hourly wages following World War II. For example, where the assembly of a U-type relay required the handling of many individual contact springs , insulators, bushings, and screws, the contact springs and insulators for an AF-type relay were provided as molded subassemblies, 288 Engineering and Science in the Bell System MAKE CONTACTS NOT ACTUATED CONTACT SPRING lal SPOOL-HEAD SPRING FORMER TYPES =====--------_::;ACTUATED § IJ~ lb) le) NOT ACTUATED J ---1 CARD SPRING I UB-TYPE ., =1 ldl (._____ CARD SPRING L ACTUATED lei D' @~ Fig. 7-2. Actuation of contact springs. (a) and (b) illustrate stud actuation of a normally open contact in a U-type relay. Without applied pressure, the spring carrying the moving contacts is positioned away from the fixed contact. A stud, moved by the armature, presses against the contact spring a short distance behind the contact to close the contacts when the relay is operated. In this case, the desired contact force of 20 to 30 grams is developed by a spring deflection of about 13 mils. (c), (d), and (e) illustrate the card-release actuation used in the UB-type relay. Here, without an ex­ ternal force, the moving contact spring is positioned against the fixed contact. To produce the desired contact force, the unconstrained spring has a deflection of about 294 mils. When the relay is not actuated, the card holds the movable contact away from the fixed contact. When the relay is actuated, the armature presses the card toward the fixed contact, allowing the contacts to close. The card no longer touches the contact spring when the relay is fully operated. so that the assembly operator had to handle only two or three wire blocks; a spring clamp took the place of the screws. Moreover, a considerable amount of individual spring bending was done to obtain the desired contact Relays and Switches 289 26 24 UB RELAY rn 22 ::;; <( a: 20 ~"" 18 w u a: 16 5: 14 u... <( 12 z... 0 u 10 w 8 "'<( ffi 6 <(> 4 2 0 0 0.002 0.004 0.006 0.008 0.010 0.012 STUD AND CONTACT WEAR IU RELAY), CONTACT WEAR IUB RELAY! IN INCHES Fig. 7-3. Stud and contact wear for the U-type relay and contact wear for the UB-type relay. An important characteristic of the UB-type relay's card­ release actuation is that wear has very little effect on contact force until the wear reaches the stage where the card fails to separate from the movable contact spring when the contact is closed. By contrast, contact force in the stud-actuated U-type relay drops off fairly rapidly as the contacts and the stud wear. U RELAY s I ~g~-1, ...... UB RELAY l:l□: I I '0.94 INCH I Fig. 7-4. Evolution of twin-contact springs from the slightly bifurcated flat spring used in the U-type relay to the completely independent twin wire springs of the AF-type relay. 290 Engineering and Science in the Bell System forces and contact gaging in U-type relays. In the wire spring relays, on the other hand, the combination of low-stiffness movable springs and card-release actuation avoided the need for any contact force adjustment, and a simple mass adjustment was sufficient to meet the gaging require­ ments.
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