Manual / Guide · 1957
Graduate Engineering Training: U and Y Type Relays — Bell System (1957)
69 pp, OCR (this is the 11 MB copy; the 294 MB original scan was skipped). The 755A is built on U-type relays (BLR 1938)
Manual / Guide · 1957
69 pp, OCR (this is the 11 MB copy; the 294 MB original scan was skipped). The 755A is built on U-type relays (BLR 1938)
Western Electric Co., Inc. Hawthorne Works Industrial Relations Branch Training Department Information Pamphlet 2.1-102 For Training Purposes only THE U-TYPE RELAY In the past, the most widely used relays In the telephone plant have been of the flat type. These have proved economical to manufacture; and experience extending over a quarter of a century has testified to their satisfactoriness in operation. During this time, however, the telephone system has changed, and upon relays there have been Imposed operations more complicated and critical than were In mind at the time of their development. Materials and manufacturing methods also have changed, so that all In all It seemed desirable a few years ago to undetake the design of a new all-purpose relay. The U-type relay - shown In the photograph at the head of this article - is the result. Reliability in relay operation has become of increasing importance In recent years. The Intricacies of the dial systems require the operation of a large number of relays on each call, and for the satisfactory functioning of the system many of these relays must operate and release at The existing types of relays do Just the right time and without fall. this, of course, with what Is really remarkable precision, but occasionally a speck of dirt will get between the contacts to prevent their closing. Also the contacts may "chatter" - rapidly opening and closing as a result either of the rebounding of the armature when the relay Is de-energlzed, or of the Independent vibration of the springs. One of the objectives of the new design, therefore, was to Improve reliability by making dirt particles less effective, and by reducing the tendency to chatter. Another objective was to secure a greater number of contact springs per relay. A study Indicated that an Increase from the twelve springs of the present relay to twenty-four would be satisfactory. To obtain the full advantage of such an Increase, however, the gain In number of contacts must not be offset by an corresponding Increase In size or In energy required. In other words, more effective and efficient use of materials was sought. 2 An increased number of contacts requires a larger magnetic flux; and if the energy consumed by the winding is not to be increased, this flux can most effectively be secured by providing a magnetic path of lower rePRING luctance. Such an improved magnetic GAR HINGE FRONT path was secured, first, by using a larger core of circular cross-section, which provides the greatest flux for a given length of wire; and second, by reducing the air-gap reluctance and making more advantageous In both the E-and R-type use of it. relays, which are of the flat type, a U-shaped armature is hinged to the yoke at the rear of the core by a piece of thin magnetic iron, as shown Fig. 1 - The armatures of the EThe spring hinge is so in Figure 1. and R-type relays are hinged to thin that only a small part of the the core by a short strip of magflux can be carried by it; the rest netic iron, which may be seen in of the flux passes through air, which the photograph of the relay, and introduces into the circuit an addiin the diagram of its magnetic tional reluctance that serves no usecircuit The gap at the hinge is ful purpose. kept as small as possible, of course, but it must be large enough to permit free movement of the armature after allowing for unavoidable unevenness at the hinge. The armature of the new relay is of the same U shape as that of the Rtype relay, but overlaps the end yoke, instead of being hinged to it, and is held loosely in place by a pin The construcin each arm of the U. In its tion Is shown in Figure 2. unoperated position, the armature pivots on the front edge of the rear bracket - leaving a long wedge-shaped air gap between this fedge and the rear end of the armature. As the armature closes, this wedge-shaped air gap narrows down until, in the operated condition of the armature, This form of it almost disappears. mounting tends to Introduce only a small reluctance, which is made still smaller by the comparatively large Fig. 2 - With the U-type relay cross-sectional area of the hinge gap a loosely pivoted hinge is as compared with former relays. Moreemployed, and the air gap at over, the flux in this gap performs a the rear becomes a long narrow useful function, since the resulting wedge. This is evident in the force of attraction on the rear end photograph and in the diagram of the armature tends to rotate the of the magnetic circuit. armature around the pivoting point in the same direction as does the relatively greater pull at the gap at the The core is milled flat at its rear where it fastens to the front end. cross yoke, and also at its front to form a broad flat surface for the armature. This front gap is limited in width by the diameter of the core, but it is made long enough to give a large area and thus a low reluctance. 3 How effective this construction is in increasing the pull is shown by Figure 3, which gives a typical relationship between the force which the armature can develop, and the power required to maintain this force, for both the R- and U-type relays. The pull provided for six make contacts is somewhat greater for the U-type relay than for the R— more contact pressure being provided - but the power required is only about a fifth. The R-type would be incapable of closing 12 make contacts, while the U-type closes them with less than half the power required by the R-type. Besides contributing to the pull of the relay, this construction tends to reduce the likelihood of chatter caused by the armature rebounding after it has opened. With the reedINPUT IN WATTS hinged armature of former relays, the full force of the rebound occurs at the front end of the relay where the Fig. 3 - Relationship between pull and watts for both the tendency to close the contacts is R- and U-type relays. great. With the U-type relay, however, the rebound divides between the front gap and the loosely mounted pivoting point ,farther back; and the net consequence at the contacts is far less than if the armature were free to recoil only at the front. Of at least equal Importance is the chatter caused by vibration of the springs. Considerable study, both theoretical and. experimental, was necessary to determine the nature and causes of this vibration and the best method of its elimination. It was finally found that by properly proportioning the dimensions of both the stationary and the movable springs this form of chatter could be eliminated as a source of serious trouble The other major Improvement incorporated in this new relay is the use of twin contacts. Each contact spring carries two separate contacts in parallel, so that even though one of them should be held open by a speck of dust, the other would close. As already noted, the failure of a relay to make contact is of comparatively rare occurrence, but the provision of double contacts very greatly decreases this likelihood. The probability of a random failure when two parallel contacts are employed is the square of the probability for a single contact. If, for example, a single contact fails to make once in ten thousand times, then a pair of contacts in parallel under similar conditions will fail only once in a hundred million times. With these various Improvements a relay has been made available that, besides being able with less power to close twice as many contacts as the R-type, is practically free from chatter and from contact failure. In designing the U-type relay every advantage has been taken of past experience and of recent developments, not only to make it more effective but also to keep its manufacturing cost low. With this in view a striking change was incorporated in the method of winding. In previous relays the front and rear of the core are larger than the section on which the winding is placed, and the wire for the coil is wound in place With the new relay the core has been made of the same dion the core. ameter throughout Its length so that it is possible to wind the coils J 4 separately and then slip them over the cores. This change In the core was adopted largely to take advantage of a method of winding colls In multiple which was developed by the Western Electric Company. On a single arbor, as shown In Figure 4, eight colls are wound at the same time from eight spools of wire. Between adjacent windings are slight separations so that they may be cut apart when completed. Between successive layers of wire are very thin layers of cellulose acetate sheet, which run the full length of the arbor and hold the wire In place. The collection of spring contacts consists of stationary and movable springs, separated In their mounting by strips of Insulation. The stationary ones are considerably thicker than the movable, and The do not bend appreciably as the relay operates. movable springs are controlled by the armature thru Insulating rods, which are fastened to alternate springs and pass through openings In the stationary springs. The general appearance of a spring assembly for twelve make contacts, which Is the full complement of a relay. Is shown at the head of this article. Each stationary spring Is a single piece with the two contacts near the ends. The movable springs are forked a little beyond the point of attachment to the operating rod, with each branch carrying one contact. This gives comparative Independence between the two contacts of a pair, so that If one Is held open the other will still be free to make contact. The soldering terminals of these springs fan out at the rear In the usual manner. For the windings, soldering terminals are similarly arranged, but they are also extended toward the front of the relay. This arrangement gives access to the windings when testing from the front. The headpiece Illustrates a relay provided only with make contacts but, as with most relays, other combinations may be built up, such as break-contacts, make -be fore -break, break-before-make, or ipake-bef oremake; several hundred combinations may be obtained. For the windings also there Is a large number of possible ratings depending on the voltage of the circuit In which the relay Is to operate and on other circuit characteristics or on the type of operation required. Other optional features are copper and aluminum sleeves which are slipped over the core when slower operation Is desired, and a spilt permalloy sleeve to give the winding a high Impedance to voice frequencies. The cores are usually of magnetic Iron, but permalloy may be used when conditions warrant It. Because relays are used In large numbers, their space requirement becomes of considerable Importance. The U-type requires Vertically the same spape as the flat relay; and horizontally, space determined by Fig. 4 - For the U-type relays eight separate windings are wound on a single arbor and then cut off and assembled over the core. 5 When equipped to capacity, it has a spring-pair its number of springs. density in the plane of the mounting rack of 4.2 pairs per square inch, while the greatest density before, which was with the R-type, was 3.4 pairs. Development of this relay had its inception in the need for more contacts and for lower battery drain; but in the course of its design it was found possible to add many other desirable features, such as twin contacts and freedom from chatter. As a result, it has become of much wider utility than was originally anticipated. BIBLIOGRAPHY Bell System Publications 5-38 . s Western Electric Co., Inc. Hawthorne Works Industrial Relations Branch Training Department Information Pamphlet 2.1-108 For Training Purposes Only Improved U, UA and Y type relays , As telephone traffic increases and costs of providing service increase, concentrated efforts are being made to hold down operating costs without impairing the quality of the service. One area of attention has been that of U, UA, and Y type relays, where recent circuit needs have imposed more difficult requirements on these relays. The new No. 5 crossbar system, automatic message accounting, and greater use of unattended offices have produced a demand" for higher operating speeds, longer life, and greater adjustment stability of these relays. Prior to, and during World War II, the relays used in the No. 1 crossbar system were not required to have operate times of less than Fig. 1 (at top of page)— The reed type hinge on the left now replaces the pin type hinge, right. Fig. 2 (a)—After 100 million operations, wear of the armature pins has allowed the armature to drop and rub on the front spoolhead. (b) After bending the armature pin bracket, the relay was good for another 100 million operations. Fig. 3 —Wear of the stop pins and core pole-face after 50 million operations. ten or fifteen milliseconds and did not have to operate in any case more than two or three hundred million operations during their life —in most cases, only about one-tenth as many. In the No. 5 system, however, the useful life of some of the relays must be as many as one billion operations over a period of 40 years, and with speeds as fast as five milliseconds. To attain the increased life, faster speeds, and greater numbers of operations, several changes have been made in design. One of these was the development of a reed type hinge suspension for the armature to replace the older pin type, as shown in Figure 1. While the pin type armature hinge is fully satisfactory when the U relay is used in the original No. 1 crossbar circuits, in the new the much larger number of operaand the higher speeds of operation circuits, tions made it necessary to use the reed type hinge. With the pin type hinge, the rubbing action of the pins wore grooves in the holes of the hinge bracket, causing the front end of the armature to drop until eventually the top Fig. 4— Improved relay life is obtained with a Teflon separator. inner surface of the armature rubbed on the top surface of the front spoolhead, resulting in faulty operation of the relay. This is shown in Figure 2(a), on a U relay after approxi- mately one hundred million operations. The associated relay, Figure 2 ( b ) is one that had failed at about the same number of operations, but was made serviceable by bending the hinge bracket so that the front end of the armature was restored to its normal position. Then, after operating the relay another 100 , million times, the additional wear of the hinge bracket holes allowed the armature to drop and touch the front spoolhead again. By this time, the useful life of the relay had ended. Similar relays provided with the new reed hinge have been operated over a billion times under the same conditions, without showing fatigue of the reed hinge and with no dropping of the armature. This hinge is now in regular production on U, UA, and Y relays. Another effect of the increased number of relay operations and higher speeds de- Fig. 5 — Motion limiting fits 2Y washer for relay coils snugly over the core. manded by new circuits, lies in the wear at lay either during its assembly in the factory or on the relay in the central office. Figure 4 the pole faces of the armature and core. Most shows a relay with the separator installed, and a separator alone. The slot is provided to clear the stop discs already in place on the relays are provided with small, non-magnetic discs welded to the armature pole-face, used to provide a space between the armature and core when the relay is operated. These “stop armature. discs” serve two purposes— to reduce the ef- Laboratory tests have shown that the relay fect of residual magnetism that tends to hold may be operated more than 500 million times the relay operated when power is removed from the winding, and to provide a desired release time of the relay. On relays equipped occurs. When the wear reaches an amount before any appreciable wear of the separator that might affect the release time of the re- with low resistance windings and having large armature travels, the armature strikes the core at a relatively high velocity, with lay, it is a simple matter to remove the old the result that the stop discs not only show wear themselves, but also pound into the of the stop discs has been achieved by chang- separator and install a new one. Besides the new separator, improved life ing the metal of which they are made. Previously, the stop discs were of nickel silver, but tests have shown that the use of an alloy of gold, silver, and platinum— previously used only for electrical contacts— doubled the useful life of the stop discs. For some core pole-faces. Under the most severe operating conditions, the air gap may be entirely gone on some of the relays after 50 and the armature pole- million operations face then strikes the core pole-face— a condi- relays, this change in stop disc material en- abled the relay to meet the new life requirements without adding the Teflon separator. The spring studs used to operate the relay springs are also subject to wear. This causes loss of contact “follow,” reducing the force between contacts that close on relay operation, and making the separation less between contacts that open when the relay operates. If this continues far enough, the relay will function. The studs were made originally of hard natural rubber, but fail in its circuit studies have shown that a synthetic rubber containing a graphite mixture has wearing characteristics two or three times better than HOURS AGING 6— Comparison of standard “pot” anneal and hydrogen anneal. Fig. Fig. 7— The magnetic separator is fastened to the pole-face by using a special wrapping tool. tion that increases the effect of residual magnetism in slowing the release of the relay, or even causing failure to release. Figure 3 illustrates such a relay, showing the impressions in the core pole-face made by the stop discs and also indications of actual contact be- tween armature and core pole-faces. The latter may be seen as bright spots at the tips of the pole-faces in the photograph. To overcome this source of trouble, a wear pad made of a new plastic called “Teflon” has been developed. This pad or separator is designed so that it can be applied to the re- 3Y of fatigue. To eliminate this movement, a motion limiting washer has been provided to fit snugly over the core and which is bonded to the end of the coil. Figure 5 illustrates the washer and the way it is used. In assembly, the washer and front spoolhead are pushed over the knurl on the core and the washer is bonded to the front end of the coil. The tight fit of the washer on the knurl is the feature that prevents lateral movement of the coil. There is always some slight shrinkage of the cellulose acetate in the coil, but the washer can move with the coil, although eliminating lateral movement. Use of this washer has practically eliminated lead longitudinal direction of the OPERATE AMPERE TURNS Fig. 8 — Increase in pull of a typical UA relay due to annealing the magnetic parts in hydrogen. breakage from fatigue. those of the natural hard rubber. The graph- The Y type relay was designed to provide time delay on release. Over a period of years, it was found that the release characteristics ited synthetic rubber is now being used for of some of these relays had changed— in some all cases, the relays would fail to release at all. new relays. With the increased speed of operation of Investigations showed that over a period of the relays, the coils are subject to much greater vibration than heretofore, and this has introduced the possibility of lead break- time, the coercive force of the magnetic iron age. Such breakage is due principally to fatigue, when there is some looseness of the coil on the relay core, and the impact of the armature against the core pole-face produces lateral displacement of the coil at the front or non-terminal spoolhead. This movement has a tendency to bend the leads sufficiently to cause them eventually to break as a result increases, and with the relatively closely coupled magnetic circuit of this relay, the residual magnetism increased to a value in excess of the spring tension restoring force. This aging effect can be practically eliminated on new relays by heat treating the magnetic parts in an atmosphere of hydrogen, and this is being done on UA and Y type relays. Figure 6 compares the effects of aging on residual magnetic force of the BIBLIOGRAPHY Bell System Publications Issued by: W. C. Slauson Telephone Systems Training Section BLR 10-51 9-1-52 4Y standard “pot” anneal with the newer hydrogen anneal. For /Y relays already in service, however, and therefore subject to aging effects, a magnetic separator has been developed that will reduce the residual force sufficiently to enable the relays to meet their adjustment requirements. This is a small nickel shown in Figure 7, that can be wrapped around the core pole-face, using the special tool illustrated. Besides greatly reducing the effect of aging, hydrogen annealing increases the permeability of the magnetic iron by a large amount. This is shown by the typical curves of Figure 8, where the pull of a UA relay has been increased as much as 50 percent. This gain is of particular importance on the UA detail as /n * relay, since this relay, having a larger pole- face area than that of U and Y type relays, and consequently having a lower reluctance magnetic circuit, is more susceptible to variations in permeability of the iron. The importance of aging or of poor quality iron is thus reduced by means of hydrogen annealing. Telephone central office switching apparatus is unique in many respects. Designs are based upon requirements in existence, or anticipated, at the time the designs are prepared, but very often, advances in technical developments introduce unforeseen difficulties. Besides, increased telephone traffic may demand speeds of operation not previously considered, and problems of life and wear of the apparatus appear. The several improve- ments described in this article, therefore, indicate only a few of the objectives that engi- neers are constantly working toward to maintain and to improve telephone service. 5Y • . ' "• • ... ' V ' : ‘ • - • ; : . ' ’ . i . Xi •• ' . . . . Western Electric Co., Inc. Hawthorne Works Personnel Service Branch Training Department Information Pamphlet 2.1-107 For Training Purposes Only The UB relay In a crossbar dial office there are in the neighborhood of 70,000 relays for 10,000 subscribers’ lines. More than half of these are of the U type. The UB relay has re- cently been introduced, on a limited basis, U where freedom from locking contacts and stability of adas a replacement for the justment are of particular importance. Its major field of application will be where the number of operations is high— of the order of several millions a year or more. At the present time, it is used extensively in the new automatic message accounting system, where relay operations exceeding twentyfive millions annually are quite common. Although in appearance and major structural features the UB is essentially the same Figure 1, and the two relays are interchangeable physically, they are not always the same circuitwise. Because of this, the UB is being restricted initially to as the U, those applications where its characteristics are of particular advantage. The advantage of the UB lies in the method of actuating the contacts. With make contacts of previous relays, the spring carrying the moving contacts is tensioned away from the fixed contact on the spoolhead spring, and exerts a force to hold the armature against the back-stop when the relay is unoperated. A stud, moved by the armature, presses against the contact spring a short distance back of the contact to close the contacts when the relay is operated. This is indicated in the upper part of Figure 2 for a single set of contacts. In contrast with this arrangement, the UB relay employs a phenol fibre card instead of a stud to operate the contacts, as shown in the lower part of Figure 2. The moving contact spring itself is pretensioned against the fixed contact to give the desired contact force of between 20 and 30 grams. The card is held by two card springs that are tensioned away from the fixed contact, and in slightly greater amount than the moving contact spring is tensioned toward it. As a result, when the relay is unoperated, the card holds a make contact away from the fixed contact. When the relay operates, the armature pressing against the top of the card pushes it toward the fixed contact, and thus allows the contact to close. A break contact on the UB relay is identical with the make, except that the card opens the break on operation of the relay and permits it to close on release. No balancing force is needed in this case. It will be noticed that with previous relays, as indicated in the upper part of Figure 2, the deflection of the contact spring in closing slightly tilts the moving contact with respect to the fixed contact. This is also true when the contact opens, and thus in both closing and opening there is a small component of relative motion of the contacts perpendicular to the direction of motion of the armature; the contacts slide in being pressed together. Fig. 1 —U relay at the left and UB at the right 2 . and the bottom of the slot in the card, and thus when the card hits the spring in opening, it is already moving and spring MAKE CONTACTS UNOPERATED has acquired kinetic energy. This energy overcome any available as impact to tendency of the contacts to stick. The comparative freedom from dirt troubles obtained with the UB relay is an additional advantage that card operation is TYPES made FORMER possible. The moving springs of double-contact relays are forked, as shown in Figure 3, and each tip of the fork carries a contact—thus providing two contacts for UNOPERATED each spring. Should a small particle of dirt hold one contact open, the other contact, because of the independent flexing of the two tips, should still close with substantially the same force. With stud-operated relays, however, the possible length of the tip is limited by the position of the operating stud, indicated in the’ diagram, and thus the amount of independent flexing of the UB-TYPE two tips is similarly limited. With the UB relay, on the other hand, there is no operating stud, and thus the slot between the tips can be made longer. With the U-type relay, for example, the Fig. 2— Simplified diagram indicating the contact oper- ation of former types of relays, above, type, below and of the UB As a result of slight arcing as the contacts open an electrical circuit, there is a transfer of metal from one contact or electrode to the other. This building up and wearing away leaves both contacts roughened. If the opening motion were along a perpendicular to the contacts, this roughening would, ordinarily, have little effect; but with a sliding motion at the contact, small projections on one contact may bind or lock mechanically in a cavity on the other and thus prevent the contacts from opening when they should. In the UB relay, where the contacts are operated without sliding motion, this action is to a large extent avoided. The likelihood of locking with the relay is further decreased because the re- UB storing force is and is applied and because of the greater closer to the contact, impact of the card on the spring when the With the contact closed, a clearance between the contact relay releases. there is while with the UB it has been made 0.94 inch. Since stiffness varies inversely as the cube of the length, the independence of the tips on the UB relay slot is 0.33 inch, twenty-three times greater than that of relay. In the Philadelphia accountonly half as much trouble from center, ing experienced with the UB been dirt has is the U relay as with the U relay. One of the major advantages of the UB relay With greater stability of adjustment. any relay, the pressure of the con- is its tends to decrease with wear—wear at the contacts themselves and at certain other critical points in the structure. With the UB relay, however, both the wear and the decrease in pressure with wear are tacts less than with previous relays. With most types of relays, wear of approximately the same amount takes place at the contacts. Although there is some wear at the card on the UB relay, particularly at the top where the armature pushes it, it is small and does not appreciably affect the contact pressure, which is determined by the pretensioning of the spring. With the former types of relays, on the 3. other hand, there is wear of the operating studs as well as of the contacts, and since the pressure at the studs is relatively high, and there is some POSITION U-TYPE OF STUD x~^ — sliding because of the flexing of the spring, the wear is relatively UB-TYPE large. Moreover, with a large spring pileup, there may be several wearing points series and the wear adds up in its effect ligible. in ->lo.33 k in on the last spring. For a U-type relay with six make contacts, the loss in travel because of stud wear after 30,000,000 operations is over 6 mils, while for a U relay with six break contacts, it is over 9 mils. Card wear for the UB relay similarly equipped, and for the r * C i) 1 0.94" |< > Fig. 3— The springs of double-contact relays are forked at the end and have a contact on each tine but the length of the tines is much greater with the UB than with the U relay same number of operations, is negThe effect of this wear in requiring readjustment is obvious in Figure 4. Although this reduced wear of the UB is an important factor in decreasing the need for readjustment, a factor of even greater importance is the much smaller rate with which pressure falls off with wear. Force is applied to the contact spring relay UB relay only at or near the free end; spring is thus a cantilever and the ratio of force to deflection is small— about 0.09 gram per mil of deflection. With previous relays, on the other hand, the spring, once contact is made, acts as a beamsupported by the pileup at one end and by the fixed contact at the other. The operating stud deflects the beam by pressure applied a short distance back of the contact. Under these conditions the ratio of force to deflection is much higher, being of a the 4— Change Fig. in contact “make” contacts of the wear with pressure for U relay and of the UB relay about 2 grams per mil of deflection for the U relay. This is illustrated in Figure 5. The shape of the preformed spring of the UB relay is shown at (c)— the end is down u-j i / V 1 about 294 mils below the position it will assume when assembled, indicated at (d). In moving through this 294 mils, a force of 25 grams has been developed, which will be essentially the pressure on the fixed contact when the card releases the spring. When the contacts have worn 10 mils, which is about the maximum permitted, the pressure will still be above 24 grams, since it drops 0.09 gram per mil of 5 l U-TYPE f- i n 1 * 1 \ 13 MILS deflection. With the U-type relay, there is no preswhen contact is first made as shown at (a). The pressure is built up to 25 grams by deflecting the spring as shown sure 5— Simplified diagram indicating spring actions of “make” contacts of the U relay and the UB relay Fig. 4. at (b). The much higher ratio of contact pressure to deflection with this beam action—2 grams per mil—requires a deflection of only about 13 mils to build up the pressure to 25 grams. When the combined wear at the stud and contacts is 10 mils, only about 5 grams of pressure will remain. A combined wear of only 2M mils will reduce the pressure to 20 grams. The drop off of pressure with wear of the UB relay, on the other hand, is so slight as to be negligible, until the contact wear has been great enough to allow this spring to rest against the lower lip of the card in the operated position. This accounts for the sudden drop of the curve for the UB relay Figure 4. Since it is desirable to maingood contact pressure, it is obvious that stud-operated types will require adjustment at much more frequent intervals than the card-operated UB type. A still further advantage of the UB relay is that it requires less hand effort in adin tain justing. The contact springs are preformed and thus the hand ten- in dies for tension, sioning after assembly is avoided. An important contribution in this connection is the method of preforming the springs suggested by D. C. Koehler. During the early development stages, the springs were formed between two smooth, curved surfaces shown in the upper part of Figure 6. Considerable variation in tension was encountered, however, because of differences in the spring-back of the various springs when removed from the forming tool. This effect was minimized by using two sharp bends, as shown in the lower part of the illustration. The bends are so located that Fig. early 6— Frequency distribution of contact force with and springs of final methods employed for preforming UB relay the resulting shape of the spring approximates that of the elastic curve for a canti- This was done so that the spring will be reasonably straight and parallel with the mating spring when it is held in position in the assembled relay. The contact force distribution of springs formed by the two methods is shown at the right lever spring. of the illustration. BIBLIOGRAPHY Bell System Publications Issued by: Telephone Systems Training Section 11-11-49 H. M. Knapp BLR 10-49 Western Electric Co., Inc. Hawthorne Works Industrial Relations Branch Training Department Information Pamphlet 2,1-103 For Training Purposes Only THE Y-TYPE RELAY In many of their applications in the Bell System, relays are required to act as rapidly as possible. They should operate promptly when voltage is applied to them, and release promptly when the circuit to their winding is opened. There are numerous applications, however, where it Is necessary for the relay to remain operated for an interval after the circuit to its winding is opened, and this release Interval is often specified within very close limits. Precise behaviour of this kind is not easy to secure; only careful design and accurate control of manufacture make It possible. Relays that are most economical for general uses will not meet the very exacting requirements laid down to secure a precise slow-release period. When the general utility U-type relay was developed, it seemed desirable, therefore, to develop at the same time a slow-release relay that so far as possible would use the same parts, manufacturing tools, and processes. The result was the Y-type relay, which is shown in the photograph at the head of this article. The need for dependable slow-release relays can be well illustrated by one of their applications In the panel systems. A group of relays in the sender records each digit dialed, and it is necessary to switch the dialing circuit from one group to the next after each digit of the called number. As the dial returns after having been pulled around to one of the digits, it opens and closes the circuit in rapid succession to indicate the digit dialed. Thus in dialing 2 the circuit will be opened and closed twice, and in dialing 4 the circuit will be opened and closed four times. Immediately after its return, the dial is pulled around for the next digit, and during this short interval between digits the circuit must be switched to the next group of recording relays. This is accomplished through a slow-release relay that does not release during the short open periods of each digit, but does in the slightly longer period between digits. It must do this regardless of the commercial range in dial speeds, of voltages and line variations, and in the speed with which the dial is pulled around. In the latest type sender the actual requirements for the Y-type relay are that it shall release in not less than 0.080 second nor in more than 0.120 second. The ordinary relay, however, releases in from 0.005 to 0.015 second. In designing a slow-release relay, therefore, there are two objectives sought: first to provide the slow-release action, and second to provide for an accurately controlled release time. 4 The effects of these structural differences on the reluctance are shown In Figure 4 for relays with flat surfaces and for those with the spherical and cylindrical surfaces of the Y-type relay. With perfect alignment the reluctance of the flat surfaces Is very low, but It Increases rapidly with an Increase In the angle between core and armature. The reluctance of the rounded surfaces Is much higher for perfect alignment, but It Increases only very slightly as the deviation In the alignment Increases, and thus gives the constancy of reluctance particularly desirable for a slow-release relay. The reasons for this are obvious. whii a ij-aL sunace in perrect alignment - zero angular deviation - the contact Is spread over the entire surface, and the reluctance as a result Is very low. When the armature meets the pole face at an angle, however, there Is a wedge-shaped air cAn f’AT'n.a/id -i» ^ ater ' the an ele of contact, the greater will be g the w?d?h f® lucta ce - of this gap. with the spherical o? cynJSrlc d ? contact Is always In a point or line with a nS?row a r Ln II regardless of the angle with which the armature meets Su fa es The reluctance of such a conta?t is hlehe? gner JK tK? of two ? than that flat2 surfaces whore the ane*le ie v • tut o? s. eases steadily as the angle widens. Fig. 4 - Variations in reluctance of relay with angle at which armature meets the core for both flat and embossed pole pieces. 4 X ^ ^ . ' n« w U e ene 10 struct ure to prevent cor?ol"5 ot ?L°f?o5 ne bJt SlSIlM?.*?* ? t S a ?? S what Is effectively an air van ?n th| li ne ? ? non-magnet 1c, It Introduces the ?ag netlc circuit. Variations In the thickness of 1 coa t would affect the release time by varying the reluctanre nf Fv, circuit. The closeness with which the release I J ?~ type relay therefore, requires that the thickness of the 1 8 h ld wlthln very close limits; the actual range i“??oS o SnnS 0003 t0 S °' nnn« 0006 ?inch. Chromium is used for the outer In L!,,?; 1 ® 6 ® t0 r ® SlSt W ® ar at polntS of contact between the > ^ DktL a^MtSrrJL^Se!” t eluctance “«'«< *r theso various »lalsr?Se vlJlluoSr?; Jl!1 fLi”J m6 a S 1 ™ ^ considerably reduced. For the Y-type relay the varlat?^ ?! re Li Se ^* mes are shown by the shaded area of Figure 5 For a r nrroc. ^A? dlng lay wlth flat s^faces, the release tl^ SlSht fall aSvSh^f Sf? between the curves a and B. With rounded surfaces the variation ° Ut fr ° m ° n ®- half t0 ° n ®third of what it woul°3 le Sit^fla^SrScS” n r a C S S the Y-type relay that the release can be closely predlcted f ro II C h V 1Ue 0f the curren t necessary to hold the relay operated HerAhnf orI ?i- ^ has v been necessary to check the operation of slow-rei pa ap by 1 y actuall measuring the release Y times, which is a Jathpr q^0 andH ex P enslve procedure, with the Y-type relay such measur P^ntfJo ls sufficient to measure the holding -current because of the^ e ^f ssary ' close correspondence between holding current and release time t lme • / 5 0 4 8 16 20 24 HOLDING AMPERE TURNS 12 28 32 36 Except for these differences, the Y-type relay is the same as the Utype, and to the casual glance one could not be told from the other. Moreover, the copper sleeves designed for the Y-type relay may be employed with the U-type to secure