Manual / Guide · 1958
AE Training Bulletin 801: Mechanical Principles of Telephony
Mechanics of relays and Strowger switches.
Manual / Guide · 1958
Mechanics of relays and Strowger switches.
This is one of the helpful booklets in the AUTOMATIC ELECTRIC TRAINING SERIES on STRBWGER AUTOMATIC TELEPHONE SYSTEMS E l e c t r i c a l Principles of Telephony Mechanical Principles of Telephony Fundamentals of Apparatus and Trunking Distributing F r a m e s The Plunger Lineswitch and Associated Master-Switch The Rotary Lineswitch The Connector The Selector Pulse R e p e a t e r s Power and Supervisory Equipment Party-Line Connectors and Trunk-Hunting Connectors Reverting -Call Methods The T e s t and Verification Switch T r a i n Toll Switch T r a i n Switching Selector-Repeater P r i v a t e Automatic Exchanges with PABX Appendix 820 Manual Switchboards 8 2 1 Linefinder Switches May we send you o t h e r s pertaining to equipment in your exchange? TABLE OF CQNTENTS TELEPHONES Page Telephones . Dial Hands et Transmitter . Receiver . Cradle Switch Capacitor . Induction Coil Ringers RELAYS 10. Relays . 11. Typical Relays . 12. Timing . 13. Spring Assemblies 14. Mountings . . 6 7 7 . . 8 8 PLUNGER LINESWITCHES 15. Lineswitches . 16. Plunger Lineswitch 9 9 . STEPPING SWITCHES . 17. Basic Mechanisms 18. Direct Drive 19. Indirect o r Spring Drive 20. Homing Type . 2 1. Non-homing Type 22. Non-bridging Wipers 23. Bridging Wipers 24. Rotary Switch 25. Minor Switch 26. Strowger Switch . 11 . 11 . 12 . . . . . . . . . . . . . 12 12 12 12 13 13 14 STROWGER-SWITCH FUNCTIONS 27. Connector 28. Selector 29. Linefinder . . . . . AUXILIARY SWITCHES 30. Auxiliary Switches 19 19 20 # . . 23 ADJUSTMENTS AND MAINTENANCE 31. Adjustment Standards 32. Routines 33. Lubrication , . . . 23 . 24 . 24 Type 80 desk Monophone. 7Ype 50 wall Monophone. Figure 2. lhree types of autohatic telephone. TELEPHONES 1. TELEPHONES A telephone is a combination of parts assembled into a complete unit having facilities f o r voice transmission, voice reception, signaling, and s w i t c h i n g . In t h i s d i s c u s s i o n , telephones r e g u l a r l y s u p p l i e d by A u t o m a t i c E l e c t r i c Company a r e used a s examples. Telephones of other makes may v a r y i n g e n e r a l design and in details of their components, The components usually c o m p r i s e a handset, including the transmitter (voice transmission) and receiver (voice reception), a base assembly on w h i c h a r e mounted t h e telephone signal ( r i n g e r ) , induction coil, c a p a c i t o r s , c r a d l e switch, and t e r m i n a l s ; t h e i n t e r i o r wiring; the handset cord; and, i n t h e c a s e of a desk telephone, the desk-set cord and terminal block. Telephones may be c l a s s e d a s t o service requirements; for example, telephones ordinarily supplied for commercial and domestic uses a r e desk o r wall handset types. Automatic Electric Monophones T y p e s 43, 50, and 80 shown i n Figure 1 a r e typical examples. Other designs f o r m o r e specific purposes include operators' telephones, test telephones, weatherproof telephones, and telephones for hazardous locations. Telephones may also be classed a s local battery o r common battery. In the f o r m e r type, the t r a n s m i t t e r of e a c h telephone r e c e i v e s its c u r r e n t f r o m a local source such a s a pair of dry cells. . Common-battery telephones receive t r a n s m i t t e r c u r r e n t f r o m a central-office storage battery which serves a l l telephones in the exchange. released a d is returned to its normal position by a coiled 'spring, i t s s p e e d of r e t u r n being controlled by a worm-driven governor. Figure 2. D p e 51 d i a l with l e t t e r s a d figures on number plate. As the dial finger plate returns to i t s normal position, i t successively opens and r e c l o s e s the telephone line circuit a riumber of times corresponding to the figure dialed. F o r example, if the finger plate is turned from the figure " 5" on the number plate and then released, the line c i r c u i t w i l l b e opened f i v e t i m e s in quick succession, by a small fibre cam geared to the m a i n dial s h a f t and acting against t h e pulse springs. Each circuit interruption l a s t s about 1/16 second. These brief interruptions operate central-office switches. Escutcheon ring Escutcheon lock Finger plate Screw Telephones may be further classed a s automatic o r manual. Each automatic telephone has in addition to t h e components a l r e a d y listed a dial-type calling device to control automatic s w i t c h i n g e q u i p m e n t in t h e c e n t r a l office. Pulse spring aseem. - S h u n t Spg.assem. 2. DIAL Shunt springe The dial (see Figures 2 and 3) is a pulse-sending calling device which, when wound up and then released, interrupts the electric current to control switching mechanisms at the central office, To operate the dial the calling subscriber inserts a finger in the appropriate hole in the finger plate and t u r n s the plate clockwise until the finger strikes the finger stop. The finger plate is then Main spg. assem. Middle pulse epg. Main pulse spg. Stop Spg. -Governor shaft Bearing screw Hex. nut Pulse cam Figure 3. 7 j p e 51 dial assembly ( f r o n t artd rear views). - TELEPHONES T h e dial consists essentially of: (a) T h e f i n g e r plate which is rotated and released by the caller f o r each letter o r digit dialed. which bears the figures (b) The number l 11" t o l 1 9 " and l 10" (and often t h e alphabet too) under the ten holes i n the f i n g e r plate. 3, HANDSET The handset (Figure 4) houses the transmitter and receiver units a t spacings and angles which provide optimum efficiency for the average user. The handset is made of molded plastic and the tEmsmitter and receiver units a r e mounted in p o s i t i o n a t e i t h e r end by m e a n s of plastic retaining Cups. Handset-cord connections to the transmitter and receiver units a r e made through s p r i n g c o n t a c t s at e a c h end of the handset. (c) The governor and its driving mechanism. (d) The pulse cam which, a s it rotates, opens and recloses the pulse springs. (e) The pulse springs, which open and reclose the line circuit. (f) The shunt springsi which short-circuit the transmitter and receiver of the telephone while the dial is off -normal. T h e g o v e r n o r a s s e m b l y c o n s i s t s of a worm wheel which is on the s a m e shaft a s the pulse cam and which drives the governor shaft through a worm. Attached to the governor shaft a r e two wings, each with a weight on the end. During rapid rotation of the worm shaft, centrifugal f o r c e causes the wings to move outward and causes friction between the governor weights and the inside of the governor cup. The pulse speed of t h e dial is adjusted by positioning the flyballs w i t h r e s p e c t to the g o v e r n o r cup. During operation, the speed is controlled by the friction r e s u l t i n g f r o m t h e c e n t r i f u g a l f o r c e of the flyballs against the inside of the governor cup. T h e governor is adjusted s o the pulse springs send about 10 pulses p e r second. 4. TRANSMITTER T h e t r a n s m i t t e r ( F i g u r e 5) is of the sealedcapsule type consisting of: (a) The main diaphragm which vibrates in r e s p o n s e to a i r waves c r e a t e d by the p e r s o n speaking, and (b) The carbon-granule cell which changes the mechanical vibrations of the diaphragm into e l e c t r i c a l pulsations. T h e plastic cup which holds t h e t r a n s m i t t e r in the handset has perforations which permit a i r waves t o reach the diaphragm. The design of the carbon-granule cell is such a s t o a s s u r e continuity of t h e c i r c u i t through the carbon granules irrespective of the position in which the handset is held; that is, in a l l positions. Paper bellows Diaphragm Granules Our modern dials provide a delay between the final circuit interruption of the s e r i e s and the completion of the r e t u r n motion of the finger plate. A c a m attached to the main dial shaft moves the two pulse .springs out of lengagement with the pulse cam during the last 2 revolution of the cam. This delay allows time for centraloffice equipment to perform switching functions between successive pulls of the dial. Sealed protective shell Figure 5. Moisture excluding membrane Cross section of transmitter capsule. 5. RECEIVER Figure 4. Handset components. The r e c e i v e r ( F i g u r e 6 ) converts incoming voice currents back into sound waves. It consists of a powerful permanent magnet associated with two pole pieces on which a r e windings carrying the voice current. The flow of voice current through t h e windings c a u s e s the r e c e i v e r diaphragm to reproduce the original vibrations of t h e t r a n s m i t t e r diaphragm a t the distanr telephone. - TELEPHONES Magnetic shunt /- 8. INDUCTION COIL The induction coil shown in Figure 8 inductively couples the transmitter and receiver units t o the line, increases the telephone output volume, and decreases the effect of speaker hearing his own v o i c e i n h i s r e c e i v e r while talking into t h e transmitter. The induction coil consists of t h r e e insulated windings wound on a common laminated-iron core. This type of induction coil has its windings connected to serve as an autotransformer (part of the winding is common to both the input and output c i r c u i t s ) . Two s m a l l a i r gaps i n the magnetic circuit prevent magnetic saturation by the direct-current supply to the transmitter. 1 I Sealed protective shell Figure 6. Acoustic damping plate Cmss section of receiver capsule. 6. CRADLE SWITCH The c r a d l e switch is a n a s s e m b l y of contact springs to connect the transmitter, receiver and dial into the circuit when the handset is removed, and t o connect the ringer into the circuit when the handset is replaced. 7. CAPACITOR (or "condenser") Capacitors like that shown in Figure 7 a r e used in each telephone-one in the transmission circuit and one in the ringing circuit. The capacitor in the transmission circuit improves the telephone output c h a r a c t e r i s t i c s . T h e capacitor in the ringing circuit permits ringing current to pass through the ringer, and stops d i r e c t current. Terminals 'T\Winding No. 2-7 Spool head Winding Capacitor I / r W i n d i n g No. 1 Silicon-iron laminations . Figure 8. Induction coil (photo and cross s e c t i o n ) . Armature rCan cover 9. RINGERS Call signals may consist of various visual o r acoustic signals for c a 11i n g a subscriber. Orinarily, however, bell ringers (Figures 9 and 1 0 ) a r e used. Ringers may be of two types: (a) Straight -line ringers I td Figure 7. Capacitor construction. T h e capacitor consists of a r o l l of alternate l a y e r s of paper and aluminum foil sealed in a metal container. Each layer of foil is separated from the next layer by two layers of paper. The terminals of the assembly a r e put between the foil and paper during the time the paper and foil a r e rolled. Thenthe assembly is impregnated with wax, and inserted into a container where it is f u r t h e r sealed. (b) Harmonic (mechanically and electrically tuned) ringers for party-line use. The principal parts of the two types of ringer are essentially the same: two gongs, an armature carrying a rod and bell-clapper, a n electromagnet consisting of two coils, and a permanent magnet to p o l a r i z e t h e two e l e c t r o m a g n e t c o r e s . When the "straight-line r i n g e r " c o i l s a r e energized with alternating current, the armature vibrates s o that the clapper is moved from side TELEPHONES AND RELAYS Reed c Armatureadjusting nut Electromagnet Figure 9. Polarized straight- 1ine ringer. to side, striking each gong alternately. A spring "biases" the ringer-i.e., holds the armature against one coil core-end. This prevents ringer "tapping" when another party dials on the line, etc. (Also, when pulsating o r superimposed ringing is used, the biasing spring restores the armature between pulses.) The biasing spring d o e s not hold the c l a p p e r against a gang, however; ringer adjustment keeps the clapper (when a t r e s t ) away f r o m the n e a r e r gong, so both gongs a r e f r e e to vibrate alike. Figure 10. High-impedance harmonic ringer. The "harmonic ringer" has a "reed" (spring) a t t h e end of a weighted bell clapper. This harmonic ringer is mechanically and electrically tuned. The capacitor used with the ringer is chosen to be resonant with the ringer coils a t the frequency of the ringer. In general, a weak spring and a heavy clapper permit vibration a t low frequencies; a strong spring and a small clapper a t high frequencies. RELAYS 10. RELAYS A r e l a y is a n electromagnetically operated s w i t c h t o open and c l o s e e l e c t r i c circuits. F i g u r e 11 shows the plan and elevation of a typical class A relay. The relay performs its s w i t c h i n g function by electromagnetically attracting an armature, which, in turn, actuates mechanical contact springs, to which the electric circuits to be switched a r e connected. r B a c k stop Armature lever fl mounting screw Coil terminals (for two windings) In general, a relay mechanism consists of two o r more circuits: (a) The e l e c t r i c coil circuit . f o r the relay contr 01. Insulator ,Contact spring terminnla (b) The connected electric circuit or circuits which a r e switched by the opening o r c l o s i n g c ontacts . Relays are used for remote control or timing, as: (a) when a weak or variable signal must have strong o r unvarying control over a local circuit. (b) when one c i r c u i t - c h a n g e m u s t cause changes in mofe than one other circuits. (c) to obtain and utilize delays o r sequential operation(s) o r both. L ~ r m a t u r e bearing pin Figure 11. Typical class A single-arm relay. (df when several causations occurring only s i m u l t a n e o u s l y o r only in a certain sequence must change another circuit. -;i." I I 1 RELAYS 11. TYPICAL RELAYS 12. TIMING The principal parts of a typical relay a r e the o p e r a t i n g c o i l with a s o f t - i r o n c o r e , the armature, and the spring assembly mounted on its heelpiece. The relay's operating and release time may be changed by adding a copper collar around the coil c o r e . The r e l a y is m a d e slow both t o operate and to release when the collar is on the armature end of the core, and slow to release (only) when on t k heel end of the core. A' copper sleeve around and along the entire core length also makes the relay somewhat slow to operate and quite slow to release. These relays a r e known as slow-acting relays. Such relays a r e used when the contacts must remain operated while their coil circuits are repeatedly interrupted for short periods. The relay in Figure 1 2 is Automatic Electric Company's class A relay, used in most selectors, connectors, etc. The r e l a y in F i g u r e 13 is Automatic Electric Company's type 57A relay, used in toll switchboards, Directors, SATT equipment, etc. Figure 12. Typical c l a s s A double-arm relay. Figure 14.' Slow-release r e l a y with short- lever m a t m e . When greater release delay is required than can be provided by a copper collar or sleeve, different armature construction can be employed. In this design, the distance from coil-core end to pivot is about the same a s the distance from spring-actuating bushing to pivot (see Figure 14). This short-lever (1:1 ratio) armature eliminates the mechanical advantage (about 2.25: 1 for class A relays, figure 15; 1.65:l for type 57A r e l a y s ) of the contact s p r i n g s acting on the standard a r m a t u r e and tending to r e s t o r e it Figure 23. Typical type 57A relay. The relay is one of the most important single pieces of apparatus used in telephone switching. Nearly e v e r y telephone circuit u s e s one o r m o r e relays. In appearance the relay is a simple mechanism. Its design and manufacture, however, involve many complex considerations. Proper engineering and experienced judgment, however, assure an efficient and durable product. The high sensitivity of Automatic Electric Company relays results from practical design both of the coil windings and of the magnetic structure. The exceptional durability of our relays is due to the unique pin-type armature bearing and to the quality of the specially selected materials used. In figure 12, the armature has a non-magnetic screw which, when the relay operates, determines the residual air gap remaining between armature and core. After the coil circuit is opened, the s p a c e thus left between a r m a t u r e and core prevents residual magnetism keeping the relay operated. It insures release. If. when relay i s operated, contact-spring load on armature bushing i s 100 grams - I Coil must pull at least 225 grams to hold relay operated L~dn~-lever armature"' If, when relay i s operated. contact-spring load on armature bushing i s 100 grams I - Coil need pull only . I 0 0 grams to hold L ~ h o r t - l e v e rarmature . relay operated Figure 15. Comparison o f lever e r a t i o s of long- and short- lever ammatures ( Z o s s A relays). t o n o r m a l . T h u s , the s p r i n g s a r e unable to r e s t o r e the a r m a t w e until the magnetic field has died down t o a considerably lower value, which r e s u l t s i n a release delay which may be as much a s half a second. F o r a given springcontact load and r e l e a s e t i m e , t h e 1: 1 ratio also permits a larger residual setting, which is more permanent and avoids armature sticking. 13. SPRING ASSEMBLIES T h e r e l a y s p r i n g a s s e m b l y is m a d e up of a number of metallic springs, insulated from each other and from the heel piece. The r e a r ends of these springs a r e terminals f o r the leads from t h e c i r c u i t s to be switched. Each spring has one o r m o r e precious-metal electric contacts through which circuits (electrical connections with o t h e r s p r i n g s ) can be established. The spring assemblies a r e made i n various combinations the circuits require. The armature springs all move together because of mechanical coupling through t h e i r bushings. The spring a s s e m b l i e s u s e d with c l a s s A r e l a y s a r e illustrated individually in Figure 16. The springs a r e a s s e m b l e d between laminated paper-base phenolic insulators, which have been heated to exclude a l l moisture. Form A - "Make" Fom B - "Break" (singlethrow, (single throw, normally open) normally closed) - Form D "Makebefore-break" - Form C "Break-make" (double-throw) 14. MOUNTINGS Relay mountings a r e made f r o m sheet s t e e l shaped to provide the most practical arrangement in every case. The individual r e l a y bracket consists of an electrogalvanized s t e e l right angle (Figure 17) having punched holes for the coil terminals, mounting s c r e w s , a n d insulating bushings. Sometimes a slip-on sheet-steel cover with a guide is used for electromagnetic and electrostatic shielding between relays, mechanical protection, and a s a dust cover. Figure 17. Typical individual relay bracket and cover. When r e l a y s a r e mounted in a row, electrogalvanized steel channels (Figure 18) provide a light and rigid support. Covers for such mountings a r e not individual but cover a l l the relays o r s p a c e s in the e n t i r e row. - Form % "Breakmake-before-break" Figure 16. &sic contact assemblies. (shorn in normal [moperatedl p o s i t i o n ) . T h e c o m p l e t e a s s e m b l i e s a r e then placed in s p e c i a l c o m p r e s s i o n jigs and held by "hightensile" steel screws tightened with power-driven screwdrivers. Figure 18. Typical relay mounting s t r i p . When relays a r e mounted in groups (Figure 19), a sheet-metal base simplifies their mounting, a s s e m b l y , and maintenance by providing a s t a n d a r d i z e d mounting on which apparatus easily may be added, rearranged, o r removed. The number of spring combinations which can be operated depends upon the number of ampere turns developed in the coil. This depelids upon the number of turns of wire used to wind the coil, the resistance of the coil, and upon the voltage drop available in the circuit. Twin contacts a r e specified sometimes. They have two contacts on each spring. Each contact is m o u n t e d on a s e p a r a t e s e c t i o n of spring t o permit each to operate independently. Thus even if one contact were held owen bv dust, the other contact would complete the cl'rcuit. All type 57A relays (Figure 13) have twin contacts. Figvre 19. Qpical relay-gruq "munting w r t h jack-in base (showing front and rear covers and the jack). RELAYS AND PLUNGER LINESWITCHES T h e pulse r e p e a t e r ( F i g u r e 20) shows relays mounted on a group base. F i g u r e s 39, 4 1, and 43 show r e l a y s mounted similarly on two-motion-switch bases. Equipment mounted on each base is connected t o a s w i t c h j a c k ( m a d e up of m a n y contact springs mounted on the back of the base), which engages similar springs of a shelf jack to extend electrical connections. Sometimes relays a r e mounted on plates which a r e not jacked in but a r e bolted permanently to the central-office iron framework. Figure 20. Pulse repeater. PLUNGER LINESWITCHES 15. LINESWITCHES All types of telephone system require some kind of electrical device associated with each line f o r the purpose of giving an individual subscriber a c c e s s to selecting equipment when (and only when) he originates a call. During the early Strowger automatic telephone practice, individual connector switches were employed in telephone systems up to one hundred l i n e s . In s y s t e m s of m o r e than one hundred l i n e s , e a c h l i n e had i t s individual selector switch. A more advanced arrangement consists of employing a s m a l l e r switch permanently associated with each subscriber's line and made up of simpler and more economical design. Such a switch is known as a lineswitch and automatically extends the calling line to either an idle selector or connector prior to dialing. It is this characteristic of being independent of dial pulses for its operation that classifies it a s a non-numerical switch. (A n u m e r i c a l switch may be defined a s a switching mechanism in which dial pulses control a t least some functions .) When Strowger automatic telephone s y s t e m s a r e required to handle traffic originating from a l a r g e n u m b e r of l i n e s , f u r t h e r u s e of the lineswitch a s a secondary lineswitch has still m o r e economical operating advantages. The s e c o n d a r y l i n e s w i t c h e s combine t h e traffic f r o m a p r i m a r y group of s u b s c r i b e r s ' lineswitches and direct i t to a group of numerical s w i t c h e s a r r a n g e d to handle t h e combined traffic. This a r r a n g e m e n t makes each trunk leading to a numerical switch easily accessible f r o m a n y s u b s c r i b e r ' s l i n e s e r v e d by the combined group of trunks. T h e s e c o n d a r y lineswitch s e r v e s a purpose similar to that of the p r i m a r y lineswitch since i t permits greater use of more costly switches by us e of t h e s i m p l e r and m o r e economical lineswitches The local secondary lineswitch is connected between the primary lineswitch and the f i r s t selector. Lineswitches may be of the plunger or rotary (see $24) type. . Sometimes multi-office automatic exchanges also employ secondary lineswitches connected to interoffice trunks at the outgoing end. These lineswitches combine the traffic directed to a distant office which originates from the various s u b s c r i b e r lines in t h e originating office. It gives a c c e s s to a l l the inter-office trunks in each office group. Whenever lineswitches a r e s o used they a r e connected between the local selector banks and the pulse repeaters. 16. PLUNGER LINESWITCH Two t y p e s of p l u n g e r l i n e s w i t c h have been developed, the e a r l i e r type called the fan-tail (also called pick-up and non-aligning, Figure 2 1) and t h e l a t e r type c a l l e d self-aligning (see Figure 22). Plunger lineswitches a r e electromagnetically operated switches and a r e grouped under control of a masterswitch. The group usually consists of from twenty-five to one hundred lineswitches - per master-switch. Each lineswitch has access to a maximum of ten trunks. When the fan-tail type switch is released by a calling subscriber, the plunger is not aligned with the other lineswitch p l u n g e r s until the m a s t e r - s w i t c h in i t s oscillation p a s s e s the trunk previously used by the lineswitch, whereupon PLUNGER LINESWJ!I'CHES - -- ~- The lineswitch bank (Figure 21) is the complete a s s e m b l y of contact s p r i n g s with which the plunger connects when a call is made. Each bank is composed of ten s e t s of contacts, each s e t terminating one end of a trunk leading to the next succeeding switch. the s p l i n e of the m a s t e r r s w i t c h guide shaft engages a slot in the fan-tail on the end of the plunger. The fan-tail plunger is now primarily used i n making additions to existing automatic t e l e p h o n e i n s t a l l a t i o n s of the s a m e type. When the self -aligning lineswitch is released by a calling subscriber, the plunger is immediately aligned with the other idle lineswitch plungers on the master-switch guide shaft. Figure 23. Figure 22. Self-aligning plunger lineswitch. The master-switch (Figure 23) consists of a locking segment by which the switch shaft is held opposite various trunks. A large solenoid furnishes electromagnetic power f o r moving the s h a f t f r o m r i g h t t o left. A centrifugal governor r e g u l a t e s the speed of the switch movement. Relays and tripping a r m s a r e used to control the operation of the master-switch. Master.-swi tch and self-aligning plunger liheswi tches. .?- * ) STEPPING SWITCHES 17. BASIC MECHANISMS High-speed, multi-contact switches are commonly used to perform three kinds of control operations: (a) Select one circuit f r o m many circuits. e (b) C a r r y out operations successively in s eparate individual circuits * (c) Furnish timed pulses a t equal or unequal intervals. . Electromagnetically operated stepping-switch mechanisms can be classified according to the way their wipers move during: (a) The selecting function, rotated by the pawl-and-ratchet mechanism. This mechanism is operated by an electromagnet responding to current pulses. Operation of the switch can b e controlled either manually o r automatically. T h e switch c a n b e operated manually (Figure 24) by transmitting current pulses from a dial, a push button, o r any other remote or local pulsing device. The switch can be operated automatically (Figure 25) by selfcycling i n t e r r u p t e r s p r i n g s mounted on the switch and connected in s e r i e s with the switchoperating circuit. The pawl and ratchet mechanisms used consist of two types of drive: (a) Direct (b) Spring (1) Direct drive (2) Indirect drive 18. DIRECT DRIVE (See figures 24 and 25) T h e d i r e c t - d r i v e stepping s w i t c h u s e s its stepping m a g n e t t o d r i v e t h e pawl into the ratchet to rotate the wiper assembly. A detent (1) R e l e a s e by r e t r a c i n g path of holds the w i p e r s in place when the stepping operation magnet is de-energized and a tensioned spring r e t u r n s the d r i v i n g pawl t o n o r m a l . The (2) Restoration by stepping ahead to switching operation is complete the instant the home position magnet operates: the circuit of the -stepping electromagnet may be held closed for any length of time after the final step. With this method of operation, the pawl is ordinarily disengaged Two s chernatic diagrams of the direct-drive from the ratchet; thus the wiper assembly can stepping switches a r e shown: F i g u r e 24 is f o r remote-control (or pulse) operation, and . be returned to n o r m a l by simply having the release electromagnet disengage the holding Figure 25 (page 12)for automatic (self - interrupt ed) detent. Direct drive is used in the minor switch operation. ($25) and in Strowger two-motion switches ($26). Three basic stepping mechanisms a r e used f o r switching operations: (b) Restoration to normal h J- Contacts (a) Single-motion selection (rotary, step-bys t e p ) and r e v e r s e - m o t i o n r e s t o r i n g ( r o t a r y travel f r o m last step backward to s t a r t i n g position) (Minor switches) (b) Single-motion selection and restoring (Rotary switches) (c) Two -motion s e l e c t i o n (vertical, then r o t a r y stepping) and restoring (rotary r e v e r s e t r a v e l , then gravity v e r t i c a l drop) (Strowger Switches) Single-motion stepping switches a r e of ratchettype construction. These switches a r e made up basically of one or more wipers fixed on a shaft To circuits to be controlled Figure 24, Direct-drive switch. Remote-contro l (pulse) operation. STEPPING SWlTCHES 2 1. NON-HOMING TYPE Wiper The wipers remain on the last contact utilized, until the switch is again put into operation. -" '*I To circuits to be controlled Figure 25. Direct-drive switch. Sel f-interrup ted operation. TO CIRCUITS TO BE CONTROLLED 19. INDIRECT OR SPRING DRrVE (See Figures 2 6 and 27). When a n indirect- o r spring-drive switch is e n e r g i z e d , the c o r e a t t r a c t s t h e a r m a t u r e , causing the driving spring to compress and the pawl t o move out from the ratchet and over to the succeeding tooth. Upon de -ener gization of the electromagnet, the stored mechanical energy i n t h e compressed driving s p r i n g d r i v e s the wiper assembly forward one step over a bank of contacts. Figures 26 and 27 show schematic drawings of spring-drive switches. Figure 27. Indirect- or spring-drive switch. Self-interrupted operation. 22. NON-BRIDGING WIPERS Stepping switches a r e equipped ordinarily with one wiper or more for switching simultaneously a l l legs of the circuit being controlled. This control may be of a closed- or open-circuit type. Non -bridging w i p e r s ( f i g u r e 28) l e a v e one contact before touching the next. i DEVICE I TO CIRCUITS TO BE CONTROLLED Figure 26. Indirect- or spring-drive switch. Remote-control (put se) operation. 20. HOMING TYPE Since the pawl is engaged in the ratchet when t h e c o i l is not energized t h i s switch can be adapted to homing o r r e s e t operation, that is the wipers a r e automatically returned to their i n i t i a l s t a r t i n g point a t the completion of a switching operation by electrical means (see bulletin 47 3, "Rotary Stepping Switches" for f u r t h e r details .) Figure 28. Rotary-switch wiper assembly with bridging (long-tip) wipers and non-bridging wipers. 23. BRIDGING WIPERS Sometimes the circuit through the switch wipers must be maintained without interruption a s the wipers rotate from one contact to the next. Then long-tip wipers a r e used so that the successive contacts a r e momentarily bridged a s the wipers m o v e f r o m one contact t o t h e next. These long-tip wipers (figure 28) a r e called bridging wipers. When a t r e s t , e a c h wiper touches only one contact. STEPPING SWITCHES 2 4 . ROTARY SWITCH 3 The rotary switch (Figures 29 and 30) is a single motion electromagnetically operated indireetdrive stepping mechanism. one wiper rotating over one level on the f i r s t 25 rotary steps, then the other wiper travels over the adjacent level f o r the next 25 steps. Thus, in one revolution the switch has access to 50 circuits). Rotary switches always step the wipers when the operating electromagnet is de-energized. The manner in which the rotary switch is used determines whether i t is a numerical o r nonnumerical type. The switch may be used in either manner. The u s e s of the r o t a r y switch in automatic telephony a r e usually as lineswitch, linefinder, linefinder distributor, o r connector. Figure 29. m i c a 1 rotary switch. The switch transmits rotary motion to its doubleended wipers for selecting a circuit from among thoss terminated on its s emi-cylindrical bank of contacts. This single-direction rotary motion may be used also for returning the wipers to the " home" position, when wanted. Ratchet spring 25. MINOR SWITCH Ratchet wheel Interrupter spring The minor switch is an electromagnetically operated stepping mechanism of the reset type designed for either self- or remote-controlled operation. Figure 31 shows a minor switch mounted on a bracket. F i g u r e 3 2 provides switch nomenclature. - rRelease Rotary magnet armature y R e l e a s e magnet Rotary armature Rotary armature Arm. stroke adj. lever Figure 30. Typical rotary switch. The switch wipers a r e rotated by a ratchet and pawl actuated by an electromagnet operating from current pulses. The switch may therefore either be operated by remote control, o r stepped automatically over the bank contacts by interrupting i t s operating circuit through contact springs mounted on the switch base and actuated by the switch operation. I p .-< Rotary switches vary in construction, and may have 11, 25, oy 50 positions for the wipers on the banks. In most rotary switches the wipers a r e double-ended and in one half-revolution p a s s over 25 bank-contact positions. (The "50-point" r o t a r y switch u s e s two s e t s of single-ended wipers, staggered 180' apart, Pawl guide block Restoring spring Restoring spring Rotary pawl guide Bank assembly Spring assembly ank contact terminal Wiper assembly restoring spring J LConnecting lug Figure 32. Minor switch. The switch rotates its wipers clockwise over a ten-point bank assembly during its stepping, and releases the wipers f o r counterclockwise rotation to r e t u r n to t h e i r n o r m a l position thereby r e s e t t i n g the switch f o r repeated operation. STEPPING SWITCHES Movement of the wiper assembly is controlled by two different electromagnets, One is f o r r o t a r y stepping and one f o r r e l e a s i n g . The rotary-magnet armature c a r r i e s a pawl which operates the rotary mechanism. When the dial pulses energize the rotary-magnet its armature is a t t r a c t e d , and the pawl engages one of the teeth of the ratchet wheel, causing the ratchet to turn on its shaft. The number of steps taken corresponds to the number dialed (except that when 0 is dialed, 10 steps a r e taken). The holding detent is complete in one piece with the release-magnet armature. It engages the ratchet wheel and retains the wipers on any of the .contacts. As the w i p e r s s t e p , a spring attached to the wiper assembly increases its tension. When the release-magnet operates and disengages the holding detent from the ratchet wheek this tensioned spring restores the wipers to their normal position. T h e bank a s s e m b l y h a s t e n s e t s of contacts a r r a n g e d i n an a r c . The bank assembly on a minor switch may be one, two, o r three levels deep depending on the circuit requirements. E a c h l e v e l has associated with it one p a i r of wiper springs. Usually, the upper surface of each level consists of ten individual contacts, while the bottom surface is one solid segment (common contact). The pair of wiper springs m e r e l y closes the circuit from the particular top contact upon which it stops, to the common contact. This common contact is sometimes r e p l a c e d with individual contacts when the circuits require such an arrangement. backward (on any level) under the power provided by a n unwinding helical shaft spring, and then letting it drop by gravity. T h i s up, a r o u n d , and r e t u r n m o t i o n i s the characteristic which distinguishes the Americanm a n u f a c t u r e d Strowger-switch mechanism. Double dog spring Vertical magnets Vertical ratche Release link Rotary back stop Rotary pawl guide Rotary magnets Switch frame Figure 33. Strowger- swi tch mechanr sm. The Strowger switch consists of banks and wipers, vertical, rotary and release magnets and controlling relays. Figure 34 shows Strowger-switch banks. T h e most common use of the minor switch in automatic telephony is to select the party on a p a r t y line. When the switch is s o used, it i s ordinarily mounted a t the top of the Strowger connector switch. 26. STROWGER SWITCH The Strowger switch (Figure 33) is an electrom a g n e t i c a l l y operated two-motion stepping mechanism designed to provide selectivity in connecting its wipers with circuits that terminate on its bank contacts. The Strowger switch is the basic switch of the "step-by-step' ' automatic telephone system. The Strowger switch gives the following movements to the shaft and wipers: (a) R a i s e s the shaft vertically any number of s t e p s f r o m one to ten, then (b) R o t a t e s t h e shaft in on any one of ten contacts of the t e n levels, after which (c) The shaft is released back to its normal position by f i r s t letting the shaft rotate Figure 34. Stkowger-swi tch wipers and banks. 7he upper wiper and bank are the control wiper and bank. me lower wipers und bank are the and + l i n e wipers and l i n e bank. 'Ihe control-bank contacts are wide; as the control wiper passes from one contact t o the next i t 'bridges'; i . e . , before Leaving one contact i t touches the n e x t (useful in absence-of-ground-searching circuits). - The 4Strowger-switch mechanism can be made to function equally well a s a connector, selector, or linefinder by changes in its electric circuit and mechanical components. 1 , STEPPING SWITCHES The switch has a spring assembly fastened to the top end. Underneath this assembly is a normalpin (see Figure 36 right-hand view) which r e s t s against the normal-pin stop when the shaft is a t ormal. This arrangement prevents the shaft f r o m t r a v e l i n g too f a r t o the l e f t due to the torque applied to the shaft by the shaft spring. When the shaft is in its n o r m a l position, the n o r m a l pin r e s t s on an a r m which causes the off -normal springs t o operate. b wipers a r e in line with the desired bank level. The vertical tooth of the double detent engages a tooth of the vertical ratchet to keep the shaft in i t s elevated position. With each de-energization of the vertical-magnet circuit, the vertical pawl is withdrawn from the ratchet. The weight of the shaft r e s t s on the normal-pin clamp (Figure 36 left-hand view) which s t r i k e s on t h e u p p e r s h a f t b e a r i n g when t h e shaft is normal. The shaft is guided by two bearings, one at the top and one at the bottom .of the switch frame. T h e hub of the shaft consists of two sections, the upper section is the vertical ratchet and the lower section the rotary ratchet. The vertical ratchet consists of ten horizontal teeth cut into the upper p a r t of the hub. The r o t a r y ratchet consists of eighteen v e r t i c a l teeth cut into the lower p a r t of the hub. The vertical ratchet i s f o r lifting the shaft and t h e r o t a r y ratchet is f o r rotating the shaft. Auxiliary mechanisms such a s detents (lock o r \unlock a me chanical movement) a r e associgated with the ratchets. The detents engage teeth i n t h e r a t c h e t s when t h e r a t c h e t s move off n o r m a l and hold t h e r a t c h e t s in t h e position l a s t operated. Two such detents a r e used; the s t a t i o n a r y d e t e n t and t h e double detent (see F i g u r e 36 l e f t and r i g h t views respectively). The stationary detent, a s the name implies, does not move. It is auxiliary t o t h e double detent and is used to hold the vertical ratchet elevated when the s