Short description on working principle of rotary mechanism is given in this presentation. Diagrams with proper labeling are provided for better understanding of the topic.

1. SUBJECT: Telematics
TOPIC REVIEW: “Rotary Mechanism”
By: Juilee Deshmukh
G.H Raisoni College of
Engineering, Nagpur

2. •Rotary dial telephone system was a switching system made by Almon B.
Strowger to replace human operator by automatic exchange.
•A formal numbering plan is required to identify the subscribers.
•The digits are arranged in a circular layout. This method is sometimes
called decadic dialing.
•In pulse dialing, a train of pulses is required to represent a digit in
the subscriber number.
•The number of pulses in a train is equal to the digit value it
represents except in the case of zero which is represented by 10
pulses.
•Two successive trains are distinguished from one another by a
pause in between them, known as inter-digit gap.

3. A rotary dial telephone uses the following for implementing pulse
dialing:
(a) Finger plate and spring
(b) Shaft, gear and pinion wheel
(c) Pawl and ratchet mechanism
(d) Impulsing cam and suppressor cam/ trigger mechanism
(e) Impulsing contact
(f) Centrifugal governor and worm gear
(g) Transmitter, receiver and bell by pass circuits

5. •The digits are arranged in a circular layout so that a finger wheel may be
rotated with one finger from the position of each digit to a fixed stop
position, implemented by the finger stop, which is a mechanical barrier to
prevent further rotation.
•When released at the finger stop, the wheel returns to its home position by
spring action at a speed regulated by a governor device.
• During this return rotation, the dial interrupts the direct electrical current
of the telephone line (local loop) a specific number of times for each digit
and thereby generates electrical pulses which the telephone exchange
decodes into each dialed digit.
•Each of the ten digits are encoded in sequences of up to ten pulses.

6. 1. When the dial is in the rest position, the impulsing contacts are kept away from the
impulsing cam by the suppressor cam.
2. When the dial is displaced from its rest position (off normal position), impulsing
contacts come near the impulsing cam.
3. Rotation of finger plate causes rotation of the main shaft. Pawl slips over the ratchet
during clockwise rotation.
4. The ratchet, gear wheel, pinion wheel and the governor are all stationary during the
clockwise movement of the dial.
5. When the dial returns, the pawl engages and rotates the ratchet. The gear wheel,
pinion wheel and the governor all rotate.
6. The governor helps to maintain a uniform speed of rotation. The impulsing cam
attach to the pineal shaft now breaks and makes the impulsing contacts which in turn causes
the pulses in the circuit.
7. When the dial reaches the rest position, the suppressor cam moves the impulsing
contacts away from the impulsing cam.
8. This action provides the required inter-digit gap timing independent of the pause that
may occur between 2 successive digits due to human dialing habit.

7. •Trigger is sprung away from the impulse contacts during the
clockwise motion of the dial, thus preventing pulsing at this stage.
•The trigger is sprung back to the operative position during the initial
return motion of the dial and thereafter operates the pulse contacts.
•The precision of operation in the cam dial is affected by the wear and tear
of the cam elements and other friction members in the mechanism.
•The trigger dial design eliminates friction members and helps to achieve
more uniform impulse ratio, larger inter-digit pause and better stabilization
of the return speed of the dial.