The dialling mechanism is that part of the telephone instrument which converts the information of the dialled telephone number into electrical impulses which are thus transmitted to the exchange for the required connection.
The impulses as mentioned above are electrical in nature and are generated by the alternating loss of contact and making of contact of the electrical leaves. The contact is broken as many times as the dialled digit.( the explanation of which will follow shortly)
The electrical leaves act as leaf springs, which are prestressed such that normally remain in contact. The inner leaf presses against the cam throughout the dialling cycle and thus as the cam rotates , the inner leaf oscillates along with it. If the outer leaf is free to move , it would remain presses against the inner leaf , thus moving along with it and maintaining electrical contact. But , if the outer leaf is held back physically;as the inner leaf oscillates it would alternately make and break contact with the outer leaf, thus generating the impulse signal. This , precisely forms the basis for the working of the dialling mechanism.
Having understood the basic principle behind the dialling process , we now proceed towards a logical , step by step explanation of the actual process from the dialling of the number through dialler to the generation of the electrical impulses at the electrical leaves.
The main gear meshes with the smaller one of the transmission gear,the larger transmission gear meshes with the cam gear and the reducer gear.The reducer gear shaft is attached with a reducer, which consists of two half disks.The plugger is attached with the cam gear shaft.There is a spring attached to the base of the main gear shaft which brings back the dialler to its original position after the number has been dialled.
Let us at this point, divide the dialling cycle into two parts:
(a) The dial stroke during which the cradle rotates the dialler.
(b) And the release stroke during which the dialler returns to its original position by virtue of the spring torque of the dialler spring .
Consider the dialing mechanism from the BACK side , thus revealing the internel mechanism.During the dial stroke the main gear rotates anticlockwise which thus rotates the transmission gear clockwise which in turn rotates the cam gearanticlockwise.This leads to the following:
(a)the plugger, which is situated on the cam gear and is linked to it by friction thus tries to rotate anticlockwise too and hence remains at one extreme position - the position at which it doesn't hinder the motion of the outer leaf.
(b) and since the plunger doesn't interfere with the outer leaf , it continues to move along with the inner leaf, thus maintaining contact throughout the dial stroke . The inner leaf ofcourse maintains contact with the cam and thus oscillates along with the rotation of the cam.
The dialling stroke is complete when the caller has turned the dial by the appropriate amount.This information is stored in the dialler spring and gets converted into the electrical signal finally , to be sent to the telephone exchange
Next as soon as the caller releases the dial ,the release strokecommences.As mentioned earlier , during this stroke the dial is brought back to its original position by the torsional dial spring. This rotates the main gear in the clockwise direction which moves the transmission gearin the anti clockwise direction which in turn rotates the cam gear in the clockwise direction.This leads to the following:
(a) the plugger, which is located on thecam gear and linked to it by friction also rotates in the clockwise direction along with the cam gear until it comes to its clockwise extreme position wherein it fixes the position of outer leaf , constraining it.
(b) this leads to the making and breaking of contact between the leaves as explained in the "basic principle of impulse generation"
The amount of turn of the dial dictates the number of revolutions of the cam gear and hence of the cam.nd this guides the number of oscillations of the innerr leaf and thus the number of pulses. And the accordance betwwen the number of dials and the pulses is maintained.
The release strokeis completed when the knocker on the main gearknocks the plugger to make it to rotate anticlockwise, thus releasing the outer leaf, bringing the impulses to an end.Finally the release stroke is completed with the knocker separating "New number electrical leaves".Thus passing the information that the process of dialling of digit is complete and hence bringing the dialling mechanism to its original state,ready to accept a new digit in the form of a new dialling cycle.
Finallly to explain the role of the "reducer."The reducer is designed from dynamic consideration.When the dial is released , during the release stroke ,it is the dialler spring that rotates the system.If allowed to unwind on its own it would lead to the attainment of a high velocity(kinetic energy) by the system leading to a more severe collision at the end of the cycle,leading to greater stresses and quicker wear and tear.To avoid this is precisely the function of the reducer.It essentially acts as a coloumb damper(friction damper)and thus dissipates some of the enenrgy stored in the spring during the release stroke.
It works as follows:
Motion is transmitted to the reducer by the reducer gear which is rotated by the transmission gear which in turn is imparted motion from the main gearThis gear train ensures that the reducers rotates at 24 times the angular velocity of the main gear. At such a higher anguler velocity the centrifugal force on the half discs, tends to move them apart, bringing them outward and thus leading to the pressing of the friction heads upon the metal ring,thereby dissipating energy in the form of friction.