CLOCK No. 28

B 1322
Issue 2, 27.5.59

Clock No. 28

1. Description
Clock No. 28 is a seconds indicating clock; the general appearance and the movement are illustrated below.  These Clocks are used on trunk exchange switchboards driven from a Clock No. 12 or 36. The mounting is made of Brass.

These clocks were originally controlled by a Relay No. 90A which is operated every second by an impulse from a master clock.

Overall diameter


Diameter of dial


Depth (back to front)


2. Operation
The current pulse operates electromagnet A, attracting a spring-controlled armature B.  From the upper extremity of armature B, the extension C carries a spring-controlled pawl D engaging with the ratchet wheel E, which steps on release of the electromagnet.  Ratchet wheel E has 60 teeth and is fixed by the 'seconds' arbor carrying the 'seconds' hand.  The ratchet wheel drives the 'minutes' hand by means of the 60 to 1 reduction wheels F and G.  A further 12 to 1 reduction gear, situated between the pillar plate and dial (not shown in the illustration), controls the 'hour' hand.  A gravity-controlled retaining detent H is provided to prevent backlash, and to maintain the 'seconds' hand in position during the operation of the electromagnet.  Felt or rubber washers J are inserted between the movement and the dial, to reduce the sound of the clock action.

The felt buffer K is provided to absorb the blow occasioned by the action of the armature lever against the limiting spring L.

3. Operating current
The normal operating current for Clock No. 28 is 200mA and care should be taken to ensure that a current of 250mA is not exceeded.

4. Mounting
When Clocks No. 28 are fitted on service observation panels, it is essential that they should operate with a minimum of noise.  In some of the older telephone exchanges, Clocks No. 28 are mounted on top of the panels and, as these act as resonators, the normal sound due to the operation of the clock movement is accentuated, and has given rise to complaints.  In all cases where the operating noise is considered by the Traffic Superintendent to be excessive, arrangements should be made for a rubber sponge pad, one inch in thickness, to be inserted between the clock and the mounting.

The Clock No. 28 is superseded by Clocks No's 66A or 68A.

Drawing - 7537.

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Last revised: January 07, 2024