AUGUST, 1950
A CLOCK-ACTUATED INTERVAL TIMER CLYDE P. BROCKETT University of Toronto, Toronto, Canada
THE history of master-clock control contacts, actuated by pendulum clocks, is not alone interesting and varied but is marked by numerous shoretime successes and long-time difficulties. By and large the basic cause of many eventual setbacks is that the contact devices, however ingenious, have almost invariably operated within the confines of an uncomfortable compromise: they have either (1) tended to impede the free-running of the clock train in order to attain a sufficiently failure-free electric contact, or (2) in avoiding interference with the clock train, they have sacrificed electric contact merit. These are the adverse limits within which the cam-action master control contact, so widely used with much success for several decades now, has operated, deriving its contacting action from the rather small torque of the escapement wheel of the clock.
The description which follows deals with the working principle of a master contact which shifts the mechanical burden from the wheel train over to the pendulum, where it properly belongs. By taking direct advantage of the swinging mass of the pendulum to bring the contact points together, contact pressure may he made a s heavy as desired, and the wheels are left to themselves. This device simply selects which stroke of the pendulum will trigger the action. Thus, the limiting compromise referred to above is obviated, to the prolongation of reliable performance. The use here suggested is for time distribution in systems employing minute or half-minute electric impulses t o control secondary clocks, program clocks, time signals, time recorders, laboratory chronographs, etc., such as are ordinarily found in schools and universities nearly everywhere.
JOURNAL OF CHEMICAL EDUCATION
Timer Mashanism
As may be deduced from the diagram, the device may readily be built into clockworks, without even necessarily taking them apart. At negligible expense and with moderate skill it may convert a pendulum clock possessing a 1-r. p. m. escapement wheel into a master control clock; or it may a master contact now . replace giving faulty service. From the forked end of a rigid rocker arm R, a5xed t,n t,he esca~ement shaft S. a thin metal s t r i ~ E-F is sus~~-~= pended so as to hang freely in a vertical position. This strip terminates a t F in a shallow upturned lip, or hook, which, in its up-and-down motion as the pendulum swings, rather narrowly misses engaging the corresponding hook W which is mounted on the lower contact spring. The best position of W relative to F , for proper functioning of the contact, is found by experiment. For this purpose, contact mount A should be provided with both vertical and horizontal adjustment. A highly flexible strip of shim brass B, affixed to E-F, is carefully shaped so that its free end is touched, once a minute only, - . by - a fine displacing pin P mounted on arm D which is fastened t o t h e escapement wheel shaft. When this occurs, F is displaced sufficiently t,nwa,rrl .... ... .. +,he ---- rizht on the down-stroke of R to cause the lip a t F to lock with the wedge W on the following up~~
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stroke and bring the contact points a t C together as the pendulum, with its considerable mass, swings from left to right. Near the end of this pedulum stroke the escapement allows P to pass spring B. Next, the succeeding downstroke of E-F then not only breaks the electric contact but frees F from W, so that E-F once more dangles freely in its up-and-down excursions until, with the next revolution of the escapement wheel, the cycle of events is repeated and another contact made and broken. Since the mass of the pendulum imparts considerable force a t F , i t is easy to realize the principal virtue of this pressure-rub contact, viz., certainty of circuit closing and opening. And, since the wheel train is relieved of mechanical burden, a second distinct advantage is gained: no loading or friction in the most sensitive part of the clock, the escapement. Within the operating principle of this device a number of variations may suggest themselves, such as: (1) break-make contact sequence, (2) half-minute contacts, by providing a second displacing pin, 180 degrees removed from the first, (3) subdivision of a minute into any desired program of contact intervals, depending only upon how displacing pins may be arranged around the circumference traveled by P. Experience with this master contact on a half-second pendulum clock shows that by adjusting the position of the contact mount A relative to the at-rest position of E-F, the contacting duration may be varied up to 0.5 second or more. Further, it is found that the 1.75-inch arc of the pendulum is diminished by less than 0.01 inch whenever the contact is made, and the clock remains a precision timepiece. Conforming to good practice, this control-clock contact is intended to operate a d.-c. master relay, rather than itself to carry heavy current loads. However, it should be able to handle easily from 50 to 100 milliamperes a t 6 to 8 volts with long and unfailing satisfaction and, through proper relaying and adequate shunt capacitance, take control over almost any existing electric-impulse time system which depends upon a clock-actuated interval contact. This deviceis sufficiently versatile to allow the use of a snap-off-snau-on microswitch in place of the t,ype of contact sholk, if desired.
