The manufacture of these clocks, has entirely ceased; there are still a large number in use, however, which occasionally require cleaning and repairing. Two styles are met with : in one, the wheels are set within a square frame formed of several pieces, and known as "the birdcage"; in the other, the wheels are between 2 plates similar to the 8-day. There are 2 points of difference which require attention - the endless chain, and the striking mechanism. The endless chain must be put upon the spiked pulleys in such a manner that the wheels turn the right way when the weight is put on, and the part that requires pulling to raise the weight should always come to the front, so that the weight passes quite free behind it, Fig. 218. Sometimes the chains will be found to be twisted, and the links, gathering up into a knot, stop the clock. The way to rectify this is to draw up the weight, separate the chain at the lowest part, let it hang free, straighten both pieces, and then unite again, when it will be found to work properly. A leaden ring, of sufficient weight to keep the chain just tight, is used to prevent the liability to twist. When a chain breaks from wear or rust, or jumps from being the wrong size, it becomes necessary to put a new one.
The most common kind of striking mechanism in 30-hour clocks is known as the "locking plate," and though it is more liable to derangement than the rack movement, still it is very largely used in French, American, and German clocks. It is much more simple than the rack, and one explanation of its construction will be sufficient for every case. The various parts are shown in Figs. 219 and 220: a, hoop wheel; 6, lifter; c, hoop-wheel detent; d, warning detent; e, locking plate; f, lockingplate detent; g, lifting pin to raise hoop-wheel detent; A, spring; i, warning pin. In testing the relative positions of the striking wheels when put together, proceed by moving the wheels round very slowly until the hammer tail drops off a pin; at that moment the hoop-wheel detent should fall into the hoop, so as to allow the hoop wheel about 1/4 in. run before it reaches the end of the detent and stops the striking When the hoop is resting against the detent, the warning pin should have half a turn to run, the same as in the 8-day clock.
The locking-plate detent / is connected by an arbor with the hoop-wheel detent c, and must be adjusted so that the latter can fall in the hoop wheel sufficiently far to stop the striking only when the end of the locking-plate detent falls into one of the notches of the locking plate. This is easily done by moving round the wheel to which the locking plate is attached, a tooth at a time, in the pinion that drives it, until it is in the correct position, and slightly bending the detent /, if necessary. When a clock with a locking-plate striking arrangement strikes till it runs right down, it is generally because the hoop-wheel detent does not fall freely, or the locking-plate detent does not enter the notches properly. It sometimes happens that the edge of the end of the hoop becomes worn and rounded by long use, and if the weight is excessive, it will cause the detent to jump out, and the clock to continue striking until run down. The remedy is obvious - file the end square. The locking plates are often cut irregularly; but on no account interfere by filing or spreading the edges, or perchance greater difficulties may arise, and there is always a position where it will answer well, which can easily be found by trial.