Another arrangement is the “maintaining spring.”
It will be remembered that during the process of winding the clock the hand twisting the key takes the pressure of the ratchet wheel off the pall, so that during that operation no force is at work to drive the clock. In consequence the pendulum receives no impulse, but swings simply by virtue of its former motion. If the process of winding were done slowly enough the clock might even stop. To avoid this, a very ingenious arrangement is made to keep the cogwheel mounted on the winding shaft going during the winding-up process. This is called a maintaining spring.
The arrangement shown in [Fig. 53] will explain it.
Fig. 53.
The cogwheel a and the ratchet wheel are both mounted loosely on the arbor carrying the drum. a is linked to b by a spring c. The ratchet wheel b is engaged by a pall fixed to some convenient place on the body of the clock frame. When the weight pulls on the drum the pull is communicated to the ratchet wheel b, and this acts on the spring c and pulls it out a little. As soon as the spring c is pulled out as far as its elasticity permits, a pull is communicated to the cogwheel a, and the clock is driven round. When the clock is wound the pressure of the weight is removed, and therefore the ratchet wheel e no longer presses on the pall, and thus no pressure is communicated to the ratchet wheel b, or through it to the clock. But here the spring c comes into play. For since the ratchet wheel b is held fast by the pall d, the spring c pulls at the wheel a, and thus for a minute or so will continue to drive the clock. This driving force, it is true, is less than that caused by the weight, but it is just enough to keep the pendulum going for a short time, so that the going of the clock is not interfered with.
If the reader can get possession of a clock, preferably one that does not strike, and, with the aid of a small pair of pincers and one or two screwdrivers, will take it to pieces and put it together again, the mechanism above described will soon become familiar to him. Not every clock is provided with maintaining spring and overwinding preventer.
The cause of stoppage of a clock generally is dirt. Where possible, clocks should always be put under glass cases. “Grandfather” clocks will go much better if brown paper covers are fitted over the works under the cases. In this way a quantity of dust may be avoided. To get a good oil is very important. It will be noticed that pivot-holes in clocks are usually provided with little cup-like depressions. This is to aid in keeping in the oil. The best clock oil is that which does not easily solidify or evaporate. Ordinary machine oil, such as used for sewing machines, is good as a lubricant, but rapidly evaporates. Olive oil corrodes the brass.
It is best to procure a little clock oil, or else the oil used for gun locks, sold by the gunsmiths. The holes should be cleaned out with the end of a wooden lucifer match, cut to a tapering point. The pivots should be well rubbed with a rag dipped in spirits of wine. If the pivots are worn they should be repolished in the lathe. If the cogs of the wheels are worn, there is no remedy but to get new ones. Old clocks sometimes want a little addition to the driving weight to make them go.
The weight necessary to drive the clock depends on its goodness of construction, and on the weight of the pendulum. If the clock is driven for eight days with a cord of nine feet in length with a double fall, then during each beat of the pendulum that weight will descend by an amount =