Part 14 (2/2)

Before we proceed to take up the details of constructing a chronometer escapement we had better136 a complete plan of a chronometer escapement as if seen from the back, which is in reality the front or dial side of the ”top plate”

The chronometer escapement consists of four chief or principal parts, viz: The escape wheel, a portion of which is shown at _A_; the i roller _C_, and the detent _D_

These principal parts are made up of sub-parts: thus, the escape wheel is co the portion of the escape wheel sunk, to enable us to get wide teeth actions on the impulse pallet The collet is a brass bush on which the wheel is set to afford better support to the escape wheel than could be obtained by the thinned wheel if driven directly on the pinion arbor The impulse roller is composed of a cylindrical steel collet _B_, the impulse pallet _d_ (some call it the impulse stone), the safety recess _b b_ The diameter of the impulse collet is usually one-half that of the escape wheel This impulse roller is staked directly on the balance staff, and its perfection of position assured by resting against the foot of the shoulder to which the balance is secured This will be understood by inspecting Fig 137, which is a vertical longitudinal section of a chronometer balance staff, the lower side of the irinder and finished a ball polish

[Illustration: Fig 136]

[Illustration: Fig 137]

It will be seen the iainst the hub _E_ of the balance staff The unlocking roller, or, as it is also called, the discharging roller, _C_, is usually thinner than the impulse roller and has a jewel similar to the impulse jewel _a_ shown at _f_ This roller is fitted by friction to the lower part of the balance staff and for additional security has a pipe or short socket _e_ which e_ The pipe _e_ is usually flattened on opposite sides to ad roller in adjusting the jewel for opening the escapement at the proper instant to permit the escape wheel to act on the io to make up the detent _D_ consist of the ”detent foot”

_F_, the detent spring _h_, the detent blade _i_, the jewel pipe _j_, the locking jewel (or stone) _s_, the ”horn” of the detent _k_, the ”gold spring” (also called the auxiliary and lifting spring) _ht and thin as possible and stand careful handling

We cannot i a chronoht Very few detents, even froht be We should in such construction have very little care for clumsy workmen who may have to repair such mechanism This feature should not enter into consideration

We should only be influenced by the feeling that we are working for best results, and it is acting under this influence that we devote soprinciples involved in adirectly to the drawing of such an escapeth of this or the dia the detent spring _h_, suppose we read in text books the spring should be reduced in thickness, so that a weight of one pennyweight suspended from the pipe _j_ will deflect the detent ” This is a rule well enough for people eical student such fixed rules (even if remembered) would be of se of the ”whys,” in order that he hly master this escapement

FUNCTIONS OF THE DETENT

We can see, after a brief analysis of the principles involved, that the functions required of the detent _D_ are to lock the escape wheel _A_ and hold it while the balance perfor _h_ th and power to perfor stone _s_ back to the proper position to arrest and hold the escape wheel; (2) the spring _h_ , the thrust of the escape wheel, represented by the arrows _p o_ Noe can readily understand that the lighter we_h_ can be You say, perhaps, if we make it too weak it will be liable to buckle under the pressure of the escape wheel; this, in turn, will depend in a great_h_

Suppose we have it straight e put it in position, it will then have no stress to keep it pressed to the holding, stop or banking screhich regulates the lock of the tooth To obtain this stress we set the foot _F_ of the detent around to the position indicated by the dotted lines _r_ and _n_, and we get the proper tension on the detent spring to effect the lock, or rather of the detent in ti _h_, instead of being perfectly straight, is bent and consequently not in a condition to stand the thrust of the escape wheel, indicated by the arrows _o p_

OBTAINING THE BEST CONDITIONS

Now the true way to obtain the best conditions is to give the spring _h_ a set curvature before we put it in place, and then when the detent is in the proper position the spring _h_ will have tension enough on it to bring the jewel _s_ against the stop screhich regulates the lock, and still be perfectly straight This ive further explanation Suppose we bend the detent spring _h_ so it is curved to the dotted line _t_, Fig 136, and then the foot _F_ would assuine the foot _F_ to be put in the position shown by the full lines, the spring _h_ will becoain and in perfect shape to resist the thrust of the escape wheel

Little ”ways andbeen known to the trade, but for some reason are never2/1000” thick and 80/1000” ill stand the thrust for any well-constructed marine chronometer in existence, and yet it will not require half a pennyweight to deflect it one-fourth of an inch It is a good rule to th of the detent fro jewel pipe _j_ equal to the dia _h_ two-sevenths of this distance The length of the horn _k_ is deterraphic plan and can be taken from the plotted plan The end, however, should approach as near to the discharging jewel as possible and not absolutely touch The discharging (gold) spring _m_ is attached to the blade _i_ of the detent with a small screw _l_ cut in a No 18 hole of a Swiss plate

While there should be a slight increase in thickness in the detent blade at _here the gold spring is attached, still it should be no_m_ from the detent blade _i_

IMPORTANT CONSIDERATIONS

It is i should be absolutely free and not touch the detent except at its point of attachainst the end of the horn _k_, and the extre rests, should only be e e The end of the horn _k_ (shown at _y_) is best ant construction, square--that is, the part _y_ turns at right angles to _k_ and is made thicker than _k_ and at the same time deeper; or, to make a comparison to a clumsy article, _y_ is like the head of a nail, which is all on one side Some makers bend the horn _k_ to a curve and allow the end of the horn to arrest or stop the gold spring; but as it is iht as possible, the square end best answers this purpose The banking placed at _j_ should arrest the detent as thrown back by the spring _h_ at the ”point of percussion” This point of percussion is a certain point in a reatest effort is produced and would be so on a pivot at _z_, Fig 138 It will be evident, on inspection of this figure, if the bar _G_ was turning on the center _z_ it would not give the hardest impact at the end _v_, as parts of its force would be expended at the center _z_

[Illustration: Fig 138]

DECISIONS ARRIVED AT BY EXPERIENCE

Experience has decided that the impulse roller should be about half the diameter of the escape wheel, and experience has also decided that an escape wheel of fifteen teeth has the greatest nues; also, that the balance should make 14,400 vibrations in one hour We will accept these proportions and conditions as best, from the fact that they are now almost universally adopted by our best chronoh it would seem as if these proportions should have established thes confine ourselves to the graphic plan, considering it preferable In the practical detail drae advise the e an escape wheel 10” in dias which accompany the description are one-fourth of this size, for the sake of convenience in copying

With an escape wheel of fifteen teeth the irees, and of course the periphery of the impulse roller must intersect the periphery of the escape wheel for this arc (24)

The circles _A B_, Fig 139, represent the peripheries of these two mobiles, and the problem in hand is to locate and define the position of the two centers _a c_ These, of course, are not separated, the su), as these circles intersect, as shown at _d_ Arithraphically, sih After we have swept the circle _A_ with a radius of 5”, we draw the radial line _a f_, said line extending beyond the circle _A_