Part 8 (2/2)

HOW TO DESIGN A DOUBLE-ROLLER ESCAPEMENT

We have already given very desirable forms for the parts of a double-roller escape principles as regards the rollers, but will give, at Fig 82, a very well proportioned and practical form of fork The pitch circle of the jewel pin is indicated by the dotted circle _a_, and the jewel pin of the usual cylindrical form, with two-fifths cut away The safety roller is three-fifths of the diameter of the pitch diameter of the jewel-pin action, as indicated by the dotted circle _a_

The safety roller is shown in full outline at _B'_, and the passing hollow at _E_ It will be seen that the arc of intersection embraced between the radial lines _B c_ and _B d_ is about sixty-one and a half degrees for the roller, but the angular extent of the passing hollow is only a little over thirty-two degrees The passing hollow _E_ is located and defined by drawing the radial line _B c_ froh the intersection of radial line _A i_ with the dotted arc _b_, which represents the pitch circle of the safety roller We will nauard point _C_ ter hollow _E_ extends on _b_ sixteen degrees on each side of the radial line _B c_

[Illustration: Fig 82]

The roller action is supposed to continue through thirty degrees of angular motion of the balance staff, and is embraced on the circle _a_ between the radial line _B k_ and _B o_ To delineate the inner face of the horn _p_ of the fork _F_ we draw the short arc _g_, frorees fronate the upper angle of the outer face of the jewel pin _D_ as the point _s_ and, froh this point _s_ the short arc _n n_ Parallel with the line _A i_ and at the distance of half the diameter of the jewel pin _D_, we draw the short lines _t t'_, which define the inner faces of the fork

The intersection of the short line _t_ with the arc _n_ ill designate the point _r_ With our dividers set to embrace the space between the point _r_ and the point _f_, eep the arc which defines the inner face of the prong of the fork The space we just made use of is practically the same as the radius of the circle _a_, and consequently of the sauard point _C'_ isas will, with certainty, clear the safety roller _B_ in all positions While we set the point _f_ at two degrees from the center _B_, still, in a well-constructed escaperees should be sufficient, but the extra half degree will do no haruard point _C'_ properly fitted, the fork will not have half a degree of play

The reader will remember that in the escaperee, being less by half a degree than advised by Grossree of lock In the perfected lever escapes for the construction of, we shall describe a detached lever escaperees fork and pallet action, with only three-fourths of a degree drop and three-fourths of a degree lock, which we can assure our readers is easily within the limits of practical construction by modern machinery

HOW THE GUARD POINT IS MADE

[Illustration: Fig 83]

The guard point _C'_, as shown at Fig 82, is of extremely simple construction Back of the slot of the fork, which is three-fifths of the diameter of the jewel pin in depth, is uard point _C_ is fitted to this hole so that it is rigid in position This uard point is equally efficient as that of attaching it with a screw, and hest ied About the best uard points is either aluold and very rigid and strong At Fig 83 we show a side view of the essential parts depicted in Fig 82, as if seen in the direction of the arrow _v_, but we have added the piece which holds the jewel pin _D_ A careful study of the cut shown at Fig 82 will soon give the horological student an excellent idea of the double-roller action

We will now take up and consider at length why Saunier draws his entrance pallet with fifteen degrees draw and his exit pallet with only twelve degrees draw To make ourselvesthe lever escape 1, plate VIII, of his ”Modern Horology,”

in which heface of the entrance pallet fifteen degrees and his exit pallet twelve degrees In the cut shown at Fig 84 we use the same letters of reference as he employs We do not quote his description or directions for delineation because he refers to so iven in the book just referred to Besides we cannot entirely endorse his methods of delineations for84

[Illustration: Fig 84]

MORE ABOUT TANGENTIAL LOCKINGS

Most writers endorse the idea of tangential lockings, and Saunier speaks of the escapes, which is not the case He defines the position of the pallet staff froth of the teeth; drawing the radial lines _A D_ and _A E_ to e the center of his pallet staff _C_ at the intersection of the lines _D C_ and _E C_, which are drawn at right angles to the radial lines _A D_ and _A E_, and tangential to the circle _t_

Here is an error; the lines defining the center of the pallet staff should have been drawn tangent to the circle _s_, which represents the locking angle of the teeth This would have placed the center of the pallet staff farther in, or closer to the wheel Any person can see at a glance that the pallets as delineated are not tangential in a true sense

[Illustration: Fig 85]

We have previously considered engaging friction and also repeatedly have spoken of tangential lockings, but will repeat the idea of tangential lockings at Fig 85 A tangential locking is neutral, or nearly so, as regards engaging friction For illustration we refer to Fig 85, where _A_ represents the center of an escape wheel We draw the radial lines _A y_ and _A z_ so that they erees of the arcs _s_ or _t_, which correspond to si 84, and represent the extrele of such teeth In fact, with the club-tooth escapement all that part of a tooth which extends beyond the line _s_ should be considered the saential locking made to coincide with the center of the impulse plane, as recommended by Saunier, would require the pallet staff to be located at _C'_ instead of _C_, as he draws it If the angle _k'_ of the tooth _k_ in Fig 84 was extended outward fro face of the entrance pallet as shown at Fig 84, then the draw of the locking angle would not be quite fifteen degrees; but it is evident no lock can take place until the angle _a_ of the entrance pallet has passed inside the circle _s_ We would say here that we have added the letters _s_ and _t_ to the original drawings, as we have frequently to refer to these circles, and without letters had no le _k'_ of the tooth can engage the pallet, as shown in Fig 84, the pallet ular rees We show the situation in the diagra the sa 84

[Illustration: Fig 86]

As drawn in Fig 84 the angle of draft _G a I_ is equal to fifteen degrees, but when brought in a position to act as shown at _G a' I'_, Fig 86, the draw is less even than twelve degrees The angle _C a I_ remains constant, as shown at _C a' I'_, but the relation to the radial _A G_ changes when the pallet le _'_, as itin the true sense of theset so that a pallet with its face coinciding with a radial line like _A G_ would be neutral, and the thrust of the tooth would be tangent to the circle described by the locking angle of the tooth Thus the center _C_, Fig 86, is placed on the line _hich is tangent to the circle _s_; said line _w'_ also being at right angles to the radial line _A G_

The facts are, the proble to the club-tooth lever escapement are very intricate and require very careful analysis, and without such care the horological student can very readily besuch problems, lead to no end of errors, and practical s lead to the popular phrase, ”Oh, such a ht in theory, but will not work in practice” We should always bear in ht, must lead practice_

CORRECT DRAWING REQUIRED

If we delineate our entrance pallet to have a draw of twelve degrees when in actual contact with the tooth, and then construct in exact confor the banks”