US2946183A

US2946183A - Self-starting magnetic escapement mechanisms

Info

Publication number: US2946183A
Application number: US589782A
Authority: US
Inventors: Clifford Cecil Frank
Original assignee: Horstmann Clifford Magnetics Ltd
Current assignee: Horstmann Clifford Magnetics Ltd
Priority date: 1955-06-14
Filing date: 1956-06-06
Publication date: 1960-07-26
Anticipated expiration: 1977-07-26

Description

July 26, 1960 c. F. CLIFFORD SELF-STARTING MAGNETIC ESCAPEMENT MECHANISMS Filed June 6, 1956 3 Sheets-Sheet 1 INVENTOR ATTORNEYS July 26, 1960 c. F. CLIFFORD 2,946,183

SELF-STARTING MAGNETIC ESCAPEMENT MECHANISMS Filed June 6, 1956 3 Sheets-Sheet 2 INVENTOR ATTORNE Y5 July 26, 1960 c. F. CLIFFORD 2,946,183

SELF-STARTING MAGNETIC ESCAPEMENT MECHANISMS Filed June 6, 1956 3 Sheets-Sheet 3 FIG. 7 (/voN- OPERATIVE) (/va/v- OPERATI r) INVE N TOR United States Patent 0 SELF-STARTING MAGNETIC ESCAPEMENT MECHANISMS Cecil Frank Clifford, Bath, England, assignor to Horstmann Clifford Magnetics Limited, a British company Filed June 6, 1956, Ser. No. 589,782

Claims priority, application Great Britain June 14, 1955 3 Claims. (Cl. 58-116) This invention relates to magnetic escapement mechanisms and is particularly, though not exclusively, applicable to such mechanisms as described in my United States Patent 2,690,646.

In magnetic escapement mechanisms, as distinct from the previously known mechanical escapements such as the English lever or Swiss cylindrical escapement, the complementary rotational and oscillatory movements are coupled by a magnetic loc usually between the escape wheel and the controlling oscillator instead of by mutual physical engagement of such parts. The parts providing such magnetic lock are an undulating magnetic track (continuous or interrupted) and complementary pole salients, the track and pole salients being arranged for relative longitudinal and transverse movement, the longitudinal movement (i.e. in the direction of the neutral axis of undulation of the magnetic track) being provided by rotation of the escape wheel and the transverse movement being provided by an oscillatory system having a natural frequency of oscillation, whereby, under normal working conditions, the magnetic pole salients relatively follow the undulations of the magnetic track and the said natural frequency of oscillation controls the speed of rotation.

In the more usual construction of a magnetic escapement, the magnetic pole salients are carried by the oscillator in jaw-like manner relatively embracing the magnetic track, and oscillatable transversely of the neutral axis of such magnetic track, the magnetic flux path between the pole salients passing through the track. A magnetic force, therefore, exists which tends to resist displacement of the magnetic track from between the pole salients in any manner which causes change of reluctance of the magnetic flux path. Such magnetic force prevents relative jumping movement of the pole salients across the gap between adjacent undulations of the magnetic track and therefore compels the pole salients to follow the undulations of the track as above explained. Such magnetic force is termed a magnetic lock.

One problem connected with magnetic escapements as above described is to make them self-starting, and the primary object of the present invention is the solution of such problem.

Experience has shown that it is comparatively easy to construct a magnetic escapement of the kind referred to which will work reliably under normal conditions, so that some problems, such as regulation of, and isochronous compensation for, the escapement have already been the subject of earlier applications for patent in connection for example, with spring-driven timepieces where it is necessary to provide regulating means to adjust the escapement for correct time-keeping, and

Patented Jul 26, 1960 to ensure isochronism within the normal amplitude of oscillations varying with the range of operating torque applied to the escape wheel by the spring between the fully wound and unwound conditions thereof.

Self-starting for magnetic escapements has not hitherto been satisfactorily obtained, the torque normally required to start rotation of the escape wheel having usually to be so great as to cause the escape wheel to accelerate too rapidly so that it does not come under the control of the oscillator. Self-starting, however is in itself, a separate problem, the object of the present invention, and for its comprehension it is necessary to consider the controlling factors. The expression self-starting is used herein to mean not merely the commencement of rotation of the escape wheel but the impulsing thereby of the oscillator in such manner that the rotation of the escape Wheel is automatically brought under the control of the oscillator for normal actuation of the escapement.

In the first place, the relative undulation of the magnetic track and the proportions and disposition of the pole salients relative thereto must be such that, when the oscillator comes to rest, with falling of the torque applied to the escape wheel below that necessary to maintain oscillation, the escape wheel must also be held at rest, just as with a mechanical escapement. Otherwise, the escape wheel could run-away at a speed uncontrolled by the oscillator. In other words, the magnetic lock which, under normal working conditions prevents the relative jumping of the track undulations by the pole salients inevitably provides a repose resistance (i.e. the resistance to movement of the escape wheel when the oscillator is stationary) which must be broken by escape Wheel torque before rotation can commence to make the escapement self-starting.

The only factor which can prevent run-away in a magnetic escapement is the magnetic lock, coupling the relative rotary and oscillatory movements, and this lock is provided by the hollow or notch of each track undulation and the air gap resistance provided thereby to prevent relative jumping of the track by the pole salients. At normal running amplitude the available air gap resistance will always far exceed the rupture force available from the escape wheel. On starting, however, the repose resistance which is provided in part at least by the magnetic lock has to be overcome to move the escape Wheel from one rest position to the next, i.e. the available magnetic lock must be broken. After initial rupture, and until normal running conditions are established, the magnetic lock provides only negligible resistance to acceleration of the escape wheel. This is because displacement or impulsing of the pole salients is at first negligible due to the wide divergence between the frequency of impulses from the escape wheel and the natural frequency of the oscillator. Myklestadt in his book entitled, Vibration Analysis has shown that on an oscillator such as the one with which we are concerned, there is no appreciable magnification factor, i.e. magnification of displacement of the oscillator, until the escape wheel has accelerated to a speed at which the impulses from the undulations of the track come within of the natural frequency of the oscillator. The magnification factor even at is only four times that of static deflection but thereafter it rises steeply until synchronism is reached. If the escape wheel accelerates beyond synchronisation, the magnification factor decreases even more steeply and at it falls below unit value.

Consequently, there is only a narrow range of speed of the escape wheel on each side of synchronised speed (hereinafter termed the critical impulsing speeds) during which the escape wheel accelerating after initial starting rupture of the magnetic lock can impulse the oscillator to an amplitude which will provide an air gap resistance of sufficient size to establish a controlling magnetic lock between the pole salients and the magnetic track. The only magnetically based possibility of a selfstarting magnetic escapement is therefore to have a low repose resistance so that a small torque will effect initial starting rupture of the magnetic lock and initiate movement with a resultant relatively slow acceleration of the escape wheel, and at the same time to have a high magnetic impulsing factor which will be a function of the magnetic lock, so as to induce a controlling amplitude of oscillation before the escape wheel can be accelerated too far through the critical speed aforesaid.

It is a further object of the present invention to obtain the longest possible running from a given available amount of driving energy, so that the relation between length of run and self-starting is clearly dependent also upon similar conditions which contribute towards reliable selfstarting.

Accordingly to the invention the pole salients and complementa'ry magnetic track of the magnetic escapement are so shaped and proportioned as to provide a low repose resistance with initial displacement of the oscillator, and the strength of the restoring force for the oscillator, the mass of the oscillator and the moment of inertia of the escape wheel are so proportioned that a torque applied to the escape wheel sufficient to overcome the repose resistance can accelerate the escape wheel to the critical impulsing speeds and whilst its speed of rotation is within the range of such speeds the magnetic track will induce an amplitude of oscillation of the pole salients sufficient to establish a controlling magnetic lock.

By the present invention the advantageous result of a self-starting escapement is achieved capable of reliable self-starting over a reasonably wide range of available starting torques.

If the starting torque is of the magnitude most frequently encountered in the normal operation of a spring motored clock, the effect of time on the power requirements is less significant, and self-starting calculations can be based upon the force theoretically transmitted from the escape Wheel to the oscillator during the initial movement of the escape wheel.

Of particular importance to the present invention, it was discovered that a very light escape wheel, having a very low moment of inertia, accelerated too rapidly through the critical impulsing speeds and did not allow time within the critical speeds aforesaid for inducing a controlling amplitude of vibration in an oscillator having its mass and the stillness of its mounting reed Within proportions generally recognized as suitable on account of other considerations. On the other hand, there are limitations to increasing the moment of inertia of the escape wheel or to decreasing the mass of the oscillator, and stiffness of the mounting reed.

A further important object of the present invention is the proportioning of the magnetic track and the pole salients to provide in effect a starting track adapted to produce a low repose resistance and an efiFective impulsing of the oscillator during the acceleration aforesaid of the escape wheel. The magnetic track is desirably so designed that the sinusoidal path of the projection of the pole salient may be in complete engagement with the magnetic track at any amplitude except small amplitude vibrations when there will only he partial engagement of the sinusoidalstarting track.

In the development of the present invention, it was discovered that the effectiveness of an oscillating magnet in controlling the acceleration of an escape wheel was influenced by the extent to which magnetic interenga'gem'ent between the salients and the magnetic track was maintained. The magnetic tracks of the present invention maintain continuous but sometimes incomplete magnetic engagement, i.e. the magnet poles overlap the magnetic track a little, not only when the mechanism operates at standard speed, but also when it accelerates from repose, as such displacement provides impulsing of the oscillator.

It 'was established that particularly beneficial results were achieved by providing a magnetic track having an imaginary non-undulating median path of a width constituting a significant portion of the width of the pole salients. Thus, if the pole salients were fixedly held at their centered position and if the escape wheel with its magnetic track were rotated past such fixedly held pole salients, the centre of'the magnet would always maintain magnetic interengagement with the centre of the said imaginary median path. H V V v In normal operation of the escapement mechanism of the present invention, the pole salients are not fixedly held at their centered position, but instead are displaced slightly toward the teeth on alternate sides of the magnetic track when the escape wheel begins to move. The progressive magnitude of such displacement has already been explained. A preferred embodiment of the present invention employs an imaginary starting track inscribed within the magnetic track and consisting of a slightly undulating wavy path having undulations which are several times smaller than half the full amplitude of the oscillator. 'The starting track may be defined as an imaginary track of substantially constant width, the boundaries of which are defined on alternate sides by the notches between the teeth of the magnetic track. Such a starting track would have its inner boundaries tangential to the boundaries of the median path aforesaid and such inner boundaries would not embrace all the fringing fiux on that side of the salient magnetic poles when in the repose position. Embodied within the starting track is the imaginary non-undulating median path aforesaid. The starting track consists'of said median path and marginal portions having starting apertures or notches alternately on either side of it, which starting apertures are the portions of the operating track forming the undulating magnetic track. A salient magnet pole projected over the entire median path and deflected sufiicie'ntly that the projection of one edge would coincide with an edge of the median path would be positioned with the projection of its opposite edge coinciding or nearly coinciding with the outermost edge of the undulating path, that is, just clearing the'aperture. In other words, the starting track which maybe imagined as having uniform width is usually slightly wider than the pole salient, and consists of the median path with successive undulations on opposite sides thereof. The crosswise distance of the starting track between the successive apices on the successive undulations is several times smaller than half the full amplitude of the oscillator. The starting track is adapted to s'ta'rt' the oscillator into oscillation from the repose condition and is characterised by a sufficiently small change of cross-sectional area of magnetic interengagement (i.e. suiiiciently small undulations) to provide low repose resistance to movement. Hence, moderate starting torques can break the magnetic lock to move the escape Wheel from repose to its first quarter-cycle position. The starting track also employs suificiently large undulations so that the magnetic shift imparts a sufficient force to the oscillator' to overcome the centering force and moment of inertia, of the oscillator, thereby initiating a very sm'all'v'ibration during the first cycle movement of the rotatable member. The controlling magnetic lock between the escape Wheel and the oscillator is dependent on the air gap energy at each undulation and is necessarily low for low amplitudes of oscillation owing "to the fiat characteristics of thestarting'wavy track, but such lock must transmit su-filcient energy to the oscillator to absorb some of the escape wheel energy which would otherwise be available for too rapid acceleration of the escape wheel and prevent re-building of the magnetic lock within the critical speed range above explained. Thus the mechanism prevents the escape wheel from racing or developing a speed so far beyond its normal or standard speed as to be beyond the regulatory control of the available magnetic lock which is re-established as synchronisation is reached.

The starting track has a band width which is usually slightly more than the width of the pole salient. The median path has a width normally slightly smaller than that of the pole salient but due to the eiiects of magnetic fringing a median path slightly wider than the pole salient can sometimes be effective in impulsing the oscillator.

In shifting from complete engagement with one starting tooth to complete engagement with another starting tooth during a half cycle operation, the magnetic pole salient is subjected to a calculable force. If the undulations are relatively large compared to the cross-sectional area of the pole salient, the locking action of the magnetic interengagement between the pole salient and the magnetic track prevents the initiation of rotation of the escape wheel unless an excessively high starting torque is applied. If the undulations are excessively small, the forces transmitted to the oscillator by the initial rotation of the escape wheel are insufiicient to impulse the oscillator, quickly enough to the regulatory amplitude within the critical speed of rotation, thereby permitting the escape wheel to develop sufiicient acceleration to attain a speed beyond the control of the oscillator.

The present escapements are self-starting when the escape wheel is first moved by any one of a Wide range of starting torques.

It is an object of the present invention to so control the size of the starting teeth or undulations relative to the cross-sectional area of the pole salients that the initial starting torque of the escape wheel, will be suiiicient to initiate and increase vibration of the pole salient during the relatively uncontrolled acceleration of the escape wheel particularly as its speed approaches the critical speed aforesaid, to an amount so at all times to transmit enough energy to the oscillator to prevent excess storage of enengy in the escape wheel beyond that controllable by the available magnetic lock as that becomes reestablished on attaining synchronisation of impulsing and oscillation.

It is an object of the present invention to so control the width of the median path that this adventageous result of self-starting is achieved.

In the accompanying drawings:

Fig. 1 is a perspective View of one example of a self-starting magnetic escapernent made in accordance with the present invention;

Fig. 2 shows an alternative embodiment of a self starting magnetic escapement according to the invention;

Fig. 3 is a schematic diagram in developed form from Fig. 1 showing the area relationships aliecting the selfstarting operation;

Fig. 4- is a schematic edge View related to Fig. 3;

Fig. 5 is a perspective view of an apparatus having a stationary magnetic track and made in accordance with the present invention;

Fig. 6 is a schematic view in developed form of the track and showing the successive positions of a pole salient in a self-starting operation embodying a magnetic track similar to Fig. 5

Fig. 7 is similar to Fig. 6 but shows a magnetic track which is inoperative and incapable of achieving self-starting because the small magnitude of a normal operating torque is insuflicient to overcome the repose resistance due to the magnetic lock attributable to the large starting teeth or undulations and narrow median path; and

Fig. 8 is similar to Figs. 6 and 7 but shows a magnetic track which is inoperative and incapable of achieving proper self-starting by reason of the small impulsing force imposed upon the pole salient due to the small magnetic locking action attributable to the small undulations and wide median path.

Referring now to the drawings, and particularly to Figs. 1 and 3, there is shown an oscillator 1. Advantages result from mounting the oscillator in such a manner that the axis of oscillation passes through the centre of gravity of the oscillator and in providing the oscillator with a centering force powerful enough to be shockresistant and to operate uniformly without regard to the position of the oscillator with respect to gravitation, as explained in said United States Patent 2,690,646.

An escape wheel 3 is mounted for rotation so that it is influenced by pole salients 5 of the oscillator mag net 1, which is carried by a flexible reed 1a secured to a post lb of non-magnetic material at the end of a bracket is of non-magnetic material. The escape wheel 3 is provided with outwardly extending magnetic teeth 8 and with inwardly extending teeth 9.

Particular attention is called to an imaginary undulating starting track 18 shown dotted having a path width slightly greater than the width of the pole salients 5 as explained below in more detail. The starting track 18 is a slightly undulating path which is adapted to start the pole salients to oscillate from their reposed condition upon the initial movement of the escape wheel. The overall width of the starting track 18 between the apices on opposite sides of the undulating path is such that it constitutes only a small portion of half the full amplitude of the oscillator. The maximum intended amplitude of the oscillator is less than that which would result if the magnet vibrated to a position at the extremity or" the magnetic teeth 8. Thus the distance which corresponds to half the full amplitude of the standard oscillation of the mag at is several times greater than the full width of the starting track. An imaginary median path 26 of a non-undulating nature is shown inscribed Within the starting track.

it will be noted that the median path 20 is of smaller width than the overall width of the starting track. Moreover, the median path 20 is narrower than the width of the magnet pole salients 5 by an extent which corresponds to the width of a starting tooth or overlap of an aperture. The centered salient with its magnetic fringe overhangs the median path on each side by one half the radial width of a starting tooth.

In Fig. 2, there is shown another embodiment of a selfstarting magnetic escapement mechanism. An oscillator 2G1 is mounted so that the axis of oscillation passes through the centre of gravity of the oscillator. Pole salients 2G5, 295 are in magnetic engagement with a dual-escape wheel 263 provided with outer and inner magnetic teeth 2.08 and 299 respectively. During the normal operation 0: the magnetic escapement, the escape wheel 233 is permitted to rotate only at a speed controlled by the natural period of vibration of the pole salients 2&5 by reason of the magnetic lock between the pole salients 2% and the undulating operating track 219. When the magnetic escapement mechanism of Fig. 2 is at the repose position, as shown in Fig. 2, and the escape wheel is given an initial rotary movement, the magnet pole salients 205 are urged into a small amplitude vibration by the imaginary starting track 218. The progressive nature of such urge and the negligible magnitude of displacement until the normal or critical speed is approached has been fully explained above. Inscribed within the starting track 218 is an imaginary non-undulating median path 220 having a width which is narrower than the pole salients 23:; by the extent of starting teeth of the starting track 213.

An embodiment of the invention particularly suited for a schematic illustration of the self-starting principles is shown in Fig. 5. Magnetic pole salients 5% of an oscillator 501 are in magnetic engagement with a stationary magnetic track member 521. The magnetic track member 521 is a cylindrical ring constructed of soft iron or other magnetic pole salients 505 oscillate up and down, i.e.

parallelto the vertical axis. The pole salients 595 are on opposite sidesof the stationary magnetic track 521. The braking force attributable to. the magnetic lock between the pole salients 505 and the magnetic track 521 is sufficient that when the escape wheel is operating at its predetermined speed, consistent with thenormal period of vibration of the oscillator, and the relationship thereto of the magnetic track 521, effective speed control is achieved. Because of such magnetic lock, great power is required to accelerate the escape wheel beyond its standard speed. When no force is applied to the escape wheel, it continues to rotate at its standard speed until the amplitude of the oscillator becomes too small to allow the momentum of the wheel to carry it past the magnetic lock of the next tooth of the starting track. The oscillator is thus capable of controlling the speed of an escape wheel, by reason of the magnitude of the forces of the magnetic lock between the pole salients and the magnetic track. The power necessary to accelerate the escape wheel from its predetermined speed is much greater than the power required to operate theescape wheel at its predetermined speed because such higher speeds are attainable only by overcoming the magnetic lock between the vibrating pole salients and the magnetic track, i.e. forcing relative jumping by the pole salients of the air gap between adjacent teeth. Obviously for larger amplitudes the available air gap reluctance is larger.

In Fig. 6 there is a schematic diagram in developed form showing a portion of a magnetic track member 621 similar to the member 521 of Fig. but as if of infinite diameter. The pole salients 605 at repose maintain a position which is substantially centred on the magnetic track 621, as indicated at 605a, thus being centred over the median path 620. When the escape wheel is rotated counter-clockwise by an initial starting torque of suitable power, the magnet is moved to the right according to Fig. 6 thereby bringing into action a starting tooth 623. If there is sufficient power to accelerate the escape wheel from the repose condition, there is suflicient power to deflect the pole salient a small amount from its reposed position. If the starting tooth 623 is of a size which is neither too large nor too small relative to the width of the pole salient, then the initial power for deflecting the magnet can be suflicient, While the escape wheel speed is within the critical impulsing speeds, to re-establish the magnetic lock after it has been broken for starting rotation of the escape wheel.

The deflecting force imposed on the pole salient by the quarter cycle movement above described of the escape wheel is a function of the strength of the magnetic field, the permeability of the magnetic track, the distance between the magnetic poles, the related factors. However, assuming that the only variable concerns the undulating shape of the magnetic track and the relative shape and size of the pole salients, certain principles are applicable. Thus, the starting teeth or undulations must be of suflicient size that the pole salient is urged to deflect a small amount during the first quarter-cycle of movement. In some of the magnetic escapements heretofore available, difliculty has been encountered by reason of a repose resistance. aforesaid. If the median path has Zero width, so that the starting teeth are as wide as one-half of the width of the pole salient, a relatively strong deflecting force is imposed on the magnet in moving from a reposed position to a quarter-cycle position but a relatively stronger torque is required for the rupture of the magnetic lock to enable the rotation of the escape wheel to commence. The escape wheel thus can be locked at repose if the initial starting torque is too small or can require a starting torque so great as to produce excessive acceleration of the escape wheel after initial rupture of the magnetic lock. Suchan inoperable condition is diagrammatically shown in Fig. 7, in which the starting teeth 723 are relatively large, and in which the median path 720 is less than of the width of the pole salient 705.

In Fig. 8 there is shown a schematic diagram of a magnetic track 819 having a median path 820 which is about twice the width of the pole salients thereby rendering the starting teeth 823 nonefiective from repose. The starting torque required to break the magnetic lock will be negligible and the impulsing force imparted to the pole salient 805 during the initial rotation of the escape wheel will be insufiicient to establish controlling vibration of the magnet, thereby permitting the starting torque on the escape wheel to accelerate the escape wheel to a speed beyond that for which it was intended, and after which there is negligible impulsing of the oscillator at its naturalfrequency and, hence, little energy is spent and the oscillator is ineffective restraining and regulating the speed of the escape wheel.

In the development of the present invention, it was establishedthat the width of the median track should be at IeasL30% and not more than 120% of the width of the pole salient in order to provide a self-starting escapement.

Data relating to the area relationships are shown in Figs. 3 and 4. Thus the median path GH is narrower than the width of the pole salient AD. The magnet salient ABCD is shown in an unstable position of equilibrium for the escape wheel where it has a maximum downward urge, but the resultant displacement is so small that it is shown centered over the median path GH. In moving from such position to an intermediate repose position the escape wheel would go through a quarter-cycle rotation and the pole salients would take up a position between two teeth one on one side and one on the other side where.

a larger portion of the track is covered by a projection of the pole salients. The change of projection (in making this movement) of the centered salient ABCD onto the magnetic track is proportional to the area ABEF, assuming that the magnet is fixedly held in centered position. In normal operation, however, the rotation would initiate the deflection of the magnet away from the aperture between its jaws. In determining the amount of force necessary to move the escape wheel during the first quarter-cycle of the operation, however, it is convenient to consider only the forces involved in the magnetic lock, assuming that none of the force is utilised for the deflection of the magnet, which in any case is negligible for the initial acceleration of the escape wheel. On this basis, the starting torque must be suflicientto bring about a change in the area ABEF of the flux member in the magnetic field.

In Fig. 4, the thickness of the escape wheel MM and the distance AA (or BB) between the pole salients are diagrammatically represented. These distances are signifi cant because they affect the energy necessary to bring about a reduction of the area of the flux member in the magnetic field. Expressing the change of area (a) in square centimeters, the energy (e) in crgs, the thickness (t) of the escape wheel in centimeters, and the intensity of the magnetic field (B) inlines of force per square centimeter or gauss, the following equation shows the relationships:

in tB As an example of the calculations, reference can be made to estimating the appropriate change of area of projection of a magnetic track on a centered pole salient in moving from a repose position to a quarter-cycle position. If the escape wheel had a diameter of 1 centimeter, and if it revolved revolution per quarter cycle, and if the initial torque (T) was 20 dyne cms., the energy (e) would be 11' crgs, as indicated.

Thus, the area difference would be about 0.0126 mm.

If, in the pole salient ABCD, the distance AB was about 0.5000 mm. and the distance AD was about 0.4500 mm. the distance AF would be about 0.0250 mm. in order to achieve the area (a) which could be modified by the available 3.1416 dyne cms. of initial torque on the escape wheel and, hence 3.1416 ergs of energy in moving ,4 revolution. Such a torque could rupture the magnetic lock in moving the escape wheel from a reposed position to a quarter-cycle position adjacent a tooth as shown in Fig. 3. If the initial starting torque was only half of that employed in the calculations, then the repose resistance would be sufficient to prevent movement of the escape wheel. If the change in area ABEF were twice as great as calculated, then twice as much initial torque would be necessary for overcoming the repose resistance. However, if the area ABEF were eight times that of the numerical example, then the median path would be so narrowed that the advantageous magnetic track of the present invention would not be attained. Only if the median path is at least 30% of the width of the pole salient, and not more than 120% of such width, can reliable self-starting be achieved.

It will be noted that the midpoints of the successive teeth are spaced longitudinally from each other a distance corresponding to several times the length of the pole salient, whereby the centering forces of the oscillator can urge the displaced pole salient toward the center before the starting track can advance to the center of the next tooth. The starting track is not only of sufiiciently small amplitude, but is also sutficiently elongated to assure initial vibration of the oscillator during the acceleration of the escape wheel and to assure the establishment of a controlling amplitude of vibration by the escape wheel as it.

reaches the critical speeds aforesaid.

In one example of the invention the following are the essential details for a spring driven clock:

(1) The oscillatory system has a weight of 0.3 grms. approximately and a natural frequency of 100 cycles per second.

(2) The supporting spring reed for the oscillatory system has a rating of 20 grs. per mm. displacement of the poles.

(3) The magnet in open circuit between the pole salients has a strength of approximately 4,000 lines per sq. cm.

(4) The magnet gap is 0.5 mm. Wide with an escape wheel (of Mumetal) 0.25 mm. thick, hence there is an air gap of 0.125 mm. between each magnet pole and the escape wheel.

(5) The moment of inertia of the rotary system is 0.1 X gm. cm. sec.

(6) The median path has a width of 0.2 mm.

(7) The thickness of the magnet is 0.35 mm.

(8) Hence the ratio of the median path to pole thickness is A magnetic escapement having the essential details above given was found to require a torque of .05 mm. grs. to start rotation of the escape Wheel and provided good self-starting for a spring-driven clock-Work mechanism designed to give a maximum torque of 0.2 mm. grs. falling to about 0.1 mm. grs. over a running period of 24 hours. The escapement was in fact capable of selfstarting with a torque as low as .05 mm. grs. and as high as 0.22 mm. grs.

i 0 A slow motion film has beentaken of an experimental escapement, the analysis of which is as follows:

Percent of Percent of proper Time Teeth Portions Revs. per speed speed, (secs) moved of revs. see. final corrected for film speed error 0 0 0 0 0 .131 )4 Mo .19 3.8 4.2 .154 1 A0 1.08 21.6 24 .l78 2 M0 208 41.6 46 .2l7 4 it 2.56 51. 2 56 .244"--. 6 340 3.7 74 81 .2680. 8 340 4.16 83.2 92 .290 10 $5 4. 55 91 400 20 1 4. 55 91 100 At .290 seconds, /2 rev. movement, there was still no visible amplitude of the magnet. At .400 seconds, one rev. movement, the amplitude was 0.010 inches or one third of the normal amplitude (.030 inch).

The above analysis shows that, until the rotor reaches substantially its normal speed after half a revolution, there is no appreciable impulsing of the oscillator, but that by that time and thereafter the impulsing must have been sufficient to establish a magnetic lock between the rotor and oscillator as the rotor ceases to accelerate while the amplitude of oscillation builds up to normal.

I claim:

1. A self-starting escapement comprising a rotatable member, an oscillator member urged toward a centered position by forces substantially proportional to the amplitude of displacement from the centered position, mag netic pole salients carried by one of said members, a magnetic track presented by the other of said members, means associating the pole salients and said magnetic track to regulate the standard operating speed of the rotatable member by the normal period of vibration of the oscillator member, said magnetic track comprising a median path constituting a significant portion of the width of the pole salients and comprising an undulating starting track whereby the power necessary to move the rotatable member from a stationary magnetically locked position is significantly less than the power exerted by the quarter cycle rotation of the rotatable member relatively to the oscillator by reason of the starting track providing a deviation of the magnetic track from the median path between points on the magnetic track corresponding to such quarter cycle rotation, the path of magnetic interengagement including said median path and said other member having staggered teeth on either side thereof, the spaces between which provide apertures, said undulating starting track including small starting undulations on opposite sides of the median path of the magnetic track to initiate small amplitude displacement of the oscillator first to one side and then to the other While the rotatable member initially rotates an amount corresponding to one normal cycle of vibration of the oscillator, and said magnetic track providing at successive oppositely disposed teeth a shift of projection from the fixedly centered pole salient upon the magnetic track from one side to the other of the median path more than 1% but less than 70% of the Width of the pole salient.

2. A self-starting escapement comprising a rotatable member, an oscillator member urged toward a centered position by forces substantially proportional to the amplitude of displacement from the centered position, magnetic pole salients carried by one of said members, a magnetic track presented by the other of said members, means associating the pole salients and said magnetic track to regulate the standard operating speed of the rotatable member by the normal period of vibration of the oscillator member, said magnetic track comprising a median path constituting a significant portion of the width of the pole salients and comprising an undulating starting track whereby the power necessary to move the rotatable member from a stationary magnetically locked position is significantly less than the power exerted by the quarter cycle rotation of the rotatable member "rela tively to the oscillator by reason of the starting track providing a deviation of the magnetic track from the median path between points on the magnetic track corresponding to such quarter cycle rotation, the path of magnetic interengagement including said median path and said other member having staggered teeth on'either side thereof, the spaces between which provide apertures, said undulating starting track including small starting undulations on opposite sides of the median path of the magnetic track to initiate small amplitude displacement of the oscillator first to one side and then to the other while the rotatable member initially rotates an amount corresponding to one normal cycle of vibration of the oscillator, and the path of a centered pole salient on the oscillator, at positions corresponding to successive oppo- 12 sitely disposed teeth, projecting on the median path portion and a starting tooth portion, said starting tooth portion constituting at least 1% but not more than 50% of the area of the pole salient. I

3. An escapement according to claim 2 in which the median path of the track is 30% to 90% of the width of the pole salient whereby the pole is held always in magnetic engagement with the track and tends to vibrate when the rotatable member rotates an amount corre- 10 sponding to two teeth on the magnetic track.

References Cited in the file of this patent UNITED STATES PATENTS 2,654,989 Nichols a Oct. 13, 1953 2,690,646 Clifford a Oct. 5, 1954 FOREIGN PATENTS 183,020 Austria Jan. 15, 1955