US2847534A

US2847534A - Stepping switch

Info

Publication number: US2847534A
Application number: US496505A
Authority: US
Inventors: Frank R Edgarton
Original assignee: General Dynamics Corp
Current assignee: General Dynamics Corp
Priority date: 1955-03-24
Filing date: 1955-03-24
Publication date: 1958-08-12
Anticipated expiration: 1975-08-12

Description

F. R. EDGARTON Aug. 12, 1958 STEPPING SWITCH 3 Sheets-Sheet 1 Filed March 24, 1955 V a mm mm g on INVENTOR FRANK R. EDGARTON ATTORNEY 12, 1953 F. R. EDGARTON 2,847,534

STEPPING SWITCH Filed March 24, 1955 3 Sheets-Sheet 3 v INVEN TOR.

FRANK R. EDGARTON ATTORNEY STEPPIN G SWITCH Frank R. Edgarton, Webster, N. Y., assignor, by mesne assignments, to General Dynamics Corporation, a corporation of Delaware Application March 24, 1955, Serial No. 496,505 9 Claims. (01.200-105 The present invention relates to ratchet drive mechanisms, and more particularly to ratchet drive mechanisms for operating electric stepping switches or the like.

Electromagnetically operated stepping switches have used various types of ratchet drive mechanisms in order to impart step-wise movement to the switch contact brush. One particular form of an electromagnetically driven stepping switch is referred to as a minor switch and is provided with a ratchet drive mechanism operated by a stepping magnet for stepping the switch contact brush, or brushes, in one directionfrom a normal starting position to a total possible movement of approximately eleven equal and successive steps. The ratchet drive mechanism includes a spring which tends to move the ratchet mechanism and switch contact brush back in a second direction when it is desired to release the. switch and restore it to its normal starting position. A conventional ratchet retainingpawl normallyengages the ratchet teeth to prevent movement of the ratchet and switch brushes in the second direction back to thenormal position and a release magnet is provided, to disengage the retaining pawl from the ratchet teeth when it is desired to return the switch to normal. .Such an arrangement does not provide for returning the switch brush and ratchet mechanism to the normal position by a series of steps.

It is an object of the present invention to provide an improved ratchet driving mechanism operable for stepping movements in either of two selected directions and connected to drive an electric switch brush contact, or contacts, in accordance with the ratchet stepping movements.

Another object of the present invention is to provide an improved ratchet drive mechanismhaving a simplified single driving pawl arrangement for stepping the ratchet mechanism in either of two selected directions.

The invention, in its basic concept, consists of a ratchet member movable against the tension of a ratchet return spring. A driving pawl is movable towardsthe ratchet member in a manner to engage a ratchet tooth andthereby step-wise move the ratchet member in a first direction against the tension of the ratchet return spring. A retaining pawl is. provided to normally engage the ratchet member teeth and prevent movement of the ratchet ,memher in a second direction, as a return movement, for example, in compliance with the tension of the return spring. According to the invention, a ratchet shift member is provided to engage the ratchet driving pawl in a manner to cause the ratchet drive pawl to engage the retaining pawl instead of the ratchetteeth when thedrive pawl is th reafter moved towards the ratchet member. In such manner, the ratchet member is allowed to move in the second direction in compliance with the tension of the return spring so long as the retaining pawlis disengaged from the ratchet teeth by reason of the engagement of the drive pawl with the retaining pawl during the movement of the drive pawl towards the ratchet member.

United States Patent 2,847,534 Patented Aug. 12, 1 7958 In the particular embodiment of the invention to be described, a preferred arrangement includes an escapement cog wheel and an escapement lever actuated simultaneously with the movement of the drive pawl to limit the motion of the ratchet mechanism in either the first or the second direction to an incremental amount for each successive movement of the drive pawl towards the ratchet member while either in the normal ratchet driving position or in the shifted ratchet releasing position.

A further feature of the invention, in the preferred form to be described, includes a ratchet shift control mechanism arranged to actuate the previously described shift mechanism whenever the ratchet mechanism reaches a predetermined driven position. For example, after the ratchet mechanism has been moved in the first direction by a predetermined number of successive ratchet stepping movements, the shift control mechanism operates the shift mechanism to cause the next series of successive movements of the drive pawlto disengage the retaining pawl from the ratchet teeth and thereby produce a series of ratchet stepping movements in the second or return direction. After a similar movement of successive return ratchet stepping movements, a shift control mechanism is again operable to return the ratchet shifting mechanism to the normal position causing the next seriesv of successive ratchet steps to be again produced in the first direction by reason of the engagement of the driving pawl with the ratchet teeth.

Further objects, features, and the attending advantages of the invention will be apparent with reference to the following specification and drawings in which:

Fig. 1 is a perspective elevation of the ratchet mechani'sm of the invention as applied to a stepping switch assembly;

. Fig. 2 is, an exploded view showing the details of the essential elements of the ratchet drive mechanism ofthe invention;

Fig. 3 is a detailed view of the ratchet mechanism with the drive pawl in a position about to engage the ratchet toothford'riving the ratchet member in a first direction;

Fig. 4 is a view similar to Fig. 3 but showing the drive pawl at the moment of engaging the ratchet tooth;

Fig. 5 is a view similar to Fig. 3 but with the drive pawl shifted to a position about to engage the retaining pawl; and

I Fig.' 6 is a view similar to Fig. 5 but showing the drive pawl in the position of engaging the retaining pawl to allow, a stepping movement of the ratchet member in the second direction.

' Referring now to Figs. 1 and 2 of the drawings for a general description of the invention, the ratchet drive mechanism and switch contact brushes are mounted on a suitable base or frame member 10'. A pivot post 11 is secured to extend perpendicularly from the base 10, as. shown. A ratchet member 12, in the preferred form of. the invention being described, is actually a rotary ratchet wheel rotatably journaled on the pivot post 11, and is provided with a depending shank portion 16 to which is secured an escapement cog wheel 13 and a pinion gear 14. A helical spring 15 surrounds the lower end 16 of the depending portion of the ratchet gear 12. Oneend 17 of the spring 15 is adjustably secured by means of the adjusting lock washer 18 and clip19 to the base member 10. The other end 20 of the spring 15 is secured to the depending shank 16 of the ratchet gear 12 in any suitable manner such that rotation of the ratchet gear 12 in the counterclockwise direction will tendto wind the spring 15. The spring 15 is wound to an initial tension by an adjustment of the position of the lock washer 18 such that the ratchet gear 12 would tend to move in the clockwise direction to a normal or starting position whenever the retaining pawl, to be described, is disengaged from the ratchet gear teeth.

A drive pawl is pivotally journaled at 26 on a driving lever arm 27. in the preferred embodiment of the invention, the driving lever arm 27 is an extension of the armature 28 of an electromagnet 29. The driving armature 28 is pivotally supported at 38 and is normally urged to an outer position relative to the magnet coil 29 by means of the coil spring 31, as is conventional. It should be mentioned that other means of actuating the ratchet drive pawl 25 may be used in place of the electromagnet 29, described above. Whenet er the electromagnet 29 is energized, the armature 28 is attracted to move the drive arm 27 and the driving pawl 25 towards the teeth of the ratchet gear 12 to impart a single stepping movement to the ratchet stepping mechanism, which will be later described in detail.

A second pivot post is fixed to the base member 10 to extend perpendicularly therefrom and at a distance spaced relative to the previously described pivot post 11. A ratchet retaining pawl 36 is pivotally journaled on the pivot post 35 and normally urged by the spring 37 to engage the teeth of the ratchet gear 12 and thereby prevent clockwise movement of the ratchet gear 12. The coil spring 37 has a lower end 38 fixed to the base member 10 in any suitable manner, and an upper end 39 received in the notch 40 of the pivoted retaining pawl 36 for purposes of resiliently urging the retaining pawl 36 into engagement with the teeth of the ratchet gear 12.

The escapement cog 13 is provided with a plurality of spaced cog teeth, such as those shown at and 46. The driving lever arm 27 is provided with an extended end portion 47 to form an escapement lever for engaging the successive ones of the

escapement cog teeth

45, 46, etc., whenever the drive arm 27 and drive pawl 25 are moved toward the ratchet gear 12. It will be later described in detail how the escapement lever 47 engages the successive escapement cog teeth to limit the stepping movements of the ratchet gear 12 in either direction of movement of the ratchet member and escapement cog.

A ratchet shift lever 50 is also rotatably journaled on the pivot post 35. The shift lever 50 is provided with a lower control arm extension, generally shown at 51, to which are fastened

coil springs

52 and 53 in an over-center arrangement for maintaining the shift arm 58 in a selected one of two rotatable positions about the pivoted post 35. With the shift lever 50 in the position shown in Fig. 1 of the drawings, the depending end 54 of the shift lever 50 contacts the drive pawl 25 to position the drive pawl, which is pivoted at 26 on the drive arm 27, in a manner to engage the teeth of the ratchet gear 12 when advancing toward the ratchet gear. A coil spring 55 is fastened between the upper end of the drive pawl 25 and the drive arm 27 to normally pivot the drive pawl 25 in a counterclockwise direction to the position as shown in Fig. 1 of the drawings. When the shift lever 50 is moved in a counterclockwise direction from the position shown in Figure 1 of the drawings, the drive pawl 25 will be moved in a clockwise direction about its pivot 26 to a position for engaging a shoulder 56 of the retaining pawl 36, as shown by the dotted lines of Fig. 2 of the drawings. Thus, with the shift lever 50 moved to the counterclockwise position, as described, driving movements of the drive pawl 25 toward the ratchet gear 12 will cause the drive pawl 25 to contact the retaining pawl 36 at the shoulder 56 instead of the teeth of the ratchet gear 12 thus disengaging the retaining pawl 36 from the ratchet gear teeth and allowing clockwise motion of the ratchet gear 12 in response to the wound tension of the helical spring 15.

The shift lever 50 may be operated in any suitable manner at any time when it is desired to shift the operation of the ratchet drive mechanism. In the preferred form of the invention being described, the shift .4 lever 50 may be operated by either or both of the following two described mechanisms.

The first shift control mechanism to be described is comprised of a magnet coil 60 having a pivoted armature 61 with a free end 62 engaging 21 depending portion 63 of the shift lever 50. The mechanical moment in the arrangement shown is such that the energization of the magnet coil 60 to attract the armature 61 will produce clockwise rotation of the shift lever 50 to the position shown in Figure 1 of the drawings.

In order to move the shift lever 50 in a clockwise direction, a second shift control mechanism including a cam and roller is provided. The cam and roller mechanism consists of

cam surfaces

64 and 65 on the lower end 51 of the shift lever 50 against which is slidably received the cam roller 66 of a slidably movable control arm 67. When the control arm 67 is moved toward the bottom of the drawing shown in Fig. 2, the cam roller 66 is caused to ride out of the cam groove 64 into the cam groove 65 causing the clockwise movement of the shift lever 50. On the other hand, when the control arm 67 is moved upward from the bottom of the drawing as shown in Figure 2, the cam roller 66 moves from the cam surface 65 into the cam surface 64 permitting the

springs

52 and 53 to move the shift lever 50 in the counterclockwise direction.

A rack gear 68 is slidably journaled on an overhead support rod 69, which is in turn supported on the upstanding flanges 70 and 71 secured to the base 10. The rack gear 68 is provided with gear teeth engaging the drive pinion 14 so that as the ratchet gear 12 is driven with a step-wise movement in either direction the rack gear 68 is correspondingly moved in either direction along the overhead support rod 69. One end of the rack gear 68 is journaled on the overhead support rod 69 at 72 in a position extending between opposing

gears

73 and 74 of the control lever 67. This arrangement is such that as the rack gear 68 reaches its lower limit of travel, the ear 73 of the control lever 67 is engaged to move the control lever 67 and cam roller 66 into engagement with the cam surface 65, thus pivoting the shift lever 50 in a clockwise direction and allowing the drive pawl 25 to pivot to a position for engaging the ratchet gear teeth. Subsequent driving movements of the drive pawl 25 will cause counterclockwise rotation of the ratchet gear 12 and advance the gear rack 68 in the direction away from the lower portion of the drawings. When the rack gear 68 reaches the upper limit of its travel, the ear 74 of the control arm 67 will be engaged to move the cam roller 66 back into the cam surface 64, thus pivoting the shift lever 50 in a counterclockwise direction and moving the drive pawl 25 to a position for subsequently engaging the retaining pawl 36 in subsequent driving movements. Thereafter, the ratchet gear will be step-wise rotated in a clockwise direction during each driving movement of the drive pawl 25 to again move the gear rack 68 toward the lower end of the drawings as shown in Figure 2.

During the movement of the drive pawl 25 into engagement with either the teeth of the ratchet gear 12 or the shoulder 56 of the retaining pawl 36, the escapement lever 47 is moved into a position for engaging a tooth of the cog wheel 13, thus limiting the rotation of the ratchet gear 12 in either direction to a predetermined amount as required for the particular stepping movement to be obtained. The gear rack 68 may carry a plurality of movable switch contact brushes such as shown at 75- 78. Each of the switch brushes 7578 may engage successive ones of a plurality of bare wire contacts such as the one indicated on Figure l of the drawings at 80. Each of the bare wire contacts is spaced from another bare wire contact by an amount equal to the movement of the gear rack 68 by each stepping movement of the ratchet drive mechanism, as described.

Reference will now be made to Figures 36 of asamsss re n the drawings for a more detailed description of the operation of the ratchet drive mechanism of the invention. In Fig. 3 of the drawings, the drive arm 27' and drive pawl 25 are shown in the position about to engage a tooth of the ratchet gear 12 when the drive pawl 25 s moved towards the ratchet gear 12. The shift lever 50 1s shown in its extreme clockwise position with the shift control arm 67 in the lower position and the gear rack 68 also in the extreme lower position. Thus, the parts are shown in the position at the moment of shift of the shift lever 50 to the extreme clockwise position. In Fig. 4 of the drawings, the drive arm 27 is shown in the pos1tion as attracted by the magnet coil 29 with the drive pawl 25 engaging a tooth of the ratchet gear 12 and the escapement lever 47 contacting a teeth 46 of the escapement cog 13. Thus the gear rack 68 is moved upward by an incremental stepping amount as determined by the geometric arrangemente of the rack gear 12 and the escapement cog 46. The actual position of the parts shown in Fig. 4 of the drawings is a position which assumes that the rack gear 68 has already been stepping upward a number of successive steps from the position shown in Figure 3 of the drawings.

When the rack gear 68 has been stepped a predetermined number of steps upward to the position shown n Figure 5 of the drawings, the shift control lever 67 s moved upward, moving cam roller 66 towards cam surface 64, allowing spring 53 to pivot the shift lever 50 in a counterclockwise direction thereby pivoting the drive pawl 25 to the position shown where it will engage the shoulder 56 of the retaining pawl 36 on subsequent driving movements of the drive arms 27. Figure 6 of the drawings shows the parts intheposition assumed when the drive arms 27 is attracted by the magnet 29, while the shift arm 50 and drive pawl 25 are in the position to disengage the ratchet retaining pawl 36 from the teeth of the ratchet gear 12. The clockwise movement of the ratchet gear 12, in response to the wound tension of the helical spring while the retaining pawl 36 is disengaged from the teeth of the ratchet gear 12, is limited by engagement of the escapement lever 47 with the cog tooth 45. Again it is assumed anumber of successive stepping movements of the ratchet gear 12 in the clockwise direction have been performed to move the gear rack 68 to the intermediate position shown by Figure 6 of the drawings.

While the ratchet drive mechanism of the invention has been specifically described in connection with a preferred embodiment employing a rotatable ratchet gear, it should be understood that the principles of the inven tion may be applied to a ratchet drive mechanism of the type embodying a linearly moved ratchet element or gear. In such an arrangement, of course, the helical spring 15 would be replaced with a coil spring or other suitable energy storing device for urging the ratchet element to move in one direction opposite to the driven direction as provided by the positively actuated driving pawl. It should also be apparent that the particular type of control mechanism for actuating the driving pawl shift lever is a preferred form and that other means for moving the shift lever such as an electromagnet or the like may be provided, as would occur to anyone skilled in the art, within the spirit of the invention. Various modifications may be made within the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. A stepping switch comprising, a movable switch contact, a ratchet gear, means interconnecting said contact and said gear for moving said contact as said gear is moved, an escapement cog having a plurality of spaced cog teeth on its periphery, said gear and said cog being secured together, means including a spring and disposed for tensioning said spring as said gear and cog assembly is moved in a first direction and to urge movement of said gear and cog assembly in a second direction, a drive a pawl, a retaining pawl, resilient means to urge said retaining pawl into'engagement with the toothed surface of said ratchet gear to thereby prevent second direction motion of said gear, means for moving said drive pawl to and from said ratchet gear to engage a ratchet tooth and impart first direction motion to said gear and tension said spring, escapement lever means disposed for engaging a cog tooth of said cog during first direction movement of said gear to thereby limit the first direction movement of said gear for each movement of said drive pawl towards said ratchet gear, and shift means movable from a normal position to a second position for engaging and moving said drive pawl to a position for engaging said retaining pawl as said drive pawl is moved towards said ratchet gear and thereby disengage said retaining pawl from the toothed surface of said ratchet gear to permit said gear to move in the second direction by the tension of said spring, said escapement lever means being disposed for engaging a cog tooth of said cog during second direction movement of said gear to thereby limit the amount of second direction movement of said gear for each movement of said drive pawl into engagement with said retaining pawl.

2. A stepping switch comprising, a movable switch contact, a ratchet gear, means interconnecting said contact and said gear for moving said contact as said gear is moved, means including a spring and disposed for tensioning said spring as said gear is moved in a first direction and to urge movement of said gear in a second direction, a drive pawl, a retaining pawl, resilient means to urge said retaining pawl into engagement with the toothed surface of said ratchet gear to thereby prevent second direction motion of said gear, means for moving said drive pawl to and from said ratchet gear to engage a ratchet tooth and impart first direction motion to said gear and tension said spring, shift means movable from a normal position to a second position for engaging and moving said drive pawl to a position for engaging said retaining pawl as said drive pawl is moved towards said ratchet gear and thereby disengage said retaining pawl from the toothed surface of said ratchet gear to permit said gear to move in the second direction by the tension of said spring, and shift control means for moving said shift means to the second position in response to said ratchet gear being moved in said first direction to a first predetermined position and for moving said shift means to the normal position in response to said ratchet gear being moved in said second direction to a second predetermined position.

3. A stepping switch comprising, a movable switch contact, a ratchet gear, means interconnecting said contact and said gear for moving said contact as said gear is moved, an escapement cog having a plurality of spaced cog teeth on its periphery, said gear and said, cog being secured together, means including a spring and disposed for tensioning said spring as said gear and cog assembly is moved in a first direction and to urge movement of said gear and cog assembly in a second direction, a drive pawl, a retaining pawl, resilient means to urge said retaining pawl into engagement with the toothed surface of said ratchet gear to thereby prevent second direction motion of said gear, means for moving said drive pawl to and from said ratchet gear to engage a ratchet tooth and impart first direction motion to said gear and tension said spring, escapement lever means disposed for engaging a cog tooth of said cog during first direction movement of said gear to thereby limit the first direction movement vof said gear for each movement of said drive pawl to wards said ratchet gear, shift means movable from a normal position to a second position for engaging and moving said drive pawl to a position for engaging said retaining pawl as said drive pawl is moved towards said ratchet gear and thereby disengage said retaining pawl from the toothed surface of said ratchet gear to per mit said gear to move in the second direction by the tension of said spring, and shift control means for moving said shift means to the second position in response to said ratchet gear being moved in said first direction to a first predetermined position and for moving said shift means to the normal position in response to said ratchet gear being moved in said second direction to a second predetermined position, said escapement lever means being disposed for engaging a cog tooth of said cog during second direction movement of said gear to thereby limit the amount of second direction movement of said gear for each movement of said drive pawl into engagement with said retaining pawl.

4. A ratchet drive mechanism including in combination, a supporting frame, a toothed ratchet gear, an escapement cog having a plurality of spaced cog teeth on its periphery, said gear and said cog being secured to gether and movably mounted on said frame, means cluding a spring and disposed for tensioning said sprg as said gear and cog assembly is moved in a first direction t urge movement of said gear and cog assembly in a second direction, a drive pawl, a retaining pawl, resilient means for urging said retaining pawl into engagement with the toothed surface of said ratchet gear to thereby prevent second direction motion of said gear, means for moving said drive pawl to and from said ratchet gear to engage a ratchet tooth and impart first direction motion to said gear and tension said spring, escapement lever means disposed for engaging a cog tooth of said cog during first direction movement of said gear to thereby limit the first direction movement of said gear for each movement of said drive pawl towards said ratchet gear, and shift means for engaging and moving said drive pawl to a position for engaging said retaining pawl as said drive pawl is moved towards said ratchet gear and thereby disengage said retaining pawl from the toothed surface of said ratchet gear to permit second direction movement of said gear by the tension of said spring, said escapement lever means being disposed for engaging a cog tooth of said cog during second direction move-- ment of said gear to thereby limit the amount of second direction movement of said gear for each movement of said drive pawl into engagement with said retaining pawl.

5. A ratchet drive mechanism including in combination, a supporting frame, a toothed ratchet gear, said gear being movably mounted on said frame, means including a spring and disposed for tensioning said spring as said gear is moved in a first direction and to urge movement of said gear in a second direction, a drive pawl, a retaining pawl, resilient means for urging said retaining pawl into engagement with the toothed surface of said ratchet gear to thereby prevent second direction motion of said gear, means for moving said drive pawl to and from said ratchet gear to engage a ratchet tooth and impart first direction motion to said gear and tension said spring, shift means movable from a normal position to a second position for engaging and moving said drive pawl to a position for engaging said retaining pawl as said drive pawl is moved towards said ratchet gear and thereby disengage said retaining pawl from the toothed surface of said ratchet gear to permit said gear to move in the second direction by the tension of said spring, and shift control means for moving said shift means to the second position in response to said ratchet gear being moved in said first direction to a first predetermined position and to move said shift means to the normal position in response to said ratchet gear being moved in said second direction to a second predetermined position.

6. A ratchet drive mechanism including in combination, a supporting frame, a toothed ratchet gear, an escapement cog having a plurality of spaced cog teeth on its periphery, said gear and said cog being secured together and movably mounted on said frame, means including a spring and disposed for tensioning said spring as said gear and cog assembly is moved in a first direction and to urge movement of said gear and cog assembly in a second direction, a drive pawl, a retaining pawl,

resilient means to urge said retaining pawl into engagement with the toothed surface of said ratchet gear to thereby prevent second direction motion of said gear, means for moving said drive pawl to and from said ratchet gear to engage a ratchet tooth and impart first direction motion to said gear and tension said spring, escapement lever means disposed for engaging a cog tooth of said cog during first direction movement of said gear to thereby limit the first direction movement of said gear for each movement of said drive pawl towards said ratchet gear, shift means movable from a normal position to a second position for engaging and moving said drive pawl to a position for engaging said retaining pawl as said drive pawl is moved towards said ratchet gear and thereby disengage said retaining pawl from the toothed surface of said ratchet gear to permit said gear to move in the second direction by the tension of said spring, and shift control means for moving said shift means to the second position in response to said ratchet gear being moved in said first direction to a first predetermined position and for moving said shift means to the normal position in response to said ratchet gear being moved in said second direction to a second predetermined position, said escapement lever means being disposed for engaging a cog tooth of said cog during second direction movement of said gear to thereby limit the amount of second direction movement of said gear for each movement of said drive pawl into engagement with said retaining pawl.

7. A reversible ratchet drive mechanism including in combination, a supporting frame, a toothed ratchet wheel, an escapement cog having a plurality of spaced cog teeth on its periphery, a ratchet shaft rotatably mounted on said frame, said wheel and said cog being secured to said shaft, a coil spring, said coil spring being positioned around said shaft with one end secured to said frame and the other end secured to said shaft in a manner to wind said spring as said shaft is rotated in a forward di: rection and to cause rotation of said shaft in a reverse direction as said spring is unwound, a drive pawl, a retaining pawl, resilient means for urging said retaining pawl into engagement with the periphery of said ratchet wheel to thereby prevent reverse rotation of said wheel, means for moving said drive pawl to and from said ratchet wheel to engage a ratchet tooth and impart forward rotation to said wheel and wind said spring, escapement lever means disposed for engaging a cog tooth of said cog during movement of said wheel in the forward direction to thereby limit the forward rotation of said wheel for each movement of said drive pawl towards said ratchet wheel, and ratchet reversing means for engaging and moving said drive pawl to a position for engaging said retaining pawl as said drive pawl is moved towards said ratchet wheel and thereby disengage said retaining pawl from the periphery of said ratchet wheel to permit said wheel to rotate in the reverse direction by the unwinding of said spring, said escapement lever means being disposed for engaging a cog tooth of said cog during movement of said wheel in the reverse direction to thereby limit the amount of reverse rotation of said wheel for each movement of said drive pawl into engagement with said retaining pawl.

8. A reversible ratchet drive mechanism including in combination, a supporting frame, a toothed ratchet wheel, a ratchet shaft rotatably mounted on said frame, said wheel being secured to said shaft, a coil spring, said coil spring being positioned around said shaft with one end secured to said frame and the other end secured to said shaft in a manner to wind said spring as said shaft is rotated in a forward direction and to cause rotation of said shaft in a reverse direction as said spring is unwound, a drive pawl, a retaining pawl, resilient means for urging said retaining pawl into engagement with the periphery of said ratchet wheel to thereby prevent reverse rotation of said wheel, means for moving said drive pawl to and from said ratchet wheel to engage a ratchet tooth and 9 and impart forward rotation to said wheel and wind said spring, ratchet reversing means movable from a normal position to a second position for engaging and moving said drive pawl to a position for engaging said retaining pawl as said drive pawl is moved towards said ratchet wheel and thereby disengage said retaining pawl from the periphery of said ratchet wheel to permit said wheel to rotate in the reverse direction by the unwinding of said spring, and shift means for moving said ratchet reversing means to the second position in response to said ratchet wheel being rotated in a forward direction to a first pre determined position and for moving said ratchet reversing means to the normal position in response to said ratchet wheel being rotated in a reverse direction to a second predetermined position.

9. A reversible ratchet drive mechanism including in combination, a supporting frame, a toothed ratchet wheel, an escapement cog having a plurality of spaced cog teeth on its periphery, a ratchet shaft rotatably mounted on said frame, said wheel and said cog being secured to said shaft, a coil spring, said coil spring being positioned around said shaft with one end secured to said frame and the other end secured to said shaft in a manner to wind said spring as said shaft is rotated in a forward direction and to cause rotation of said shaft in a reverse direction as said spring is unwound, a drive pawl, a retaining pawl, resilient means for urging said retaining pawl into engagement with the periphery of said ratchet wheel to thereby prevent reverse rotation of said wheel, means for moving said drive pawl to and from said ratchet wheel to engage a ratchet tooth and impart forward rotation to said wheel and wind said spring, escapement lever means disposed for engaging a cog tooth of said cog during movement of said wheel in the forward direction to thereby limit the forward rotation of said wheel for each movement of said drive pawl towards said ratchet wheel, ratchet reversing means movable from a normal position to a second position for engaging and moving said drive pawl to a position for engaging said retaining pawl as said drive pawl is moved towards said ratchet wheel and thereby disengage said retaining pawl from the periphery of said ratchet wheel to permit said wheel to rotate in the reverse direction by the unwinding of said spring, and shift means to move said ratchet reversing means to the second position in response to said ratchet wheel being rotated in a forward direction to a first predetermined position and to move said ratchet reversing means to the normal position in response to said ratchet wheel being rotated in a reverse direction to a second predetermined position, said escapement lever means being disposed for engaging a cog tooth of said cog during movement of said wheel in the reverse direction to thereby limit the amount of reverse rotation of said wheel for each movement of said drive pawl into engagement with said retaining pawl.

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