US2905304A - Rotary proportional escapement - Google Patents

Description

I Sept. 22, 1959 h R. D. DODGE ETAL 2,905,304

ROTARY PROPORTIONAL ESCAPEMENT Filed D60. 30, 1955 4 sheets-sheet 1'.

AFIGJI' INVENTORS RONALD D. DODGE NEIL D WALTON 8 ATTORNEY p 22, 1959 R. D. DODGE ET AL 2,905,304

' ROTARY PROPORTIONAL ESCAPEMENT Filed Dec. 50, 1955 4 Sheets-Sheet 2 FIG.2

INVEN TOR RONALD D. DODGE NEIL D. WALTON ATTORN EY Sept. 22, 1959 R. D. DODGE ETAL 2,905,304

' ROTARY PROPORTIONAL ESCAPEMENT Filed Dec. 30, 1955 4 Sheets-Sheet 3 FIG.4

INVENTORS RONALD D. DODGE NEIL WALTON a, Mun

ATTORNEY p 1959 i R- D. D ODGE ET AL 2,905,304

ROTARY PROPORTIONAL ESCAPEMENT Filed Dec. 30, 1955 4 Sheets-Sheet 4 INVENTORS RONALD 0. DODGE 76 77 NEIL D. ALTON (M "a luv-7 ATTORNEY nited States atent Patented Sept. 22, 1959 ROTARY PROPORTIONAL ESCAPEMEN'I' Ronald D. Dodge and Neil D. Walton, Poughkeepsie, N.Y., assignors to International Business Machines gi orporation, New York, N.Y., a corporation of New ork Application December 30, 1955, Serial No. 556,504

4 Claims. (Cl. 197-84) This invention relates to a typewriter escapement mechanism, and more particularly to an improved rotary proportional escapement mechanism.

Rotary proportional escapement mechanisms utilizing flying pawl carriers are well known in the art as shown in Reid Patent 2,202,565. In conventional installations, however, unit selection is made and then the pawl is released from a stopping position to fly to the selected positions, reengage the rotary rack or ratchet wheel and then travel with the wheel, as driven by the carriage, back to its stopping position. In such installations, however, the timing is critical and the selection must be made a predetermined time before the carriage can be released. This results in a slow operation, and accordingly, it is a first object of this invention to provide an improved rotary proportional escapement mechanism.

It is a still further object of this invention to provide an improved flying pawl rotary escapement mechanism.

It is a still further object of this invention to provide an improved selecting mechanism for a rotary pawl proportional escapement mechanism wherein the carriage may be released concurrently with, or even slightly before the selection is made.

It is a still further object of this invention to provide an improved flying pawl rotary escapement mechanism wherein the flying pawl is engaged with the rack before the carriage is released for escapement.

It is a still further object of this invention to provide an improved flying pawl. rotary proportional escapement mechanism wherein the flying pawl is engaged with the rack before a selection is made.

It is a still further object of this invention to provide an, improved flying pawl rotary escapement mechanism wherein a pair of pawls, pawl carrier mechanisms and selecting mechanism are provided with each pair being used for alternate escapement operations.

It, is a still further object of this invention to provide an improved flying pawl rotary escapement mechanism wherein the. flying pawl is engaged with the rack and wherein the pawl and carriage may be released.

Other objectsof the invention will be pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principle of the invention and the best mode, which has been, contemplated of applying that principle.

' In the drawings:

Fig. 1 is a-perspective view of the improved 2 pawl rotary escapementmechanism.

Fig. 2 is asection view taken.through the supporting shaft, for the rotary escapement mechanism.

Fig. 3 is, a plan View of one,o f.the interposers.

Fig. 4. is atop section through the shaft of the rotary escapement mechanism.

Fig, 5 .isa front elevation of a portion of the rotary escapement mechanism.

Fig, 6 is a front elevation of the, rotary; escapement I mechanism along with its control.

Fig. 7 is a front elevation of the escapement control device.

Fig. 8 is a front elevation of the rotary escapement mechanism shown in Fig. 5.

Referring to Fig. l, a typewriter carriage is shown having an escapement rack 2 which moves with the carriage (in direction of the arrow in Fig. 1) under control of a conventional spring motor as controlled by a new and improved escapement mechanism. The escapement mechanism is connected to the rack via a pinion 4 and a shaft 6 (Fig. 2) and a ratchet wheel 8. Actually, the ratchet wheel 8 is driven by shaft 6 through a pin 10, Figs. 2 and 4, which is movable relative to the ratchet wheel within the limits of a notch 11 in ratchet wheel hub 12. A spring 14 biases the ratchet wheel 8 ahead of the shaft 6. That is, with reference to Fig. 5, the escapement wheel 8 and shaft 6 are biased clockwise by the carriage spring motor and the ratchet wheel 8 is biased ahead of shaft 6 by means of the spring 14 (Fig. 4).

Control for escapement rack With reference to Fig. 5, the escapement of the ratchet wheel 8 is controlled by a pawl, such as pawl 16, moving from a home position 18 (solid line) to a stop position (dotted line). In the present embodiment, four settable interposers 20 and one permanently set interposer 20 are provided. The smallest character occupies two units of space whereas the largest occupies five units of space. The first settable interposer 20 is further designated 202 to indicate that it provides two units of space. Succeeding settable interposers are designated 20-3, 20-4 and 20-5. The permanently set interposer is designated 20-6 and provides six units of space. On the left-hand side of Fig. 5, corresponding interposers are designated 20:1-2, 20a3, 20a-4, 20a-5 and 20a-6. The interposer 20-5 or 20a-5, in normal escapement, is automatically set. In an expand operation, described hereinafter, the interposer 20-5 or 20a-5 is not automatically set and instead its function is performed by the permanently set interposer 20-6 or 20a-6. The degree of escapement is controlled by a selected interposer 20-which is moved into the path of-a pawl carrier 22. Pawl carrier 22 is pivotally supported for movement about shaft 6, and is biased counterclockwise toward home'position 18- by a spring 23. Pawl 16 is pivotally mounted onthe pawl carrier 22 and biased into engagement with ratchet wheel 8 by means of a spring 25.

It is apparent, assuming the ratchet wheel 8 is locked in the position shown in Fig. 5, that if pawl 16 is pulled into a tooth in the ratchet wheel by its spring 25 as the ratchet wheel is released for clockwise movement, this clockwise movement will continue until pawl carrier 22 strikes an interposer stop 20 thereby blocking further rotation of the ratchet wheel 8.

The interengagement of the pawl .16 and the ratchet wheel 8 is under controlof a link 24, Figs. 5 and 8, which is connected between a pin 26 and pawl 16. Pin 26in turn is slidable within-a slot 28 and when in the radially inner end of slot 28, pawl' 16 is in engagement with ratchet wheel 8, and when in the radially outer end of slot 28, pawl 16 is lifted out of the ratchet wheel 8. Spring 25 urges pin 26 to its radially inner position shown inFig. 5.

For illustrative purposes only, the escapement mechanism will. be considered as having even and odd cycles. That is, a duplicate pawl and pawl carrier mechanism on the left side of the shaft 6 is employed with that on the right side of shaft'6 (Fig. 5); the mechanism on the right side controlling the even cycles while that on the left side controls the odd cycles. This is, while the even cycle pawlmechanism is controlling the rotation of the ratchet wheel, the odd cycle pawl mechanism is restored to home position ready to control the next cycle of operation and vice versa.

It follows then that for every part of the even cycle pawl and pawl carrier mechanism there is a corresponding part on the odd cycle side and these latter parts have the same identifying numbers as the even cycle parts followed by a subscript a.

With this background information, a complete escapement operation (first even then odd) will be described. Starting with the parts in the solid line position of Fig. 5 and assuming the pins 26 and 26a have just been moved concurrently to the left by a mechanism later described, then pawl 16a will be disengaged from and pawl 16 will be engaged with ratchet wheel 8. The pawl carrier return spring 23a will have pulled carrier 22:: counterclockwise to its home position 18a. Then, since there is nothing blocking the pawl carrier 22, the spring 14 will first drive the ratchet wheel 8 clockwise ahead of shaft 6 to the extent permitted by the notch 11 (since ratchet wheel 8 has less mass than the carriage and rack 2) and then the carriage spring motor will drive the ratchet Wheel 8 further clockwise via the shaft 6 until the carrier 22 strikes one of the interposer stops 20. This will stop the rotation of the ratchet wheel 8, but the shaft 6 will continue to rotate the limited distance between the pin 10 and the notch 11 in hub 12. This movement of the ratchet wheel 8 ahead of the shaft 6 permits the carriage to be released simultaneously with release of the ratchet wheel and to permit determination of the extent of escapement prior to arrival of the carriage. Then for the odd escapement cycle, the pins 26, 26a will be moved concurrently to the right as viewed in Fig. 5, by means described hereinafter, whereupon simultaneously the spring 25a pulls pawl 16a into engagement with the teeth of the ratchet wheel 8 while pin 26, acting through link 24, pushes pawl 16 out of engagement with the ratchet wheel 8 and the ratchet wheel is again free to rotate until the carrier 22a strikes an interposer stop 20a.

Proportional escapement It is obvious that the degree of rotation of the ratchet wheel 8 is controlled by the interposer stops 20, 20a and these, in turn, are selected in accordance with a predetermined pattern (i.e., the size of the character that has previously been printed). The selection of the interposers 20, 20a can best be explained with respect to Fig. 1 wherein it will be understood that conventional key lever 36 is illustrated as being pivotally supported for engagement with a trip lever 38 which, in turn, is supported on a cam lever 40 of a well known IBM type bar drive mechanism. The rocking of the trip lever 38 pushes on the tail of a cam 42 which is pivotally supported on the cam lever 40 to rock the cam into engagement with the power roll (not shown) to initiate a type bar print stroke. The rotation of the cam lever 40 pulls on a selector bar 46 which is equipped with selecting fingers 48 of the type shown and completely described in the Dodge Patent 2,547,449. As the selector bar is moved, one selector bail 50 is pulled forward to rock the corresponding actuating lever 52 clockwise (as viewed in Fig. 1) about a fulcrum rod 54. Upper selecting fingers 48a are associated with a case shift mechanism (not shown) for efiecting different amounts of escapement for upper case characters.

Each actuating lever 52 carries a link 56, further designated 562, 563 and 564, corresponding respectively to 2, 3 and 4 units of space, and including rod ends 58-2, 583 and 584 that ride through suitable apertures in a selecting plate 60. Any movement of a rod end 58 pushes on a corresponding selector lever 62, further designated 62-2, 62-3 and 624 to rock the same about a fulcrum point 64 (see also Fig. 2). The upper end of each selector lever 62 is connected by means of a corresponding link 66 to the tail of a corresponding interposer stop 20.

In view of the even-odd escapement cycles, it is apparent that a duplicate set of selector levers 62a is required on the left side of shaft 6 (see Figs. 1 and 6). The interposers 20a are in inverse order on opposite sides of shaft 6. Corresponding pairs of selector levers, for example 562 and 56a2, are moved in response to a type bar print stroke.

All the interposers 20, 20a (one for each unit of escapement, e.g., 202=2 units; 20-3=3 units etc.) are slidably mounted in an interposer guide 70, 70a. The idea is to pull an interposer stop, for example 205, Figs. 1, 5 and 6, from a rest position along the direction of link 66 until a latch shoulder 68 (see Fig. 4) books onto the guide 70. This structure is better shown in Fig. 3 with reference to an interposer 20a, guide 70a, shoulder 68a, etc. In this position, the interposer acts as a positive stop controlling the degree of rotation of the pawl carrier 22, 22a as previously described. In order to do this, a spring 72, 72a is provided for each interposer stop 20, 20a which spring is connected between the interposer stop 20, 20a and a spring anchor 74 (see Fig. l). The spring 72, 72a tends to pull the interposer stop 20, 28a to the left (or right) after it has been pulled through the interposer guide 70, 70a a distance suflicient to permit the latch shoulder 68, 68a to engage the interposer guide 70, 70a.

Actually, a pair of interposer stops, for example 20-2 and 20a2, are selected each time there is a type bar print stroke, but only one interposer, either 20, 20a is latched in engagement with the interposer guide 70, 70a. The reason for this is that a tripping plate 76 (later described in detail) is connected by means of links 78, 78a to each one of the interposer stops 20, 20a andwhen the trip plate 76 is moved to the right as viewed in Fig. 1, then the interposers 20 are held to the right and cannot latch on the guide 70 while the interposers 20a (Fig. 3) on the opposite side of the shaft 6 can latch on the interposer guide 70a. Conversely, when the trip plate 76 is moved in the opposite direction, the selected interposer 20 can latch on interposer guide 70, while the selected interposer 20a cannot latch. Only one interposer 20, 20a of a pair can be latched, otherwise, the free pawl carrier 22. 22a would be blocked from returning to its home position.

Tripping operation The tripping operation for this improved escapement mechanism is under control of a conventional U bar 80 (Fig. l), which is moved in the direction of the arrow during each type bar print stroke. The U bar 80 is connected by means of a link 82 to an escapement trip lever 84. The trip lever 84 is alternately engagement with trip bell cranks 86, 88 and is pivotally mounted on a bracket 85 by a pin 87. Each of the trip bell cranks is pivotally supported for rocking movement about support pins 90, 92 respectively, carried by the typewriter frame. The movement of the trip link 82 in the direction of the arrow rocks the trip latch 84 into engagement (Fig. 1 position) with one arm of the trip bell crank 86 rocking it counterclockwise about its pivot 90 to exert a pull on a connecting link 94 which provides parallel motion between the two bell cranks 86, 88. The bell crank 88, in turn, is connected by means of a link 96 to the trip plate 76. The actual construction of the trip plate will be later described, but for the present, let it be understood that the trip plate moves as a unit to the right or to the left, relative to shaft 6, in response to the movement of link 96. With the rocking of bell crank 86 counterclockwise, the link 96 will be pulled, thereby urging trip plate 76 to the right relative to shaft 6. The bell cranks 86, 88 respectively, have extensions 98, 100 respectively, which are connected by means of springs 102, 104 respectively, to a trip boss 106 integral with link 82 (see also Fig. 6). It is understood that, as bell crank 86 is rocked counterclockwise to the dotted position in Fig. 4, with 88 following in parallel, then the extension 98 will rock towards the link 82 thereby relieving tension on the spring 102. Accordingly,

extension 100 of hell crank 883150; moves to; the left thereby biasing spring 105}. Then, when the U, bar 80 is restored. to, itsnorrnal position, it pulls on the trip link 82 and the biased spring 104, will toggle the link 82 to the left of the dotted position in Big. 4. and,consequent1y, the trip lever 84 is moved, into the Path of the bell crank c r i glv, o he, ex ype bar.v pr nt s ke, the U bar will, act through link 82. todrive the trip lever 84 into. engagement with bell; crank 88, rocking the same clockwise thereby pushing on a link 96 to move the trip plate 76 to the left relative to shaft 6,. It follows that the bell crank 86wil1 move parallel to the bell crank 88; and produce a converse action on the springs 104, 102v to toggle the lever 84. back to its Fig. 1. position, sothat on thenext type bar print stroke, the trip lever will again engage the bell crank 8 6 and this action will continue in alternate cycles for every type bar print stroke. In order to allow for the described toggle action, a loose fit is pro: vided between the U bar 80 and,the link 82 and between the lever 84 and its supporting bracket 85'. A spring 95 holds the bellcrank assembly 86-88 in either position.

The trip plate 76 actually comprises two mirror image yokes 77 and 77a, Figs. 6 and 7, whichare slidably supported. for transverse movement relative to the shaft 6 within predetermined limits, The slidable support comprises bearings 108, 110, at the bottom of the yokes 77, 77a and a complementary pair of bearings 11-2 and 114 at the top of the yokes 77, 77a thereby providing bearing spools for frictionless movement of both yokes; The two yokes 77, 77a are interconnected by means of a pin 97 and spring 118 to form the trip plate 76 which is moved in pull-push fashion, That is, when one yoke 77 ;is pulled by the link 96 (which is connected to pin 97 which is passed throughoverlapping slots; 99, 99a) the spring 118 will act to pull the yoke, 77a thereby moving the two yokes as a unit, and converselywhen the link96 pushes on pin 97, the pin willpush on the periphery of slot 99 to move yoke 77a to the left and spring. 118 will pull yoke 77 in unison with it.

The shape of theyokes 77, 77a is such that each has a contact portion 120;, and l20g respectively which is, engageable respectively with, the,pi ns 26, 2641 toperform the escapementoperation heretofore described. The contact portions 120, 120a, are concentric with the path of movement of pins 26, 26a from active to home position, and therefore the pins 26, 26a ride on the peripheral surfaces of contact portions 120, 120a to act through the links.24, 24ato. holdone pawl .16, l6aout of engagement with theratchet. wheel, depending on. whetherfthe. trip plate has beenmovedctolhe. right or left relative toshaft 6.

The yokes 77, 77a are also provided with plural slots 122, Fig. 7, for carrying the interposer links 78. It is apparent from Fig. 6 that when the trip plate 76 has been moved to the left from the dotted position of the yoke 77, the links 78a will hold all the interposer stops 20a to the left so that they cannot latch on the interposer guide 70a and thereby hold the interposers out of the path of the pawl carrier 22a as it is being restored to its home position. Conversely, the links 78 are relaxed to permit a selected interposer 20 to be moved into its latched position by its spring 72.

It should be explained at this point that a selected interposer must be restored after each escapement action and while its corresponding pawl carrier is being restored to home position. This restoration is accomplished by its control spring 72, as follows:

That is, with reference to Fig. 3, interposer 20a would reach its latched position as trip plate 76 moved to the right, then as hip plate 76 is moved to the left, link 78a pulls interposer 20a free of guide 7011 whereupon its spring 72a restores the interposer 20a to its dot-ted rest position. Since the spring 72a is cocked with respect to the axis of link 66a, the spring performs the dual function of latching the interposer on the selecting cycle, and then restoring it on the restore cycle.

With respect to the inverse order of the selector levers 62, referenee ismadeto the links 56 of Fig. 1 and the interconnection between, levers 52 and selector levers 62- i.e., the, outside lever 62-2. corresponds to the inside selectorlever62g-2. The reason for this is that the outside lever 62-2 operates the topinterposer. stop 20-2 and it is apparent that on. the opposite side, the corresponding interposer is, thebottom one 20 1-2 and this one is controlled by the inside lever 624-2, remembering that during the actual; escapement. operation, the pawl always moves clockwise and the, shortest number of units of escapement must be closest to the home position.

Referring to Fig. l, anexpand key is pivotally mountedon a bracket; 132 fixed to. the machine frame. It. is sometimes desirable. to expand the printing, that is, to use additional units of. space between characters, for example, to justify the right-hand margin. This escapement mechanism is adapted for adding, selectively, one additional unit of space between characters.

As described hereinbefore, the interposers 20-5 and 20a-5 normally are setso that the pawl carrier 22 or 22a willcome to rest thereagainst unless one of the earlier interposers 20. or 20a is set. The interposers 20-6 and 2011-6 are permanently set. Referring to Figs. 1 and 6, the selecting plate 60 which guides the rod ends 58, 58a carries cam surfaces 61 and 61a.which are adapted, in normal (as distinguished from expanded) escapement, to hold the selector levers 62-5 and 6241-5 respectively, in their actuated positions, to holdthe interposers 20-5 and 20a-5, through their interposer links 66, clear of the guide members 7 0, 70a whereby the interposers 20-5 and 20a-5 move alternately totheir actuated positions as the trip plate 76 is shifted left and right. The selecting plate 60- is shiftably mounted by studs 134, Figs. 1 and 6, and slots 136 ona bracket 1318, When it is desiredto go into an expandedprint operation, the expand key 130 is pivoted counterclockwise on the bracket 132 and, through a member -anda link 142,- rocks a crank 144 counterclockwiseabout a pivot rod 146, pulling on alink 148v to pivot a bell crank 150 clockwise thereby, pushing on a link 152 and shiftingthef selecting plate,60 to the left as well as rocking it to the dotted position. in Fig. 6. In the dotted positionthe cam surfaces 61 and 61a are clear of all selecting levers 62. and the rod ends 58v and 58a are shifted to the left so thatIthe rod ends and selecting levers are aligned as follows.

Aligned with Rod end: selecting lever 58-2 62-3 58-3 62-4 ss-4 62-5 Similarly the rod ends 58a are shifted relative to the selecting levers 62a. It is apparent that a normal 2 unit space is now expanded to 3 units; 3 units are expanded to 4; 4 units are expanded to 5; and 5 units are expanded to 6 as controlled by the permanently set interposers 20-6 and 20a-6.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. A typewriter comprising in combination, a type bar, key lever actuating means for driving said type bar to printing position, a carriage having means urging it in letter spacing direction, and an escapement mechanism controlling the extent of carriage spacing comprising a control shaft, means positively interconnecting said carriage and control shaft to urge the latter in a direction corresponding to the letter spacing direction of the carriage, a ratchet wheel, means interconnecting said ratchet wheel and said control shaft whereby the extent of carriage movement can be regulated by controlling the escapement of said ratchet wheel, said interconnecting means providing limited relative movement between said shaft and said ratchet wheel, spring means urging said ratchet wheel to one limit of relative movement in the direction of its normal letter spacing movement, a pawl carrier, a pawl mounted on said carrier for peripheral movement therewith relative to said ratchet wheel, resilient means biasing said pawl normally into engage ment with said ratchet wheel, means responsive to type bar print strokes for cyclically moving said pawl carrier from a stopping position relative to said ratchet wheel to said home position and with said ratchet wheel back to a stopping position, and additional means responsive to said type bar print strokes for determining said stopping position.

2. A typewriter comprising in combination, a type bar, key lever actuating means for driving said type bar to a printing position, a carriage having means urging it in letter spacing direction, and an escapement mechanism controlling the extent of carriage spacing comprising a control shaft, means positively interconnecting said carriage and said control shaft to urge the latter in a direction corresponding to the letter spacing direction of said carriage, a ratchet wheel, means interconnecting said ratchet wheel and said control shaft whereby the extent of carriage movement can be regulated by controlling the escapement of the ratchet Wheel, a pair of pawls, a pair of pawl carriers, means mounting said pawls one on each carrier respectively, means biasing said pawls normally into engagement with said ratchet wheels, second resilient means biasing each of said pawl carriers to a home position, and means responsive to a type bar print stroke for simultaneously releasing one pawl from said escapement wheel permitting said pawl to be moved by its pawl carrier back to home position and engaging the other pawl with said escapement wheel for movement therewith to a stopping position, and additional means responsive to said type bar print stroke for determining the stopping position of said other pawl.

3. A typewriter comprising in combination, a type bar, key lever actuating means for driving said type bar to a printing position, a carriage having means urging it in letter spacing direction, and an escapement mechanism controlling the extent of carriage Spacing comprising a control shaft, means positively interconnecting said carriage and said control shaft to urge the latter in a direction corresponding to the letter spacing direction of said carriage, a ratchet wheel, means interconnecting said ratchet wheel and said control shaft whereby the extent of carriage movement can be regulated by controlling the escapement of the ratchet wheel, said interconnecting means providing limited relative movement between said shaft and said ratchet wheel, spring means urging said ratchet wheel to one limit of relative movement in the direction of its normal letter spacing movement, a pair of pawls, a pair of pawl carriers, means mounting said pawls one on each carrier respectively, means biasing said pawls normally into engagement with said ratchet wheels, second resilient means biasing each of said pawl carriers to a home position, and means responsive to a type bar print stroke for simultaneously releasing one pawl from said escapement wheel permitting said pawl to be moved by its pawl carrier back to home position and engaging the other pawl with said escapement wheel for movement therewith to a stopping position, and additional means responsive to said type bar print stroke for determining the stopping position of said other pawl.

4. In a typewriter having a type bar, selectively controlled means for driving said type bar to printing position, and a carriage including an escapement rack biased in letter spacing direction, an improved escapement mechanism comprising a control shaft, means positively interconnecting said control shaft and said carriage, a ratchet wheel, driving means permitting relative motion between said ratchet wheel and said control shaft, spring means biasing said ratchet wheel in the direction of normal travel and towards one limit of said relative motion, and an improved pawl control mechanism for regulating the extent of rotation of said ratchet wheel comprising a pawl carrier, resilient means urging the same to a home position, a pawl mounted on said pawl carrier for selective engagement with said ratchet wheel, means responsive to a type bar print stroke for simultaneously engaging said pawl with said ratchet wheel and for freeing the same for rotation, and interposcr stop means positioned in response to said type bar print stroke to block the movement of said pawl and hence said ratchet wheel whereby said control shaft is not stopped until said ratchet wheel and control shaft driving means reach the opposite limit of said relative motion.

References Cited in the file of this patent UNITED STATES PATENTS 585,235 Sears June 29, 1897 895,238 Brown Aug. 4, 1908 2,220,185 Waite Nov. 5, 194-0

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