US2497798A - Printing press having curved impression surface - Google Patents

Description

c. F. ROOT 2,497,798

PRINTING PRESS HAVING CURVED IMPRESSION SURFACE Feb. 14, 1950 5 Sheets-Sheet 1 Filed July 26, 1946 C. F. ROOT Feb. 14, 1950 PRINTING PRESS HAVING CURVED IMPRESSION SURFACE I 5 Sheets-Sheet 3 Filed July 26, 1946 C/zar/es Feb. 14, 1950 c. F. ROOT- 2,497,793

PRINTING PRESS HAVING CURVED IMPRESSION SURFACE Filed July 26, 1946 5 Sheets-Sheet 4 1 fiveizzor Clzczi-Zesfffadi" Patented Feb. .14, 1950 umrso srArEs PATENT OFFICE PRINTING PRESS HAVING OUBVED HHPBESSION SURFACE came- F. in... Cleveland, om, aalignor to Chandler & Price Company, Cleveland, Ohio, a

corporation of Ohio Application July 2a. 1946, Serial No. cause a cam. (01. 101-497) This invention relates in general to platen or job type printing presses, and in particular to a printing press wherein the platen has a convex curved surface whereby cylinder press printing is simulated on a platen press.

and type form will cause a printing impression to take place.

Heretofore it has been common practice to employ type beds and platens both having flat surfaces thereon. This has resulted in a tremendous amount of pressure being necessary between the type bed and platen in order to obtain satisfactory' printing results. This pressure may range up to about 300 pounds per square inch.

The advantages of a practical curved platen press have long been recognized. It has been estimated that the amount of pressure necessary to obtain satisfactory printing results by the use of a curved platen may be reduced to approximately 3 or 4% of the total pressure necessary for the same printing results where both the type bed and platen are flat. A practical machine of the curved platen type could thus be manufactured more economically by having the frame much lighter. Many other advantages have been set forth in earlier patents where an attempt has been made to produce a practical curved platen press. For example, Clark Patent No. 1,034,083, recognized many of the advantages of this type of press as early as 1010 and has clearly set them forth in his specification.

The major difllculty in producing an operable and practical curved platen press stems from the inability to obtain what may be termed pin- 2 tween a curved platen and a flat type bed, it is necessary to provide some mechanism whereby the actual path travelled by one end of the curved platen during its printing operation follows exactly the path which the same edge would make if it were free to "climb or roll along th flat surface of the type bed without any slipme therepoint contact" between the type bed and the true circle. This becomes increasingly evident as the radius of curvature of the curved member becomes smaller. For perfect pin-point contact bebetween. v

To the best of my knowledge there has never been a practical curved platen press designed or in use at this time which will accomplish this re- I suit. There have been attempts in the past to reciprocate vertically the type bed during the printing operation in order to obtain this pinpoint contact. This, however, involves the use of extra mechanism which I have found to be unnecessary in my invention. It has also been suggested that the curved platen member be pivatally secured at one end thereof to the frame on which it is mounted. This is a step in the right direction, but here again it is still necessary to slide the type bed during the printing operation inorder. to prevent slippage between the platen and bed.

I have been able to overcome these dillculties previously encountered in the operation of a curved platen press by so locating the pivotal points of the pivoted member (whether it be the platen or the bed) and the pivoted frame with respect to each other and with respect to the nonpivoted member that the actual path travelled by the ends of the pivoted member will so nearly approximate the path which these ends would naturally travel to produce perfect pin-point contact that any discrepancies which there might be between the two paths will be negligible. A printing operation performed in accordance with my invention will, for all practical purposes, be the result of perfect pin-point contact. The reason, of course, that this perfect pin-point contact might not be accomplished in a practical machine is because the ends of the curved platen will describe substantially the arc of a true circle, whereas the path travelled thereby for perfect pin-point contact will not be the arc of a true circle. By providing the curved platen with a large radius of curvature and moving the ends thereof a very short distance, as well as so arranging the various pivotal points as above outlined, the discrepancy, if any, between the actual path of the ends of the curved platen and the natural path which should be travelled thereby for perfect pin-point contact cannot be measured and hence will be negligible. Because of the fact 3; that no difference can be detected between these As a practical example, .I have found thatif the radius of curvature of the curved platen is approximately 7 /2 feet and the cord of the are taken is 13 inches, then the actual length of the curve bounded by the cord will be 13.018 inches. This means that, for perfect pin-point contact between the curved platen and the flat type bed, it will be necessary for the platen to climb .018 inch as it rolls over the type bed from one end thereof to the other. I have been able to make the platen climb this extra distance without any slippage between the platen and type bed by properly arranging the relative positions of the two main pivotal points. For example, in the Clark patent just mentioned, pin-point contact impression is attempted through a curved slot. In the Waters Patent No. 1,360,063, this result is attempted by providing male and female spikes meshing with each other along the outer ends of the platen and on each side of the type bed. In

the well known cylinder type of printing press a flat straight rack is mounted on the side of the reciprocating type bed, and meshes with a gear mounted on the end of the printing cylinder, thus obtaining pin-point contact on impression.

It is, therefore, one of the principal objects of the present invention to provide a platen printing press with a curved platen which will produce pin-point contact between the platen and type bed during the printing operation with but slight alteration of a conventional Golding type platen press, and avoiding huge cumbersome uncanny means, which are not only impractical, but would be expensive to manufacture, and would be most uncertain and slow in their action, and which, if it were possible to construct, the various parts would be subject to rapid wear, adding to the problems of irregular and uncertain operation.

Another object of the invention is to provide a platen printing press wherein a curved platen is caused to climb" on the type bed during the printing operation to thereby obtain pin-point contact between the platen and type bed from one end thereof to the other.

' A further object of the invention is to provide a curved platen printing press whereby the platen is so mounted that the actual path travelled by the ends thereof will so nearly follow the natural path thereof to obtain perfect pin-point contact between the platen and type bed, that any discrepancies which might occur between the actual and natural paths will be negligible for all practical purposes.

Still another object is to provide a, platen printing press wherein a curved platen is pivotally mounted to an oscillating frame and wherein the pivotal points of the platen and the frame are so located with respect to each other and to the type bed as to obtain pin-point contact between the platen and bed from one end tthereof to the other during the printing opera- A-still further and more particular object is to provide a platen printing press wherein a curved platen is pivotally mounted at its upper end to an oscillating frame and wherein the pivotal points of the platen and the frame are so located with respect to each other and to the type bed as to obtain pin-point contact from the bottom to the top thereof between the platen and bed during the printing operation.

Another object is to provide a curved platen printing press with pressure equalizing means whereby a substantially constant pressure can be obtained at every point of contact between the platen and the type bed during the printing operation.

A further object is to provide a curved platen printing press with means to prevent a second printing impression as the platen and type bed move away from each other by holding the platen in its final printing position until after the oscillating frame on which it is mounted has moved a predetermined distance away from the type bed.

Still another and more specific object is to provide a curved platen printing press with hydraulic pressure equalizing means to produce a substantially constant pressure between the platen and type bed at every point of contact thereon during the printing operation.

Other objects and advantages of the invention will become apparent upon a reading of the following description taken in connection with the accompanying drawings in which:

Fig. 1 is a vertical section through a curved platen printing press embodying the invention showing the platen in open position;

Fig. 2 is a vertical sectional view similar to Fig. 1, but showing the curved platen in its initial printing position;

Fig. 3 is a vertical sectional view similar to Fig. 2, but showing the curved platen in its final printing position;

Fig. 4 is a front elevatlonal view of the press;

Fig. 5 is a fragmentary horizontal section taken substantially along the plane of line 55 in Fig. 3; and

Figs. 6 and 7 illustrate diagrammatically the path travelled by one end of the curved platen to obtain pin-point contact between the platen and the type bed.

Referring now more particularly to the drawings, the invention is illustrated herein as applied to a printing press commonly known as the Golding type, wherein the main frame supports a stationary type bed and the platen is mounted on the second or auxiliary frame which is pivoted to the main frame for oscillation toward and away therefrom. In this form of the invention, I may provide a supporting base i on which the main frame 2 is located and to which the second or oscillating frame 3 is rotatably or pivotally mounted on the shaft 4, which extends between the sides of the main frame 2. The general operation of this type of press is well known to those skilled in the art,. and a brief description thereof will be sumcient for present purposes.

Suitable driving means such as an electric motor 5 may be provided with a shaft 6 on which a pulley 1 is located, and around which a belt 8 passes to drive the large pulley 9 mounted on the shaft I0. Two pinions II are keyed to the shaft III which mesh with and drive the main driving gears I! which are mounted for rotation on each end of shaft l3. This shaft l3 extends across between the sides of the main frame 2.

An idler gear I is mounted on a stub shaft 15 to be driven by one of the gears I2 and which in turn will drive the gear I6 mounted onthe shaft I1. The oscillating frame 3 is caused to move about its pivotal point 4 for movement toward and away from the type bed by means of a linkage mechanism at each side of the machine.

link II. The other end of each link It is pivoted,

as at 25 to a toggle connecting arm II which is,

in turn, pivotally mounted on a shaft 22 on the main frame. Links 23 are pivoted to the outer ends ofthe toggle 2| and adjacent th pivotal point 24 by means of the pivot pin 24, the other end of each link 28 being pivotally mounted on a pin 25 on each side as at. 26 of the second frame 3. From the position of the platen shown in Fig. 1, it will be moved toward the type bed upon rotation of the gear I! in a clockwise direction through the linkage mechanism Just described. One revolution of the gears I! will carry the platen forwardly to the type bed, complete a printing operation and return the platen to its open or sheet receiving position shown in Fig. l.

The manner of inking the type form for each printing impression may comprise the usual and well known mechanism shown in Fig. 3. I Briefly, this comprises a rotatable ink cylinder 21 mounted at the upper end of the main frame 2 which is caused to rotate by means of a motor 28 having a chain belt 29 passing around a suitable pulley 30 on the motor shaft and another pulley 3| mounted on the shaft 32. This latter shaft is provided with a gear 33 in mesh with a suitable gear at one end of the'cylinder 21. The motor 23 operates continuously'to rotate the roller 21.

The inking 'rollers 34 are caused tomove downwardLv over the type form during the period that the platen has moved away from the type bed. These rollers 34 are rotatably mounted on the outer ends of rods (three in number) 35 which are slidably mounted in roller frames 350 at each side of the press which are caused to move from the position shown in Fig. 3 to the position shown in Fig. l by reason of their keyed connection to shaft 36, which shaft also has keyed thereto an arm 31 pivotally secured at 35 to the upper end of an arm 39. This latter arm is mounted at its other end on the crank pin l3.

An elongated link 40 is also pivotally mounted to the upper end of the arm 39 at}! and has its upper end 4! pivoted to an arm 42 which carries a pawl 43. The outer end of the arm 42 is mounted on a shaft 44 to which is secured a rachet wheel 45 at the end of a fountain ink roll (not shown). Thus, each rotation of the gear l2 through the connecting arms 39 and link 40 will cause the rachet wheel 45 and the fountain ink roll to rotate and feed ink to one of the rollers 34, which in turn places ink on the cylinder 21. This mechanism is old and well known in the art and need not be further described at this time. 1

The usual form of type bed 46 may be provided and suitably mounted on the main frame 2. This type bed supports a chase and is of the, usual construction secured to the side members of the main frame 2 in front of a separator brace 41 (see Fig. Ink roller tracks 48 are provided at each side thereof adjacent which the platen bearers 49 are located on each side of the chase 50. The type form is adapted to be located and mounted within the chase 50.

The preferred embodiment of my invention contemplates the use of a curved platen member 5| provided with suitable supporting side members 52 pivotally mounted on pins 53 to the upper end of the second or oscillating platen carrier frame 3. It will be clear that either the platen or the type bed can be the pivoted member, but

. 8. for purposes of illustration the curved platen is the one which has been shown to be pivotally mounted in this embodiment of the invention.

In the actual printing operation, the platen moves forwardly toward the type bed and into its initial printing position as shown in Fig. 2. Further movement of the frame 3 toward the type bed will cause a rolling action to take place between the curved platen and the type form whereupon the platen will assume simstantially its ilnal printing position as shown in Fig. 3.

Referring now to Figs. 6 and 7, these figures show somewhat diagrammatically the principle underlying the invention. Fig. 6 shows the curved platen II in its initial printing position against the yp bed 46 where the lower end 54 of the curved platen first contacts the type form. It

will be obvious that any suitable curvature, regular or irregular, can be used for the curved platen, but to illustrate the principle involved herein, 1 have chosen a regular curve having a radius of 7% feet and I have taken a cord of the arc of 13 inches. The upper edge 55 of the curved platen 5i will assume the position shown in Fig. 6,in the initial printing position thereof. In order' to obtain perfect pin-point contact throughout the entire length of the are on the curved platen from its original printing position shown in Fig. 6 to its final printing position shown in Fig. '7, it will be necessary that the upper edge 55 of the platen follow a path substantially like that indicated by the dotted line 55. I have determined that, using the radius of 7 feet anda cord of 13 inches, the actual length of the curved surface from the lower edge 54 to the upper edge 55 will be about 13.018 inches. Therefore, the ideal situation would be for the upper edge 55 to follow the path 56 and to climb up the type bed a distance of .018 inch. As stated before, this path 55 will not be an arc of a true circle. However, Ihave found that by pivoting the platen to its -oscillating carrier or frame at substantially the location indicated by the numeral 53, and by pivoting the oscillating frame at substantially the point indicated by the numeral 4, the combination of those two pivotal points with respect to each other and with respect to the type bed, will permit the upper edge 55 of the platen to climb" the required distance and to closely follow the path 56. In actual practice this path appears to be followed exactly. No discrepancies can be detected, and if there are any, they are negligible. Thus, by properly positioning the two pivotal points, I am able to obtain pin-point contact between the curved platen and the type bed from one end thereof to the other during the printing operation.

Another problem which has prevailed in the past with respect to curved platen presses is that of providing some means to insure a substantially constant pressure between the platen and the type form. This problem has been increasingly diiilcult to meet in the use of a curved platen which is pivoted at one end thereof. For example, in the illustrated embodiment of the invention where the curved platen is pivoted at its upper end, some means must be provided adjacent the lower free end thereof whereby the platen will exert a pressure against the type form during the printing operation for satisfactory printing results. Obviously, if this pressure exerted at the lower end of the platen were to remain constant, then the pressure between the platen and the type form would gradually increase as the contact neared the upper end of the assigns lized must also be so arranged as to permit the platen to swing about its pivotal point during its printing operation.

If yieldable resisting means only are used, then there must be some provision to prevent a' double printing operation from taking place. the curved platen must not be allowed to roll back along the type form after the completion of the printing operation, but the type form and the platen must be permitted to separate without further contact therebetween.

To overcome this difficulty I have provided pressure equalizing means which-will operate to exert a suflicient pressure against the lower edge of the platen when in its initial printing position, but which pressure will be progressively decreased as the platen moves to its final printing position, thus maintaining a substantially constant printing pressure during the entire operation. While the pressure equalizing means may assume any desired form, this phase of the invention is illustrated as being hydraulically controlled and operated. A hydraulic cylinder 51 is mounted at the lower part of the machine on a suitable base 58 and has a flexible conduit 59 connected at one end thereof. The other end of the conduit 59 may be secured to a suitable connection 60 loosely mounted on the shaft 4, and a rigid conduit 6| may be connected at one end to the member 60 and at its other end to a joint 62 which has conduits 63 extending outwardly therefrom toward the sides of the machine.

The outer ends of the conduits B3 are suitably connected to cylinders 64 mounted on brackets 65 which, in turn, may be suitably secured to a cross-bar 65a on the oscillating frame 3. A piston rod 68 is adapted to carry a piston at its inner end and reciprocate within the cylinder 64. The forward end of the piston rod 66 may then be pivotally secured as at 61 to the rear of the curved platen 5|.

A similar piston rod 68 has a suitable piston connected therewith and is adapted to reciprocate within the cylinder 51. The outer end of the piston rod 68 is connected to a rocker arm 89 which has a cam follower mounted at its upper end and which is pivotally mounted on a pin H at its lower end. A cam 12 is mounted on the shaft l1 and rotates therewith and is provided with a suitable configuration which will reciprocate the piston rod 68 in timed relation with the printing operation.

The operation of this phase of the invention will be substantially as follows:

A hydraulic fluid is located for movement within the conduits 59 and GI and in the nor,- mal open position of the press as shown in Fig. 1, the cam 12 will have been rotated so as to force the piston rod 68 inwardly within the cylinder 51, thus forcing the hydraulic fluid through the conduits 59 and Si, into the cylinder 64 to force the piston rod 66 outwardly and thus move the lower end of the curved platen II to its initial printing position. The platen will remain in this position as long as the piston rod 68 is at its extreme inner position within the cylinder 51. As soon as the printing operation begins, however, the cam 12 will reach its position as shown in Fig. 2 where it is about to permit the piston rod to be moved toward the left or outwardly from the cylinder 51 by reason of the inward movement of the piston rod 66 forcing the fluid out of the cylinder 84. This That is,

, 8 movement of the piston rod II is caused by pressure between the type bed and platen.

the B! the time the final printing position of the platen is reached, as shown in Fig. 3, the cam follower 5 10 will have reached its lowermost position, the

piston rod 88 will have reached its innermost position within the cylinder u and the platen will have been rotated about its pivotal point 58. If the pressure were not thus progressively relieved at the free end of the platen, it would progressively increase toward the upper end thereof as the printing operation continued, but by thus relieving the pressure at the bottom, it remains constant as the printing contact point moves upwardly.

The curved platen is limited in its swinging movement by means of the limiting links II which are pivotally mounted at 14 to the rear side of the platen. Each is provided at its outer end with an opening 15 adapted to receive a rod 16 mounted between the sides of the oscillating frame 3. Thus, the full forward movement of the platen with respect to the frame 3 as shown in Figs. 1 and 2 will position the rod 16 at the rear end of the opening 15, and the final printing position of the platen as shown in Fig. 3 will position the rod 16 at the opposite end of the slot 15.

A coiled tension spring 11 is suitably secured at one end thereof to the rear of the platen II as shown at 18, and atits other end to the cross member 65a. The spring 11 will tend to pull the lower end of the platen about its pivotal point in a counter-clockwise direction. The hydraulic fluid, however, holds the platen in its outer or initial printing position against the tension of the spring H. The printing pressure will rotate the platen about its pivotal point as the fluid leaves the cylinder 64, but when the platen reaches its final printing position, it will be held there by the tension of spring ll until the platen and type bed separate, otherwise a double printing impression would take place. After the type bed and platen separate, the fluid acts to move the platen back to its initial position against the tension of spring 11.

It will thus be seen that the cam operated hydraulic means provide pressure equalizing means which will exert suflicient pressure against the platen in its initial position to cause printing to take place, but by the progressive release of this pressure in the hydraulic line, the printing pressure will remain substantially constant throughout the printing process.

It will be understood that actually the pressure contact between the platen and type form will be at points along the bearers ll of the chase and similar bearers 49s at each side of the platen, and this is the intended meaning when discussing no pressure or contact between the platen and type form.

In many instances the heavier types of printing presses are also used for cutting and creasing operations. That is, a cutting and/or creasing form is substituted for the type form, and it is intended and understood that any reference in the description and claims herein to a printing press, necessarily includes such a press when used .either for printing or cutting and creasing. 1 From the foregoing, it will be evident that I have provided novel and practical means whereby a curved platen printing press may perform a printing or cutting and creasing operation wherein pin-point contact may be obtained be- 75 tween the platen and the type form during the entire working operation. It will be obvious that numerous changes may be made in the form, construction and arrangement of parts from that disclosed herein without in any way departing from the spirit of the invention or sacrificing any of the attendant advantages thereof. For example, either the bed or the curved platen may be the pivoted member. Likwise, either the frame on which the bed is mounted or the frame on which the platen is mounted may be the oscillating frame pivoted to the main frame. Furthermore, the pressure equalizing means need not be hydraulically operated, but mechanical means may be utilized as well, and any other suitable mechanism may be provided to control the movement of the pivoted member and to prevent a double impression.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United- States is:

l. A printing press comprising a main frame, a second frame pivotally mounted on said main frame, means to move said second frame toward and away from said main frame for a printing operation, a type bed member mounted on one of said frames, a curved platen member mounted on the other frame, means pivotally mounting one of said members adjacent its upper end to its frame, means urging the lower end of the pivoted member away from its frame while said main frame and second frame are in non-printing position, means including said pivotal means to obtain line contact between the two members progressively from one end thereof to the other as the lower end of said pivoted member is moved toward its frame during the printing operation, means to provide a substantially constant pressure between the two members from the point of initial contact to the point of final contact therebetween during the printing operation, and means to retain said pivoted member in the final printing position for a predetermined period of time during movement of the second frame away from the main frame to prevent double impression.

2. A printing press comprising a main frame, a second frame pivotally mounted on said main frame, means to move said second frame toward and away from said main frame for a printing operation, a type bed member mounted on one of said frames, a curved platen member, means pivotally mounting said curved platen member adjacent its upper end to the other of said frames, means urging the lower end of said platen away from-its frame while said main frame and second frame are in non-printing position, means including said pivotal means to obtain line contactbetween the two members progressively from one end thereof to the other as the lower end of the platen is moved toward its frame during the printing operation, means to provide a substantially constant pressure between the two members during the printing operation, and means to retain said curved platen member in the final printing position for a predetermined time during movement of the second frame away from the main frame to prevent double impression.

3. A printing press comprising a main frame, a second frame pivotally mounted on said main frame, means to move said second frame toward and away from said main frame for a printing operation, a type bed mounted on said main frame, F

a curved platen, means pivotally mounting said curved platen adjacent its upper end to said second frame, means urging the lower end of said platen away from its frame while said main second frame away from the main frame to prevent double impression.

4. A printing press comprising a main frame,

r a second frame pivotally mounted on said main frame, means to move said second frame toward and away from said main frame for a printing operation, a type bed member mounted on one of said frames, a curved platen member mounted on the other frame, means pivotally mounting one of said members adjacent its upper end to its frame, a hydraulic system connected at one end to the lower end of said pivoted member, a plunger in said hydraulic system, means to actuate said plunger to urge the lower end of said pivoted member away from its frame while said main frame and said second frame are in non-printing position, means including said pivotal means to obtain line contact between the two members progressively from one end thereof to the other as the lower end of said pivoted member is moved toward its frame during the printing operation, said hydraulic system providing a substantially constant pressure between the two members from r the point of initial contact to the point of final contact therebetween during the printing operation, and means to retain said pivoted member in the final printing position for a predetermined period of time during movement of the second frame away from the main frame to prevent double impression.

5. A printing press comprising a main frame, a second frame pivotally mounted on said main frame, means to move said second frame toward and away from said main frame for a printing operation, a type bed member mounted on one of said frames, a curved platen member mounted on the other frame, means pivotally mounting one of said members adjacent its upper end to its frame, a rotatable cam having a portion thereon urging the lower end of the pivoted member away from its frame while said main frame and second frame are in non-printing position, means including said pivotal means to obtain line contact between the two members progressively from one end thereof to the other as the lower end of said pivoted member is moved toward its frame durmg the printing operation, said rotatable cam having another portion thereon to provide a substantially constant pressure between the two members from the point of initial contact to the point of final contact therebetween during the printing operation, and means to retain said pivoted member in the final printing position for a predetermined period of time during movement of the second frame away from the main frame to prevent double impression.

6. A printing press comprising a main frame, a second frame pivotally mounted on said main frame. means to move said second frame toward and awayfrom said main frame for a printing operation, a type bed member mounted on one of said frames, a curved platen member, means pivotally mounting said curved platen member adjacent its upper end to the other of said frames,

a rotatable cam having a portion thereon urging the lower end of said platen away from its frame while said main frame and second frame are in non-printing position, means including said pivotal means to obtain line contact between the two members progressively from one end thereof to the other as the lower end of the platen is moved toward its frame during the printing operation, said rotatable cam having another portion thereon to provide a substantially constant pressure between the two members during the printing operation, and means to retain said curved platen member in the final printing position for a predetermined period of time during movement of the second frame away from the main frame to prevent double impression:

7. A platen printing press comprising a stationary frame, an oscillating frame pivotally mounted on said stationary frame, means for moving said oscillating frame toward and away from said stationary frame for a printing operation, a type bed member on one of said frames,

a curved platen member on the other of said frames, cooperating means on one of said members and its frame for floatingly mounting the point of final contact therebetween during the 35 printing operation, and means to retain said rockable member in the final printing position for a predetermined period of time during movement of the oscillating frame away from said stationary frame to prevent double impression.

8. A platen printing press comprising a staber away from its frame while said stationary frame and second frame are in non-printing'position, means to provide a substantially constant pressure between the two members from the point of initial contact to the point of final contact therebetween during the printing operation, and means to retain said rockable member in the final printing position for a predetermined period of time during movement of the second frame away from said stationary frame to prevent double impression.

CHARLES F. ROOT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Harrington Feb. 2, 1943

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