US3926355A

US3926355A - Apparatus for intermittently feeding lengths of continuous sheet material at a constant velocity

Info

Publication number: US3926355A
Application number: US516371A
Authority: US
Inventors: George N Bonnell
Original assignee: Bobst Champlain Inc
Current assignee: Bobst Champlain Inc
Priority date: 1974-10-21
Filing date: 1974-10-21
Publication date: 1975-12-16
Anticipated expiration: 1992-12-16

Abstract

Apparatus for intermittently feeding lengths of continuous sheet material moving at a constant velocity includes a metering roll rotating at a constant speed to accept the material, a feed roll intermittently engaging and disengaging the material and reciprocating brake means for stopping the movement of the material during each disengagement thereof by the feed roll are provided to intermittently feed given lengths of material. The feed roll and brake means are reciprocated through the use of a multiple cam system and the brake means is designed to exert a high brake force on the sheet material without distorting the normal path of movement of the material. A looper roll is located along the material length between the metering roll and the feed roll. Means are provided for cyclically moving the looper roll to compensate for material buildup which occurs during engagement of the material by the brake. The moving means comprises means for oscillating a first member which is connected by means of a pivotal joint to an elongated second member which in turn is connected to the looper roll. The first member consists of a first part and a second part. Means for adjusting the position of the second part relative to the first part are provided such that the distance between the oscillating means and the pivotal joint may be varied to vary the path of movement of the roll, thereby permitting the apparatus to be adjusted to intermittently feed different lengths of material.

Description

United States Patent 1191 Bonnell [4 Dec. 16, 1975 [54] APPARATUS FOR INTERMITTENTLY FEEDING LENGTHS OF CONTINUOUS SHEET MATERIAL AT A CONSTANT VELOCITY [75] Inventor: George N. Bonnell, Summerville,

[73] Assignee: Bobst Champlain, Inc., Roseland,

22 Filed: on. 21, 1974 21 Appl. No.: 516,371

[52] US. Cl. 226/114; 226/117; 226/145 [51] Int. Cl. B65I'I 17/42 [58] Field of Search 226/113, 114, 117, 35, 226/39, 195, 145, 155

[56] References Cited UNITED STATES PATENTS 3,608,804 9/1971 Swansen, Jr. 226/114 3,702,673 11/1972 Zernov et a1. 226/114 Primary ExaminerAllen N. Knowles [57] ABSTRACT Apparatus for intermittently feeding lengths of continuous sheet material moving at a constant velocity includes a metering roll rotating at a constant speed to accept the material, a feed roll intermittently engaging and disengaging the material and reciprocating brake means for stopping the movement of the material during each disengagement thereof by the feed roll are provided to intermittently feed given lengths of material. The feed roll and brake means are reciprocated through the use of a multiple cam system and the brake means is designed to exert a high brake force on the sheet material without distorting the normal path of movement of the material. A looper roll is located along the material length between the metering roll and the feed roll. Means are provided for cyclically moving the looper roll to compensate for material buildup which occurs during engagement of the material by the brake. The moving means comprises means for oscillating a first member which is connected by means of a pivotal joint to an elongated second member which in turn is connected to the looper roll. The first member consists of a first part and a second part. Means for adjusting the position of the second part relative to the first part are provided such that the distance between the oscillating means and the pivotal joint may be varied to vary the path of movement of the roll, thereby permitting the apparatus to be adjusted to intermittently feed different lengths of material.

12 Claims, 2 Drawing Figures U.S. Patent Dec.16,1975 SheetlofZ 3,926,355

US. Patent Dec. 16,1975 Sheet 2 of2 3,926,355

APPARATUS FOR INTERMITTENTLY FEEDING LENGTHS OF CONTINUOUS SHEET MATERIAL AT A CONSTANT VELOCITY The present invention relates to apparatus for handling continuous sheet material and more particularly to an apparatus for intermittently feeding lengths of sheet material moving at a constant velocity which includes an adjustable cyclically moving looper roll.

Material handling problems often arise on high speed assembly lines forming products from continuous sheet material which utilize equipment requiring intermittent feeding as well as equipment performing operations on sheet material moving at a constant velocity. For example, operations such as cutting, creasing, bending, etc. are performed on continuous sheet material in sheet or web form as a portion of the manufacturing process for the production of a variety of different products. One method of performing these operations on the material is to pass a given length of material between a pair of relatively movable dies. The dies are provided with the appropriate contours to perform the desired operation when the dies are moved together to engage the material therebetween. To prevent tearing of the material, the material must be stationary relative to the dies when the dies are closed. Thus, in order to properly perform operations with equipment utilizing a pair of relatively movable dies, the material must be fed to the dies intermittently. More specifically, the material must be moved in place between the dies when the dies are apart and then held stationary relative to the dies as the dies engage the material. However, since equipment requiring intermittent feeding is often used on an assembly line in conjunction with other apparatus which handles the sheet material in a continuous fashion, difficulties arise in accomplishing a smooth transition between the two types of mechanisms without reducing the overall speed and thus the efficiency of the line.

In order to eliminate this difficulty, material handling apparatus having a looper roll, which is cyclically moved along a predetermined path, is placed between the continuous feed metering rolls (which receive the sheet material traveling at a constant velocity) and the intermittent feeding mechanism (which has a reciprocating brake to intermittently stop the material) to vary the path of travel of the continuous sheet material therebetween. Thus, when the sheet is engaged by the brake, the slack in the material which normally occurs between the brake and the metering rolls is compensated for by the movement of the looper roll as the looper roll effectively lengthens the path of travel of the material between the metering rolls and the brake. On the other hand, when the brake is not in engagement with the material, the looper roll effectively shortens the path of travel of the continuous sheet material between the brake and the metering rolls such that the slack accumulated during the period when the brake engages the material is eliminated without a jerking motion. This system has worked effectively to interface equipment requiring intermittent feed into an assembly line performing operations on sheet material moving at a constant velocity.

From this brief explanation it is obvious that the. movements of the looper roll must be synchronized with the movements of the reciprocating brake and of the other portions of the apparatus. In addition, the length of the path of travel of the looper roll, referred to as the looper roll excursion, must be adjusted in accordance with the length of material which is being intermittently fed. For instance, if carton blanks of a given length are being cut from a continuous sheet material on apparatus having relatively movable dies, the cyclic operation of the looper roll must be adjusted to intermittently feed lengths of material in accordance with the size requirements of the finished carton blank. Should the apparatus be required to cut carton blanks of a different length, then the looper roll excursion must be varied to intermittently feed different lengths of material in accordance with the size of new blanks.

Prior art systems incorporating looper roll mechanisms with adjustable excursions have required an adjustable looper roll linkage on each side of the looper roll, each of which is connected to the oscillating mechanism. This was due to the particular structure of the members of the linkage and the placement of the adjustable linkage member within the structure. It is important to eliminate as much mass as possible in such a mechanism in order to reduce the inertia of the oscillating parts. Having dual adjustable linkages, of course, increases the total mass of the system. The present invention eliminates the necessity for dual adjustable linkages thereby reducing the total inertia of the linkage as well as eliminating the necessity for careful coordination of linkage adjustments.

In addition, prior art systems utilized a rotating cam to cause the feed roll and brake means to reciprocate. The brake means was designed to move the sheet material out of the normal flow path as the brake arm was moved, by action of the cam, upwards from underneath the flow path to clamp the sheet material between the arm and a stationary abutment located a small distance above the flow path. This cam system, although it provided the necessary synchronization, was not sufficient to accurately control the movement of the feed roll and brake means resulting in a loss of feeding time or jamming under the die. Moreover, accurate braking of the sheet material depends also upon the application of a high brake force almost instantaneously as the brake contacts the material. However, if the sheet material is moved out of the normal flow path by the brake, the application of this force may cause rupture of the sheet material.

It is, therefore, a prime object of the present invention to provide apparatus for intermittently feeding lengths of sheet material moving at a constant velocity wherein the excursion of the looper roll may easily and quickly be adjusted to feed different lengths of material.

It is a second object of the present invention to provide apparatus for intermittently feeding lengths of continuous sheet material moving at a constant velocity wherein the looper roll excursion adjustment mechanism has reduced inertia because of the elimination of the necessity for dual adjustable linkages, one on each side of the looper roll.

It is a third object of the present invention to provide apparatus for intermittently feeding lengths of continuous sheet material moving at a constant velocity wherein only a single adjustable linkage is required thus eliminating the necessity for coordinating linkage adjustments.

It is another object of the present invention to provide apparatus for intermittently feeding lengths of continuous sheet material moving at a constant velocity wherein the looper roll is connected to an oscillating shaft by means of a mechanical linkage which is adjustable to vary the looper roll excursion.

It is a further object of the present invention to provide apparatus for intermittently feeding lengths of sheet material moving at a constant velocity wherein the feed roll and brake means are reciprocated through a multiple cam system.

It is still another object of the present invention to provide apparatus for intermittently feeding lengths of sheet material moving at a constant velocity wherein the brake means exerts a high brake force on the sheet material without moving the sheet material from the normal flow path.

In accordance with the present invention, the apparatus includes a metering roll rotating at a constant speed to accept the sheet material moving at a constant velocity. A feed roll intermittently engaging and disengaging the material and reciprocating brake means for stopping the movement of the material during each disengagement of the material by the feed roll are provided to intermittently feed given lengths of material. A looper roll is located along the material between the metering roll and the feed roll. Means are provided for cyclically moving the looper roll. The moving means comprises a shaft, means for oscillating the shaft and mechanical linkage operatively connecting the shaft to the looper roll. The linkage includes first and second members which are operatively connected together by a pivotal joint. The first member has a first part which is mounted to the shaft and a second part which is connected to the pivotal joint. Means are provided for adjusting the position of the second part relative to the first part such that the distance between the shaft and the joint may be varied. Variation of the relative position of the parts serves to vary the looper roll excursion thereby permitting the apparatus to intermittently feed different lengths of material.

The feed roll and brake means are reciprocated by a multiple cam system to assure accurate timing. The linkage arm connecting the cam system to the brake means includes a compression spring and piston arrangement. The displacement of the linkage arm, caused by the cam system, is greater than the movement of the brake arm itself. The spring and piston permit the length of the linkage arm to be slightly re duced to compensate for this difference assuring that a relatively high brake force is applied to the sheet material immediately upon braking. The brake arm is designed so that the sheet material is not moved out of the normal flow path when the brake engages the material such that the large braking force can be applied without rupture of the material.

To the accomplishment of the above and to such other objects as may hereinafter appear, the present invention relates to apparatus for intermittently feeding lengths of continuous sheet material moving at a constant velocity as defined in the appended claims and described in the specification taken together with the accompanying drawings in which:

FIG. 1 is a side elevational view of a preferred embodiment of the present invention; and

FIG. 2 is an isometric view of the cam system of the present invention utilized to reciprocate the feed roll and brake means.

FIG. 1 shows the apparatus er the present invention which is adopted to receive a c' oi'itiiillous sheet of material 10, in either sheet or web ff'rii, :vvhich is fed to the apparatus at a constant speed and alter the movement thereof such that given lengths of material are intermittently fed from the apparatus to the work station (not shown). The work station may consist, in part, of a pair of relatively movable dies or the like which intermittently engage the material to perform an operation. Work stations of this nature are conventional in the art and since the work station forms no part of the present invention, it is not described or shown herein. However, it should be appreciated, that the apparatus of the present invention can be used with any work station which requires intermittent feeding of a continuous sheet material.

As the sheet material 10 enters the apparatus, it moves past an idling roll 12 to a set of metering rolls 14, 16, and 18. One or more of the metering rolls 14, 16, and 18 is positively driven at a constant speed by a drive mechanism (not shown) of the type well known in the art. The metering rolls 14, 16, and 18 feed the material 10 at a constant velocity to looper roll 20. The material 10 travels around looper roll 20 to an intermittent feed means comprising a reciprocating brake mechanism, generally designated 22, and through a feed mechanism, generally designated 24, which intermittently engages and disengages the material 10. From the intermittent feed mechanism 24, given lengths of material 10 intermittently exit the apparatus and may be fed directly to the work station.

Intermittent feed mechanism 24 and brake means 22 are driven by a cam system, generally designated 26 as seen in FIG. 2. Cam system 26 consists of four

cams

26a, 26b, 26c, and 26d, all of which are mounted to a shaft 27. Shaft 27 is rotated by a standard drive mechanism (not shown). Reciprocating brake mechanism 22 includes an abutment or support 28 located beneath the path of travel of material .10. Support 28 has a planar surface directly underneath the fiow path of material 10. Located on the upper side of material 10 in alignment with support 28 is a reciprocating brake bar 30. The movement of brake bar 30 is controlled so that it moves along a path substantially perpendicular to the path of travel of material 10 from a position adjacent support 28 to a position remote therefrom. Thus, the movement of brake bar 30 serves to alternately engage and disengage material 10 between the bar 30 and support 28.

Brake bar 30 is connected to a bracket 32 which in turn is connected to a second bracket 34 by means of a shaft 33. Bracket 34 is pivotally joined to one end of a connecting rod, generally designated 36. The other end of connecting rod 36 is pivotally mounted to a triangular bracket 38 which in turn is pivotally mounted to the housing 40 of the apparatus by a pin 39.

The contour of cam 26a determines the point of initial movement of bar 30 towards support 28. When roller 42 encounters the protrusion of the surface of cam 26a,

rollers

42 and 43, which are connected by the shaft 45, are moved to pivot bracket 38 and thus connecting rod 36 to cause bar 30 to engage material 10 between bar 30 and support 28. As shaft 27 rotates, the protrusion on cam 26b is moved underneath cam 43, and both

cams

42 and 43 are supported. As shaft 27 continues to rotate, the surface of cam 26a drops below the roller and only roller 43 is supported. When cam 26b has rotated to the point where the protrusion thereon stops,

roller

42 and 43 move downwards thereby pivoting bracket 38 to the original position and moving bar 30 away from support 28 to release material 10. Bar 30 is spring loaded to return to the release position by a spring (not shown). Thus, cams 26a and 26b control the closing and opening respectively of brake means 22.

It should be appreciated that the movement of brake bar 30 does not cause material to deviate from the normal flow path. Since support 28 is placed beneath the normal flow path and normally supports the material, when the material is engaged between the brake bar 30 and support 28 the position thereof is not altered. This prevents rupture of the material when the force of the brake bar is applied to the material.

The size of the protrusions of cams 26a and 26b, and thus the movement of

rollers

42 and 43 is greater than the clearance between bar 30 and support 28 when the material is not engaged. This difference is compensated for by the connecting rod 36 in a manner which provides an almost instantaneous application of force to material 10 when the brake is closed. This enhances the accuracy of the timing of brake means 22 while preventing tearing.

Connecting rod 36 consists of a spring 37 adjusted by a nut 39 to a high compression load, for instance 600 pounds, which will be applied to a cylinder 41. During installation, the length of connecting rod 36 is adjusted by a turn buckle 47 such that the distance between bar 30 and support 28 is less than the height of the protrusion on cam 26a and 26b. A piston 49, situated within cylinder 41 moves relative to cylinder 41 when the brake bar 30 is moved adjacent support 28 and compensates for the difference between the movement of the

rollers

42 and 43 and bar 30 by causing the effective length of connecting rod 36 to decrease. As a result, the spring force is almost instantly applied to the sheet material when bar 30 is moved.

The intermittent feed mechanism 24 comprises a stationary roll 48 positively driven at a constant velocity by a conventional drive mechanism (not shown) and a movable roll 50 which is rotatably mounted to a bracket 52. Bracket 52 is pivotally mounted on a shaft 54. A second bracket 56 is also mounted on shaft 54 and fixedly connected to bracket 52 such that bracket 52 moves with bracket 56. One end of an adjustable connecting rod 58 is pivotally mounted on bracket 56 such that the movement of connecting rod 58 moves moves

brackets

56 and 52 as well as roll 50. The other end of connecting rod 58 is connected to a triangular bracket 60 similar to bracket 38. Triangular bracket 60 is pivotally mounted to housing 40 by a pin 61 and carries

rollers

62 and 63 in freely rotatable fashion at a position such that

rollers

62 and 63 communicate with the surface of

cams

26c and 26d respectively. Roller 62 follows the contours of cam 260 such that bracket 60 is pivoted relative to housing 40 and the closing of the nip between rolls 50 and 52 is initiated. Cam 26d causes the opening of this nip, in the same manner as described above with respect to cams 26a and 26b.

The material 10 is thus intermittently engaged and released between rolls 48 and 50. When the material 10 is engaged between rolls 48 and 50, a force is exerted on the material by the rotation of roll 48 thereby causing it to exit the apparatus at a predetermined speed. On the other hand, when rolls 48 and 50 are moved apart, no force is exerted on material 10. The action of reciprocating brake mechanism 22 is synchronized with the movement of intermittent feed roll mechanism 24 such that material 10 is engaged by the feed mechanism 24 only when the reciprocating brake means 22 is not acting on material 10. On the other hand, when the material is engaged by the reciprocating brake mechanism 22, rolls 48 and 50 are apart such that no force is exerted on material 10. In this manner, the movement of material 10 is alternatively stopped and started.

Since material 10 is fed from metering rolls 14, 16, and 18 at a constant velocity but is stopped and started by means of reciprocating brake mechanism 22 and intermittent feed mechanism 24, it is obvious that during the time when the material 10 is stopped a certain amount of slack will accumulate along the path of material 10 between the metering rolls and the brake mechanism. On the other hand, when the brake mechanism 22 releases the material and the intermittent feed roll engages it, the material will be fed from the apparatus at a given exit speed. The exit speed of the material must, of necessity, be greater than the speed imparted to the material by the metering rolls 14, 16, and 18. Otherwise, the slack between the metering rolls and the brake mechanism would continually build up.

The slack forms a loop of matarial between the metering roll and the brake mechanism. Looper roll 20 is utilized to take up this slack as it is being formed and prevent breakage of the material caused by a jerking motion as the slack is eliminated from the path of material by the feeding of material 10 by intermittent feed roll mechanism 24 at a relatively high exit speed. The contours of

cams

26a, 26b, 26c, and 26d control the movements of brake 22 and feed mechanism 24. Once the relative positions of the cams within cam system 26 are set, they always remain the same. In order to feed different lengths of material, the intake speed and speed of metering rolls 14, 16, 18 are adjusted such that the material moves through the apparatus at a greater speed if longer lengths of material are required and lesser speed if shorter lengths are required. Thus, by simply adjusting the speed of the material, different lengths of material can be intermittently fed.

An oscillating shaft 68 is utilized to drive looper roll 20. Oscillating shaft 68 is operably connected to looper roll 20 by means of a mechanical linkage which comprises a first member, generally designated 70, and a second member, generally designated 72. Member 70 and member 72 are connected by means of a pivotal joint at 74. The looper roll 20 is rotatably mounted on a bracket 76 which, in turn, is fixedly mounted to rotatable shaft 78. A second bracket 80 is also mounted to shaft 78 and forms a connection between shaft 78 and member 72. First member 70 of the linkage mechanism is comprised of a first part 82 and a second part 84. Part 82 is operably connected to shaft 68 by means of bracket 86. Part 84 is connected to member 72 by means of pivotal joint 74.

Parts

82 and 84 are movable relative to each other varying the effective distance between oscillating shaft 68 and pivotal joint 74. Part 84 interlocks with and is slidably mounted on part 82. A screw 88 extends from part 84 along the length of part 82 and past a rotatable worm wheel 90 mounted on part 82. Worm wheel 90 is externally threaded to mesh with the threads on screw 88 such that the rotation of worm wheel 90 causes the movement of screw 88 and thus part 84 relative to part 82. The rotation of worm wheel 90 can be controlled by any convenient means. Herein, the control means is shown as a flexible cable 92 which extends to the exterior of the housing for ease of manipulation. It is necessary that cable 92 be flexible as first member 70 will reciprocate in accordance with the oscillations of shaft 68 and part of cable 92 must follow along with this 7 movement. The rotation of flexible cable 92 will serve to adjust the excursion of looper roll 20.

In addition, member 72 is provided with a plurality of apertures 94 such that the effective length of member 72 between pivotal joint 74 and bracket 80 may also be adjusted. The adjustment of the effective length of member 72 is a gross adjustment of the path of movement of looper roll 20 whereas the adjustment of the effective length of first member 70 is a fine adjustment therefor. Normally, only fine adjustments of the path of looper roll 20 are required.

The oscillations of shaft 68 serve to reciprocate members 70 and 72 thereby causing looper roll 20 to move along an are shown by arrows 96 whose center is shaft 78. The movement of looper roll 20 is synchronized with the remainder of the mechanism. This may be accomplished by utilizing the same drive mechanism for oscillating shaft 68 as is utilized to rotate cam 26. Looper roll 20 is shown in an intermediate position. When brake mechanism 22 stops the movement of material 10, the looper roll 20 will be moved towards the right (as seen in the drawing) in order to take up the slack in the material between the metering rolls l4, 16, 18 and the reciprocating brake mechanism 22. On the other hand, when reciprocating brake mechanism 22 is not engaging the material and intermittent feed mechanism 24 is exerting a force on the material, the looper roll will be moved towards the left (as shown in the drawing) such that the slack in the material is eliminated in an even fashion, and tearing of material 10 is prevented. When the length of material which is intermittently fed by the apparatus is changed, worm wheel 90 will be rotated to adjust the excursion of looper roll 20 such that movement thereof is sufficient to compensate for the different amount of slack formed in the material.

The present invention eliminates the necessity of having dual adjustable linkage mechanisms, one on each side of the looper roll, as in prior art systems, thereby reducing the inertia of the moving parts of the mechanism. In addition, since the adjustable member is required only on one side of the looper roll,'there is no necessity for coordinating two separate adjustments, thereby reducing machine set-up time. Moreover, the present invention includes brake means which do not alter the normal flow path of the material and which by means of a spring loaded connecting rod having a reducible length provide for the application of a high braking force to reduce rupture of the material. Also, the feed roll and brake means are driven by a multiple cam system to assure accurate timing.

While but a single preferred embodiment of the present invention has been described herein for purposes of illustration, it is apparent that many variations and modifications can be made thereon. It is intended to cover all of these variations and modifications which fall within the scope of the present invention as defined by the following claims:

I claim:

1. Apparatus for handling a continuous sheet material comprising a metering roll, a feed roll intermittently engaging and disengaging the material, reciprocating brake means for stopping the movement of the material during each disengagement of the material by said feed roll, a looper roll located along the material between said metering roll and said feed roll and means for cyclically moving said looper roll, said moving means comprising a shaft, means for oscillating said shaft and mechanical linkage operatively connecting said shaft to said looper roll, said linkage comprising a first and a second member, said members being operatively connected together by a pivotal joint, said first member comprising a first part mounted to said shaft and a second part connected to said pivotal joint and means for adjusting the position of said second part relative to said first part such that the distance between said shaft and said joint may be varied.

2. The apparatus of claim 1 further comprising a support and wherein said linkage further comprises a first and a second bracket, said looper roll being mounted on one end of said first bracket, the other end of said first bracket being connected to said second bracket and pivotally mounted on said support, said second bracket being operatively connected to said second member for movement therewith.

3. The apparatus of claim 2 further comprising means for changing the effective length of said second memher.

4. The apparatus of claim 1 wherein said adjusting means comprises a rotatable worm wheel mounted on said first part and a screw mounted on said second part and extending outwardly therefrom adjacent said worm wheel for engagement therewith.

5. The apparatus of claim 2 wherein said adjusting means comprises a rotatable worm wheel mounted on said first part and a screw mounted on said second part and extending outwardly therefrom adjacent said worm wheel for engagement therewith.

6. The apparatus of claim 3 wherein said adjusting means comprises a rotatable worm wheel mounted on said first part and a screw mounted on said second part and extending outwardly therefrom adjacent said worm wheel for engagement therewith.

7. Apparatus for handling a continuous sheet material comprising a metering roll, a feed roll intermittently engaging and disengaging the material, reciprocating brake means for stopping the movement of the material during each disengagement of the material by said feed roll, a looper roll located along the material between said metering roll and said feed roll and means for cyclically moving said looper roll, said feed roll and brake means being driven by a multi-cam system, said cam system comprising a first cam means for causing said feed roll to engage the material, second cam means for causing said feed roll to disengage the material, third cam means for causing said brake means to stop the material, and fourth cam means for causing said brake means to release the material.

8. The apparatus of claim 7 wherein each of said cam means is mounted on. the'same rotating shaft.

9. The apparatus of claim 7 wherein each of said feed roll and said brake means is operably connected to said cam means by a connecting rod and a pair of cam following elements operably connected thereto.

10. The apparatus of claim 9 wherein each of said pairs of elements is mounted on a single shaft.

11. The apparatus of claim 9 wherein the connecting rod for said brake means comprises means compressible to reduce the effective length of said rod.

12. The apparatus of claim 11 wherein said compressible means comprises a spring loaded piston and cylinder arrangement.

Claims

3. The apparatus of claim 2 further comprising means for changing the effective length of said second member.

8. The apparatus of claim 7 wherein each of said cam means is mounted on the same rotating shaft.