Log Pre-centering Apparatus For Veneer Lathes

Abstract

Each of a pair of independently, vertically movable log-supporting bases mounts a pair of opposed knees for confining a log between them. The knees are movable horizontally inward in direct proportion to the vertical movement of the associated base, so that when the confronting faces of the knees are brought into abutment with a log supported on the bases the axial centerline of the log is aligned with the axial center of log transfer mechanism which, in turn, is aligned with the rotational axis of veneer lathe chucks in the log transferring position of said mechanism.

Description

BACKGROUND OF THE INVENTION

This invention relates to veneer lathes and more particularly to novel means for pre-establishing the axial centerline of a log prior to transfer to veneer lathe chucks.

Log pre-centering apparatus provided heretofore generally are characterized by structures in which opposed log-centering members are movable vertically above and below the log to be centered. Such structures not only utilizes excessive headroom, but also require complex and costly transfer apparatus capable of circumventing the centering components above the log. Such apparatus also are slow in operation and therefore restrictive of production, and inaccurate to a degree which results in significant waste of peeler log material.

SUMMARY OF THE INVENTION

In its basic concept the log pre-centering apparatus of this invention utilizes the coordinated vertical movement of a pair of log-supporting bases and the horizontal movement of a pair of opposed log-confining knees on each base to establish the axial centerline of a log for transfer to the rotational axis of veneer lathe chucks.

It is by virtue of the foregoing basic concept that the principal objective of this invention is achieved; namely, to overcome the above enumerated disadvantages of prior apparatus.

The foregoing and other objects and advantages of this invention will appear from the following detailed description, taken in connection with the accompanying drawings of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary view in side elevation of log pre-centering apparatus embodying the features of this invention.

FIG. 2 is a fragmentary view in end elevation as viewed from the right in FIG. 1.

FIG. 3 is a foreshortened, fragmentary view in side elevation of a modified form of log pre-centering apparatus embodying the features of this invention.

FIG. 4 is a fragmentary plan view as viewed from the top in FIG. 3.

FIG. 5 is a fragmentary side elevational view, similar to FIG. 3, illustrating by solid and broken lines the operation of the log precentering apparatus.

FIG. 6 is a foreshortened fragmentary view in longitudinal section taken on the line 6--6 in FIG. 7 of log transfer apparatus for association with the log pre-centering apparatus of this invention.

FIG. 7 is a foreshortened plan view of the log transfer apparatus of FIG. 6.

FIG. 8 is a transverse sectional view taken on the line 8--8 in FIG. 6.

FIG. 9 is a transverse sectional view taken on the line 9--9 in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For purposes of explanation, reference first is made to FIGS. 6-9 of the drawings which illustrate apparatus by which a log is transferred from pre-centering apparatus to a veneer lathe. The transfer apparatus illustrated incorporates two types of transfer mechanism, one preferably for the transfer of relative large diameter peeler logs and identified generally by numeral 10 and the other preferably for the transfer of relatively small diameter peeler logs and identified generally by the numeral 12. Conveniently, both mechanisms may be mounted upon a common framework for selective use, although it will be understood that either one of them may be omitted, if desired.

The framework includes a pair of laterally spaced longitudinal I-beams 14 secured together at their opposite ends by transverse channel beams 16 and supported by vertical posts 18 at a desired elevation.

The transfer mechanism 10 normally employed for the transfer of relatively large diameter peeler logs includes a transverse carriage frame 20 supported between the longitudinal beams 14 for reciprocation relative thereto by rollers 22 which track on the lower inner flanges of the I-beams 14. Motive power for the carriage is provided by a pair of laterally spaced chains 24 which are secured at their opposite ends to the longitudinal ends of the framework. Intermediate their ends the chains are trained over longitudinally spaced idler guide sprockets 26 and looped about drive sprockets 28 secured to a common drive shaft 30. The drive shaft extends transversely of the carriage and is coupled through a drive chain 32 and appropriate sprockets and gear reduction mechanism to an electric drive motor 34 mounted on the carriage.

A pair of laterally spaced log clamping members 36 are supported by the carriage and mounted for lateral adjustment toward and away from each other. Thus, a pair of transversely elongated support units are spaced apart longitudinally on the carriage. One of the clamping members is secured to the outer sleeve member 38 of each unit and the other clamping member is secured to the inner rod member 40 of each unit. Each rod member is mounted for longitudinal movement relative to the carriage frame, and each sleeve member is mounted for longitudinal movement relative to its associated rod member. A hydraulic piston-cylinder unit 42 mounted on the carriage engages each clamping member for moving the latter.

Accordingly, by appropriate application of fluid pressure to the drive units 42 the pair of clamping members may be moved toward each other to engage the opposite ends of a peeler log for transfer from log pre-centering apparatus to a veneer lathe, or may be moved away from each other to release the log after it has been secured between the rotary chucks of a veneer lathe.

The carriage 20 functions to reciprocate the clamping members 36 between a log pick-up position at log precentering apparatus and a log transfer position at a veneer lathe. Referring to FIG. 6 of the drawings, let it be assumed that the point X represents the axial centerline of a pre-centered log at pre-centering apparatus described hereinafter and that the point Y represents the rotational axis of laterally spaced chucks of a veneer lathe. It will be understood that limit switches or other conventional control mechanism are provided for stopping the carriage at these positions.

The lower log-engaging portion of each clamping member is provided with log-engaging spikes 44 surrounding an opening 46 in the side facing the direction of movement toward the veneer lathe, the opening being of sufficient size to receive the veneer lathe chuck therein. Thus, when the log has been secured between the lathe chucks, the clamping members may be retracted and returned to the log precentering apparatus, as will be understood.

The transfer mechanism 12, normally used with relatively small diameter peeler logs, includes a rock shaft 50 which extends transversely of the framework and is journaled in bearings mounted on the framework. The rock shaft supports a pair of laterally spaced log clamping arms 52 which, like the clamping members previously described, function to releasably engage the opposite ends of a peeler log. Thus, the arms are mounted pivotally on brackets 54 projecting from the rock shaft and are interconnected intermediate their ends by the transversely extensible fluid pressure piston-cylinder unit 56.

The rock shaft is connected through a gear reduction unit 58 to an electric drive motor 60 mounted on the framework. The electric motor functions to move the clamping arms arcuately between a log pickup position at log pre-centering apparatus and a log transfer position at a veneer lathe.

The lower ends of the clamping arms are provided with spikes 44 and openings 46 similar to and for the same purpose as the spikes and openings previously described.

It will be understood that when the transfer mechanism first described is to be used, the electric drive motor 60 is operated to rotate the clamping arms 52 upwardly above the framework to permit reciprocation of the carriage 20 between the log pick-up position and log transfer position previously described.

Since the transfer mechanism 12 functions to transfer logs of relatively small diameter, it is positioned closer to the veneer lathe than the transfer mechanism first described. Accordingly, the point Z represents the axial centerline of a precentered log at pre-centering apparatus described hereinafter.

Referring now to FIGS. 1 and 2 of the drawings the log precentering apparatus illustrated therein includes a framework of horizontal beams 70 supported above a foundation base 72 by means of spaced vertical beams 74 and hollow posts 76. Supported on the horizontal beams of the framework is a box frame 78 which supports a pair of laterally spaced, upwardly projecting log supports 80. The upper portions of these log supports are interconnected for stability by means of transverse braces 82. The upper surfaces of the supports form shallow, aligned Vs for centering a log L across them as the log is delivered thereto from an infeed conveyor illustrated by the broken line 84.

Disposed laterally outward of each log support 80 is a longitudinally elongated elevator base 86. Each base is supported centrally on the upper end of a hollow post 88. The post preferably is of square or other non-circular cross section and is guided for vertical reciprocation within the correspondingly non-circular hollow guide tube 90. The guide tube is secured to the box frame 78 and projects vertically upward and downward therefrom.

The lowermost position of each post 88, and hence each elevator base 86, is defined by abutment of the under side of the base with the upper end of the guide tube.

Vertical reciprocation of each post is provided by an elongated hydraulic piston-cylinder unit. The elongated cylinder 92 is supported at its lower end on a base plate 94 on the foundation 72 and extends vertically upward into the hollow post 88. The piston rod 96 extending vertically upward from the cylinder is connected at its upper end to the post by means of pivot shaft 98. Thus, each elevator base 86 is movable independently of the other from the lowered position illustrated in FIGS. 1 and 2 vertically upward by application of hydraulic pressure to the bottom end of the associated cylinder.

Mounted on each elevator base is a pair of log-abutting knees 100 and 102. Each knee includes a horizontal mounting member 104 and an upstanding log-abutting member 106 secured thereto and reinforced by the angular brace 108. Each mounting member is provided with a rectangular opening 110 slidably receiving the elevator base 86. A slot 112 in the bottom side of the mounting member receives the downwardly extending flange 114 of the elevator base. The pair of knees thus are movable on the elevator base toward and away from each other.

Means is provided for moving the pair of knees associated with each elevator base simultaneously toward each other in direct proportion to the vertical upward movement of the elevator base. In the embodiment illustrated in FIGS. 1 and 2 a pair of elongated chains 120 are connected at one end to laterally spaced lugs 122 projecting downward from the mounting member 104 of each knee. The chains extend inward toward the longitudinal center of the elevator base and are reeved over sprockets 124 mounted for rotation on a shaft carried by the flange 114 of the elevator base. The lower end of each chain is connected to an elongated rod 126 which extends downward slidably through an opening in an abutment block 128 secured to the vertical beam 74 of the framework. The lower end of each rod carries an enlarged head 130 arranged to engage the associated abutment block at a predetermined elevation of the elevator base 86, whereupon further upward movement of the latter effects movement of the pair of knees toward each other, as explained more fully hereinafter.

It will be apparent that the chains 120 may be replaced with cables or other suitable forms of flexible lines, in which case the sprockets 124 will be replaced with appropriate forms of pulleys.

Means also is provided for retracting the pair of knees simultaneously away from each other as the associated elevator base is lowered toward the position illustrated. In the embodiment illustrated such means is provided by a pair of cables 132 connected at their upper ends one to each of the mounting members 104 of the knees. Each cable is reeved about a pulley 134 mounted on a longitudinal extension of a flange 114 of the elevator base, thence about a pulley 136 mounted rotatably on a tab extending downward from the flange and thence downward through the associated hollow post 76 where it is connected at its lower end to a heavy weight 138 confined freely in the post.

A central longitudinal slot 140 in each knee mounting member above the rectangular opening receives the associated cable above the elevator base during reciprocation of the knees, as will be understood.

The operation of the log precentering apparatus illustrated in FIGS. 1 and 2 is as follows: With both elevator bases 86 lowered to the position illustrated, a log L is delivered by the infeed conveyor 84 and deposited and centered on the spaced log supports 80. Hydraulic pressure then is applied to the bottom ends of the cylinders 92, whereupon the pair of elevator bases are caused to move upward into abutment with the bottom of the log at longitudinally spaced points on the latter (FIG. 2) and to lift the log therefrom.

It is to be noted, particularly from FIG. 1, that as the log-abutting surface of each elevator base 86 moves upward through the distance a, which is the same as the distance between the head 130 of each rod 126 and its associated abutment block 128, the associated pairs of knees are not caused to move inward toward each other. However, when the elevator bases reach the level 86', the rod heads 130 also engage the abutment blocks 128. Thereupon, further upward movement of the elevator bases and, of course, the sprockets 124, causes the pairs of knees to move inward toward each other. Each knee moves inward the same distance as its associated elevator base moves upward, and therefore the point of intersection of the knee and base traverses a line which is disposed at an angle of 45.degree. with respect to vertical or horizontal.

Accordingly, it can be seen that if each elevator base moves upward from its position 86' a distance equal to one-half the distance between the associated knees at their illustrated positions of extreme retraction, the log-abutting members 106 of the associated pair of knees will have been brought into mutual abutment on the plane of the vertical line 142 (FIG. 1). The point X of intersection of this line with the horizontal line 144 representing the log supporting surface of the associated elevator base defines the axis on which the axial centerline of a log is to be precentered.

In FIGS. 1 and 2 of the drawing a log L to be precentered is shown in full lines with an exaggerated taper, merely to better illustrate the relative differences of movement of each elevator assembly to effect centering of a tapered log on its axial centerline.

It will be apparent that the pair of knees 100, 102 adjacent the smaller diameter end of the log must move closer together to engage that end of the log than must the pair of knees 100, 102 associated with the larger diameter end of the log. In order for the first mentioned pair of knees to move closer together, the associated elevator base 86 must also move upward a corresponding distance higher than the elevator base 86 associated with the larger diameter end of the log. Accordingly, when the pair of knees have engaged the smaller diameter end of the log and the other pair of knees have engaged the larger diameter end of the log the axial centerline of the log is established precisely on the axis X in the manner illustrated in broken lines.

Since the pairs of knees are in firm abutment with the log and therefore cannot move farther inward toward each other, the associated elevator bases are restrained against further upward movement. The increased hydraulic pressure in the cylinders 92 thus may be utilized, for example, through a pressure sensitive switch (not shown), to actuate the log transfer apparatus to pick up the precentered log and transfer it to the veneer lathe. In this regard, it will be understood that the point X of intersection of the lines 142 and 144 illustrated in FIG. 1 also corresponds to the axis Y of rotation of the veneer lathe chucks (FIG. 6), as previously explained.

It will be further understood that when the transfer mechanism has engaged the opposite ends of the precentered log, manual or automatic control mechanism (not shown) is operated to retract the piston rod 96 of the cylinder 92 and thus lower the elevator bases 86. Lowering of these bases simultaneously effects retraction of the knees 100, 102 to the position illustrated in FIGS. 1 and 2, with the assistance of the weights 138.

Referring now to FIGS. 3-5 of the drawings, the log precentering apparatus shown therein is employed primarily with logs of relatively small diameter and thus is associated with the log transfer mechanism 12 illustrated in FIG. 6. The apparatus includes a pair of laterally spaced log supports 150 similar to the log supports 80 previously described. Each of the log supports is supported by a plurality of longitudinally spaced beams including the pair of beams 152 illustrated.

Outward of each pair of beams is a longitudinally elongated elevator base 154 in the form of a box frame. A T-shaped guide bar 156 secured to the inner side of each base 154 is slidably received in a pair of longitudinally spaced, vertical guides 158 secured to the beams. The guide bar and guides thus serve to guide the elevator base for vertical reciprocation.

Each base is supported for vertical reciprocation by an elongated hydraulic piston-cylinder unit. The elongated cylinder 160 is supported at its lower end on a base plate 162 on the foundation 72 and extends vertically upward therefrom. A piston rod 164 extending vertically upward from the cylinder is connected at its upper end to the base by means of a pivot shaft 166. Accordingly, each elevator base is movable independently of the other from the lowered position illustrated in FIG. 3 vertically upward by application of hydraulic pressure to the bottom end of the associated cylinder.

Mounted on each elevator base is a pair of log-abutting knees 170 and 172. Each knee includes a pair of laterally spaced side plates 174 and an upstanding log-abutting member 176 secured thereto. The lower ends of the log-abutting members extend downward into the hollow box frame of the elevator base sufficiently to prevent debris from lodging between the lower edges of the side plates 174 and the upper log-supporting surfaces of the elevator base.

Means is provided for moving the pair of knees associated with each elevator base simultaneously toward and away from each other in direct proportion to the vertical movement of the elevator. Thus, each knee is connected to its associated elevator base by means of a pair of links 178 and 180 (FIG. 5) which are confined within the elevator box frame base and between the side plates 174 of the knee. The upper ends of the links are connected to the knee by means of the longitudinally spaced pivot shafts 182 and 184, respectively, and the bottom end of the link 178 and an intermediate point of the link 180 are connected to the elevator box frame base by means of the longitudinally spaced pivot shafts 186 and 188, respectively. The pivot shafts are arranged to form a parallelogram, whereby the vertical log-abutting member 176 of the knee remains vertical throughout the range of movement of the knee toward and away from its associated knee.

The link 180 is extended from the pivot shaft 188 arcuately inward and upward and terminates in an arcuate end 190. This end is located such that a line 192 extended from the axis of pivot shaft 188 perpendicular (angle b) to a line 194 extending between the axes of pivot shafts 184 and 188 terminates at a point 196 on said arcuate end which is the same distance from the axis of pivot shaft 188 as is the axis of pivot shaft 184.

One end of a chain 200 is wrapped about the arcuate end 190 of each link 180 and is secured to the latter. The lower end of the chain is connected to an elongated rod 202 which extends downward slidably through an opening in an abutment block 204 secured to the vertical beam 152. The lower end of the rod carries an enlarged head 206 arranged to engage the abutment block at a predetermined elevation of the elevator base, whereupon further upward movement of the latter effects movement of the knee inward toward its associated knee.

In similar manner, retraction of each knee is effected by lowering the elevator base, whereupon the parallelogram linkage pivots clockwise (FIG. 5) to return the knee to its fully retracted position.

As in the embodiment first described, the knees of each pair reciprocate toward and away from each other during vertical movement of the associated elevator base in such manner that the point of intersection of the vertical log-abutting member 176 and the log supporting surface of the elevator base 154 moves along a line which forms an angle c of 45.degree. with vertical or horizontal. Accordingly, each knee moves horizontally through the same distance that the elevator base moves vertically.

The operation of the log pre-centering apparatus illustrated in FIGS. 3-5 is as follows: With both elevator bases lowered to the position illustrated in FIG. 3, a log L is delivered, deposited and centered on the spaced log supports 150. Hydraulic pressure then is applied to the bottom end of the cylinders 160, whereupon the pair of elevator bases are caused to move upward until they abut the bottom of the log at longitudinally spaced points on the latter (FIG. 4) and lift the log therefrom.

It is to be noted from FIG. 3 that as the log-abutting surface of each elevator base moves upward through the distance a, which is the same as the distance between the head 206 of each rod 202 and its associated abutment block 204, the associated pairs of knees are not caused to move inward toward each other. However, when the elevator bases reach the level 154' the rod heads 206 also engage the abutment blocks 204. Thereupon further upward movement of the elevator bases causes the links to pivot clockwise and move the pair of knees inward toward each other, each knee moving forward the same distance as its associated elevator base moves upward.

Accordingly, as in the previous embodiment, it can be seen, particularly from FIG. 5, that if each elevator base moves upward from its position 154' a distance equal to one-half the distance between the associated knees at their illustrated position of extreme retraction, the log-abutting members 176 of the associated pair of knees will have been brought into mutual abutment on the plane of the vertical line 210. The point Z of intersection of this line with the horizontal line 212, representing the log supporting surface of the associated elevator base, defines the axis on which the axial centerline of a log is to be precentered.

In FIG. 4 of the drawing, a log to be precentered is shown in broken lines in an exaggerated taper, for the same purpose as previously explained. It will be apparent that the pair of knees adjacent the smaller diameter end of the log must move closer together to engage that end of the log than must the pair of knees associated with the larger diameter end of the log. In order for the first mentioned pair of knees to move closer together the associated elevator base must also move upward a corresponding distance higher than the elevator base associated with the larger diameter end of the log. Accordingly, when the pair of knees have engaged the smaller diameter end of the log and the other pair of knees have engaged the larger diameter end of the log, the axial centerline of the log is established precisely on the axis Z in the manner illustrated in broken lines in FIGS. 3 and 5.

Since the pairs of knees are in firm abutment with the log and therefore cannot move farther inward toward each other, the associated elevator bases are restrained against further upward movement. As in the previous embodiment, the increased hydraulic pressure in the cylinders 160 thus may be utilized to actuate the log transfer mechanism 12 to pick up the precentered log and transfer it to the veneer lathe. In this regard it will be understood that the point Z of intersection of the lines illustrated in FIGS. 3 and 5 also corresponds to the axis Y of rotation of the veneer lathe chucks, as previously explained.

From the foregoing it will be apparent that the log precentering apparatus of this invention provides no obstruction above the log. This simplifies the construction and enhances the speed and mode of operation of transfer apparatus and correspondingly increases veneer production. The degree of precision of centering afforded by the log precentering apparatus of this invention also minimizes waste of peeler log material, thereby further decreasing the cost of veneer production.

It will be apparent to those skilled in the art that various changes may be made in the structural details described hereinbefore. For example, if the peeler logs are untapered, a single elevator, designed to support the log, may replace the pair of independently movable elevators.

As another example, the fixed log supports 80 or 150 may be omitted, the log to be centered being deposited directly upon the elevator base 86 or 154. Use of the log supports is preferred, however, particularly in an embodiment illustrated in FIGS. 1 and 2 in order to avoid jarring and other adverse effects on the elevator assembly which might occur when large diameter logs are dropped directly upon the elevator bases 86.

As a further example, the arrangement of chains 120 and headed rods 126 may be replaced by rack and pinion assemblies or other suitable drive means interconnecting the elevator bases and knees and the knee-retracting weights 138 may be replaced with springs, or other suitable mechanism.

If it is desired to have each pair of knees move toward each other as the associated elevator moves downward, rather than upward as illustrated, the lower ends of the chains 120 may be connected to the vertically movable post 88. Such an arrangement might be useful, for example, under circumstances in which it is inconvenient to provide a pit for the lower portion of the apparatus.

The foregoing and various other changes in the size, shape, number and arrangement of parts may be made without departing from the spirit of this invention.

Claims

Having now described my invention and the manner in which it may be used, I claim:

1. Log pre-centering apparatus for veneer lathes, comprising

a. a frame,

b. elevator means supported by the frame for vertical reciprocation and adapted to support a log thereon,

c. a pair of spaced log-engaging knees supported by the elevator means for movement vertically therewith and toward and away from each other, and

d. connector means interengaging the knees and elevator means for moving the knees toward and away from each other by vertical reciprocation of the elevator means,

e. the connector means being arranged to move each knee a distance equal to the distance of vertical movement of the elevator means between a predetermined log-supporting position of the elevator means and a predetermined log-centering axis.

2. The apparatus of claim 1 including drive means engaging the elevator means for reciprocating the latter.

3. The apparatus of claim 1 including log support means adjacent the elevator means for supporting a log initially in position for engaging support by the elevator means.

4. The apparatus of claim 1 wherein the elevator means comprises a pair of spaced elevators supported for independent vertical reciprocation and each supporting a pair of knees.

5. The apparatus of claim 4 wherein the connector means comprises flexible line means connected at one end to each knee and engaging the frame at its opposite end, and guide means on the associated elevator means engaging the line means intermediate its ends.

6. The apparatus of claim 4 wherein the connector means comprises parallelogram link means pivotally interengaging each knee and associated elevator means, and connecting means interengaging the link means and frame.

7. The apparatus of claim 6 wherein each parallelogram link means includes an extension of one of the links having a point thereon arranged on a line extending perpendicular to a line through the pivots of said link and spaced the same distance from the pivot common to both lines as the distance between said pivots, the connecting means interengaging said extension and frame.

8. The apparatus of claim 1 including abutment means on the frame and connector means arranged for mutual engagement at a predetermined elevation of the elevator means, whereby to prevent movement of the knees toward each other until the elevator means reaches said predetermined elevation.

9. The apparatus of claim 1 wherein the connector means comprises flexible line means connected at one end to each knee and engaging the frame at its opposite end, and guide means on the supporting elevator means engaging the line means intermediate its ends.

10. The apparatus of claim 1 wherein the connector means comprises parallelogram link means pivotally interengaging each knee and supporting elevator means, and connecting means interengaging the link means and frame.

11. The apparatus of claim 3 including abutment means on the frame and connector means arranged for mutual engagement at a predetermined elevation of the elevator means above the log support means, whereby to prevent movement of the knees toward each other until the elevator means reaches said predetermined elevation.

12. The apparatus of claim 3 wherein the elevator means comprises a pair of spaced elevators supported for independent vertical reciprocation and each supporting a pair of knees, and wherein the apparatus includes abutment means on the frame and each connector means arranged for mutual engagement at a predetermined elevation of the elevators above the log support means, whereby to prevent movement of the knees toward each other until the elevators reach said predetermined elevation.

13. Log precentering apparatus for veneer lathes, comprising

a. a frame,

b. a pair of spaced elevators supported by the frame for independent vertical reciprocation and adapted to support a log thereon,

c. a pair of spaced log-engaging knees supported by each elevator for movement toward and away from each other, and

d. connector means interengaging the knees and associated elevator for moving the knees toward and away from each other by vertical reciprocation of the elevator, the connector means comprising parallelogram link means pivotally interengaging each knee and associated elevator, and connecting means interengaging the link means and frame,

e. the connector means being arranged to move each knee a distance equal to the distance of vertical movement of the associated elevator between a predetermined log-supporting position of the elevator and a predetermined log-centering axis.

14. The apparatus of claim 13 wherein each parallelogram link means includes an extension of one of the links having a point thereon arranged on a line extending perpendicular to a link through the pivots of said link and spaced the same distance from the pivot common to both lines as the distance between said pivots, the connecting means interengaging said extension and frame.