US3256725A

US3256725A - Stretching method and apparatus

Info

Publication number: US3256725A
Application number: US267393A
Authority: US
Inventors: Keller Julius Stefan
Original assignee: Individual
Current assignee: Individual
Priority date: 1956-08-06
Filing date: 1963-03-21
Publication date: 1966-06-21
Anticipated expiration: 1983-06-21
Also published as: DE1182277B

Description

June 21, 1966 J. a KELLER 3,256,725

STRETCHING METHOD AND APPARATUS Filed March 21, 1963 2 Sheets-Sheet 1 r l I l 4 I j I A l J. g i O -5 I f 1 z T p l I I" I/ 4 7 2; 2 17 n GEI/T: 2444 4, We

June 21, 1966 J. s. KELLER 3,

STRETCHING METHOD AND APPARATUS Filed March 21, 1963 2 Sheets-Sheet 2 FIGS INVENTOR.

JULIUS STEFAN KELLER way/L A TTORNE Y5 United States Patent 3,256,725 STRETCH-TIN G METHOD AND APPARATUS Julius Stefan Keller, Irmgardstrasse 18, Munich-Solln, Germany Filed Mar. 21, 1963, Ser. No. 267,393 Claims priority, application Germany, Aug. 6, 1956, K 29,547 11 Claims. (Cl. 72-164) This application is a continuation-in-part of co-pending application Serial No. 674,678, filed July 29, 1957, now abandoned.

The present invention relates to a method and a machine for carrying out the method of increasing the elastic limit or yield point and the strength of rolled steel, especially of steel for reinforced concrete such as structural steel wires or bars, or rods and the like, and, more particularly to cold-drawing of such material, which operation is distinguished by simplicity and economy. Another advantage of the operation is that always uniform and faultless steel products are obtained.

It has been known to improve the elastic strength of structural steel in form of wires, rods or the like, particularly of steel for reinforcing concrete structures, by drawing steps carried out in cold condition, so as to increase the tensile strength of such structural steel. This has been done by drawing the material either through perforated die plates or to twist it, i.e., subject it to torsion. The first mentioned method is mostly used in the structural steel industry, while the second method is employed in the manufacture of so-called tor steel.

The drawing method has the principal disadvantage that it is relatively costly, because the dies are relatively expensive. A further disadvantage of the drawing method lies in that a forced transverse contraction occurs when the material is forcedly drawn through a narrowed cross section with the exertion of great forces. This method differs from pure straightening caused by stretching of the material in that a forced compression is added to the stretching forces, said compression acting transversely with respect .to the axis of the material. The quality of the material is thereby impaired due to an undesirable and unavoidable decrease in the breaking dilatation.

The cold rolling method is analogous to the drawing of I a wire or rod to a smaller diameter in that the aforementioned disadvantages are likewise present.

The inventor has tried to obtain better results by cold rolling round wires or rods first to an approximately oval shape and, thereafter, to roll them to a circular shape of smaller diameter. For example, starting from a diameter of 14 mm., the rods were rolled to an oval cross section of 12 x 14 mm. and then circularly to a diameter of 12 mm.

The mentioned twisting method of wires or rods in cold condition has the disadvantage that such operations, in case of large lengths of material, require a correspondingly large and expensive apparatus and the twisting steps can be carried out only with considerable difficulties, so that the economy of production with such method is open to doubt. In addition to this, it has proven disadvantageous that the material is not stretched uniformly throughout its entire cross section, i.e., the elastic limit increases towards the outside, rather than the property characteristic of the material being uniform between the center and the periphery.

The method according to the invention has the advantage over the known cold-Working methods for structural steel wires or rods that it can be continuously carried out with material of long length in a simple and inexpensive manner, whereby material may have non-uniform cross Patented June 21, 1966 section, such as steel having a cross section with transverse ribs.

It is an object of the present invention to draw or pull the material through a guideway formed by bending rollers, said guideway having at least two sharp bends which are oppositely directed to one another and oifer such resistance to the movement of the material during the drawing step as a result of the curvature of these bends, that the material is stretched to the desired degree.

It is another object of the invention -to sharply bend or deflect the wire or rod to be improved in one or more,

preferably in two, planes perpendicular to one another, continuously and progressively at least twice in one plane and each time about one center, i.e., first in one and directly thereafter in the opposite direction. As a result of this, the material is simultaneously stretched in such a way, that the outer fibers of the material are stretched to a greater extent than the inner fibers. The bending or deflection radii of the.bending rollers should be relatively small. However, these radii should notbe too small, i.e., they may be about 4 to 10 times the diameter of the wire. Tests have shown that bending radii in the upper range are particularly recommended to obtain the desired results. The bending or deflection radii are suitably selected in such a manner, that the fibers at the periphery of the wire or rod facing the bending or deflection center are not stretched or only to a lesser extent, so that the stretching action increases uniformly in the cross section of the material from the center to the periphery. The reverse actions take place during the subsequent bending or deflecting of the wire or rod in opposite direction, so that finally a material is obtained which is absolutely uniform throughout its whole cross section. Such material is distinguished by a uniformly superior quality and improved resistance values.

The angles of the loop of the wire or rod about the bending or deflecting center should be as large as possible. These angles are dependent upon the desired stretching action and suitably lie between 15 and 180".

If the wire or rod is bent or deflected only twice in opposite directions, looping angles between and will be suitable. If the material is bent or distorted consecutively four times or even six times, the looping angles should be decreased to about 15 to obtain the desired stretching action, because the resistance in case of multiple bending at smaller looping angles results in the same value as bending twice at larger looping angles.

It has been known to bend wires or round steel rods slightly in several directions in order to descale them. This is conventionally done by passing the wire or the like over two rollers, the axes of which are placed 90. It has been further known to obtain strengthened material by bending its repeatedly in several directions. Such operations are different from the process according to the present invention, because they lack the important and characteristic stretching steps. Finally, straightening of wires and rods by means of wellknown straightening rollers belongs to the state of the art. The method according to the invention is carried out by means which have a remote resemblance to these known roller straighteners, because in the latter only very slight bending action takes place. Such operation is principally different from the stretching of the material according to the invention for the purpose of improving the strength characteristic of the material to obtain a product absolutely uniform throughout its entire length.

The process according to the present invention can be explained as follows: 7

The wire drawn over the first roller is not only bent due to the suitably large looping angle along a correspondingly large arc, but it offers considerable resistance to the sharp bending action, requiring considerable traction mutually disforces in the longitudinal direction. Such conditions are present to a still greater extent in the second roller over which the wire or rod is then drawn and by which it is bent in opposite direction. A further increase is obtained at the third and fourth rollers. During drawing or pulling over the first roller, the neutral fiber is displaced from the center of the wire or rod towards the periphery thereof in direction toward the bending or deflection center. Accordingly, the outer fibers are stretched to a greater extent.

If this operation is repeated in opposite direction, the fibers which have not been previously stretched, or stretched only to a limited extent, are subjected to a maximum stretching action. As a result of the bending and stretching in different directions, the material is absolutely uniformly stretched throughout its entire cross section and, thereby, an elongation in the desired ratio of, for example, 5, 10, 20% is obtained. If the quality of a material having an elastic limit of about 2,500 kg./cm. is to be improved to about 5,000 kg./cm. the traction force amounts to about 2,500 kg./cm. i.e., is in the range of the elastic limit or yield point of the starting material.

As tests have shown, any degree of stretching can be obtained within narrow tolerance limits, so that practically any desired degree of improvement of the quality of the material can be obtained. The extent of stretching is dependent upon the numbers of rollers over which the wire is drawn, upon the diameter of these rollers and their looping angles. In carrying out tests, the diameters of the rollers remain suitably fixed, while their looping angles are varied and a suitable adjustment of the rollers with respect to one another and suitable looping angles are then empirically ascertained to obtain the desired degree of stretching. In such tests, it has to be considered that the composition of the material and its thickness influence the operation to a certain extent.

Various kinds of machines may be used for carrying out the method according to the invention. Particularly suitable is a device in which a pair or set of two deflection rollers with axes which are parallel to one another and perpendicular with respect to the main traction direction of the wire are used. These rollers are disposed one behind the other and can, preferably, be adjusted perpendicularly with respect to the main traction direction, so that the looping angles can be varied or adjusted as desired. If two pairs of rollers are provided, the roller axes of the second pair are displaced 90 with respect to the roller axes of the first pair. At least one of these pairs or sets of rollers is mounted on a rotatable plate or disc which can be locked at desired positions. Thus, by turning this plate or disc, the looping angles of the rollers can be readily adjusted.

It will be seen from the above that the present invention thus resides, basically, in a method for stretching a material such as rolled steel wires and rods, of given diameter, for permanently reducing the cross section of the material, which method comprises the step of passing the material over at least two freely rotatable bending rollers of substantailly equal radius, particularly radii which are approximately four to ten times the diameter of the material, which rollers are arranged such that the material is first bent sharply in one direction and thereafter in another direction so that the material will follow a zigzag path from the point at which it first contacts the first of the rollers to the point at which the material last contacts the last of the rollers. The rollers are spaced sufiiciently far apart so that no point along the length of the material will be contacted simultaneously by any two rollers, and each of the bending rollers is provided with two mutually inclined side walls which form a peripheral groove between themselves, these walls together constituting a means for clamping the material of the given diameter, thereby to prevent twisting of the material. Furthermore, the motive force which moves the material over the rollers is constituted solely by a pulling force which acts on the material at a point located past the last roller, considered in the direction of movement of the material.

The present invention further resides in an apparatus for carrying out the above method. According to one feature of the invention, the apparatus incorporates bending rollers having peripheral grooves which clamp the material so as prevent twisting of the material.

Additional objects and advantages of the present invention will become apparent upon consideration of the following description when taken in conjuction with the accompanying drawings in which:

FIGURE 1 shows schematically a plan view of a machine for stretching of wires by means of two bending rollers according to the inventive method.

FIGURE 2 illustrates schematically such machine, having two pairs or sets of bending rollers.

FIGURE 3 is a diagrammatic illustration by means of which the nature of the invention will be explained.

FIGURE 4 shows the behaviour of a material passing over a bending roller.

FIGURE 5 is a fragmentary sectional view of a grooved bending roller incorporated in an apparatus according to the present invention.

Referring now specifically to FIGURE 1 of the drawing, a carrier such as a plate or disc 1 rotatably mounted on a stationary support or base 20 supports two freely

rotatable bending rollers

2 and 3 over which a wire 4 can be drawn in the direction P by known pulling or traction means, this wire being guided in front of the first bending roller 2 between a pair of guide rollers 21 and 22 and behind the second bending roller 3 between a pair of

guide rollers

23 and 24. The positions of these guide roller pairs 21, 22, and 23, 24, with respect to the bending

rollers

2 and 3 can be readily adjusted by displacing

slides

25 and 26 on which the guide roller pairs are respectively mounted across the base 20, i.e., the

slides

25 and 26 are guided along recessed,

straight edges

27, 28, and 29, 30, respectively, provided in this base 20 for this purpose. A graduation 5 is provided along the circumference of the plate or disc 1 to be read by means of a mark 6 provided on the stationary base 20, so that an exact angular position of this plate or disc 1 can be readily determined.

The rotating plate or disc 1 can be locked in position on its base 20 in a manner known per se, for example, by means of pawls (not shown) engaging in suitable detents on the circumference of this disc 1. In place of this type of locking means, a worm gear (not shown) rotatably mounted in the base 20 and engaging screw threads on the periphery of the disc 1, or other locking means, may be provided for locking the disc 1 to the base 20.

In the second embodiment, shown in FIGURE 2, a wire 7 is drawn in the direction P over two pairs of freely

rotatable bending rollers

9, 10, and 12, 13, arranged one behind the other considered-in the direction of movement of the wire. The wire 7 is passed with the aid of front rollers 8 to the

rollers

9 and 10, constituting the first pair of bending rollers mounted on a disc 11 rotatably and adjustably mounted on a base 31. The wire 7 is then passed over the second pair of bending

rollers

12 and 13, secured to a disc 14 which is mounted on the base 31 in the same manner as the disc 11. The wire 7 is subsequently guided between straightening rollers 15 provided behind the second pair of bending

rollers

12, 13. Circular graduations or dials 16 and 17 are provided on the base 31 around the periphery of the

discs

11 and 14, respectively, cooperating with

marks

18 and 19 on these

discs

11 and 14, respectively, to adjust their angular position. The two

discs

11 and 14 are disposed in the same plane.

It is possible to arrange the discs with the second pair of rollers displaced with respect to the first pair of rollers, whereby two guide rollers are to be provided between these two discs.

In practice, the radius of the bending rollers will be between 4 to 10 times that of the material being treated; this will produce a 4 to 40% stretching. In a preferred embodiment, the radius of the bending rollers will be equal to 5 to 7 times that of the material; this will produce a stretching of between 5 and 20%.

The theoretical considerations underlying the permanent reduction in cross section, and therefore stretching, obtained by the present invention will be explained with reference to FIGURE 3. The latter shows the various rollers depicted in FIGURE 2, identified by the same reference numerals.

In essence, thanks to the fact that the motive force which causes the material to travel along the .zig-zag path over the bending rollers is constituted solely and exclusively by a pull which is exerted at a point past the last bending roller, the resultant of the bending and tension forces which are brought to bear on the material will vary from roller to roller.

Consider first the force P with which the material has to be pulled over roller 8. Since there is (at least in theory) nothing which prevents the rightward movement of the material, the force P has simply to be sufficiently great to effect bending of the material over the roller 8. However, the force P with which the material must be pulled over roller 9 has to be greater, because it will also be the force which is required to bend the material over the roller 9. Thus, P will have'to be equal to P plus the force required to effect bending over roller 9. Next, force P has to be even greater, because the material now has additionally to be bent over roller 10. Thus, each sucessive force P must be greater than the preceding force P because it has to do everything that the preceding force did plus bend the material over yet another roller. Considered intuitively, it will be appreciated that if one were to pull a rod or wire over, say,

rollers

8 and 9 only, this would require less force than pulling such rod or wire over

rollers

8, 9, 10 and 12, which, in turn, would require less force than if the rod or wire were to be pulled over all of the rollers shown in FIGURE 3. which the material is bent offers a resistance to the pulling force which must be applied in order to pull the wire or rod through the series of rollers.

Obviously, the force P, which is here shown as the force applied directly after the material leaves roller 15, must be greater than any of the preceding forces and, of course, be sufficiently great to pull the material through the machine. The relationship between the pulling forces might thus be expressed as follows:

Next, it is pointed out that the magnitude of the puling force has a bearing on the position of the neutral axis within the material being bent. To explain: When a material is drawn over a bending roller, the fibers of the material which are immediately adjacent the bending roller will be compressed, while the fibers which are furthest away from the surface of the bending roller will be lengthened. Intermediate these extremes will be neutral axis in which the fibers are neither compressed nor lengthened. This is shown in FIGURE 4 which shows a bending roller 50 and a material 51 drawn thereover in the direction of arrow 52. Before the material engages the roller 50, the fibers will be of a given length. As the material passes over the roller the outside fibers will progressively lengthen while the inside fibers will be compressed. The neutral axis 0 is the region at which the fibers retain their original length.

Attention is directed to the distance N between the neutral axis 0 and the surface of the roller 50 whereat the fibers are compressed. This distance is a function of the force P with which the material is drawn over the roller, in that the greater this force, the closer will the In each case, then, a roller over 6 neutral axis 0 be to the compressed roller surface, i.e., the smaller will be the distance N.

Referring back to FIGURE 3, it will be remembered that the material is moved with increasing force over the successive bending rollers. For this reason, the neutral axis 0 of the material will be closer to each bending roller than it was to the preceding bending roller, or, conversely, further from the stretched side of the material. In this way, each portion of the material is successively more stretched at any one bending roller than it was compressed at a previous roller, and it is this progressive stretching which brings about an overall stretching of the material as a whole, this, of course, being a concomitant of a permanent reduction in the cross section. Note that this phenomenon would not occur if the various forces P acting throughout the zigzag path were equal to each other, because then each compression would be accompanied by a corresponding stretching and vice versa. It is for this reason that it is essential that the material be pulled through the zigzag path, i.e., that the motive force be constituted solely and exclusively by a pulling force acting at a point past the last of the bending rollers. If there were, additional-. 1y, a pushing force acting ahead of the zigzag path and preventing the various forces P from assuming different values, no true stretching, i.e., permanent reduction of cross section, could result.

For completeness, FIGURE 3 also shows the progressively smaller diameters D D etc., which the material has at various points along the zigzag path, as well as the distances N N etc., which the neutral axis 0 of the material is spaced from the respective bending rollers It will be appreciated that the relationship of these dimensions can be expressed as follows:

s 9 1o 12 1a 15 and Ni H2 h N12 Na D8 D9 D10 Da D191)...

FIGURE 5 shows a bending roller 60 which may be used in a machine according to the present invention for stretching a rod 61. The roller is provided with a. peripheral groove 62 having slanting side walls 63 and 64 which form with each other an angle a of approximately 60, so that a line 65 which joins the points 66 and 67 whereat the rod 61 engages the side walls is spaced from the central axis 61a of the rod a distance equal to approximately one quarter of the diameter d of the rod. As is apparent from FIGURE 5, the cross section of the groove 62 is such that the rod 61 is not in contact with the bottom of the groove, but is in contact only with the side walls 63, 64, which clamp the rod, i.e., the cross section of the groove causes the material of a given diameter, which material is to be stretched, to be out of contact with the bottom of the groove and in contact only with the side walls forming the groove.

The purpose of the groove is to clamp the material so as to prevent it from twisting. It will be appreciated that if a steel rod of suitable diameter is bent, say, by hand, the forces applied will be exclusively, or almost exclusively, bending forces, and there will be compressed and stretched fibers on opposite sides of the neutral axis. If it is then attempted to straighten out the bent rod, i.e., to stretch the previously compressed side and to compress the previously stretched side, the rod will have the tendency to twist into a position in which the forces by means of which it is attempted to straighten the rod will actually tend to bend the rod further.

The same thing may tend to occur in an apparatus incorporating a series of bending rollers, i.e., the material will tend to snake over the rollers so as to produce a corkscrew-like eifect, rather than the desired stretching or permanent reduction in cross section. This, however, is avoided by the clamping-type bending roller shown in FIGURE which, in the preferred form, is so dimensioned with respect to the material being treated that the neutral axis is shifted by approximately one quarter of the diameter of the material.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

What is claimed is:

1. An apparatus for stretching a material such as rolled steel wires and rods of given diameter for permanently reducing the cross section of the material, said apparatus comprising, in combination:

(a) supportmeans;

(b) a plurality of freely rotatable bending rollers of substantially equal diameter arranged on said support means such that the material passing over said rollers is first bent sharply in one direction and thereafter in another direction so that the material will follow a zig-zag path from the point at which it first contacts the first of said rollers to the point at which it last contacts the last of said rollers, said rollers being spaced sufficiently far apart so that no point along the length of the material will be in contact simultaneously with any two rollers, each of said bending rollers being provided with two mutually inclined side walls which form a peripheral groove between themselves and which walls together constitute a means for clamping the material of said given diameter thereby to prevent twisting of the material, said two side walls forming an angle of approximately 60 with each other, as a result of which a line which joints the points whereat the material engages said side walls is spaced from the central axis of the material a distance equal to approximately one quarter of the diameter of the material; and

(c) means for moving the material over said rollers along said zig-zag path, said moving means consisting exclusively of pulling means which engage the material, and exert a pulling force thereon, at a point located past said last r-oller, considered in the direction of movement of the material.

2. An apparatus as defined in claim 1 wherein the radius of said bending rollers is approximately 4 to times the diameter of the material.

3. An apparatus as defined in claim 2 wherein the radius of said bending rollers is approximately 5 to 7 times the diameter of the material.

4. An apparatus as defined in claim 2 wherein said rollers are so arranged that the material loops over each respective roller to encompass the same by an angle of between and 180.

5. An apparatus as defined in claim 2, further comprising guide roller means carried by said support means and arranged both ahead of and behind said plurality of rollers, considered in the direction of movement of the material.

6. An apparatus as defined in claim 5 wherein there are at least two bending rollers and wherein said support means comprise a carrier member rotatably mounted between said guide roller means, said carrier member rotatably supporting said two bending rollers in spaced apart relationship to allow said two bending rollers to occupy difierent positions with respect to said guide roller means thereby to alter the configuration of the zig-zag path along which the material moves.

7. An apparatus as defined in claim 6 wherein the guide roller means located behind said bending rollers incorporate a plurality of rollers at least some of which constitute Straightening rollers.

8. An apparatus for stretching a material such as rolled steel wires and rods of given diameter for permanently reducing the cross section of the material, said apparatus comprising, in combination:

(a) support means;

(b) a plurality of freely rotatable bending rollers of substantially equal diameter arranged on said support means such that the material passing over said rollers is first bent sharply in one direction and thereafter in another direction so that the material will follow a zig-zag path from the point at which it first contacts the first of said rollers to the point at which it last contacts the last of said rollers, said rollers being spaced sufficiently far apart so that no point along the length of the material will be in contact simultaneously with any two rollers,

' said rollers each being provided with a peripheral groove having two side walls which together constitute a means for clamping the material of said given diameter thereby to prevent twisting of the material, said tWo side walls forming an angle of approximately 60 with each other as a result of which a line which joins the points whereat the material engages said side walls is spaced from the central axis of the material a distance equal to approximately one quarter of the diameter of the material, the effective radius of said rollers being approximately 4 to 10 times the diameter of the material; and

(c) means for moving the material over said rollers along said zig-zag path, said moving means consisting exclusively of pulling means which engage the material, and exert a pullingforce thereon, at a point located past said last roller, considered in the direction of movement of the material.

9. An apparatus as defined in claim 8, further comprising guide roller means carried by said support means and arranged both ahead of and behind said plurality of rollers, considered in the direction of movement of the material, said plurality of bending rollers including at least two bending rollers, and said support means comprising at least one plate rotatably mounted between said guide roller means, said plate rotatably supporting said two bending rollers in spaced apart relationship to allow said two bending rollers to occupy ditferent positions with respect to said guide roller means thereby to alter the configuration of the zig-zag path along which said ma- References Cited by the Examiner UNITED STATES PATENTS 2,006,087 6/1935 Miller 153-102 2,163,504 6/1939 Thomas 153-54 2,347,904 5/1944 Grenlich 153-86 2,432,828 12/1947 Stone 153-85 2,746,513 5/1956 Kiesow et a1 153-104 CHARLES W. LANHAM, Primary Examiner.

R. J. HERBST, Examiner,

Claims

1. AN APPARATUS FOR STRETCHING A MATERIAL SUCH AS ROLLED STEEL WIRES AND RODS OF GIVEN DIAMETER FOR PERMANENTLY REDUCING THE CROSS SECTION OF THE MATERIAL, SAID APPARATUS COMPRISING, IN COMBINATION: (A) SUPPORT MEANS; (B) A PLURALITY OF FREELY ROTATABLE BENDING ROLLERS OF SUBSTANTIALLY EQUAL DIAMETER ARRANGED ON SAID SUPPORT MEANS SUCH THAT THE MATERIAL PASSING OVER SAID ROLLERS IS FIRST BENT SHARPLY IN ONE DIRECTION AND THEREAFTER IN ANOTHER DIRECTION SO THAT TFHE MATERIAL WILL FOLLOW A ZIG-ZAG PATH FROM THE POINT AT WHICH IT FIRST CONTACTS THE FIRST OF SAID ROLLERS TO THE POINT AT WHICH IT LAST CONTACTS THE LAST OF SAID ROLLERS, SAID ROLLERS BEING SPACED SUFFICIENTLY FAR APART SO THAT NO POINT ALONG THE LENGTH OF THE MATERIAL WILL BE IN CONTACT SIMULTANEOUSLY WITH ANY TWO ROLLER, EACH OF SAID BENDING ROLLERS BEING PROVIDED WITH TWO MUTUALLY INCLINED SIDE WALLS WHICH FORM A PERIPHERAL GROOVE BETWEEN THEMSELVES AND WHICH WALLS TOGETHER CONSTITUTE A MEANS FOR CLAMPING THE MATERIAL OF SAID GIVEN DIAMETER THEREBY TO PREVENT TWISTING OF THE MATERIAL, SAID TWO SIDE WALLS FORMING AN ANGLE OF APPROXIMATELY 60* WITH EACH OTHER, AS A RESULT OF WHICH A LINE WHICH JOINTS THE POINTS WHEREAT THE MATERIAL ENGAGES SAID SIDE WALLS IS SPACED FROM THE CENTRAL AXIS OF THE MATERIAL A DISTANCE EQUAL TO APPROXIMATELY ONE QUARTER OF THE DIAMETER OF THE MATERIAL; AND (C) MEANS FOR MOVING THE MATERIAL OVER SAID ROLLERS ALONG SAID ZIG-ZAG PATH, SAID MOVING MEANS CONSISTING EXCLUSIVELY OF PULLING MEANS WHICH ENGAGE THE MATERIAL, AND EXERT A PULLING FORCE THEREON, AT A POINT LOCATED PAST SAID LAST ROLLER, CONSIDERED IN THE DIRECTION OF MOVEMENT OF THE MATERIAL.