GB2249234A - Bi-directional magnetostrictive actuator - Google Patents

Abstract

One or more bi-directional magnetostrictive actuators 344 are used to control a manufacturing apparatus and process by varying the shape, location, or dimension of a part 320 of the apparatus, especially one used in manufacture of a thermoplastic flooring product, in particular a knife over roll coater. <IMAGE>

Description

"Method and Apparatus" This invention relates to a method of controlling processes of manufacture and to a manufacturing apparatus controlled by the method.

In the manufacture of many products, it is necessary or advantageous to control accurately the dimensions of the product or of its component parts. In many instances, the dimension of the product will correspond precisely to or be a constant proportion of some dimension of the apparatus used to make it and it is accordingly only necessary to ensure that the apparatus is correctly dimensioned to achieve the desired product dimensions.

In certain cases, however, because of the nature of the manufacturing process, or the material of which the product is made, the dimensions of the product do not bear a constant relationship to the dimensions of the apparatus, for example because of variations in the material of which the product is made. Another problem encountered is variation during the course of a manufacturing run in the dimensions of critical portions of the manufacturing apparatus.

In certain cases, also, difficulties are encountered in precisely ascertaining the important dimensions of the manufacturing apparatus directly, making it necessary to measure the dimensions of the product initially produced by the apparatus and, using the information so provided, to adjust the dimensions of the apparatus accordingly.

More than one of the difficulties mentioned above may of course arise in the same manufacturing process, the combination of them making for severe problems.

One example of a process in which such problems are encountered is the manufacture of a flooring material in which a vinyl chloride polymer layer is applied in the form of a plastisol to a substrate web by means of a knife over roll coater, the plastisol layer being gelled by passing the plastisol-covered web over a heated drum.

The web may be of a width up to 4 metres, and accordingly the knife and the roll used in the coater must be of this length also. It is very difficult to make a roll of this length of constant diameter, it is very difficult to make a knife of such a length absolutely straight, and it is very difficult to make the gap between the knife and the roll of constant width, within the tolerances required in the product.

If the resulting layer of plastisol is anywhere too thin, then the irregularities present in the surface of the substrate will not be smoothed out, and the final product will not meet the required standards of quality.

If the average thickness of plastisol applied is increased to ensure that the minimum thickness is sufficient to ensure the desired quality, then the cost of manufacturing the product is increased.

To overcome these problems, one present solution is to suspend the knife from a beam positioned over the roll by means of a large number of stainless steel bolts screwed into the top of the blade at locations uniformly spaced along its length. Each bolt is provided with an electrical resistance heater, the output of each heater being independently variable. Downstream of the heated drum, the thickness of the product carrying the now gelled plastisol is measured by a scanning device that traverses the width of the moving web, feeding the results to a computer that produces a profile of the product across its width from the average of a number of traverses or scans.If the profile indicates that the product is too thick at a particular location across the width, the output of the heater at the corresponding bolt is increased, causing the bolt to expand slightly, thereby moving the blade closer to the roll. Correspondingly, if the product is too thin at that location, the heater output is reduced, the bolt contracts slightly and the blade moves away from the roll.

This procedure suffers from the disadvantage that there is inevitably a time lag between the detection of an irregularity in the product and its correction by movement of the blade. This time lag has a number of elements. First, because the plastisol must be gelled before the product thickness is measured, the scanner is some metres downstream of the roll coater. Second, the signal to change the output of a heater is only sent out after a number of scans have confirmed that the thickness requires alteration. Third, the thermal capacity of the heater and the material in the region surrounding it is such that there is a delay before the required change in bolt length is achieved, especially when a reduction is required.

During all this time, which may be as long as 30 minutes, product is being made which, if it does not meet the required quality standard, must be scrapped.

Another disadvantage of the procedure is that adjustment of the blade over roll coater to accommodate different requirements of different products is extremely time-consuming, because of the need to allow the blade bolts and their surroundings to reach thermal equilibrium after setting up. Although scrap product is not initially produced during this time, neither is merchantable product, with consequent loss.

A further disadvantage of the present procedure is that because the blade is heated by the bolt heaters, strain gauges, which normally operate by recording changes in electrical resistance of a measuring device i,,! a Wheatstone's bridge circuit, cannot be employed on the blade because thermal effects on resistance mask the effects of mechanical movement on the device's resistance. Accordingly, the use of thermal bolts inherently prevents the rapid reaction to changes in blade configuration during operation that would reduce production of scrap material.

The above discussion has been directed to the problems of controlling the production of one particular type of product, but similar problems arise in any other manufacturing process in which a dimension of the manufacturing apparatus governs a dimension of a product.

It will be appreciated that the dimension under consideration may be that of an aperture, which may be formed by two or more components of the apparatus, as in the case of the knife and roll discussed above.

Such procedures include, by way of example only, the manufacture of sheet or film by a doctor blade technique similar to that described above, either of a thermoplastics material as described above or other fluid coating, which itself may be, e.g., a solution or dispersion from which a solvent or dispersant is to be evaporated, drained, or otherwise removed to yield a uniform solid or fibrous mass. The material may be on a substrate, e.g., a support, when it is passed through the dimension controlling part of the apparatus, and such substrate may form part of the final product, as described above, or may be removed from the product as in the case, for example, of a release sheet; the material may be unsupported, as in the case of a self-supporting sheet or film, or it may be unsupported at the relevant stage and passed onto a support subsequently. The material may be of greater dimension than would be regarded as sheet, for example a board, which may be made from fibrous material in a dispersant, e.g., a ceiling board.

It has been proposed, in GB-A-2 162 118, to control the dimensions of a slot extrusion die having resilient die lips by applying to one lip a variable force, tending to close the lips. The force is applied by a piezoelectric translator or, less preferably, a magnetostrictive translator. The control device which is a commercially available product in its piezoelectric form, as described in Reifenhäuser-News 13, November 1987, is capable of applying force in one direction only, to effect closure of the slot, relying on the resilience of the lip to open the slot when the piezoelectric translator is deactivated.

In the types of apparatus described previously, e.g., a knife over roll coater, the blade is not resilient and the principle employed in GB-A-2 162 118 is not applicable.

There remains a need for improved control of product dimensions in such types of apparatus.

The present invention provides apparatus for the manufacture of a product, in which the shape, location, or a dimension of a part of the apparatus is variable by or controlled by an actuator having a magnetostrictive element, the actuator being capable of exerting a force in either a first direction or in a second direction opposite to the first.

The invention also provides a process for the manufacture of a product in which the shape, location, or a dimension of a part of the apparatus for making the product is variable by or controlled by such a magnetostrictive actuator.

The invention further provides the use of such a magnetostrictive actuator to control or vary the shape, location, or a dimension of a part of a manufacturing apparatus.

Advantageously, the part of the apparatus controlled or varied directly or indirectly controls the shape or dimension of the product. The part controlled or varied may be a component of the apparatus or it may be an aperture formed between two or more components, in which case the shape or dimension of a component or components is not necessarily changed; movement without change in shape or dimension may suffice.

Depending on the nature and size of the apparatus, there may be provided one actuator or a plurality of actuators and where there are two or more the actuators may be the same or different.

Advantageously, a plurality of spaced apart actuators is employed to control the shape, dimension, or position of a production apparatus component, the component, either alone or in combination with another component, advantageously in turn controlling the shape or dimension of a product made on the apparatus.

More especially, a plurality of actuators are positioned along the length of a blade spaced from a roller, each actuator serving to control or vary the spacing of a portion of the blade from the roller.

In one embodiment, the or each actuator comprises a magnetostrictive ferromagnetic element, advantageously mounted so that it is always in compression, and biassing means exerting a force in a given direction on the element, and an excitation means, normally a winding carrying direct current, the arrangement being such that on energization of the winding a force is exerted by the element on a movable member, the force being in the direction opposite to, and at maximum excitation greater than, that of the bias force. The force exerted by the element is advantageously continuously variable as, for example, by continuously varying the strength of the current in the winding.

The biassing means is preferably resilient, and exerts a force that increases as its distortion increases. It may however exert a constant force.

In other embodiments, the or each actuator comprises first and second magnetostrictive elements and excitation means, normally windings as above, the elements advantageously being mounted so that they are always in compression, and first and second optionally resilient biassing means exerting a force on the first and second elements respectively, the force exerted by the first means being in a direction opposite to that of the second means, the arrangement being such that on energization of the first winding a force is exerted by the first element on a movable member in a direction opposite to and at maximum energization greater than that exerted by the first biassing means, and on energization of the second winding a force is exerted by the second element in a direction opposite to and at maximum energization greater than that exerted by the second biassing means.

In still other embodiments, the or each activator comprises first and second magnetostrictive elements and excitation means, normally windings as above, the elements being mounted in compression, and a movable member, the arrangement being such that on energization of the first winding a force is exerted by the first element on the movable member in a first direction and on energization of the second winding a force is exerted by the second element on the movable member in a second direction opposite to the first.

Ferromagnetic materials suitable to provide the magnetostrictive element are known Der se. As examples there may be mentioned nickel, alloys of nickel, cobalt and iron, lead zirconate titanate and, advantageously, rare earth-iron compounds. Of rare earth-iron compounds, there may be especially be mentioned terbium-iron compounds, in particular those containing a third element, especially dysprosium, and more especially compounds of the composition Tbx Dy1~x Foe1,6 to 1.98 more especially Tub0,27 Dye.73 Foe1.95 or Tub0,3 Dye.7 Foe1,95 Other suitable materials include compounds of the formula Sml,f Rf Fey, where R is one or more, preferably two or three, of Y, Nd, and Tb, f is up to 0.10, and y is 1.7 to 2.0. Further materials are disclosed in U.S.

Patent No. 4308474, the disclosure of which is incorporated herein by reference.

The shape and size of the element will depend on the particular application. Advantageously, however, the element is in the form of a solid or hollow rod, preferably of uniform cross-section, and is conveniently a circular cylindrical rod oriented so that one end exerts a force on an apparatus component the shape, position or dimension of which is to be controlled or varied. Advantageously, the material is crystalline, and the element is preferably a single crystal or a grainoriented polycrystalline material. Preferably, if the element is a rod and, at least if the material is a rare earth-iron alloy or compound, the 111 crystalline direction lies along the axis of the rod.

As a practical matter, the size of elements presently available is limited; but the forces available, e.g., up to about 20 mPa, mean that in the blade applica tion discussed above actuators may be substituted one for one for the thermal bolts.

One form of apparatus constructed in accordance with the invention will now be described in greater detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of part of a flooring manufacturing line, Figure 2 is an elevation of a part of a blade carrying a first form of actuator according to the invention.

Figures 3 to 7 are elevations of parts of blades carrying various further forms of actuator according to the invention.

Referring to the drawings, and more especially to Figure 1, a web substrate indicated generally by the reference numeral 10 is fed between the applicator roll 12 and a backup roller 14. Positioned above the roll 12 is a hopper 16 having an aperture 18 almost closed by a blade 20, the aperture 18 allowing a feed stream of plastisol 22 from the hopper onto the roll 12 which is rotating in the direction shown by the arrow. The plastisol is applied to the substrate 10, and the coated substrate 24 passes to an oil heated drum 26 where the plastisol is gelled. It is subsequently passed over cooling rollers 28, 30 and 32 and through a scanner 34 where its thickness is measured.

Referring now more especially to Figure 2, the blade 20 is positioned a desired distance from the roller 12 to allow a desired flow of plastisol from the hopper 16 of which the blade 20 forms the lower portion of one wall, the remainder of the wall being formed by a beam 40 pivotally mounted on bearings (not shown). A plate 42 is mounted on the beam 40, and at spaced intervals along the plate 42 are mounted actuators, of which one is indicated generally by the reference numeral 44. The actuator comprises a magnetostrictive element 46 surrounded by a coil 48 in a former 50, the coil 48 being connected by conductors 52 to a d.c. source.

The element 46 is retained in a cylinder 60 and maintained in compression by air pressure from a supply line 62 acting on a piston 64 advantageously aided by resilient means, in the device illustrated a spring 66.

Each end of the cylinder is provided with a threaded bolt 70, 72, the upper bolt 70 being threaded into the plate 42 and the lower bolt 72 likewise engaging the blade 20.

A strain gauge (not shown) is mounted on the rear of the blade 20, and is connected by lead 78 (see Figure 1) to a control device (not shown).

In operation, the configuration of the blade, as shown by the information provided by the strain gauges along the blade, one or more corresponding to each actuator, is compared to a standard configuration known to produce uniform product, as measured by the scanner 48, corresponding to an extension of the element 46 of about one third to two thirds of its maximum possible extension. If the comparison shows that the blade 20 is spaced too far from the roller 12, at a location corresponding to the actuator 44, the current through the coil 48 of that actuator is increased to increase the extension. The scanner 34 continuously monitors the product thickness, and serves as a check on the blade configuration.

Subsequently described embodiments may be employed similarly.

Referring now to Fig. 3, a blade 320 is positioned a desired distance from the surface of a roller, not shown, as in the arrangement described above with reference to Fig. 2, the blade forming the lower portion of one wall of a hopper, the wall also comprising a beam 340 on which is mounted a plate 342. At spaced intervals along the plate are mounted actuators, of which one is indicated generally by the reference numeral 344.

Each actuator 344 comprises tubular magnetostrictive elements 346, 347, surrounded by activating coils 348, 349 respectively. The coils are independently energizable. As seen in the drawing, the upper element 346 is biassed upwardly by a compression spring 366 acting through a washer 368, while the lower element 347 is biassed downwardly by a compression spring 367 acting through a washer 369.

Extending through the elements, washers and springs is a tie rod 390, which terminates at its upper end in a piston 392, against which the element 346 is urged by the spring 366, the rod being fixedly mounted at its lower end to a point on the blade 320.

In operation, each actuator 344 is set up so that with the elements 346 and 347 in equal compression, and the coils 348 and 349 not energized, the lower edge of the part of the blade beneath the actuator is at the calculated required distance from the roller surface.

If in use the blade to surface separation decreases, as shown by the results of inspection of the product by the scanner 34 - see Fig. 1 - the coil 348 is energized, expanding the element 346, raising the piston 392 and lifting the tie rod 390 and hence the blade 320.

Conversely, if the blade to surface separation increases beyond the desired value, the coil 349 is energized, expanding the element 347, and depressing the blade 320.

The movement required may be varied by varying the current in the relevant coil, any reduction being accompanied by contraction of the corresponding element under the force exerted by the relevant spring and the weight of the blade, it being noted that the spring 367 will act indirectly on the element 346, and the spring 366 on the element 347.

Referring now to Fig. 4, an actuator operating on a similar principle to that of Fig. 3 is illustrated.

In the Fig. 4 embodiment, an actuator indicated generally by the reference numeral 444 is fixedly mounted on a plate 442, which corresponds to the plates 42 and 342 of the knife over roll coater of Figs. 2 and 3, the illustrated actuator optionally being one of a plurality located at spaced intervals along the plate 442.

Each actuator 444 comprises two tubular magnetostrictive elements 446, 447 surrounded by independently energizable activating coils 448, 449 respectively. As seen in the drawing, the upper element 446 is biassed upwardly by a compression spring 466 located about the element 447, acting through a piston 492, while the lower element 447 is biassed downwardly by a compression spring 467 located about the element 446, also acting through the piston 492. The piston 492 is fixedly mounted on a tie rod 490 which passes freely through the interior of the tubular element 447. The ends of the springs 466 and 467 and of the elements 446, 447 remote from the piston 492 each abut one of the interior surfaces of ends 402, 403 of cylinders 404, 405, the opposed other ends 406, 407 of which cylinder are open and threadingly engage with each other in the area of the piston 492. The lower end 412 of the rod 490 threadingly engages the blade 420.

The actuator is set up by energizing each coil 448, 449 to half maximum strength and placing the elements 446, 447 under compression by screwing the cylinders 404, 405 together.

In operation, to lower the blade, the current in the upper coil 448 is increased, thereby extending the element 446 against the spring 466, while reducing the current in the lower coil 449, the spring 467 following the element 447, which depresses the piston 492 and hence the tie rod. The blade is raised correspondingly by reducing the current through the coil 448 and increasing that in the coil 449.

Referring now to Fig. 5, a further actuator, indicated generally by the reference numeral 544, is shown mounted on a plate 542, which corresponds to the plate 42 of the knife over roll coater of Fig. 2; the illustrated actuator again may be one of a plurality mounted at spaced intervals along the plate.

The actuator 544 comprises two threadedly interengaged cylinders 504, 505, the upper cylinder 504 as shown in the Figure having a closed upper end 502 and the lower cylinder 505 having a closed lower end 503. A tie rod 590 extends through central passages (not shown) in upper and lower ends 502 and 503, and carries a piston 592 which is located in the region of engagement of the cylinders 504 and 505. The lower end 512 engages a blade 520. A helical compression spring 566 bears against the interior surface of the cylinder end 503 and the lower surface of the piston 592, biassing it upward. In the cylinder 504, above the piston 592, is a tubular magnetostrictive element 546 surrounded by an energizing coil 548. The cylinders 504, 505 are screwed together to such an extent and the spring 566 is of such strength as to ensure the element 546 is always in compression.A port 511 in the wall of the lower cylinder 505 provides an inlet for an optional supply of compressed air.

The actuator is set up like that of Fig. 4, and operated similarly, with the uppermost limit of the blade level corresponding to zero energizing of the coil and the lowermost corresponding to maximum. In the event of the compressed air source being used, this is advantageously maintained at constant pressure, e.g., at about 6 x 105 Pa, which on a cylinder diameter of about 40 mm gives a thrust of about 8OKg on the element 546 in addition to that exerted by the spring 566.

Referring now to Fig. 6, an actuator, illustrated generally by the reference numeral 644, mounted on a plate 642, comprises a threaded tie rod 690 the upper end 611 of which is fixed relative to the plate 642, and its lower end 612 is attached to a movable blade 620. A tubular magnetostrictive element 646 is slidably mounted over the central region of the rod, and is surrounded by an energizing coil 648. The element 646 is retained in compression between upper and lower nuts 620, 621, reinforced by upper and lower locknuts 622, 623.

The material of the tie rod, and the relative crosssections of rod and element, are so chosen that the force exerted by the element at maximum excitation extends the rod by an amount within its elastic limit so that on reduction of current in the coil the rod returns toward its original undeformed length and regains it totally on complete de-energization of the coil.

Referring now to Fig. 7, there is shown a segment of a coating blade indicated generally by the reference numeral 701, the blade 701 having a lower coating edge 702. Mounted in an upper aperture 703 and a lower aperture 704 in the blade is an actuator indicated generally by the reference numeral 705, the actuator 705 comprising a first upper magnetostrictive element 746 and a second lower magnetostrictive element 747, the elements having associated independently energized coils 748 and 749 respectively. The elements 746 and 747 are maintained in compression by compression springs 766 and 767 respectively, the springs abutting against washers 791 and 792 respectively. The actuator 705 is one of a plurality of identical actuators of which only three others 706, 707, and 708 are shown, each having upper and lower elements 746 and 747.

When the coil 748 of an upper element 746 is energized, the element expands and distorts the blade 705 in the sense that makes the portion of the coating edge 702 immediately below it concave, while when the coil 749 of a lower element 747 is energized, the element expands to distort the blade 701 to make the portion of the coating edge 702 immediately below it convex. By varying the relative current strengths in the coils of the various actuators the level of the coating edge may be adjusted appropriately.

Claims (37)

Claims:

1. Apparatus for the manufacture of a product, in which the shape, location, or a dimension of a part of the apparatus is variable by or controlled by an actuator having a magnetostrictive element, the actuator being capable of exerting a force either in a first direction or in a second direction opposite to the first.

2. Apparatus as claimed in claim 1, which comprises at least two actuators.

3. Apparatus as claimed in claim 1 or claim 2, wherein the or each actuator comprises a magnetostrictive element formed of a ferromagnetic rare earth-iron compound.

4. Apparatus as claimed in claim 3, wherein the element is formed of a terbium-iron compound.

5. Apparatus as claimed in claim 3, wherein the compound is of the formula Tbx Dy1~x Foe1.6 to 1.98.

6. Apparatus as claimed in any one of claims 3 to 5, wherein the element is a rod.

7. Apparatus as claimed in any one of claims 3 to 5, wherein the element is crystalline.

8. Apparatus as claimed in claim 7, wherein the element is a single crystal or a grain-oriented polycrystalline material.

9. Apparatus as claimed in any one of claims 1 to 8, wherein the or each actuator comprises a magnetostrictive element mounted in compression, and biassing means exerting a force in a given direction on the element, and an excitation means, the arrangement being such that on energization of the excitation means a force is exerted by the element on a movable member, the force being in the direction opposite to, and at maximum excitation greater than, that of the bias force.

10. Apparatus as claimed in claim 9, wherein the biassing means is resilient and exerts a force that increases as its distortion increases.

11. Apparatus as claimed in claim 9 or claim 10, wherein the biassing means is a compression spring.

12. Apparatus as claimed in any one of claims 1 to 8, wherein the or each actuator comprises first and second magnetostrictive elements and excitation means, the elements being mounted in compression, and first and second biassing means exerting a force on the first and second elements respectively, the force exerted by the first biassing means being in the direction opposite to that exerted by the second biassing means, the arrangement being such that on energization of the first excitation means a force is exerted by the first element on a movable object in a direction opposite to and at maximum energization greater than that exerted by the first biassing means, and on energization of the second excitation means a force is exerted by the second element in a direction opposite to and at maximum energization greater than that exerted by the second biassing means.

13. Apparatus as claimed in any one of claims 1 to 8 wherein the or each actuator comprises first and second magnetostrictive elements and excitation means, the elements being mounted in compression, and a movable member, the arrangement being such that on energization of the first excitation means a force is exerted by the first element on the movable member in a first direction and on energization of the second excitation means a force is exerted by the second element on the movable member in a second direction opposite to the first.

14. Apparatus as claimed in any one of claims 1 to 13, which is an apparatus for the manufacture of a surface covering product.

15. Apparatus as claimed in claim 14, wherein the surface covering product is a flooring material.

16. Apparatus as claimed in claim 15, wherein the flooring material comprises a thermoplastic layer.

17. Apparatus as claimed in claim 16, wherein the thermoplastic layer is applied to a substrate in the form of a plastisol.

18. Apparatus as claimed in claim 17, wherein the part of the apparatus, the location, dimension or shape of which is controlled or varied by the actuator, is the blade of a knife over roll coater.

19. Apparatus as claimed in any one of claims 1 to 13, wherein the part of the apparatus the location, dimension or shape of which is controlled or varied by the actuator is a blade.

20. Apparatus as claimed in claim 1, substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.

21. A process for the manufacture of a product in which the shape, location, or a dimension of a part of the apparatus for making the product is variable by or controlled by an actuator as defined in claim 1.

22. A process as claimed in claim 18, wherein the actuator is as defined in any one of claims 3 to 13.

23. A process as claimed in claim 21 or claim 22, which is a process for the manufacture of a surface covering product.

24. A process as claimed in any one of claims 21 to 23, wherein the surface covering product is a flooring material.

25. A process as claimed in claim 24, wherein the flooring material comprises a thermoplastic layer.

26. A process as claimed in claim 25, wherein the thermoplastic layer is applied to a substrate in the form of a plastisol.

27. A process as claimed in claim 26, wherein the part of the apparatus, the location, dimension or shape of which is controlled or varied by the actuator, is the blade of a knife over roll coater.

28. A process as claimed in claim 21, carried out substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.

29. The use of an actuator as defined in claim 1 to control or vary the shape, location, or a dimension of a part of a manufacturing apparatus.

30. The use as claimed in claim 29, wherein the actuator is as defined in any one of claims 3 to 13.

31. The use as claimed in claim 29 or claim 30, wherein the apparatus is apparatus for the manufacture of a surface covering product.

32. The use as claimed in claim 31, wherein the surface covering product is a flooring material.

33. The use as claimed in claim 32, wherein the flooring material comprises a thermoplastic layer.

34. The use as claimed in claim 33, wherein the thermoplastic layer is applied to a substrate in the form of a plastisol.

35. The use as claimed in claim 34, wherein the part of the apparatus, the location, dimension or shape of which is controlled or varied by the actuator, is the blade of a knife over roll coater.

36. A process for the manufacture of a surface covering product which comprises applying a plastisol to a substrate web using a knife over roll coater, said knife comprising a blade separated from the surface of said roll by a spacing appropriate to lay down a predetermined thickness of said plastisol, gelling said plastisol, monitoring the thickness of gelled plastisol, and correcting any deviation from the predetermined distance at a given location across the width of said substrate web by causing a portion of said blade corresponding to said location to vary its spacing from said roll surface under a force applied by an actuator having a magnetostrictive element, said actuator being arranged to exert a first force whereby said spacing is increased and a second force whereby said spacing is reduced.

37. Any new and novel feature hereindescribed, or any new and novel combination of hereindescribed features.