The invention relates to a web tension meter of the gene¬ ral kind set forth in the preamble of Claim 1.

The non-contact measurement of moving webs is known in the 5 art and has been employed, for example, for regulating tension when winding paper onto reels and unwinding paper therefrom. Examples of this methodology are found in U.S. Patent Specification No. 2,945,637. In accordance here¬ with, the web is drawn in a curved path over a perforated

10 curved plate through the holes of which air is blown so as to produce a kind of air-cushion bearing between the plate and the paper. The greater the tension in the paper, the less air consumed, so that the air pressure increases in- ' wardly of the perforated plate. A similar arrangement is

^' 15 known from the Swedish Patent Specification No. 207 513, although in this case the pressure is measured via one or more measuring apertures located in the wall in which the air-exhaust orifices are seated. It is possible herewith to measure the web tension at a number of positions across

20 the width of the web.

These known non-contact web-tension measuring apparatus, however, have the disadvantage that it is necessary to change the direction of the web, by drawing the same over 25 a plate or like element, admittedly while supporting the same on an air cushion and therewith in a manner which is at least contactless in principle, but not reliably so in practice. Although, as far as the arrangement disclosed in the aforesaid Swedish Patent Specification is concerned, 30 it is possible to measure web tension at mutually diffe¬ rent locations across the width of the web, these measure¬ ments can only be taken at fixed locations determined by the construction.

It is an object of the invention to provide a non-contact web tension meter which can be placed against a web without needing to change the direction of the web and the position of which-, moreover, can be freely moved to any part of the web whatsoever across the breadth of the web, and which is therewith suitable for incorporation in a transversable measuring head, for co-action with other measuring devices operative in measuring thickness, moisture content and other variables of interest with regard to process controls and quality controls. The mea¬ suring process shall preferably provide a linear, or near linear, result.

In order to elucidate this object, it may be mentioned that during my work with thickness gauges for measuring the thickness of moving paper webs, I discovered through experiment that the thickness of a paper web, at least when measured in a contactless manner or with but low sur¬ face pressure, surprisingly changes when a change in web tension occurs. One might presume that an increase in ten¬ sion would result in a thinner paper, due to transverse contraction. What was found during experimentation, how¬ ever, was something far more complicated, a matter which, moreover, is contingent on the kind of thickness gauge or meter used. When a non-contact meter of the kind disclosed in Swedish Lay-Open Print .3**1 997 is used, stretching of the web will initially result, in the case of newsprint, in the measured thickness increasing by 3-4 micron (force 0-0.2 N/m) and then increasing more gradually by some further microns (force 0.2-1.0 N/m) and only then, when this point is reached, decreasing when tension is increa¬ sed. Although I cannot be sure, I assume that this effect, is due to the "pile" of the paper rising as the paper is placed under tension. Consequently, in order to obtain a

true value in respect of the thickness of the paper, it is necessary to know the tension therein, in order to make a suitable correction. Newsprint can have a standard thickness of 80 micron, and it is desirable to be able to maintain control of this thickness to within about one or two micron, or. better. In addition, it should be noted that contacting thickness meters have an even greater dependency on web tension than do present day non- contacting thickness meters.

Measurements of the web tension, correlated with other measurements, is desirable in continuous manufacture con¬ trol processes, for controlling a paper machine. This con¬ trol is normally effected by heating a roller-located in • the machine to varying degrees of temperature at different locations along the length of the roller, to achieve a uniform thickness. A web of uniform thickness is a requi¬ rement from the printers. Data relating to the web tension is also important when the paper is to be wound on reels, and there is reason to suspect that irregularities in the tension of the wound web are the cause in many cases of unforeseen web fractures when, for example, printing news¬ print. A paper roll which has been wound properly with respect to web tension is also less likely to become non- round.

The aforementioned objects and advantages are achieved in accordance with the invention by means of a non-contact web tension meter having the characterizing features set forth in the following Claim 1.

The meter is suitably of a telescopic construction con¬ trolled by pressurized gas, such as to be withdrawn when pressure gas falls away. This is achieved with an embodi- ment according to Claim 2. A preferred embodiment in this

respect is set forth in Claim 3, in which embodiment the meter can only be extended telescopically when the pressu¬ re exceeds a given minimum pressure. A further improvement in this respect, -and an improved function during a measu- ring operation, is obtained with an embodiment according to Claim 4 or Claim 5. The width of the pressure aperture should be small in relation to the width of the web and preferably not greater than one twentieth thereof, and it is suitable to permit solely the lips which define the pressure aperture and the measuring aperture, to face towards the web.

The invention will** now be described with reference to an embodiment thereof illustrated in the accompanying drawing.

Fig. ^' 1 is a side view of a web tension meter. Fig. 2 illustrates the same web tension meter in cross- section. Fig. 3 illustrates a stationary testing station. Fig. 4 illustrates a test result obtained in the testing station shown in Fig. 3«

Fig. 5 illustrates variations in thickness with web ten¬ sion in respect of two mutually different papers. Fig. 6 illustrates the effect of a correction made to a paper profile without taking web tension into account.

Fig. 1 ^" is a schematic side view of a web tension meter according to the invention. Seated in a holder 3 is a mea-

• suring head, which is substantially of cylindrical shape and has a diameter of 30 mm. A pressurized air-inlet is shown at* 4, and a pipe 5 is connectable to a pressure

. meter. Such pressure meters are known to the art in many - variations. In the illustrated embodiment there has been used a low pressure transducer designated Mod. 261

from Setra Systems, 45 Nagok Park, Acton, Mass. USA. As clearly shown in Fig. 2, the measuring head is divided into two parts, a stationary part 1 and a movable part 2, these parts moving telescopically relative to one another. The stationary, upper part is hollow and has a narrow ope¬ ning 14 and a wider opening 16 and a cap 18, through which the pressurized-air pipe 4 extends. The movable part com¬ prises a nozzle part which is firmly mounted on a stem 15 fitting in the opening 14 and the other end of which car- ries a piston 13 which fits in the wider opening 16. A compression spring 12 is mounted so as to tend to draw the stem 15 inwardly to the position shown in Fig. 2. An air passage 11 extends through the stem, up to the nozzle part and is terminated, in one direction in a sealing ring 17, which is located on the upper side of the piston and which, in the illustrated position, seals against the inner surface of the cap 18, around the air inlet hole 4.

It will be seen that the piston 13 when occupying the illustrated position will not be affected by the pressuri¬ zed air introduced until there is reached a pressure of such magnitude as to free the sealing ring 17 from the cap 18, against the action of the spring 12.

The passage 11 opens into the outwardly open space 8, via a coarse sintered plate 19. The sintered plate contributes in holding up the pressure in the passage 11, so that sufficient pressure is obtained through the supply of pressurized air for pressure to be exerted over the whole of the upper surface of the piston 13, whereafter the piston forces the movable part 2 out to a working posi¬ tion, not shown.

When using the meter to measure the tension in a web, the

web is brought into position for abutment and, in use, is spaced at a distance of approximately 1 mm from the lower portion of the movable part 2 and forms a defining wall of the space 8. Extending around the space 8 is an annular chamber 7 with an annular measuring slot 9, outwardly defined by a lip 10. When the space 8 is pressurized to a pressure of 0.5-1 bar, the yielding web will move away to a greater or lesser extent, depending upon the tension prevailing in the web. The pressure measured in the pres- sure gauge connected to the measuring slot 9 through the passage 5 will vary with web tension. The measuring result, on the other hand, is not particularly influenced by the pressure of the input air, and hence quite primi¬ tive pressure control devices will suffice.

It might be possible to guess beforehand that such a dependency is to be expected, by relatively elementary considerations. However, the fact that this effect is found to be linearly or nearly linearly, dependent on the web tension, rendering the invention well suited for the aforesaid measuring function, is likely to surprise one of normal skill in this art.

It has been possible to ascertain this linearity experi- mentally. In one experiment a strip of paper 30 (type newsprint) having a length of 2 m and a width of 440 mm was hung vertically between two supports 31 and 32, and was loaded with a container 33, into. which water could be poured. With the container empty, the tension corresponded to a load of 220 grams. A web tension meter 34 was placed against the surface of the paper. The geometry will be seen from Fig. 3, in which the measurements are in milli¬ meters. The result obtained with this arrangement is shown in Fig. 4, where the values in percentage of a full-scale reading on the pressure meter used are plotted on the

X-axis. The curve obtained was practically linear.

It will be understood that the force conditions are not completely simple ^* , since the arrangement functions with a flowing gas. It will be perceived that there prevails in the space 8 (Fig. 2) a static pressure which, quite natu¬ rally, will force the paper outwardly, away from the measuring head. The air, however, also flows, at the same time, laterally in the gap formed between the head and the paper web, creating a vacuum force in accordance with Bernoulli's law, which strives to draw the paper towards the head. The measured pressure is influenced by this sub- pressure. That the result of these different effects brings about a result which is a linear, or near linear function of- the web tension, is apt to cause surprise, as is also the fact that the measuring result depends so little on the applied pressure, within wide limits.

Tests have also been carried out with moving webs, with paper wound from one roll to another. As far as could be seen, the measuring result is not influenced by the speed at which the ^' web moves. In this arrangement the web forms at a location opposite the head a little pronounced "valley", having a length in the feed direction of some twenty centimeters and a width which slightly exceeds the width of the head.

In order to illustrate the particular problem within the paper industry where the invention affords a solution, reference is made to Fig. 5, which shows the change in paper thickness in microns as a function of web tension. The curves 51 and 52 show the results obtained with tests carried out on newsprint obtained from two different Swe-

dish paper mills. The difference is not one of chance; on the contrary, paper samples obtained from the two mills consistently exhibiting each its particular curve form. It will be seen that' when stretching, there is a rapid increase in thickness succeeded by a more gradual increase in thickness with a further increase in the tension, finally followed by a decrease in thickness for web ten¬ sions above about 1 N/m. It is apparent that no accurate measurement of the thickness of paper webs is possible when they are tensioned, unless the effect of the web ten¬ sion is corrected for.

Fig. 6 illustrates the effect of a thickness measurement which is impaired by errors due to failing to compensate for web tension. Fig. 6 shows along the X-axis the width of a paper web which is assumed to be initially of uniform thickness, i.e. does not deviate from a desired "standard thickness. This standard uniform thickness is shown by the line 61. A variation in web tension exists, however, across the width of the web, shown by the broken line curve 62, which is quite representative, since web tension is often lower t the edges of the web. If the thickness of .the web is now measured, the value obtained will be in error, the paper being found thicker in the centre of the web. When correcting this supposed error until a uniform thickness .is found in the thickness meter, there will actually be obtained a manufactured paper having a true thickness curve which exhibits the deviation shown by the chain line 63.

As to the accuracy aspect of thickness measurements, the following can be said. Standard newsprint has a thickness of roughly 80 micron. Newsprint which exhibits variations in thickness in the order of 10 micron is considered to be second-rate, and a standard manufacturing tolerance is at

present 2 micron. Variations in thickness are significant both with respect to the quality of the print obtained and to the risk of web fracture. High accuracy measurement of web thickness during manufacture would be highly benefi- cial in producing a uniform dry weight of the paper across the whole of its breadth. At present, it is necessary, instead, to make corrections by drying the web to varying degrees at different locations across the width, so as in this way to reduce the variations in thickness when calen- dering the formed paper. The varying magnitudes of moistu¬ re content of the finished paper rolls are economically disadvantageous to the paper mill, which by applying better controls - -solely possible by improved thickness measurement - could reduce the consumption of raw mate- rials for each ton of paper produced without loss of quality. Winding of newsprint into rolls (diameter 2 m) can also be controlled in an improved manner, and a suit¬ able tension distribution obtained, all by accurate measu¬ ring of the tension of the web. There is a well-founded general suspicion that a poor lateral distribution of the web tension profile is an essential source of web fracture in printing shops.