INSTRUMENT FOR WEB TENSION MEASUREMENTS

Abstract: Tension in continuous webs, particularly webs that are in motion (3) is measured by bringing an elastic sensor in contact with the paper web (3) and the displacement of an elastic sensor (4) relative to a fixed point of reference (10) is measured, the displacment being a measure of the web tension. An apparatus for the measurement of the web tension comprises a measuring head (1) equipped with at least one smooth surface of contact (2), and an elastic sensor (4) fastened to a head (1) and movable relative to the head, extending through an imagined plane touching a contact area (2) and a device (10) for the determination of the position of the elastic sensor (4).

Description: Instrument for Web Tension Measurements This invention relates to an instrument for the measure¬ ment of the web tension of the type which is described in the preamble to Claim 1. 5 Most industrial companies handling materials in web form, will need an instrument or a device which can measure the web tension. The paper industry, the plastic industry, the graphic industry and the textile industry may be mentioned '" as examples of such industries. There are many reasons why people would use such an instrument, but most important is probably the fact that most materials in web form have a limited strength. The web may therefore break when the web tension exceeds a certain limit. For paper and plastic 5 material it is also important to note that the web tension will influence the properties of the finished product, and varying web tension may therefore cause problems during the conversion of paper and plastics to bags and sacks. A considerable interest therefore exists in an instrument 20 which can measure the web tension, either in connection with a permanent installation in the machines or portable types to be used on different machines, for instance in connection with trouble-shooting assignments. 25 The intention with the present invention is accordingly to provide a portable instrument for web tension measure¬ ments which is both simpler and cheaper than those previously offered on the market and which will also give accurate and reproducible results. The methods previously 30 used in order to measure the web tension, have been the following: 1. Guide rolls in the machines with axle journals carried on dynamometers.The web is given a predefined arc of contact around the rolls and from the pressure on the axle 5 journals it is then possible to obtain a measure of the average web tension across the width of the web. In some cases the pressure from the two axle journals may HEET be separately read, making it possible to detect an optional skewness in the web tension profile. 2. Acoustic methods. a) The measurement of the propagation of transversal sound waves along the web, with a no-contact sensor that will both generate and measure the sound waves. The propagating speed will be proportional to the square root of the web tension. b) The sensor is in contact with the web and with a window facing this web whereby the resonance frequency of the web can be measured. This frequency, generated by a built-in loud speaker and giving transversal oscillations, will be proportional to the square root of the web tension. c) The sensor in contact with the web, and measurement of the amplitude of the oscillation in a window facing the web where oscillations in a vacuum are generated by a loud speaker. The amplitude is a function of the web tension. Examples of mechanical web tensiometers can be found in GB.Pat.no.1.318.735, DE.Pat.no.2.819.951, DE.Pat.no.3.203.815, FI.Pat.no.42.480, US.Pat.no.4.437.352, 4.444.065 and 4.470.297. When measuring the web tension it is important to distinguish between the following main o jectives: A. "On-line" control to ensure optimum (average) web tension for a stable run without breaks. (Total web tension) Most suitable for this purpose is the guideroll/load cell system described under 1. B. A characterization of the cross machine profile of the web tension as a measure of the quality of the paper (product control) . This can be achieved with a sensor which can move across the web. C. "Trouble-shooting" and state analysis of printing presses and other types of machinery which convert materials in web form. If there are abnormally many web breaks or problems with an unstable web (across the machine and as longitudinal shocks) it is very important to determine if the reason is a poor quality of paper, plastic or any other product, or if the condition is caused by poor conditions in the machinery treating the web. Examples of the latter phenomenon may be the following: - too tight (or slack) web in some areas. - a skewness in the machinery, for instance the printing press (may show up as a skewed web tension profile in one direction) . - variable pull in the web (pulsating web tension). The present web tensiometer is primarily intended used for the purposes mentioned under B) and C) and particularly for the latter purpose where it is essential with an instrument which is mobile and easily portable. In order to study the conditions for instance on a web offset press it will be necessary to carry out measurements several places along each of 4-6 paper webs. It is sometimes difficult to get access to places where it is important to measure the web tension, and the instruments should therefore be as small and light as possible and preferably be independent of external power such as line voltage. It should also be possible to repeat the measurements several times without interfering with the production. The instrument which in the following paragraphs is described in detail, is believed to be very suitable for such purposes. The web tension at a certain position of the paper web is read digitally on the instrument which is battery-powered. No wires to the main line are therefore needed. By moving the sensor across the paper web the cross profile is recorded with good reproducibility and accuracy. The instrument must then be wired to a recorder. This may be a battery-driven small unit held in (the other) hand or an oscilloscope with digital storage and/or a recorder with access to main line voltage. According to information received an acoustical instrument is under development in Sweden, but the price is assumed to be more than 10 times as high. The present web tensiometer includes a sensor in the shape of a leaf spring (4) arranged in a measuring head (1) with a smooth surface (2) which is brought in contact with the paper web (3) during the measurements. When the measuring head (1) is brought in contact with the paper web (3), the spring (4) which extends from the measuring head (1), will be compressed in the direction of the head (1), and the displacement of the spring (4) will be related to the web tension. The web tensiometer will be described in detail with reference to Figs. 1-4. Fig. 1 shows in principle the measuring head (1) in contact with a moving paper web and where the measuring head (1) is connected to a suitable measuring instrument (6) by means of a suitable cable (5) and for instance equipped with a recorder. Fig. 2 shows in perspective the measuring head (1) from below, that is, the side being in contact with the paper web during use. Fig. 3 shows in perspective an imagined picture of the measuring head (1) in contact with the paper web (3), seen from the backside of the paper web, and indicating the bulge of the paper (7). Fig. 4 shows the measuring head (1) in accordance with Fig. 1 with greater magnification. A leaf spring giving a slight pressure against the paper web (3) thereby giving the paper a small bulge (seen- from the back side of the paper web (3) is placed within a measuring head with a smooth contact surface against the paper. See Fig. 2 and 3. The depression of the paper web by the leaf spring is somewhat dependent upon the contact pressure of the measuring head against the paper (3). If this pressure is too small, the paper web (3) will lose contact with the measuring head in the track where it passes the leaf spring (4). See Fig. 4. It is necessary to have paper contact along the entire area of the measuring head (2) in order to obtain a defined relationship between the web tension and the depression of the leaf spring (4) in the paper web (3). In the contact area (2) of the measuring head (1) a small optical device is located. This device will indicate when the paper has contact in the critical area, that is, when enough pressure has been applied against the paper web (3) See Fig. 4. Red light from a light diode (8) is reflected from the paper surface and is seen through a transparent plexiglass plug (9) only when there is no paper contact at this place. The light disappears when there is adequate contact and enough pressure has been applie . The required pressure is very low, only a few Newton. It has however no practical significance if this level is somewhat exceeded. (This may be seen from the reading on the tensiometer). If the contact pressure is exaggerated, the web tension will increase in this particular area. With a reasonable contact pressure against the paper web (3), that is, a little more than what is required to make the red light disappear, there is a very reproducible relationship between the web tension and the depression of the leaf spring in the paper web(3). This depression, that is, the position relative to the contact area (2) of the measuring head, is measured by means of a contactless, inductive displacement transducer (10) having a fixed position relative to the contact area of the measuring head. The displacement transducer is fastened to the measuring head (1) by means of two clamps (11) attached to a beam (12) to which the displacement transducer is fastened. An inductive displacement transducer has been chosen, but other types of electro-magnetic or optical sensors measuring the motion of the leaf spring contactless may be used. In order to get absolute values for the web tension in N/m., it is necessary to calibrate the instrument on a paper web where the web tension can be v aried in a known manner. This was done on a newspaper printing press and in a rewinder both of which were equipped with web tension control and with guide rolls carried on load cells. It is then necessary to compensate for the fact that the result is somewhat dependent upon the speed of the paper web. For the measurement of the web tension of non-reflecting webs such as black plastic webs, it will be difficult to observe the light reflected from the paper. In this case the optical sensor may be replaced by a mechanical device which gives a signal when the desired contact between the moving paper web and the measuring head (1) has been obtained. A simple type of this mechanical device may be a microswitch with a suitable contact area which will either open or close an electrical circuit incuding for instance a light diode.

Claims: AMENDED CLAIMS [received by the International Bureau on 01 April 1987 (01.04.87); original claims 1-7 replaced by new claims 1-4 (1 page)] 1. Apparatus for the measurement of the web tension c h a r a c t e r i z e d b y a measuring head (1) equipped with at least one smooth surface of contact (2), and an elastic sensor (4) fastened to the head (1) and movable relative to the head, extending through an imagined plane touching the contact area (2) and a device (10) for the determination of the position of the elastic sensor (4). 2. Apparatus according to claim 1, c h a r a c t e r i z e d b y means (8,9) to determine contact between the measuring head (2) and the web (3). 3. Apparatus according to claim 2, c h a r a c t e r i z e d b y a light diode (8) directing the light against the web (3), and a transparent window (9) in line with the contact area (2). 4. Apparatus according to any of the preceding claims, c h a r a c t e r i z e d b y an inductive measurement of the distance and a connection (5) between this instrument and-,. a recording voltmeter (6).

Inventors: HANSEN AOGE, HANSEN, AOGE
Applicants: PAPIRIND FORSKNINGSINST, PAPIRINDUSTRIENS FORSKNINGSINSTITUTT
Application year: 1986
Main classification: G01L5
IPC codes: G01L5