Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Utility Patent
1998-08-21
2001-01-02
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C033S533000
Utility Patent
active
06169290
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for measuring an envelope surface of an object such as a roll or cylinder of a papermaking machine.
BACKGROUND OF THE INVENTION
Substantially all machines include a larger or smaller number of machine elements which are in the shape of cylindrical bodies that rotate about an axis of rotation during operation. In a paper machine, for instance, such machine elements consist primarily of either rolls or drying cylinders such as Yankee cylinders. The rolls are usually used to influence a continuous material web in one or more roll nips through which the web runs; to transfer some form of liquid, mixture or the like to said material web; and also to support, guide and propel the web. The drying cylinders are used to dry the moving web before it is finally reeled to a finished reel of paper. The drying cylinder has a hollow, thin-walled drum, heated by steam, which has a polished envelope surface over and around which the moist material web is moved in contact with the surface so that considerable heat can be transferred to the web. In view of this heat transfer the drum has relatively large diameter in order to facilitate the necessary drying.
Common to said machine elements is that, due to their function and because they shall not influence the properties of the moving paper web in an uncontrolled and thus negative manner, they should normally be either as long as is technically possible; perfectly straight in their longitudinal extension, i.e., the envelope surface is parallel to said axis of rotation or cambered as in the specific case of a Yankee cylinder where instead of the actual cylinder being perfectly straight a carefully adjusted curvature is sought (a camber achieves the best possible uniformity in linear loading of the material web); and also perfectly cylindrical, i.e., that their physical cross section coincides exactly with the rotationally symmetrical cross section of the relevant machine element having one and the same radius around the axis of rotation for any imagined cross section along this axis. Any deviation, such as an incorrect or discontinuous camber and/or out of roundness, thus always affects the pressing, operability and, in the case of Yankee cylinders, the creping. Depending on how and where these deviations appear, the paper produced in the paper machine will not achieve the uniform paper properties striven for as far as possible.
Measurement and inspection of the envelope surface of a measured object, e.g., a Yankee cylinder, are relatively simple to perform when it is not in operation and a great deal of information can therefore be obtained. The shape of the envelope surface can be compared, for instance, with the rotationally symmetrical cross section described above in order to determine the size and any variation in the cylindricity or the straightness profile of the measured object. For the sake of simplicity the word “straightness” is used instead of the expression “straightness profile”. Furthermore, deposits, corrosion, wear patterns and the magnitude of this wear can be observed or measured. However, the shape of the Yankee cylinder's envelope surface, and thus the properties of said cylinder, are very different during operation from when it in stationary, because of the unavoidable deformations that occur during operation. Said deformations are caused primarily by three different loads, viz. linear pressure or loading from press rollers, an internal steam pressure and temperature forces.
If the measured object consists of a drying cylinder, the temperature forces mentioned above are caused by heating from the water vapor used for the drying process, which affects the shape and dimensions of the drying cylinder.
These stresses arise because the construction can never be completely homogeneous and the measured object, particularly in the case of a thin, hollow drum which is also subjected to many other forces such as centrifugal force, linear pressure, steam pressure, vibration forces, etc., is always deformed to a greater or lesser extent during operation. Even extremely slight changes in the parts of the machine element give rise to friction, wear and, in the worst case, even an increased risk of rupture in the moving material web, which always entails extremely high costs. It is therefore vital to arrive at a satisfactory method of correctly measuring the envelope surface even during full operation of the paper machine.
In a previously suggested measuring device, e.g., “Tissue Making ′89: Creping and Drying”, Oct. 5-6, 1989 in Karlstad, Sweden, VALMET Paper Machinery, Uddeholm Strip, pages 75-78, a distance transducer stated to be a “non contact eddy-current type displacement transducer” is used for measuring the envelope surface of a Yankee cylinder. This “displacement transducer” is supposed to enable measurement of the distance between the envelope surface and the transducer with an accuracy of ±{fraction (1/100)} mm when the cylinder is in full operation which, as established below, is not the case. Carried by a movable measuring carriage, the transducer is moved along a device travelling longitudinally along the Yankee cylinder and included in said measuring means, which is suitably secured to the stand of the doctor blade, or the doctor-blade beam, and transverse to the paper machine. Situated at a specific first position along and close to the envelope surface of the Yankee cylinder, the transducer emits an extremely accurate signal which is thus proportional to the distance to said envelope surface. After one complete turn of the drying cylinder, therefore, the cylindricity of the envelope surface for this first position can be determined with relatively great precision.
However, it is in practice impossible to subsequently move the transducer along the envelope surface of the cylinder to a specific second, third, and so on position absolutely in line with the first position, and at the same time completely parallel with the axis of rotation of the cylinder with an accuracy equal to or better than the resolving power of the transducer mentioned above. When measuring a straightness profile, therefore, the measurement performed for a specific first position must in some way be related to corresponding measured values for every other position along the cylinder. In said measuring device, therefore, a taut metal wire, e.g., a piano wire, is used as a straightness reference, this reference wire being arranged as best possible between two attachment devices in a measuring stand arranged in said measuring device parallel to the envelope surface of the cylinder and extending from the drive side of the paper machine across the entire length of the cylinder close to its downstream side.
Due to the unavoidable physical properties of the straightness reference, however, the measured values obtained acquire a number of unavoidable measuring errors. One of the reasons is the fact that a certain apparently slight, but due to the accuracy of the measurements still not negligible, sag occurs which is greatest at the middle of the wire and the measured value in the measuring direction of the transducer will thus naturally also be incorrect. The magnitude of this sag varies greatly depending on the tension to which the wire is subjected and also on the stretch and creep occurring in the wire as a result of this tension.
In said measuring device an attempt to solve this problem has been made by applying a specific tension in the wire firmly anchored to the measuring device by one end, with the aid of a weight attached at the other, free end of the wire. Since it is arranged to extend over and round a rotatable member arranged at the opposite end of the measuring device from the fixed attachment, the wire will always be kept taut. How much this weight weighs has thus been calculated and chosen so that the wire tension obtained will be very close to the yield point of the wire used so that its suspension and curvature are minimized maximally without risk
Rosberg Lars Tomas Ragnar
Stark Mats Ola
Alston & Bird LLP
Lee John R.
Pyo Kevin
Valmet-Karlstad AB
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