Web Tension Measurement in
the Paper Industry
Continuous paper production processes require multipoint measurements of the web tension to enable the various individual drives to be controlled in a papermaking machine. The paper web proper cannot be directly measured due to process-specific necessities. For that reason, its tension is usually determined indirectly in such a manner that the web is deflected via a roller with the resulting bearing reaction forces being detected and converted into web tension forces using known web travel geometry parameters.
Force transducers of a specific design particularly suitable for measuring bearing reaction forces are usually arranged for this purpose between the bearing housings of the pulley or roller bearings and the foundation supports. In this context, Dr. Brandt transducers have been used successfully for years by leading paper machine manufacturers in Europe and the Far East.
Especially in papermaking machinery construction great emphasis is laid on making use of highly rigid structural elements. Web tension transducers and their mounting elements are not exempt from this requirement, either. The main reason for this stringent requirement to be complied with in the paper industry is to protect the structural systems against natural frequency vibrations. Due to the very high web velocities combined with high roll speed rates and at the same time high roll weights, these requirements are well understandable from a technical viewpoint. To satisfy these requirements, Dr. Brandt transducers for web tension measuring have been deliberately developed to offer particularly high torsional stiffness.
Since the transducers’ own mass is very low, their natural frequency is usually in the kHz range. The cutoff frequency of the entire measuring point thus virtually depends on the transducer’s torsional stiffness and on the mass of the deflector roll including supporting system.
Dr. Brandt’s technology for paper web transducers is based on the principle of strain gauges (special solutions on request). These do not behave dynamically on their own and thus do not influence the frequency behavior of the entire transducer element.
Dr. Brandt’s Electronic Instrumentation for
Web Tension Measurement
Strain-gauge based transducers are passive systems that do not send out signals on their own. Their operating principle is based on the fact that the strain gauges (SG) increase their resistance at extension and decrease it at shortening. During the application work step in the manufacture of the transducers, the strain gauges are firmly attached to the spring body and thus follow each of its length changes precisely. In this way, any mechanical deformation of the spring elements due to the influence of the force to be measured (e.g. pressure, torque or torsion) is converted by the SG into resistance variations capable of being transformed into electrical signals provided the appropriate electrical circuitry is in place and then evaluated, archived or further processed by a downstream electronic system.
Computer-aided evaluations with the Dr. Brandt DMM-10 measuring electronics or similar are often used in the paper industry where a web tension measurement has to be installed directly in front of a coiler or reel. Due to the continuously increasing reel diameter, the deflection angle at the measuring roll changes constantly. By using the Dr. Brandt measuring electronics and other signals from the plant control unit, a measuring signal is nevertheless generated that corresponds to the real web tension, which is ultimately essential for reliable control of the reel drive.
Further Applications in the Papermaking Industry
Dr. Brandt transducers are in no way limited to the use of measuring systems exclusively aimed at web tension determination. By evaluating the bearing forces, Dr. Brandt's software-supported measuring electronics make it easy to also determine other variables that lead to a bearing reaction force.
For example, Dr. Brandt transducers are used for dynamic unbalance determination of working rolls in installed condition. Another application is the measurement of the contact forces of two rollers, e.g. in web coating to control the thickness or contact pressure. Moreover, measurements of line forces exerted when the web is taken up on a reel have also been realized quite often.
In addition to the „normal“ web tension transducers, the use of microprocessor-controlled Dr. Brandt measuring electronics is indispensable here, since only a complex and extremely powerful filtering and further processing of the measuring signals enables the correct representation of the desired measured variable.
If you also have requirements that cannot be met by our modular system of standard solutions, please do not hesitate to contact us. We will be pleased to advise you and, on request, also manufacture special versions of transducers to optimally support your application economically and technically.