There are several ways to increase productivity without sacrificing quality. Examples are the minimization of the scrap rate and the avoidance of downtimes. Torque sensors in extruders are playing a role in achieving these goals.
EXTRUDERS FULFILL IMPORTANT FUNCTIONS: they serve to mix, compact, and plasticize constituents and deliver high pressures required to form the goods. Twin-screw extruders have additional advantages such as enhanced mixing, and self-cleaning properties. Especially multi-screw extruders are exposed to a higher risk of jammed screws, e.g. after malfunctions in the heating systems or temperature sensors, or as a result of variations in the properties of raw materials. Over the past years, torque in extruders has been increased in order to maximize throughput, up to the point of reaching the mechanical limits of the construction materials. On the other hand, downtimes resulting from breaking screws lead to high costs and are hardly acceptable.
Such downtimes can be reliably avoided by constantly monitoring torque on every individual extruder screw. Torque sensors by NCTE (Germany) have been tailored to meet the requirements especially in multi-screw extruders. They work based on magneto-elastic principles. The existing screw, shaft or coupling is converted to the primary torque sensor by magnetic encoding. Additional components such as glued-on stain gauges are obsolete. Because of the non-contact measurement principle of the sensors, these products are robust and durable even under extreme conditions such as heat, presence of chemicals, liquids, lubricants. The torque sensor reliably detects the conditions inside the extruder in operation and allows counteraction before any damage is created. Based on the torque information, throughput can be maximized without the risk of damage; the machine can be operated safely more closely to the mechanical limitations.
The sensor is typically integrated in the couplings between gearbox and the “hot“ parts of the extruder, but can also be placed on the screws or on any shaft. The patented process of magnetically encoding steel components consist of current pulses applied to the part, which lead to remanent magnetization – and a robust and precise torque sensor without additional parts on the rotating device.
Upon application of torque, the magnetic field surrounding the primary sensor changes. These changes are detected by the secondary sensor: precise sensor coils placed near the primary sensor register changes in the magnetic fields. The higher the desired accuracy, the higher the number of coils recommended. An external shielding eliminates possible impact by external magnetic fields. A discrete electronic processing unit converts the sensor signals into operationally useful data. The user interface can be an analog signal such as current or voltage, or CAN network, frequency, or PWM-type signals, or any other format.
Due to the design of the torque sensors, the sensors are immune against disturbances such as bending forces. The system is robust, since it does not require telemetry or any electrical contact to the primary sensor. The magnetic field ranges between 0,5 mT and 0,7 mT, which is so weak that metallic particles are not attracted. The sensors can be retrofitted to existing machines and come in protection classes IP67, IP69K or ATEX.
From airplanes to extruders
The aviation industry is particularly demanding in terms of robustness requirements. In order to verify the ruggedness of NCTE’s solutions, aviation-OEM Liebherr Aerospace has subjected the sensors to the aviation robustness test in their laboratory. After 2 million load cycles, 68 extreme temperature cycles, 120 hours of strong vibration and an 8-week permanent load test the sensor signal remained perfectly constant.
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