Temperature sensors are used in different industries and applications. One of the classical applications is the monitoring of liquid circuits in equipment, machines or their components. The wide selection of temperature sensors ranges from simple Pt100, NTC units, bimetal switches to parameterisable sensors with display. In some applications, however, easy-to-use sensors are needed.
The company Uhde High Pressure Technologies GmbH from Hagen manufacturers high-pressure pumps for water jet cutting systems. For water jet cutting a hydraulic pump produces a pressure of up to 200 bar. The compressed oil is directed into the hydraulic cylinder. Here, the oil acts on a piston which compresses drawn-in process water in the high-pressure cylinder. This way, the pump generates a high-pressure water jet of up to 4,000 bar. With this jet it is possible to cut, for example, 100 mm thick steel at a feed rate of 5 to 10 mm per minute. When the process water is compressed, the flow of oil is reduced via the right-left piston movement at the change point. Because of this and the friction of the cylinder heat energy is input into the hydraulic system. „The goal is to remain below 60 °C at the top of the hydraulic cylinder because the heat also has an effect on the high-pressure unit,“ says the Project Manager Dr. Andrzej Karpinski.
When monitoring the hydraulic temperature for the new generation of high-pressure pumps for water jet cutting systems, the long-established company from Hagen uses easy-to-use sensors. Before this, the company used a mechanical thermostat. Due to the short sensor length it could not be installed into the hydraulic tank from the top. Uhde did not want to install the sensor from the side to avoid the possibility of leaks.
Easy to use
For the new generation of pumps the company decided to use the temperature sensor TK7480 from ifm electronic. TK7480 is one of the first switching temperature sensors with intuitive operation: the two radial setting rings of the sensor allow the user to set the switch points quickly and precisely. Even without previous knowledge a machine operator can adapt the switch points to different ambient temperatures, if necessary. As the high-pressure pumps are used worldwide, much higher outside temperatures could require an adaptation of the switch points. The alternative to the TK unit would be a Pt100 temperature sensor. However, the user would have to reprogram the controller to change the switch points. So changing the switch points would be much more complex. Another advantage: "The probe length of 255 mm corresponds to the level of the suction connection of the hydraulic pump in the tank. This means that the sensor detects exactly the temperature the pump is processing and this is what counts for us," says Dr. Andrzej Karpinski and continues: "The sensor has another advantage concerning installation: for the TK unit - in contrast to mechanical switches - the requested switch points can be set via the setting rings without voltage and system temperature being applied."
The switch points
The two switch points monitor the temperature of the hydraulic oil. If the current temperature is below the system temperature, e.g. before start-up of the highpressure pump, the operator is requested to carry out a warm-up program via the controller to reach the necessary oil temperature. "As a result of this the pump first starts with low pressure. The cooling circuit is switched off until the system has heated up. The pressure is gradually increased until the machine is ready for production", says Dr. Andrzej Karpinski.
Via the second switch point a prealarm is generated as soon as the oil temperature exceeds the specified values of 60°C in the hydraulic tank. If the temperature is exceeded, the display on the high-pressure pump requests the operator to check the system once more. Causes for an error message could for example be dirty radiator ribs. The operator can carry out this check while the machine is still in operation so that it is not switched off during the cutting process but only after the workpiece has been finished.
Bottom line
Water separates with maximum precision and flexibility more or less independently of the properties and type of the material. Exactly this precision is required by Uhde High Pressure Technologies for the components used for the high-pressure pumps. The temperature sensor TK7480 meets these requirements because in addition to a good setting accuracy and repeatability it has an optimum probe length of 255 mm. Moreover, the switch points can be adapted to the local operating conditions quickly and easily without previous knowledge. So the sensor is the optimum choice for monitoring tasks in hydraulic applications.
Author: Sabrina-Ulica Schuster,
Technical Editor, ifm electronic
