MONITORED COOLING FOR ROBOT SPOT WELDING UNITS
NO LEAKAGE WITH RELIEF CYLINDER
To prevent this, the modular cooling system can be equipped with a relief cylinder in the form of a MasterJet unit (see Figure 2). This cylinder is pneumatically preloaded during operation and relieved in the event of a leak or during cap replacement. The pressure inside the electrode clamps of maximum 2 bar is thus relieved through the cylinder. This ensures that no water can escape the system while the caps are being replaced, irrespective of the actual pressure in the electrode clamps. The MasterJet offers another significant advantage: when the cooling system is restarted, water is fed back from the cylinder into the pipeline, effectively preventing air bubbles in the lines. Thus, there is no need for additional bleeding measures and the system can be restarted without faulty sensor signals.
Restarting therefore takes only two to three seconds. Without the MasterJet, the restart time would be around 15 to 20 seconds. Taking into account all these advantages, the relief cylinder pays for itself within a very short period of time. In addition, MasterJet solutions are very compact in design so that the monitoring block can be installed directly at the base of the welding robot close to the actual process.
MODULAR COOLING SYSTEMS WITH ELECTRIC DRIVE ELEMENTS
The design and function of the integrated cooling blocks with electrically operated solenoid valves correspond to that of the pneumatically operated systems (see Figure 3). They also provide additional connection ports for the cooling of thyristors. The use of electrically operated cooling systems is particularly recommended for plants where the electrode clamps are equipped with electric servo motors, as the cooling blocks do not require compressed air supply lines. On request, electrically operated cooling systems can be fitted with an auxiliary pump. In the event of a leak or during the replacement of the electrode caps, the pump removes the trapped coolant to the return line, thus preventing cooling water escaping during electrode cap replacement. There are however some disadvantages to this solution.
Apart from the relatively high cost of the pump, air is trapped in the line when the machine is restarted, which is not the case with the relief cylinder of pneumatically driven systems. The trapped air causes faulty measurements of the sensory devices, which in turn delay the restart and cause longer downtimes of the robot welding units.
CUSTOMISED SENSORY EQUIPMENT AND VALVE TECHNOLOGY
The choice of the optimum sensor and valve technology for cooling systems for spot welding robots is determined by a number of different factors. One of the most important issues is thereby the quality of the cooling water. It is often contaminated and contains iron (Fe) and oxygen (O2) magnetite resulting from chemical reactions. In this case, we recommend using direct-acting valves, such as seat valves or diaphragm valves, rather than electrically driven valves, as particles in the cooling water can block the pilot channel that is crucial for the proper functioning of the latter.
Diaphragm valves also have the advantage of very low flow-rate losses and pressure drops in the valve. If the required flow rate can only be achieved with servo-assisted solenoid valves, we recommend using media-separated valves (see example in Figure 3) where the cooling water containing magnetite is not in direct contact with the magnetic coil.
Key factors for the choice of the correct sensors are the number of adjustable electric switching points (one or two), the design of the outputs (analog or frequency) and the response time in the event of a leak (normally between 300 and 500 milliseconds). In addition, operators must consider whether the sensory equipment can be properly integrated into the system. The integration of paddle wheel sensors is for example much easier than that of ultrasonic sensors, (see Figure 4), as paddle wheel sensors tend to be smaller in size. When choosing between these two sensor types, the level of contamination of the cooling water must also be considered. As an alternative to systems with summary coolant return flow monitoring of both electrode caps, operators can opt for solutions where the cooling water flow of each electrode cap is monitored separately (Figure 4). Such solutions allow operators to easily determine the level of contamination in the cooling lines.
SUMMARY
For the cooling of electrode clamps in spot welding robots, the use of compact devices with integrated sensory equipment, valve technology and controllers offers a range of advantages. Such modular systems can be individually configured to provide optimised cooling solutions for specific applications and operator requirements. In contrast to conventional solutions consisting of products from various manufacturers, integrated solutions are much more compact in size and their components are specially chosen for optimised interoperability.
Integrated systems are suitable for various water quality levels, are available with a wide range of diameters and sensor types and come with or without display. Pneumatic and electric drive units allow for perfect integration into the existing plant environment. By using a configurable control system with operator-specific software, customers can acquire intelligent tailor-made cooling systems that guarantee zero leakage in the event of loss or replacement of electrode caps in their production environment.
About the Author: Egon Huefner
