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When proper precautions are not taken, sensors placed in weld cells—to ensure proper weld fixturing and monitor quality—can experience some of the shortest life spans in industry today. Heat, slag, physical impact and general wear and tear can quickly destroy the most robust inductive proximity sensor, not to mention more vulnerable photoelectric sensors carelessly installed in locations far too hostile for their general-purpose construction.
Sensors that are not properly mounted, bunkered and protected from weld splatter will not only fail prematurely, they will cause premature maintenance stoppages, lost production and decreased profitability. While it's common for large users to expend between $5,000 and $60,000 per month on material-replacement costs, these pale in comparison to the real cost—downtime. The good news is that much of this cost is preventable.
A RECIPE FOR SENSOR PROTECTION
Slag-Resistant Coatings. Special coatings are the first step. Many sensor providers currently offer inductive sensors with a thin coat of PTFE (Teflon) applied to the sensor housing and sensor face. When weld slag strikes a PTFE coated sensor face, it may not initially stick, but it will pit the surface and give weld slag a foothold. Over time this allows slag to build up on the sensor. Eventually enough slag will accumulate on the sensor face causing the sensor to false trigger. At this point the slag will need to be removed. During this abrasive removal process the PTFE is often removed along with the slag, leaving the sensor vulnerable to further damage. Eventually, this cycle will destroy the sensor.
While PTFE coatings was a major improvement over standard nickel-coated housings with thermoset or thermoplastic sensor faces, there have been major efforts to find a better solution. With that in mind, some sensor manufacturers have developed new composite coatings specifically designed for use in weld-cell applications. These materials resist weld slag, abrasion, heat, and they offer superior impact resistance. With improved abrasion and impact resistance, the composite material will remain on the sensor, even when slag needs to be removed. As a result, the usable life of a sensor in these harsh conditions has substantially increased.
Prox Mounts. Beyond coatings, further protection is readily available in the form of special mounts and sensor bunkering. A prox mount is a special PTFE-coated metal fixture that the sensor slides into, equipped with a lock nut that locks the sensor in the exact position desired. This mount itself acts as a shield, protecting the sensor against physical damage, heat and slag while allowing quick removal of the sensor at any time.
Sensor Bunkering. Bunkering the sensor is the next level of protection. In the case of a proximity sensor, the prox mount slips into the bunker block, which is internally machined to position the sensor at just the right position within itself. Bunker blocks are also available for photoelectric sensors, which are much more difficult to protect in the weld-cell environment. This new ability to protect comparatively more fragile photoelectric sensors is a major step forward toward increasing overall weld-cell productivity.
Cable Protection. Connector cables with TPE jackets offer superior weld-slag resistance and flexibility. With their proven performance in weld cells, TPE cables are becoming a standard. While TPE cables have outperformed other cable materials such as PVC and PUR, there are additional steps that can be taken to protect connector cables. Tubular silicone jacketing, cut to length and applied to the cables back from the connector will protect the cabling from ambient temperatures of up to 500 Fas well as prevent slag buildup on the cabling.
Weld Repel wrap is a special silicone tape that, once applied like tape on a hockey stick, will protect the sensor and connector from slag and heat just as well as silicone jacketing. Once applied, the Weld Repel wrap bonds to itself, becoming a solid barrier to heat and slag. Wrapping the bunker block and the sensor protects the entire assembly to an even higher level. Because there is no slag buildup, the wrap will also protect the tubing from the weight of the slag and prevent the tubing from sliding down the cable and exposing the vital cable/connector junction to slag and heat damage.
Another trick involves slitting the tubing, gathering multiple cabling and wrapping the whole thing with Weld Repel wrap. The tubing and wrap can be easily slit and removed to change out a sensor or modify an installation.
TAKING A TOTAL APPROACH
First, here's what doesn't work. Some try to solve the problem of high sensor usage in weld cells not by attacking the core issues within the weld cell but by increasing the flow of replacement sensors, either by placing vending machines nearby or increasing the flow of sensor replacement deliveries. This makes it marginally easier for maintenance teams to replace destroyed sensors, but it does nothing to improve the overall reasons for premature sensor failure. Improving the supply chain process has little leverage on the core welding process, nor does it improve overall productivity in the weld cell. It only puts the problem behind a smoke screen, increasing base costs, and masks the real path to increased weld-cell productivity.
On the other hand, converting over to a totally protected weld environment is relatively easy. It involves little control revision or reconfiguration of existing sensor applications. Depending on sensor usage, this process can lead to the most significant increase in productivity you can make to your weld cell.
A comprehensive sensor protection solution provides total protection not only for the sensors but for the connectors and cables as well. This complete protection package approach will yield a significant increase in overall productivity because sensors and their associated cabling will last far longer than unprotected or poorly protected sensors and cabling. Migrating from a maintenance cycle that is measured in hours to one that is measured in weeks or months has a profound positive impact on the overall productivity of not just the weld cell but the entire manufacturing process.
Editor's Note: Craig Brockman is product manager at Balluff Inc., Florence, Ky., www.balluff.com. Artwork courtesy of Balluff.
author: By Craig Brockman