How often are sensors replaced in resistance welding applications? Imagine several sensors on several machines, all with a different degree of exposure to welding operations. This could be anywhere, from a few sensors to a few hundred, depending on the application and industry. Considering a sensor in relation to the weld flash, imagine this: if a sensor fails in one operation and needs to be replaced, it will cause a certain degree of downtime – maybe just a few minutes. But, if another sensor down the line also fails and needs to be replaced, and so on down the line, this spirals into a productivity issue, a gross cost concern.
One of the most common applications where resistance welding occurs is in the automotive industry, where it is used to fuse parts of the car body. The welding temperatures are very high – often in excess of 1200 degrees Fahrenheit – and currents can range from 15,000 - 35,000 Amps. Sensors are used in this industry for multiple operations, including sensing where metal car parts are located to ensure proper placement prior to welding. An automated (robotic) weld arm manoeuvres into place and welds in multiple locations around the vehicle. This causes sensors in proximity of the weld flash to experience different degrees of exposure and vulnerability to the effects of weld flash.
Snags causing breakdowns
Sensors are affected by the conditions resistance welding produces. Strong electromagnetic fields can cause a standard (ferrite core) proximity sensor to false trigger (output) or lock-on. Weld slag and/or splatter can accumulate on the sensor or melt the housing material causing small ‘pock’ holes to form. These areas are particularly vulnerable for further accumulation of weld slag/splatter. Depending on the sensor’s construction (i.e. how well their material withstands weld by-products), it will withstand different levels of accumulation before malfunctioning. This is an obvious problem when position sensing is in use, and raises concerns for downtime, maintenance and associated costs.
Sensors in severe welding environments can fail (false output) as much as three to four times a day, depending on the amount of welding involved in the application and where the sensor is located in relation to the weld tips. Sensors that are mounted very close to the weld tips are frequently subjected to the weld flash, while those located further away will be affected by the flash, but not nearly to the degree to which those that are closer. For example, a sensor that is within 10 inches from weld tips can easily experience 1,000 - 2,000 flashes per day per switch.
Increasing sensor endurance
To make a sensor with increased resistance to the weld flash, manufacturers have changed the sensor’s design to withstand varying degrees of weld slag/splatter. Some manufacturers use front caps made from teflon, stainless steel or use different materials for housing, like PTFE or copper.
Some manufacturers use proprietary weld resistant material on the housing and/or front cap. It is essential to ensure that the front cap is more resistant to the weld field, slag and splatter, while the housing can be less impervious to the slag/splatter andmore resistant to the electromagnetic field. This is because the face of the sensor is frequently directly exposed to the weld flash, and the slag/splatter will attach to the face but skid off the sides of the sensor with less likelihood of accumulation. Sensors that use stainless steel front caps are particularly prone to false outputs, as the oscillator must be tuned to a resonant frequency for the front face to sense through steel. Users commonly cope with sensor malfunctions by simply replacing the sensor. Some are ‘repaired’ using a tool (screwdriver) to chip off the built-up slag. A sensor that has been ‘fixed’ this way will probably work for a brief period but will fail again and again by fewer weld flashes until rendered useless.
Some sensors designed for welding environments incorporate technologies that make sensors resistant to the strong electromagnetic field. Factor 1 sensors that use separate, independent sender and receiver coils on a PCB and remove the ferrite core are inherently immune to the magnetic field interference that often occur during electric welding operations, lifts and electronic furnaces. The absence of the ferrite core also allows factor 1 sensors to operate at a higher switching frequency.
Many sensors designated for welding applications by their manufacturers are not truly so and fail after very few weld flashes. In fact, some sensors especially designed for weld resistance, cannot function after 5,000 weld flashes. Sensors that can withstand 10,000 - 20,000 flashes are impressive on the low end and exceptional on the high end, though a select few can function beyond 20,000 weld flashes without failure. It is good to keep this in mind when choosing a sensor for welding environments to determine what level of resistance is best suited for the application.
To determine if one’s application would benefit from sensors specially designed for welding applications, one could consider auditing the rate at which sensors are expended in the current applications. How often are the sensors failing? How often are the sensors being replaced? How much time does it take to diagnose or remedy the problem? The answers to these questions should help one determine how rugged a sensor one really needs. Keep in mind that the effects of weld slag and splatter are not just harmful for sensors, but often affect surrounding components. Furthermore, sensors in these environments may be susceptible to human and mechanical damage. Some manufacturers incorporate fitted steel covers into the sensor housing prior to sealing the sensor; so, it’s not a separate part – the sensors are impervious to physical damage from the side and weld damage from the front (when used with weld resistance front caps or coatings). In any sensing situation, it is important to examine all aspects that contribute to the success or failure of one’s sensors to make an informed decision regarding which sensor is right for the respective application.
Weld slag can also significantly affect the cordsets used to connect the sensors to higher level control systems in these locations. Weld slag build up is generally most harmful to the cordset where it mates with the sensor, or the quick disconnect area, if using a quick-disconnect sensor/cordset combination. If enough slag is present, it will effectively fuse the sensor to the cordset’s coupling nut and require the cordset to be replaced along with the sensor. This may not sound like a big deal but, replacing the cordset can be very time consuming. Imagine removing 20 feet of tie wrapped cable – some of which is covered in solidified weld slag – and lying out, installing and tie-wrapping 20 feet of new cable.
There are ways to help avoid some of these cabling pitfalls. Depending on one’s application, they can choose from various levels of protection. Cable jackets, plug bodies and coupling nuts are all components of a cordset that can be altered to provide weld slag protection. For instance, coupling nuts may be coated with PTFE to improve weld resistance.
Not just any cable jacket material can be used in these environments, as the slag will cause the cordset to melt or burn on contact. Instead, cable jackets are made from materials that are more resistant to weld slag build up. The cable jacket, most commonly used for welding environments, is rubber (chlorinated polyethylene, CPE) for its ruggedness and durability. A thermo-set CPE jacket over EPDM rubber insulation is impervious to flame and temperature extremes. CPE jackets also provide superior resistance to tears, cuts and abrasions. The drawback of this cable type is that it is more difficult to strip and is not recyclable.
If the welding environment doesn’t require cable as rugged as CPE, Thermoplastic Elastomer (TPE) jacket material may also be used. TPE cable, sometimes called TPR (Thermoplastic Rubber), provides very good resistance to weld slag build up. It is also more flexible, less expensive than rubber and easier to strip. TPE material may also be used as a moulding for the cordset plug body.
If specifying a different material is not enough to protect one’s cordsets, there are other options to increase resistance to adapt to the extreme conditions. If the weld slag is so extreme that the coupling nut is fusing to the sensor, adding a protective sleeve over the cordset should be the first option one should consider. The sleeve, often made from fibreglass, fits over the cordset and the coupling nut to where it meets the sensor to protect it from the weld slag. The sleeve is usually coated with another substance, so it is capable to withstand the brunt of the damage from the weld slag. Sleeves can be made to fit most cordsets in lengths that are specific to the application. A second protection measure involves an expandable silicone rubber coated fibreglass sleeve. This method provides a ‘heat shrink’ type fit around the cordset and the coupling nut.
Another option is to use a short extension cordset between the sensor and the second, longer cordset. Since most of the slag damage occurs near the sensor, adding the extension cordset, also called a sacrificial or shorty, to this area means one will only have to replace the extension cordset and won’t have to go through the hassle of replacing a 20 foot cable.