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Strategies that fight downtime in robotic resistance spot welding cells

Unscheduled downtime in a robotic welding line has an outsized consequence on the productivity of a manufacturing or fabricating facility. More often than not, robotic welding equipment is involved in a critical stage of manufacturing. Here, hiccups are costly.

To keep unexpected, unscheduled downtime to a minimum, manufacturers and fabricators need to understand the root causes of weld cell downtime. By doing so, they will find it easier to make changes in the process and equipment that can prevent the problem from occurring in the first place.

robotic welding
Fighting downtime in spot welding lines can be particularly challenging, considering the equipment is constantly bombarded with heat, molten metal and cooling water.

Perhaps no industry has embraced this philosophy as well as automotive manufacturers, who are at the forefront of using advanced automation to achieve quality and efficiency.

Following their example, companies in many industries are now setting goals to get as close as possible to eliminating waste, product defects, equipment breakdowns and inventory. Spot welding lines are a particular challenge for plant managers, however, because of the harsh environment that is constantly bombarded with heat, molten metal and cooling water.

When quality engineers analyze weld cells for the root causes of downtime, they see a number of factors that drive success to unacceptably low levels. Fortunately, there are four strategies to address these causes and reduce unscheduled downtime in welding lines.

Strategy 1:

Control weld spatter

To understand the problem with weld spatter, it’s necessary to explain how resistance spot welding works. High current travels through electrodes in the weld gun tips. The heat that is generated at the target creates an area of molten metal that joins the target pieces with a joint called a “nugget.”

Unfortunately, the electric current causes the metal molecules to align in polarity. The resulting magnetism repels the molten metal, propelling it up to 10 ft. away from the weld site. Flying, hot liquid metal is the top safety hazard for operators, but it should also be noted that hoses and cables can melt and sensitive electronics can be damaged.

Regarding downtime, weld spatter is perhaps the most visible cause, which is why in the past decade manufacturers have focused on reducing weld spatter with improved coatings and by reducing caustic materials. Now, the focus has shifted to the corrosive effects of cooling water.

automotive spot welding
A Parker Hannifin WRA WBB water management panel used in automotive resistance spot welding applications.

Strategy 2:

Minimize water expulsion

On robotic welding lines, weld tips are cooled by water circulating past the weld tip. This poses a problem when operators must change out the tips – a frequent occurrence. Because the water is in a closed loop system under pressure, water inevitably spurts out. Water is more than an inconvenience; it introduces safety and maintenance concerns.

A wet floor presents slip hazards to workers. According to the Industrial Safety & Occupational Health Markets 5th edition, 85 percent of worker’s compensation claims in the United States are attributed to employees slipping on slick floors. The 2016 Liberty Mutual Workplace Safety Index, which tracks the cost of the most disabling injuries in the workplace, reports that falls on the same level (slips and trips) cost U.S. employers $10.17 billion dollars annually, and falls are the second most common cause of injuries. An additional $2.35 billion in cost is attributed to slips and trips without falls.

Beyond the risk of falling, when operators are in contact with water during weld tip changing, they are at risk of skin irritation. And, as if the maintenance issues of water weren’t enough of a headache, chemicals in the water have been known to attack the coatings on plant floors.

Some manufacturing facilities deploy buckets to catch the water. Others cover the equipment to keep it dry. A better approach is to eliminate the release of water altogether.

An effective technique for combatting water expulsion during weld tip changes is to install a water retract actuator. With this technology, the control system closes the valve to the water system, but pressure is often still remaining in the water line. If so, the control system tells a pneumatic actuator to operate. Attached to the water system, the actuator creates suction in the system by withdrawing. This movement removes the water pressure from the weld gun, so water doesn’t spurt out while the operator changes out the tip.

changing weld tips
The manual changeout of a weld tip releases cooling water.

Operators appreciate the dry tip changes, and managers appreciate dry weld cells. Managers who have adopted this technology have learned that the cost savings go beyond uptime, maintenance and safety: Operators can change out tips quicker when they are dry, which increases productivity. A water retract actuator can also be used in conjunction with an automatic tip changer.

Strategy 3:

Preventive inspection, maintenance

It may seem obvious, but rigorous attention to scheduled maintenance is particularly important for weld cells. The equipment is complex, which means that there are multiple potential points of failure. Moreover, the duty cycles are demanding and the conditions are messy. To help minimize downtime, plant managers should focus on key maintenance practices:

Inspect equipment for impending power failures. Drops of molten metal and cooling water, treated with chemicals, can cause contamination of electrical connections and corrosion of electrical components. Sparks emanating from under the electrode or between the parts being welded may be a sign of impending power failure.

Keep weld cell components free from weld spatter, dust and other sources of contamination. Check equipment frequently for wear and fatigue.

Despite the impetus to lean-out inventories, managers must concede a ready supply of spare components on site. The practical reality of running a welding production line means that parts will be subject to significant levels of corrosion.

Take advantage of information and data that may predict failure. Smart machine systems can communicate health status to the equipment interface that, in turn, can publish the information to maintenance managers. If the information is made available throughout the organization, even globally via the internet, then IT systems need to be hardened against unauthorized access and must be robustly reliable. In the same way, the PLC and the software that drive automation systems need to be maintained and updated.

pneumatic water retract actuator
A pneumatic water retract actuator, like this one from Parker Hannifin, can prevent the release of cooling water, keeping weld cells dry.

Strategy 4:

Continuous training

Plants that have the best uptime rates are typically those with maintenance technicians and operators who understand how the machines function and the safety precautions. It takes an attitude of lifelong learning, as technology changes continuously. Advanced certifications and ongoing training from welding equipment manufacturers go a long way toward ensuring that equipment is operated correctly and unscheduled downtime is avoided.

It’s a good idea to cross train workers. In the event that a shift needs a fill-in worker, all operators should be fully qualified. Remember, too, that automated welders apply significant clamping force and move quickly. Refresher training will remind operators of how to work safely around robotic equipment.

Parker Hannifin

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