Manitowoc Cranes recently implemented a unique, triple-robot tandem-arc welding automation solution at its Shady Grove, Pa., factory to solve several common challenges that are faced by many U.S. manufacturers.

Despite a shortage of skilled manual welders, Manitowoc needed to find a way to improve productivity and to bring outsourced production back in-house, while providing manufacturing flexibility that was needed to perform heavy-deposition gas-metal arc welding on a wide range of large construction crane outrigger beams.

The new robotic system welds 26 different models distributed among three part families.

Mild steel parts range from approximately 5 ft. to 12 ft. in length, and are 0.25 in. to 0.75 in. thick. Outrigger beams weigh between 500 lbs. and 1,700 lbs.

“Results have been stellar,” Peter Johnson said. Johnson is the senior manufacturing engineer for Manitowoc.

“Beams with seven to 10 welds that range from approximately 2 in. to 10 ft. long that formerly took 3 hrs. to 4 hrs. to weld manually now can be welded, on average, in 17 min. to 18 min. with some parts finished in as little as 13 min. That is significantly faster than the 27 min. cycle time requirement that was our goal with this robot system,” Johnson said.

“We've also been able to bring outsourced production back in house, saving the company a tremendous amount of time and money. Having the robots also has allowed us to redeploy skilled manual welders elsewhere in the plant,” he added.

The use of tandem arc welding equipment on a triple robot system makes this installation remarkable.

“We believe that this is the first triple robot tandem-arc welding system of its kind anywhere in the United States,” Gerald Obritzberger, director of sales and marketing for Fronius USA, said. Fronius provided the welding equipment for the application.

“Long, straight welds with little fixture interference made the tandem-arc process a good fit for this application, increasing welding speed and deposition rate,” Obritzberger added.

System Equipment

Heavier-than-usual payload robots were needed to carry the tandem-arc torches and peripherals, and a relatively long reach was required to access the welds on the large outrigger beams.

The workcell includes three, six-axis Motoman HP20 robots that feature a 20 kg. payload, 67.6 in. (1,717 mm) reach and ±0.002-in. (±0.06) mm repeatability.

Welding and related equipment includes three Fronius TimeTwin TPS 5000 digital welding packages and three Fronius Robacta Twin Compact 900-amp water-cooled tandem-arc gas-metal arc welding torch packages. To allow bi-directional welding using the tandem-arc process, six 200-V ComArc seam-tracking and touch-sensing packages were required.

Peripherals include three reamer-type nozzle cleaning stations with wire cutters and anti-spatter sprayers. The Shady Grove facility uses 0.045 in., solid steel wire dispensed from bulk rolls. The shielding gas it uses is a common, 90/10 Argon/CO2 mixture.

Two Motoman MHT-3000 headstock/tailstock positioners, which are controlled as external axes by NX100 robot controllers, are used to locate the beams for welding.

This positioner model features a 3,000 kg (6,615 lb.) rated load at 6 in. (152 mm) off-center of gravity, 4.95 sec., 180-deg. sweep, and 6.7 rpm headstock speed. Motoman's trademarked MotoMount fixture mounting system is included in the system.

The cell includes a common equipment base for the NX100 controllers, with a table for the welding power supplies; two operator stations; and a total safety environment. Safeguarding complies with the ANSI/RIA R15.06-1999 safety standard, and includes 8-ft high woven-wire perimeter safety fencing, arc flash protection curtains, four sliding gates with positive-break safety switches at the positioner load/unload stations, and a three-color status beacon.

Fixtures

Motoman provided custom hydraulic clamping fixtures for the two headstock/tailstock positioners.

A large spanner between the headstock/tailstock positioner faceplates is adjustable to accommodate various outrigger beam lengths. Two pneumatic alignment clamps and two hydraulic swing arms per fixture hold each beam for welding. Fixtures include part-present and clamp open/closed sensors.

“We needed hydraulic fixtures as opposed to manual clamping, to ensure achieving the takt time required,” Johnson said.

“Our industry can be very hard on tooling, but these hydraulic fixtures are very heavy-duty, strong and robust. They just keep working the way they were designed to work,” he added.

“The integration of all of the elements in this robot system allows us to perform heavy-deposition welding very quickly. The three robots each deposit 20 in. to 26 in. of weld bead per minute, depending on the size of the bead, which is generally 5/16 in. to 3/8 in. wide,” Johnson said.

“Our experience with Motoman has shown us that they can deliver an outstanding system that gives the proper takt time we need to get out the product. Instead of proposing multiple single-robot cells, Motoman designed an innovative, multiple robot cell that needs less floor space — a high-tech solution that works,” Johnson said.

“They provided us with many viable concept alternatives and ideas, and then kept refining the concept we chose,” he added.

Manitowoc also is pleased with the training that was provided.

“Their training facility is the best we've found. We surveyed numerous companies, but Motoman offers everything we needed — programming training that ranges from beginners to advanced, maintenance training, computer-based simulation, hands-on learning and accredited classes. It's all set up very well,” Johnson said.

No stranger to robotic automation, Manitowoc also has another large, six-robot welding system for crane turntables in the same plant. That system uses four, six-axis Motoman HP50-20 robots invert-mounted from overhead transport beams and two HP50-20 robots on a linear track to weld large crane turntable frames mounted on two extra-heavy-duty (16,000-lb) payload headstock/tailstock positioners.