No weld cracks on this aluminum boom

Farmers with aluminum spray booms say benefits outweigh the drawback. That drawback, of course, is cracked welds. Once cracking commences, it plagues the boom until it’s finally sold for scrap.

The key to building an aluminum boom that doesn’t crack is to put the welds in non-stressed zones. All hinge points are fitted with rod ends made of 4140 heated treated steel that bolts to the end of 6061 t6 aluminum panels. It’s the high grade steel that handles the stress and abuse, not the aluminum welds.

That’s how Specialty Enterprises designed its new Millennium boom, which the company says will last eight times longer than its previous boom or the competitors’ comparable aluminum boom.

Company owner Kyle DeMars says Millennium is the second generation aluminum boom from the 40-year old Wisconsin manufacturer, North America’s largest supplier of aluminum booms.

Over a two-year period Specialty engineers analyzed faults in their first design, and faults in competitors’ designs. They saw that once a small crack started, welders would typically grind it out and re-weld, usually with gusset re-enforcing. But regardless of the welder’s expertise, the crack always returned to the heat-affected zone adjacent to the weld, generally not to the weld itself. That specific area of the boom will remain a weak spot because too much point loading is focused on that joint.

For extra strength, the entire top of each section is capped with a single roof of extruded aluminum. Oval-shaped holes are machined in to reduce weight. This cap eliminate welds in high-stress areas. | Specialty Enterprises photo

The engineers designed those stress points to be made with steel instead of aluminum. For easy repair without welding, the steel joint components bolt to the aluminum arms of the boom.

DeMars explains, “In any aluminum structure made of heat-treated aluminum, the heat-impacts zones, or weld zones, are your weak points. With our 6061 t6 we have a 50 ksi ultimate tensile strength. However, when you weld that piece, you lose tensile strength. In the worst case, if you heat it enough, you can take it from condition t6 all the way down to condition zero. Condition zero is half the strength of t6.

“One of the key design requirements in our new design is to fit all those mating parts together to create stronger joints. We don’t try to try heat treat those welds to bring the strength back at those points because we know that’s futile. And we don’t need to because they’re all in non-stressed locations. That’s why Millennium has eight times the fatigue life of our previous aluminum boom, which was roughly comparable to the Pommier boom.

“In the designing process, we used accelerometers and strain gauges to model the stresses we see at each joint. We run the booms over bump courses, figure eights, barrel tests and things like that to beat them up good so we can document what happens at the joints. For example, with the barrel test, the sprayer goes down the field at typical spraying speed and smashes into a barrel.”

He emphasizes that it’s not simply a matter of heavier hardware at the joints, it also comes down to geometry and force loads. The challenge is to design joints that experience less abuse, which is why there’s a lot of radiuses in the boom, and very few abrupt stress risers. They also designed all new extrusions.

Fold cylinders are situated within the structure for protection. They have built-in hydraulic pressure relief valves. All components bolt to the boom, eliminating the need for welding when making repairs. | Specialty Enterprises photo

“The data we have comes from field testing with OEMs such as Case. It’s their proprietary data, so we obviously can’t share it.

“There’s a difference between normal fatigue and smashing into a centre pivot or a telephone pole. The Pommier boom had two 2-3/8 inch tubes welded together. It rolls and twists and goes up and down. Over time those cycles just wear out the metal. Aluminum has a finite fatigue life.”

DeMars is working closely with Jesper Voois of Croplands Equipment in Alberta to install WEEDit spot spray systems on Millennium booms. Millennium booms can be ordered from the Specialty plant.

Base price for a 90-foot boom is US$36,600. Base price for a 132-foot boom is $43,600. The booms are available straight from Specialty in Wisconsin and from equipment dealers across the three prairie provinces.

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