Omer W. Blodgett, Sc.D., P.E.

Heat exchanger assembly.

More movement horizontally could result in a wavy weld.

More movement vertically could affect the arc length.

The square cross-section gives equal stiffness in both directions.

When designing a product, manufacturers face tradeoffs. To resolve those tradeoffs they typically ask themselves which feature is most important. Of course, this line of reasoning means other features are considered "less important." Although not the best grammer, sometimes it is helpful to ask: "What don't you want the worst?" Sometimes posing this question can help a designer reveal faulty assumptions and better determine the lesser of two evils.

In one situation, for example, a welding job had to be performed inside a 24-in.-diameter, 40-ft.-long heat exchanger. Three hundred pounds of welding equipment and consumables had to be attached to the end of a boom that would extend inside the heat exchanger.

The maintenance folks were ready to start fabricating the boom, but first asked: "How much deflection could be tolerated at the end of the boom?" They contacted the production department, and the answer was "none." However, this proved to be an impossible goal. Eventually it was agreed that the boom could deflect 0.010 inches without harm.

The maintenance crew made some quick computations, ignoring the weight (mass) of the beam. The required moment of inertia (I) was determined to be 36,900 in. ⁴ to support the 300 lb. load of the welding head, electrode and flux.

Assuming that two channels would be used to construct a box section, they opened their tables of section properties, searching for a channel with a moment of inertia of 36,900 divided by 2 or 18,450 in. ⁴ However, when they checked their tables of standard MC-shapes, the largest one listed, an MC18X58, had a value of only 675 in. ⁴ Out of curiosity, they next checked some wide flange sections. Even a W44X335 would not work (I = 31,100 in. ⁴ ), and of course, that would not fit into the 24-in.-diameter heat exchanger. Some type of compromise would be required.

They began a quest for the "best we can do" solution. What could be done with the C15X50 channels? Some quick mathematical checks showed that a boom made from two such channels would deflect 0.46 inches with a flange width of 3.75-in., the box made of the two channels would be 15-in. deep and 7.5-in. wide. It would fit inside the tube. But, could production live with a deflection of nearly 0.5-in once the welding equipment was added?

The team re-examined the basis for the deflection. The concern was the placement of the welding electrode with respect to the seam. No one wanted the wire moving up and down during welding. The maintenance mechanic who had been working on the project pointed out that the boom might deflect by as much as 0.5 in. when it was loaded with the equipment, but it would not move up and down — it would stay deflected. A bit embarrassed, everyone gradually realized that the 0.01-in. dimension was meaningless, and the initial 0.5-in deflection wouldn't cause any problems. Further, even if the boom did move up or down slightly, the only effect on the welding operations would be a slight change in the arc length, something that would not cause any major welding problem.

Looking at the tall and relatively narrow shape of the assembly, the people gathered started to think about deflection in the other direction, that is, horizontally. What would happen if the boom was bumped? Obviously, horizontal movement could be a problem: the arc could move off of the seam, resulting in a defective weld. It was time to ask: "What don't you want the worst?" Everyone agreed that the more problematic situation would be horizontal movement. Perhaps the boom should be rotated, and the largest dimension made to be the horizontal direction.

Still, this idea seemed a bit radical. Certainly, limiting deflection to 0.01 inches did not make sense, and was not necessary. But to forget all about horizontal deflections and concentrate only on horizontal movement didn't seem quite right either. In the end, they decided upon a square section, one that would balance the stiffness so that neither direction would be a weakness.

Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co., struck his first arc on his grandfather's welder at the age of ten. He is the author of Design of Welded Structures and Design of Weldments and an internationally recognized expert in the field of weld design. In 1999, Blodgett was named one of the "Top 125 People of the Past 125 Years" by Engineering News Record. Blodgett may be reached at (216) 383-2225.