Figures 1 and 2: The initial design specified a 1-in. fillet weld to join the shaft to the gear.

Figure 3: A combination partial joint penetration groove weld with an external fillet weld was added to increase the component's strength.

Figure 4: The weld throat of the fillet weld was located 2.42 in. from the center of the shaft.

Figure 5: The weld detail with the additional partial joint penetration weld.

Can it be true that a larger weld may actually result in a weaker connection? As strange as it may seem, the answer is yes. Listen to the "tale of two welds." A manufacturer of presses decided to build a new, larger model with twice the capacity of anything they had ever produced. The new press required larger shafts, bigger gears, and greater horsepower, and involved more torque. Everything about it was bigger than before. Everyone in the company agreed that every part of this new unit would have to be robust enough to reliably handle all the increased loads, including the increased torsional loads transmitted through a shaft to a gear.

The standard model that the manufacturer had been turning out for years featured a shaft joined to a gear with a simple fillet weld around its circumference, and this design worked successfully in the past. The new model required increasing the diameter of the shaft to 4 in. Initially, the design engineer specified a 1 in. fillet weld to join the shaft to the gear (Figures 1 and 2). As a precautionary measure, the engineer decided that the joint should be beveled to a depth of 1 in. as well. The new, larger model would thus have a combination partial joint penetration groove weld with an external fillet weld (Figure 3). In theory, a 1 in. external fillet weld alone should have been sufficient, but given the unusual size of this new machine, the engineer felt more comfortable adding the PJP groove weld that he assumed would make the assembly stronger.

The engineer was quite pleased with his design, but the manufacturing personnel were not. They asked, "Why is the bevel required?" And said, "That will require an additional manufacturing operation. What will be gained by the partial pen addition?"

When the designer explained how the connection would be stronger, the manufacturing group demanded to know, "How much stronger?" The designer returned to his office, and sat down to calculate the stresses in the two welds. He was confident the numbers would speak for themselves.

The analysis wasn't as simple as he expected. The controlling weld throat was not along the 45o plane as would normally be assumed when a weld is subjected to shear. Through a trial and error approach, he found the highest stresses occurred on a 54o plane. The weld throat was located some distance from the center of the shaft, a distance he called the effective radius. For the fillet weld detail, this occurred at 2.42 in. The resulting stress on this weld was 9,309 psi (Figure 4).

Next, the engineer considered the weld detail with the added PJP weld (Figure 5). This time, the weld throat did occur on a 45o plane, and the effective radius was 2 in., the same as the shaft radius. The resulting stress was determined to be 11,374 psi. There was the answer: the addition of the partial penetration groove weld reduced the stresses by...what? The numbers said the stresses had gone UP by 2,065 psi, or 22%! Could this be true?

After checking and double-checking, the design engineer realized that for circumferential welds loaded in torsion, beveling reduced the effective radius. This resulted in greater stresses for a given torsional load, even though the throat of the weld had increased. Rather than increasing the connection strength, his proposal actually weakened the connection. On top of that, it added to manufacturing costs.

It turned out that the fillet-weld-only design the company had used for many years on its standard model was the right one for the new, larger press, as well.

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.