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

Above, Figure 1 a and b. Below, Figure 2.

Top left, Figure 3; top right, Figure 4; above left, Figure 5; above right, Figure 6.

Whether a mechanical or structural application, designers often make a big mistake by putting welds in bending. This practice should be avoided as seen in the following three examples. Each case explores design solutions that get around the problem.

Hydraulic Motor Attachment
A heavy hydraulic motor was attached to its base using hold-down bolts (Figure 1a). The force caused by uplift on the motor pinion, multiplied by the eccentricity of the distance of the support to the hold-down bolts, gave an end moment of M = Fbe. There was no way to eliminate the moment, and the welds broke in service.

The solution was extending the metal base plate the full length of the motor, as depicted in Figure 1b. This puts the moment into the base and subjects the welds to tensile uplift only, as opposed to bending.

Pressure Vessel
Figure 2 illustrates a principle borrowed from the structural field: Bending moments can be avoided by putting welds at points of inflection, or points where the bending moment is zero.. The moment diagrams illustrating the internal pressure in the vessel show that welding at four points of inflection can result in a stronger vessel than fabricating with two welds. If internal pressure is high, the cost of doing twice as much welding may be money well spent.

Truck-Axle Arm
The sketch in Fi gure 3 shows an arm used on large concrete-hauling trucks to carry a third axle, which was put down when the truck is loaded, and raised when empty. The curved arm was made with ¹ 4-in. web and flanges connected with ¹ 4-in. fillet welds. Cracking occurred in the fillet welds. Analyzing the problem showed the applied bending moment (M) subjected the inside flange to tension. When the tension force on the flange changes direction around the curved portion, it caused a radial force directed toward the center of the curvature. The radial force made the inner flange bend, putting the weld root in tension as shown in Figure 4. With every bump in the road, the weld root was stressed, which caused it to crack.

The Welder's Solution
A welder looking at the cracked weld, and hearing the explanation for the cracking, had a simple solution: Place a set of fillet welds on the inside of the arm to eliminate tension on the weld root. The welder proposed the following: Weld the two webs to the inner flange first, followed by an inside fillet along the length of the curved section and extending beyond the point of tangency. Finish the box by welding on the top flange (see Figure 5). The welder s solution precludes the application of tension forces from being applied to the fillet-weld root.

The Engineer's Solution
The engineer had another idea: An alternate design keeps the whole weld region out of bending. Figure 6 a illustrates the cross-section of the original arm. The radial force made the flanges bend as shown, and the weld root was placed in tension. If the two webs are moved to the center of the flanges (Figure 6 b), the weld root is put into compression, but the face of the weld, and the weld toes, in tension.

The engineer reasoned at some point between these two extremes, there is no bending of the flanges. Using calculus, he determined the inflection point on the moment diagram and proposed placing the webs, and the associated welds, at that location (Figure 6c). Without any local bending, the single-sided welds are placed and the cracking problem solved because the welds are no longer in bending..

Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Company, 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, Mr. Blodgett was named one of the Top 125 People of the Past 125 Years by Engineering News Record.. Mr. Blodgett may be reached at (216) 383-2225.