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

When new designs are considered, blindly following tradition can lead to problems. While it may be very tempting to build upon successful approaches of the past, sometimes following tradition may severely handicap a new design or make progress impossible.

An old cast iron frame for a press provides a case in point. Manufacturers often created castings such as this using a trial-and-error process. If a cast component failed in service, the casting designer would beef up the casting pattern and put a modified replacement into service. The process continued until the part ceased cracking.

In this example, the user of a broken cast iron press decided to make a replacement frame from welded steel. The maintenance engineer submitted to the responsible plant engineer a design that faithfully copied the cast iron original. The design consisted of two 1-in.-thick, 13-in.-wide sections welded to a single 212-in. plate. The 1-in. plates extend one inch beyond the 2-in. plate, and the plant engineer quickly saw that the peak tensile stress would occur at the end of this projection.

Realizing that a better design would eliminate these two protruding regions, the plant engineer conceptualized an alternative solution, in which a curved, 2-in.-thick plate formed the inner radius. He would attach two webs to the inside radius of this plate by fillet welds. He calculated the bending moment that would create 20,000 psi on the outer surface of the 2-in. plate. Next, using the same bending moment, he analyzed the maintenance engineer's design, and his assumption proved to be true: The projections would have experienced an unacceptably high peak stress of 31,500 psi, an increase of 57.5% over the conceptualized alternate design. The successful design challenged tradition, but it had lower peak stresses and used less material.

The frame for a 2,000-ton press provides another example of the problems that can result when an engineer designs "by tradition." The original design consisted of a base that was made of an upper and lower plate.

Separating the two plates were a series of "egg crate" members, intersecting each other at 90° angles. When the company put the unit into service, the welds that connected the various base members together began to crack. This perplexed the designer, since the same basic design concept had been used for years. For most applications, the applied loads were distributed across the base of the press. In this particular situation, however, the 2,000-ton force was concentrated in the center of the base.

The press consisted of both a base and an upper platen, connected together by four large tension rods. A large hydraulic cylinder provided the downward force that was concentrated in the center of the base. Ultimately, the downward force was resisted by the four tension rods mounted to the corners. To under-stand what might be causing the welds to crack, the designer envision ed a diagonal section through the design. This theoretical cut showed that the design actually amounts to a Vierendel truss, where the top and bottom plates to the base were like the flanges to the truss. The tension rods on the ends acted like simple supports for the truss. The highly concentrated load in the center of the truss bent the web members, causing the welds to crack. Further, the conceptual cut showed that the applied 2,000-ton load was concentrated in a small zone — one that was not supported by any vertical member.

The challenge was to transfer the large concentrated load in the center out to the ends, where it could be picked up by the tension rods. What if the vertical members of the Vierendel-like truss were replaced with a continuous plate, like a plate girder? The designer correctly assumed that this would be a more efficient load transfer path.

To create a plate girder-like design for the base, the designer placed a large cylindrical member in the center of the base, directly under the point where the 2,000-ton load would be applied, supplying an adequate support for the load.

Next, he added a pair of diagonal supports, connecting the center cylindrical member directly to the tension rod supports. The welds joining these supports to the upper and lower plates are subject to shear, not bending, and are therefore more efficient in transferring the loads than the old design.

To check the load-transfer mechanism, the same type of conceptual diagonal cut was made in the new concept. The new design, while challenging an old tradition, not only eliminates the cracking problem but also provides for a more effective load transfer.

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.