Listen to the welder
An engineer carefully designed the welded assembly for the lifting bail of a machine. On his drawing, everything fit fine. But on the shop floor, the welder had to grapple with assembly issues that the engineer simply had not anticipated. In this case, the engineer ultimately learned a lot about his design by going down to the shop floor and listening to the welder.
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The concept was simple, but when the engineer's idea, perfect on paper, took shape in steel on the shop floor, a variety of unanticipated problems arose. While the crosssections of the C-shaped channels were drawn as having flanges of a uniform thickness, they are slightly tapered in reality. Ninety-degree intersections between the flanges and the web would be ideal, but in reality, some are slightly less while others are more. Furthermore, the overall height of the channel can vary. |
 When the various channel
sections were fitted together to make the lifting bale assembly,
every imaginable combination of tolerance variation was
encountered. When the cross-channel was too narrow, or the vertical
support too wide, a gap existed. |
 This made melt-through a
problem when welding this connection. When the crosschannel was too
wide, or the vertical support too narrow, the cross-channel
intersected at the tapered portion of the web, affecting the
overall width of the frame assembly and creating a gap against the
web. |
 Even when all the
members were ideal, tight fit-up was always impaired by the
naturally occurring radius between the web and flange. |
 The problems
weren't limited to the cross-channel to the vertical support. The
shelf connection had similar drawbacks: added to the variation in
the channel tolerance was a variation in the profile of the cut as
applied to the shelf angle. Sometimes, the channel was too wide for
the cut, and sometimes, it was too narrow. The ideal situation
almost never happened. |
 The welder,
however, was resourceful. He used a 2-lb ball peen hammer to make a
variety of "adjustments" as needed to account for all the
production variations. Banging and beating the steel together, the
welder would mutter under his breath, "it all pays the same," and
"doesn't that design engineer know anything about steel
tolerances?" |
 One day, the
engineer was on the shop floor and overheard the ruckus caused by
the day-to-day fabrication procedure associated with assembling the
lifting bail. The welder was ready to give the engineer an earful;
fortunately, the engineer was willing to listen. As the welder
explained the situation, the problem became obvious, but the
solution was not so apparent, at least to the engineer. The welder,
however, had been thinking about the problem, so when asked by the
engineer for a solution, he was ready. He suggested, "Why not just
reverse the orientation of the vertical channels?" |
 Sometimes,
the most elegant solutions are the easiest. Once the vertical
channels were rotated, the cross member always fit. |
 There was no longer a need
to notch the shelf angle (which, incidentally, had removed much of
the strength of the angle), and the fit was also consistent. |
 As the
engineer contemplated the consequences of the changes, he realized
this solution also improved the resulting load path from the base
angles to the vertical channels. While the original design required
a variety of directional changes in the load path, the new concept
permitted easy transfer of loads between the base angle's vertical
leg, through the weld, directly into the channel web. |
 A better
design, easier assembly, improved quality, and lower cost —
all these resulted from the engineer listening to the welder. |
The lifting
bale assembly was simple in concept: two angles at the base served
as the primary supports for the machine's components. The angles
were attached to two vertical channels. Another pair of angles
formed a shelf at the mid-height of the machine, and the shelf
angles also were connected to the vertical channels. Finally, the
vertical channels were tied to each other by a horizontal
cross-member, again made of a channel section. The lifting lug was
attached to this channel. All the connections were welded.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.
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