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

Any design engineer or manufacturer dreads the following scenario: A new system is installed at a customer's facility, and it immediately begins to experience service problems. That's what occurred when the manufacturer of a vibrator-conveyor installed a new unit into a customer plant. Shortly after installation, cracks developed around the welds and the disappointed customer called the manufacturer.

The vibrator-conveyor consists of a long tray.

When the tray is vibrated, parts are transported from one end to the other. A motor with an eccentric weight provides the vibratory motion. The tray is supported by four links, two on each side. The links are attached to brackets that are welded to the tray.

The service technician arrives and observes a crack between the weld that joins the bracket to the tray, and the tray.

If the unit is left in service for a few more days, the whole tray will probably break away from the supports. The serviceman reasons the tray is being overloaded and the parts being conveyed are too heavy for this unit. Thinking that excessive deflection of the tray is the problem, he decides to add a third pair of supports at the midpoint of the tray. Whatever deflection the heavy parts are causing, the added support in the middle of the tray will reduce it significantly. A quick call to his engineering department confirms his hunch: The product designer estimates that adding the extra support will reduce deflection to ¹ /16th of the current level.

Armed with this information, the service technician proceeds to install the additional supports. Working in the field, without the normal welding fixtures used in production, he carefully locates the new brackets to be welded in place, center punches new hole locations, and drills the various holes. New links are installed, and the unit vibrator is turned out. Satisfied that the customer's problem is solved, the technician heads home.

A few days later, the phone rings again: The "fix" is broken. The cracking is the same — the tray and the bracket are separating again, right where the weld joins the tray. Perplexed, the service technician returns to the customer site, this time accompanied by the design engineer. Everyone wonders why the extra support has failed to solve the customer's problem.

After removing the parts from the conveyor tray, the technician and the engineer manually shake the tray and note that it is binding. As force is added to overcome the binding, the designer notices that the tray is twisting and buckling and the crack between the tray and the bracket is opening and closing.

Conceptually, the tray was expected to operate like a four-bar linkage, allowing the tray to move while the support below remained fixed.

Four brackets were welded to the tray, and to the brackets were attached the links.

Despite the conscientious efforts of the service technician, the alignment of the third support was not accurate enough, and the three sets of links were not parallel to each other.

This arrangement introduced "constraint." The welded connections were required to transmit more load than expected, and hence the "fix" failed.

Fundamentally, the tray is not stiff enough for the length of the conveyor and the mass-weight of the parts being transported.

The sheetmetal tray is the whole structural member. Instead of adding the third support, a better solution would be to increase the stiffness of the tray, either by using thicker sheetmetal or by creating a deeper section with a larger moment of inertia. While the attempted fix might have worked if the required tolerances could have been maintained, this proved to be impossible under field conditions, and unintended "constraint" was introduced.

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.