Sixty Years of Machines, Part VII

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Issue #12251 - June 2020 | Page #10
By Joe Kannapell, P.E.

Part VII: Gantries—Running Off the Rails

One glaring gantry glitch, truss plates falling off, still needed to be addressed in the early 1990s. Missing bottom plates interrupt production, or worse, compromise quality. Sometimes an uneven table surface is the cause. But often the gantry mechanism is to blame. Tolerances are surprisingly exacting, as illustrated here [see PDF or View in Full Issue]. Note that the bottom plate typically penetrates only 0.15” into the wood, or about the thickness of two stacked quarters, after being gantry-rolled. Therefore, if the table is 1/16 (0.062”) out of level, or if there’s 1/16” of looseness in the mechanism, the plate may fall off when ejected or conveyed.

Roll-a-Master tables were conceived with an all-steel structure, assuredly level. But as trusses became more complex, so did the makeup of tables. Where setups were infrequent, steel tops with bolted hardware sufficed. Where jigs were changed often, wood tops with nailed-on blocking worked faster. In the 1980s, a “best of both” solution was attempted when high density polyethylene (plastic) became more affordable. When installed on top of solid sawn boards, it provided a uniform surface that held up better with nailed-on jigging. Later, when LVL replaced solid sawn lumber beneath the plastic surface, an even more durable tabletop was achieved, however at a very high cost. While plastic outlasted wood, it eventually broke down, especially where bottom chord joints were repetitively jigged. To address this, yet another variant was proposed, the combination tabletop, with 12”-18” steel plate along its entire bottom portion. However, even with a level and unyielding table surface, plates still came loose during conveyance. Attention then turned to the gantry head itself and its supporting structure.

In the highlighted portion of Sanford’s original design [see PDF or View in Full Issue], both table legs and gantry head are supported by the same base plate. Early in its commercialization, the Z-shaped section shown (item 125) was replaced with a railroad rail. Either system worked well when properly installed on a level floor. Over time, however, variances from the critical 1-9/16” truss clearance may develop. Wear or improper adjustment of bogey wheels (item 131) is the most common cause. Or individual tables may settle or somehow become unlevel with adjacent tables.

The Sanford gantry head itself was and is a workhorse, as illustrated by this 50-year-old system still running today [see PDF or View in Full Issue]. The end frames were cast iron, which enabled weldless fabrication, as with an automotive engine block. Castings are stiff and unyielding, subject to cracking if significantly overloaded. Sanford’s frame was fabricated with a notch (highlighted here) to provide clearance for the cantilevered Sanford tabletop (not shown here). The abruptness of this change in cross section created a stress concentration and sometimes gave way to an unrepairable fracture under an unexpected load (such as running over a nail gun).

As our business began its 17-year ascent in the early 1990s, the roller gantry in its original form and all of its variants began to dominate truss production. Existing users of hydraulic equipment and new entrants to the business seized on its versatility and its mechanical simplicity and dealt with the tolerances discussed above. By 1994, the truss business had grown by 50% and was attracting newer entrants, including private equity investors, intent on consolidation. As they purchased the latest machinery, existing CMs had to step up purchases. As a result, we enjoyed record machinery demand, which enabled us to fund the purchase of Diamond Machinery. And when Diamond’s owner, Bob LePoire, came to us, he brought with him the most significant gantry innovation, the trackless system.

Next Month:

Trackless Gantries

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