“You cannot prevent fatigue; we can only delay it.”
That’s according to Dr. Sanjeev K. Khanna, a professor in the Mechanical and Aerospace Engineering Department at the University of Missouri who’s studied stress fatigue in materials ranging from aircraft to road vehicles. By using the proper materials and structuring them correctly, Khanna said you can double or triple the amount of stress a material can handle before it fractures. For one thing, he told Ozarks Farm & Neighbor, make sure the material is inspected property, has the right mechanical properties and the right metallurgical composition. “Steel, for instance, can go from 70,000 lbs/sq in. up to about 250,000 lbs/sq in. stress capacity,” he said.
The shape of the object is also a factor. Khanna said, “If it has sharp corners or too many holes through it, or if there are frequent changes in size – if you take a long shaft and it reduces at certain times and then goes back up, there are too many variations. The holes create areas where the stresses are higher than in other places. Those are local areas where fatigue failure can initiate, and then propagate from there outwards.”
Bob Studebaker, owner of GoBob Pipe & Steel Sales in Mounds, Okla., said he has a passion for reducing stress fatigue in farming equipment.
Bob became interested in ways to prevent stress fatigue in the ‘80s when his company needed flatbeds to haul pipe; he bought the cheapest he could find, worked them hard, and discovered through annual inspections that one of the trailers’ main beams had so much metal fatigue it was cracked and could fall apart. So in 2005, “I got a couple of engineers involved and explained to them what had happened to mine; I said, ‘I want you to design me some features that will reduce or minimize that as much as possible.”
Studebaker agreed that fatigue is inevitable. “Whether it’s wire, or a beam, or a piece of angle iron, it doesn’t matter – anything that’s twisted and flexed enough will eventually break,” he said. This happens with trailers in a couple of ways; if it’s a side-loader, the load is heavier on one side when you set it down than it is on the other, which makes the trailer twist. If it’s loaded from the back, as with rolling wheeled equipment up a ramp, Studebaker said it bows the main beams longitudinally.
His trailers attack the side loading problem by transferring the twisting motion to a piece of pipe called a torsion tube. “You don’t see it unless you get underneath the trailer and look for it,” he said, “but that piece of pipe is gusseted to the frames of the main beams. If you take a pencil and try to snap it in half it’s pretty easy but if you try to twist it in half, it’s difficult. This torsion tube prevents the trailer from twisting if you set something heavy on one side, and there’s nothing balancing it on the other side.” That transfers the additional tension to the frame, which needs to be stout to handle it.
As for rear-loaded rolling stock, his trailers address that problem by transferring the weight of the load from the trailer to the ground, using a double hinged ramp that self-levels in the same degree of slop as the ground. Then, they put a stabilizing block on the ramp; when the ramp is placed in the loading position, the block drops down and comes into contact with the ground.
