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The Route That Forces Take PUBLIC ACCESS

Visualizing the Load Path in a Design Can Uncover Areas Open to Improvement.

[+] Author Notes

James G. Skakoon operates Vertex Technology LLC. an engineering design firm in St. Paul. Minn. He is the author of several books. most recently The Elements of Mechanical Design. published earlier this year by ASME Press.

Mechanical Engineering 130(09), 39-42 (Sep 01, 2008) (4 pages) doi:10.1115/1.2008-SEP-4

This article discusses that visualizing the load path in a design can uncover areas open to improvement. Planning the force transmission path during mechanical design is hardly dazzling engineering analysis, but explicitly doing so will improve your designs. By visualizing the transmission of forces, one can eliminate unnecessary parts, strengthen the design, and identify potential problems for further analysis or correction. Visualizing the path of transmitted forces for cables is pretty easy; forces follow the tension cables. But it is only slightly more complex with compression and shear involved. Although design is never a strictly linear progression, reviewing and refining the load path should be a formal part of the design process. Troubles with the load path in user-centered device design may become obvious with testing, but thinking about load paths as a human factor design issue can save time and effort. It is not a highly analytical design tool, but visualizing and refining load paths in structures and mechanisms is extraordinarily useful for designers, and it’s simple.

Gazing at a vintage 1920 kerosene-burning John Deere tractor called the Waterloo Boy, which recently fetched $162,500 at a Wisconsin auction, the engineer in me sees not the extravagant price tag, but the unusual path of forces in the drivetrain. The steel wheels have internal gears driven by pinions coming right out of a gearbox adjacent to the crankshaft. It is quite unlike the drive trains of modern vehicles.

The Waterloo Boy's drive train has obvious advantages and disadvantages: Gear reduction at the wheel perimeter means low torque on shafts, but exposing gear teeth to dirt and rocks is poor practice by anyone's standards. Most obvious to me, however, is how direct the path of forces is from engine to ground.

The short direct-force transmission path of a 1920 John Deere Waterloo Boy tractor. Did the designers visualize this direct-force path when they designed this tractor?

Planning the force transmission path during mechanical design is hardly dazzling engineering analysis, but explicitly doing so will improve your designs. As you create a mechanism or structure, visualize how loads go into, through, and out of your design. By visualizing the transmission of forces, you can eliminate unnecessary parts, strengthen the design, and identifY potential problems for further analysis or correction.

The Wright brothers' wing warping scheme, showing the load paths throughout the structure. From How We Invented the Airplane by Orville Wright, reprinted by Dover Publications Inc. in 1988.

Grahic Jump LocationThe Wright brothers' wing warping scheme, showing the load paths throughout the structure. From How We Invented the Airplane by Orville Wright, reprinted by Dover Publications Inc. in 1988.

According to David Uliman, Professor Emeritus of Mechanical Engineering Design at Oregon State University and the author of The Mechanical Design Process, a textbook used for university engineering capstone courses, design students and design engineers rarely visualize load paths for structures and mechanisms. "It's definitely lacking," he said. "In this day of automatic finite element analysis, having some of these visualization tricks is invaluable."

Mechanical engineers analyze a structure or mechanism by drawing free-body diagrams showing forces on a component or joint. But what about the overall path of these forces? Does it represent the best solution?

Only by viewing the structure as a whole can you answer that. Visualizing the complete load path is rather like the opposite of drawing a free body diagram-instead of decomposing the structure or mechanism into smaller chunks, you look at it as a simplified whole to see how loads are transmitted throughout.

The Wright brothers thought about load paths when rigging their airplanes. Their wing-warping method for banking the plane used properly routed tension cables to distort the airplane's wings.

Visualizing the path of transmitted forces for cables is pretty easy; forces follow the tension cables. But it's only slightly more complex with compression and shear involved. Once you see the overall picture, it's time to zoom in on components and joints to see more detail of how forces are transmitted.

So just what, exactly, is a good force transmission path? Above all, good load paths are short and direct.

For short load paths, the designer places all components that carry load as near to each other as possible, and makes the components as small as possible. For direct paths, the designer keeps the number and complexity of loadcarrying components to a minimum.

A clevis assembly showing the paths of loads through the joint. This is an example of what the mind's eye should see when visualizing a detailed load path.

Grahic Jump LocationA clevis assembly showing the paths of loads through the joint. This is an example of what the mind's eye should see when visualizing a detailed load path.

Although design is never a strictly linear progression, reviewing and refining the load path should be a formal part of the design process, according to Crispin Hales, a consultant, lecturer, and author on design. "It's a very specific item that you look at during the embodiment phase, in between conceptual design-after you've selected a concept- and before the detailing phase," Hales said.

Hales added, "There are some very specific guidelines about force transmission paths." Some of the load path guidelines from design authors include keeping them:

  • Short

  • Direct [minimum number of parts)

  • In a line, or, barring that, in a plane

  • Non-redundant [a single path)

  • Non-changing [or changing intentionally)

  • Symmetrical

  • Locally closed [more on this later)

  • Easily analyzed

Lower stress is one result of following load path guidelines. Ullman writes in his book, "Less stress is generally developed if, when the bodies of components are being designed, direct force transmission paths are used."

Material efficiency is another result. Because bending is less efficient than tension or compression, you can refine the load path to be more direct so stresses are either tension or compression.

Or, you can shorten the load path and eliminate parts. If the load path includes three parts, you may try to delete the middle one and connect the remaining two. Or, see if you can take fasteners, welds, or glue joints out of the load path altogether by using them only to position the parts; if they are not part of the load path, they won't fa il because ofload.

You might find obvious out-of-balance loads and large moments. The solution may be to reduce the moment arms, balance the forces, or make them symmetrica l. If you discover unstressed material, perhaps you can eliminate it altogether.

You might be able to predict and avoid component failures by including time, use, and abuse as you consider load paths in structures and mechanisms. Does the load path change with wear in normal use? Is the load path affected by corrosion? Temperature? Can catastrophic Llilure be avoided if the structure is overloaded? Visualize the load path not only for initial conditions and expected use, but throughout the design's life and for unexpected use and abuse.

So here is a summary of what you gain by visualizing and refining the load path using the preceding guidelines:

  • Strength

  • Stiffness

  • Material efficiency

  • Best stress distributions [constant tension or compression, not bendingl

  • Stability, reduced buckling

  • Simplest configuration [minimum part countl

  • Safety [Lower potential for failurel

  • Easier analysis and testing.

A useful variation of visualizing load paths is the flow lines of force method. Imagine force to be streamlines of fluid flowing between interf,lces and through components, taking the path of least resistance. The streamlines converge, diverge, and change direction as would stress: the closer together, the higher the stress. You can label areas for compression, tension, and shear.

"I have a fluid dynamics background," Ullman said, "which makes visualizing the flow of fluid through something tolerably easy for me."

But it isn't too difficult for anyone to visualize forces using force flow lines. In Ullman's experience, "We all have some feel for it. You turn on a f,l ucet over here and you've got a drain over there. What happens? That's really all it is." Analogies to heat transfer or magnetic flux work equally well.

Flow lines of force in a flanged pipe joint. Force moves through the components as if it is a flowing fluid. T, C, and S labels show areas of tension, compression, and shear, respectively.

Grahic Jump LocationFlow lines of force in a flanged pipe joint. Force moves through the components as if it is a flowing fluid. T, C, and S labels show areas of tension, compression, and shear, respectively.

Good designs have ideal force flow lines, lines that are straight rather than curved, neither converging nor diverging, and, of course, short. Any areas with bunched lines or radical direction changes deserve a closer look, either to analyze, test, or better yet, eliminate them.

Poorly designed load paths can lead to catastrophic f,lilures. Hales, whose consulting firm also does forensic accident investigations, sees this all too often. "With a lot of the cases that I've done, it may not be directly part of the case, but if you consider the load path, it seems they all have that element to them-one way or another."

Had loads been transferred as intended through this truck steering assembly. the design may have been satisfactory. The actual load path caused a critical part to fail, resulting in loss of control and a fatal accident.

Grahic Jump LocationHad loads been transferred as intended through this truck steering assembly. the design may have been satisfactory. The actual load path caused a critical part to fail, resulting in loss of control and a fatal accident.

For example, Hales investigated a fatal accident involving a passenger vehicle hit by a truck that had lost steering control. The poorly designed load path in the truck's steering system caused components to fail, a steering kingpin to fall out, and a front wheel assembly to go adrift.

As designed, the truck's vertical load goes from the axle eye, through a thrust bearing, into the steering knuckle, then to the wheel spindle. Horizontal loads are transferred by shear through the kingpin.

This design is not conspicuously unusual, except, perhaps, for the tapered kingpin being inserted from below rather than from above. Even that, although not the best design practice, might not have been a great deal of trouble.

What was a great deal of trouble is the actual load path, which changed over time from normal to dangerous with wear of the thrust bearing.

Herringbone helical gears eliminate external thrust loads because their design creates a load path that loops within the gears.

Grahic Jump LocationHerringbone helical gears eliminate external thrust loads because their design creates a load path that loops within the gears.

As the thrust bearing became thinner with wear, the kingpin's retaining nut took an ever-increasing portion of the vertical load, for which it was never intended. Eventually, the torsional friction acting on the retaining nut as the vehicle turned left and right sheared the cotter pin, allowing the nut to unscrew and the kingpin to fall out.

Had the steering assembly's load path, and especially any unintended change to it, been visualized during design, the failure might have been avoided. One glance at the assembly cross-section shows how unsound this design's load path is.

But done properly, a load path that changes with load can also prevent failures. Hales recommends another trick. "If you start to overload a system, then change the load path. Put a stop on it, so the bigger load goes through a different path and relieves the load on the other part."

The objective for a safe design is to explicitly control the load path during all of a design's life.

Newton's Law about actions and reactions makes it clear that a load path must forn1 a complete loop. Consider not only the loads into and through a component or assembly, but also how the world around reacts. The best load path designs are those that loop the action and reaction forces into a tight path-a locally closed path.

For example, helical gears have an axial thrust vector, usually reacted by a thrust bearing. But the locally closed path of a herringbone helical gear eliminates external thrust; it loops entirely within the gear pair.

Troubles with the load path in user-centered device design may become obvious with testing, but thinking about load paths as a human factor design issue can save time and effort. According to Michael Wiklund of Wiklund Research & Design Inc., a human factors design consulting film in Concord, Mass., something as simple as keypad entry can benefit.

"If a device bounces against a touch on a touch screen, for example, we can get issues with double entlies," said Wiklund. Designers could avoid this problem with a short, direct load path from keyboard to ground, or a locally closed load path (think text messaging on a cell phone).

Wiklund believes that mechanical design is often overlooked or secondary compared to data input and output. "The physical side of it perhaps gets a little less attention and a little less budget in human factors projects."

But regarding visualizing load paths to improve usercentered design, Wiklund added, "It's almost like you're halfWay there just by asking people to think about it. As a human factors engineer, I like the idea of mechanical engineers including the human as part of the load path. If mechanical engineers keep that in mind, that's a good step forward."

Locally closed load paths are often best for products that require good human factors. Wiklund often sees this in surgical instruments. "Squeezing a handle together as opposed to pushing a plunger forward could make a big difference in the stability of the tip of a surgical instrument," he said. In this case, a locally closed load path separates and isolates actuation from location, giving finer control of position to the surgeon.

It's not a highly analytical design tool, but visualizing and refming load paths in structures and mechanisms is extraordinarily useful for designers, and it's simple.

But it's easy to overlook, too. Hales laments this by saying that many of the annoying evelyday product failureshe mentioned a broken door on his laptop, among other examples-should never happen. "It's just because people' don't visualize it as a force system."

A seasoned design engineer and long-time Boston area inventor and consultant, Herbert Loeffier, once told me: "If this is where you want the load, and this is where it's applied, put them together and have nothing in between, and then you'll have nothing to fail." Early John Deere tractor designers must have known that, too.

Copyright © 2008 by ASME
Topics: Stress , Design
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