PC-BASED FINITE ELEMENT ANALYSIS HELPS SELL CRANE ASSEMBLY DESIGN

Manufacturing process of crane hanger plate assembly modeled and proven using Algor Supersap.

Finite element analysis (FEA) on a personal computer, employing mainframe-level stress calculations using yield theories such as the Von Mises Failure Criterion, has proven an engineering firm's design of a 20-foot-long crane assembly. Tri-axial stresses had to be determined at every point under such an extreme load; the Von Mises Failure Criterion showed they were well within the allowable range. The FEA prove-out was the basis on which the Tulsa-based Crosby Group was able to sell the product to their customer.

The Crosby Group is a subsidiary of American Hoist, headquartered in St. Paul, Minnesota. The Crosby Group manufactures wire rope fittings, sheaves for wire ropes, turnbuckles, lifting tackles, hooks, fittings and shackles for lifting devices used in cranes, sea vessels, tramways, railways, off-shore oil drilling and production rigs. Their customers range from oil drilling companies and crane manufacturers to the fishing industry and the United States Armed Forces.

"The impetus for buying the FEA software," relates Crandall, "was that our customer said if we could prove the soundness of our design by a method such as finite element analysis, then they would accept it. There was some question as to whether the flanges and gussets that we applied on the hanger plate assembly were effective. "We began looking at FEA software on the PC level because we had PCs running AutoCAD for design work," Crandall explains.

The FEA software that the Crosby Group purchased in January of 1987 was the Algor Supersap Full Stress and Dynamic Modeling and Analysis package that they run on their IBM AT. The software provides graphically interactive tools for creating, loading, processing, and analyzing models. "We bought Algor instead of another PC-level program because its mesh generator seemed superb; it divided up the model into a finer mesh. Another reason was Algor's service; it was clear they were really willing to work with us. And of course, the price was lower than the others.

"I also felt Algor's level of features and graphic interface came closer to a mainframe program than did the other PC programs. In my master's coursework in engineering, I've become familiar with FEA on a mainframe level, working with such programs as LESA and ANSYS. I think Algor's offering of several yield theories is a real plus for them. For every point, Algor could give us its tri-axial stresses."

Crandall says Algor's Supersap will give the results from all the popular yield theories, from the Von Mises Failure Criterion to the Tresca Theory and the Maximum Principal Stress Theory. The Von Mises Criterion, also called the Maximum Distortion Energy Theory, or the Octahedral Shear Stress Theory, is the most widely accepted theory for ductile materials, mainly because it is the only theory that results in the same answer from five independent theoretical approaches. The Tresca Theory, also called the Maximum Shear Stress Theory, is also appropriate for ductile materials. The Maximum Principal Stress Theory is best suited for brittle materials.

Graphic representation of one-half of hanger plate.

The nearly dozen yield theories in FEA are used to define tri-axial stresses and the beginning of material yield. Tri-axial stresses are combinations of six numerical values for the 3-D state of stress, three of the values being normal stresses in the X, Y and Z direction, the other three being shear stresses on three orthogonal X, Y and Z planes.

The engineer's goal is to make his or her design stress less than the yield point stresses as derived from Hooke's Law (the point under which the material will go back to its original shape when the stress is removed; beyond this is the plastic region, where the material will not go back to its original shape). Since information is generally available concerning the yield point of a uni-axial test bar - a one-dimensional stress measurement - the engineer has to decide which yield theory is appropriate for the material being used.

The Crosby Group's hanger plate assembly is part of a barge-mounted crane used for picking up large objects and setting them upright, most notably beams. The crane is a central part of off-shore construction equipment, used for building structures ranging from production equipment to crew living quarters at sea. The shape of the hanger plate assembly is rather unusual; Crandall says it is a massive fabricated beam. It is 20 feet long by eight feet deep, the beam web is 3?inches thick, the flange width is one foot wide. Crosby Group's design is similar in some respects to a conventional I-beam, except that there are additional plates welded to the bottom of the assembly because of the high tensile stress in certain small regions.

Crandall modeled the hanger plate assembly in two states: with the flanges and gussets present and then without them. To save time, Crandall modeled only half of the symmetric hanger plate assembly. She created the model in AutoCAD, then converted it to Algor's format by running Superlink, Algor's link to other CAD programs that performs an intelligent translation of drawing files into node and element tables. (Although Algor's link programs can interpret data from numerous popular CAD programs, Algor also markets their own fully 3-D, graphically interactive drawing program, Superdraw II, which provides simple tools for creating models on-screen using lines, arcs, circles, and text.)

Stress contour lines and a zoomed view of the area under highest stress.

Crandall used Algor's MSHGEN to mesh the model, in which she created 195 elements. Crandall used Layergen to generate a 3-D mesh, which developed brick element models from layers of 2-D meshes that were created in MSHGEN. The layers were in different planes and consistent numbering rules had to be followed. Crandall entered into the mesh the points in space that accurately define the model's outline, called keynodes, then divided it into polygons, called keynode regions.

For each region, Crandall specified the model's density in two directions and the locations of intermediate nodes, as well as edge curvatures and pressures. The Algor program then computed section properties, such as moment of inertia, automatically checking adjacent regions for curve and mesh compatibility. The mesh was displayed on-screen with TDraw as she built and modified it, enabling Crandall to visually check the mesh throughout the definition process.

Once the mesh was completed, Crandall used Algor's AEdit to establish the boundary conditions on the model, applying the maximum load. Next, the Algor processor program SSAPO was used to perform the calculations necessary to determine the deformations of the model caused by the loads that had been placed on it.

Algor's POST, which creates a stress output file from the processed model, was used after the analysis to give Crandall a listing of the loads and deflections. "It told me I did just fine," she says. The deflection of the hanger plate assembly with the flanges and gussets present was well within the allowable range. meanwhile, the assembly without the modifications was deflected beyond the allowable range. Crandall adds that the customer was finally satisfied.

Dithered display showing varying intensities of stress under loading.

With the hanger plate assembly design successfully proven, the Crosby Group also used FEA to aid in designing the hydraulic cylinder head of a 3000T press. Like the hanger plate assembly, Crandall made the model using AutoCAD, then meshed it in MSHGEN. "The model was so big," she relates, "we could not run it on our personal computer, so we sent it on a diskette to [Algor headquarters in] Pittsburgh; they ran it on their mainframe and sent the results back on a diskette."

TDraw was used again to examine the processed model. The attachments, made of 8620 steel, were too stressed at their thickness of 3? inches. "But with a model of it onscreen," says Crandall, "we could see that as the material got thick, it was also overstressed. Ultimately, it was a middle range that gave us the lowest stresses. At 3?inches, the stresses were 60 ksi; at 4?inches, the stress went down to 20 ksi; and at 6 inches, stress went up to 45 ksi."

Crandall says their early success with FEA has led the firm to apply it extensively in the design of its products. "Without finite element analysis, it would have been extremely time-consuming to get accurate measurements of our products' stress," Crandall concludes. Algor's Supersap Full Stress and Dynamic Modeling and Analysis package employs stress calculations using mainframe-level yield theories, such as the Von Mises Failure Criterion, enabling the engineer to optimize design, while providing a superb mesh generator and enhancing FEA with graphics.

Crosby Group's hanger plate assembly, which will support a barge-mounted crane used for moving large objects, most notably beams.