ALGOR FEA PROGRAM EVALUATES AXLE SPINDLE STRESS RISER

Prototype version of the new front axle system as mocked up on a Ford Bronco by design engineers for durability testing. - Photo by Dave Brockett

Installing vacuum-actuated locking hubs on the front axle of a South-Korean-built 4-wheel-drive truck required modifying the axle's spindles to allow the passage of a vacuum line to the hub's actuator. Finite element analysis (FEA) was used to investigate the stress-riser effect generated by the hole. The data will help the truck builder plan the type and degree of the spindle modification needed to ensure reliability and long-term durability.

A total of five analyses were made. The first, done on an unmodified spindle, provided baseline data for comparison with the results of analyses of modified parts. Four other analyses were made with the hole at different radial locations in the spindle. Engineers considered these variations necessary because the hub's design allows considerable flexibility in locating the vacuum line.

Study results demonstrated that the stress rise effect varies with hole orientation. The localized increase in stress it caused reached 15% with the hole located on the spindle's vertical axis (the load axis). A minimum stress riser effect developed (no increase) with the hole on the horizontal axis, 90^o from the load axis.

Model for FEA analysis with part geometry, meshing, boundary conditions, and loadings as created by graphic routines.

A central bore in the spindles - which are steel forgings - accommodates the front axle. Critical components of the front suspension, these forgings support around 25% of the vehicle's weight and transfer tire and wheel loads to the suspension. They have thin-walled tubular outer section cantilevered from an inner flange.

Modifying the spindles to accept a vacuum-actuated locking hub requires making a 4-mm-diameter hole in each spindle. Though small, this hole would be drilled through the base of the spindle's tubular section at a point close to its integration to the flange. Its axial location places the hole in an area of higher stress. Thus, the truck maker needed to know how much it would increase spindle stresses.

For the study, engineers used the Algor Finite Element Analysis (FEA) System. Designed for desktop computers, this system is sold outright rather than leased. It's made in versions for use on IBM AT and AT-compatible computers, for machines that use the 80386 and 80486 chips, and for computers that use these chips with the high-performance Weitek coprocessor.

Results of FEA for a spindle with a hole on its load axis show that the hole causes a 15% increase in stress level.

To make the FEA studies, a design engineer used the appropriate version of the Linear Stress analysis software package and a Compaq computer equipped with the 386 chip. As for the model, it was made using data taken from the truck builder's drawings and sample parts, and applying loads derived from vehicle weight data. These data were applied to Algor's ViziCad Plus graphical system. Stresses were determined using the package's FEA processor for linear static stresses. The results were displayed, manipulated and recorded using ViziCad Plus and the SPlot graphical plotter control program.

Warn Industries, Milwaukie, Oregon, the manufacturer of the automatic locking hubs, made the FEA study at the request of the truck manufacturer. Clark Wilson, the Warn engineer responsible for the application, performed the analyses.

Wilson says that durability questions such as this are normally evaluated by a rigorous prototype testing program. "In fact," he says, "we had already built and installed prototypes of this system on several test trucks, and one of these vehicles is now undergoing these tests. But the truck builder's stress-riser question came up before this testing had begun." Wilson used FEA to obtain a quick and accurate answer to the question. Warn Industries' prior experience with this method revealed close correlation between its results and those of test programs.

Clark Wilson, Engineer, Warn Industries

In discussing the study, Wilson says: "Using the computer and this FEA System, I was able to run five different models in about two days. To obtain the same results by hand methods would have taken much longer. The results have confirmed our original intuitive judgment and have given our customer a starting point for his own design and testing. It has also given us a better understanding of the requirements of our own system and put us in a better position for making recommendations to this manufacturer and other OEMs."