A big trend in automotive engineering is to find new ways to reduce weight to accommodate fuel efficiency and environmental standards. This article examines the weight-saving potential in using bimetallic or hollow components on heavy-duty vehicles and the obstacles to their production.

 

Increasing environmental awareness has caused many governments around the world to impose stricter regulations on heavy-duty vehicle (HDV) manufacturers. For example, this year the EPA released new greenhouse-gas and fuel-efficiency standards that require OEM manufacturers of class-7 and class-8 trucks to improve the fuel efficiency of new trucks.[1,2] One of the focal points of this standard is that the results will be achieved by using both off-the-shelf solutions and new, innovative technologies.

Classically, one of the best ways to improve the fuel efficiency of vehicles has been to reduce their weight. This strategy has been applied to lightweight passenger vehicles to improve fuel consumption and reduce emissions, but little effort has been focused on heavy-duty vehicles, which have their own unique benefits and challenges to lightweighting efforts. For instance, besides the obvious benefit of saving fuel, lightweighting would also lead to an increase in the amount of freight that can be hauled by weight-limited trucks. 

Non-Transferable Technologies

A potential challenge to lightweighting efforts for HDVs is that knowledge gained from weight-saving projects performed on light-duty vehicles cannot be directly transferred to heavy-duty applications. This is because the weight distribution in these two classifications of vehicles is fundamentally different (Figure 1). Clearly, efforts to reduce the weight of HDVs should focus on the powertrain in order to maximize results. The fact that a large number of powertrain components are forged, combined with EPA’s emphasis on new technologies, gives forging companies a unique opportunity to make money by offering high-strength and lightweight components to the heavy-duty trucking industry.  

Review of Lightweighting Techniques

During the past year, the Advanced Metal Forming and Tribology Laboratory at North Carolina State University has been tasked with investigating the lightweighting potential for class-7 and -8 heavy-duty trucks by the Forging Industry Education and Research Foundation (FIERF) and the American Iron and Steel Institute (AISI). 

The first step in this study was to conduct a survey of existing and on-the-verge research of lightweighting techniques to determine what potential methods could be used to reduce vehicle weight. This survey was conducted by looking at previous lightweighting studies carried out on LDVs, performing literature reviews and going on plant tours of forging companies and OEMs. The major finding of this study was that there are five main ways in which the weight of parts is reduced, namely geometric modification, material substitution, bimetallic construction, innovative heat treatment and process substitution.  

These insights were used to motivate several other studies into the weight reduction of heavy-duty trucking components, one of which was an investigation into the weight-saving potential of making hollow and bimetallic power transmission shafts. The examined shafts were the input, counter and main shafts in the gear box and the full-float axle shaft. 

Because the shafts are primarily torsionally loaded, most of the load is being carried by the material on the outer diameter of the shaft, leaving the material on the inside of the shaft as dead weight. This material would ideally be removed entirely by making the shafts hollow, but this may result in difficult forging conditions due to the use of less-stable tubular feedstock. Bimetallic construction with high-strength material in areas of high stress concentration and low-density material in lower stress areas may be another solution for situations in which tubular billets would make the forging of the part impractical or impossible.

Modeling

If it is assumed that these variations on power transmission shafts could be fabricated, the feasibility of a hollow or bimetallic shaft depends on its ability to carry the load observed by the conventional, solid shafts. The finite-element method (FEM) provides a quick and convenient way to analyze both the conventional and lightweight shafts. Before one can apply FEM, however, the loading conditions on the parts must be determined. 

This is done by making load maps that depict the flow of energy through the various systems and components in the HDV. One such load map, depicting the transfer of torque in a tandem rear axle, is shown in Figure 2. These loads are combined with geometry given by one of our industrial partners and/or gleaned from diagrams of parts in HDV equipment manuals to create finite-element models of the parts under realistic loading conditions using the commercial FEM software ANSYS Workbench 17.[3] These models give a baseline stress and weight for the conventional components, which serve as the basis to evaluate the various lightweight designs. An example of the baseline analysis carried out for the full-float axle shaft is given in Figure 3.

It goes without saying that, in order to accommodate these two weight-reduction techniques, allowances will need to be made for the small changes in the load-carrying capacity in the shafts. Namely, as more and more material is removed from the center of the shafts, their outer diameter will need to increase. Thus, the designer will have to choose what level of compensation is appropriate for a particular weight reduction. 

Finite-element studies were used to determine the stresses in both the hollow and bimetallic shafts for various inner and outer diameters of the high-strength shell material. For demonstration purposes, the stresses in the hollow axle shaft for one geometric variation are shown in Figure 4. Note that the minimum stress in the hollow shaft is considerably higher than it was in the baseline study, while the maximum stress is comparable. This suggests that the material is used more effectively in the hollow axle shaft designs.

To further demonstrate this point, graphs showing a summary of the results from all of the hollow axle shaft simulations are shown in Figure 5. For a particular outer shaft diameter, one can find the inner diameter for which the stress in the shaft is the same as the stress in the baseline solid shaft. This geometry can then be used in combination with the other graph to determine the weight of the hollow shaft and compare that weight with the conventional solid shaft. 

Using this methodology, it can be seen that by allowing the diameter of the axle shaft to increase 20%, a hollow shaft could reduce the weight of the component by 16.1 pounds. By carrying out similar studies on the other three shafts, it was determined that the total weight of a truck with three counter shafts and four rear-axle shafts could achieve a combined weight savings of 86.6 pounds if these four shafts were replaced with hollow shafts and 50.3 pounds if they were replaced with bimetallic shafts. Based on these results, the best solutions come from the large increases in shaft diameter and, as expected, hollow shafts are much more attractive from a weight-savings perspective than bimetallic shafts, though both are superior to the original solid shaft. 

Conclusion

Despite the potential benefits associated with adopting hollow and bimetallic shafts, there are some major hurdles that must be overcome before they can be cost-effectively mass produced. In order to produce hollow shafts, the shaft either needs to be created as a solid part and drilled or the part needs to be fabricated using a hollow billet. Drilling can be a time-consuming and impractical solution due to the length of some of the power transmission shafts (the axle shaft is 3.5 feet). 

Starting with a tubular billet can also be problematic because the tubes can be very expensive and have vastly different deformation characteristics than their solid counterparts. While the forging companies might have little control over the price of the tubes, there are many promising ways of forming tubular workpieces that are currently being developed. In fact, the Advanced Metal Forming Laboratory at North Carolina State University, which carried out this study, is currently working with industrial partners to develop new forging strategies to create hollow shafts from tubular blanks. 

Bimetallic shafts also present numerous difficulties associated with everything from creating the initial bimetallic billet to the difference in forming temperature between the two metals and the increased chance for corrosion between the two metals. These issues are currently prohibitive, but there have been some recent promising advances in bimetallic technology.

Acknowledgement

It goes without saying that this study would not have been possible without the dedicated support of FIERF, AISI and the numerous industrial partners that provided plant tours, advice and guidance. Specifically, we would like to thank Fox Valley Forge, Mid-West Forge, Sona BLW Precision Forgings, Volvo Powertrain North America (Hagerstown, Md.), GKN Driveline Precision Forming and the Daimler Cleveland Truck Manufacturing Plant. Additionally, there were three undergraduate students who worked on the project (S. Hinkle, J. Jongkind and F. Marrow) as well as three graduate research assistants (F. Aktaruzzaman, J. Lowrie and H. Pang). 

 

References

  1. U.S. Environmental Protection Agency (EPA) and Department of Transportation (DOT) National Highway Traffic Safety Administration (NHTSA), Greenhouse Gas Emissions and Fuel Efficiency Standards for Medium- and Heavy-Duty Engines and Vehicles - Phase 2, Aug. 2016, Web Oct. 2016, https://www3.epa.gov/otaq/climate/documents/2016-08-ghg-hd-final-rule-phase2-preamble.pdf
  2. U.S. Environmental Protection Agency (EPA), EPA and NHTSA Adopt Standards to Reduce Greenhouse Gas Emissions and Improve Fuel Efficiency of Medium- and Heavy-Duty Vehicles for Model Year 2018 and Beyond, Aug. 2016, Web Oct. 2016, https://www3.epa.gov/otaq/climate/documents/420f16044.pdf
  3. ANSYS Workbench Documentation, User Guide. (2016)
  4. U.S. Department of Energy (DOE), WORKSHOP REPORT: Trucks and Heavy-Duty Vehicles Technical Requirements and Gaps for Lightweight and Propulsion Materials, Feb. 2013, Web Oct. 2016, http://energy.gov/sites/prod/files/2014/03/f13/wr_trucks_hdvehicles.pdf
  5. Spicer AdvanTEK 40 Tandem Axle http://www.dana.com/commercial-vehicle/products/driveline/drive-axles/advantek-40
  6. Bennett, S., & Norman I. A. (Nov. 3, 2005). Heavy Duty Truck Systems. 4th Edition. Boston, Mass., Cengage Learning

Co-author Dr. Gracious Ngaile is Professor of Mechanical and Aerospace Engineering, Advanced Metal Forming and Tribology Laboratory at North Carolina State University. He may be reached at 919-515-5222 or gngaile@ncsu.edu. Co-author James Lowrie is a Ph.D. candidate at North Carolina State University. He may be reached at 919-515-0685 or jblowrie@ncsu.edu.