The solid axle shaft (shown) and a hollow axle shaft reveal similar stress points during simulated stress distribution. (image: NCSU)

Double-digit weight loss possible for heavy-duty truck drivelines

The solid-core axle and gearbox shafts on commercial trucks are targeted for weight reduction in an ongoing research project sponsored by two industry groups.

“This project is about increasing the fuel efficiency of heavy-duty trucks with the intent of reducing weight in the powertrain, which accounts for approximately 48% of a heavy-duty vehicle’s weight distribution,” said Gracious Ngaile, professor in the Department of Mechanical & Aerospace Engineering at North Carolina State University.

Ngaile spoke with Truck & Off-Highway Engineering during the Steel Market Development Institute’s 2017 Great Designs in Steel conference in Livonia, MI.

The weight-loss targets are the solid cores of the input shaft, output shaft and countershaft of a transmission/gearbox as well as the axle shafts used on Class 7 and 8 commercial vehicles.

For the gearbox shafts, a per shaft weight reduction of 10 to 15% is possible. “With a gearbox shaft, the principal load is the torque that’s needed to turn the gears. So why should a solid shaft be used? If you take out the central core, you reduce the weight,” he said.

According to Ngaile, the research analysis indicates that a truck with a tandem rear axle and three gearbox countershafts could have its weight reduced by a total of approximately 38.4 kg (84.6 lb) by using hollow shaft geometry.

The estimated weight loss per part is 5.6 to 7.35 kg (12.3 to 16.2 lb) for the axle shaft, 1.75 kg (3.85 lb) for the input shaft, 1.7 to 2.2 kg (3.7 to 4.8 lb) for the output shaft, and 0.5 kg (1.1 lb) for the countershaft.

In the recently completed phase one of the project, Ngaile and a team of NCSU undergraduate and graduate students used FEA (finite element analysis) to investigate different ways to reduce part weight. Forging companies and truck manufacturers provided information for the study that’s sponsored by the American Iron and Steel Institute and FIEF, a support organization of the Forging Industry Association.

NCSU’s research team has proposed a methodology for making lightweight steel axle shafts that use conventional tooling and forging presses/equipment at similar cycle times and at a competitive price. “Otherwise, it probably won’t go anywhere in terms of implementation,” he said.

A proposed forging sequence for producing a hollow axle shaft entails heating a section of a tubular blank via induction heating; upsetting the heated section into a solid rod; and shaping the solid section into a flange or a desired shape by further upsetting. A proposed sequence for forging pinion gear shafts from tubular blanks needs experimental verifications to access the feasibility of the process.

Phase two of the project is focused on prototype parts production by a forging company. “We’ll also make lab-scale prototypes at NCSU,” said Ngaile.

Lightweighting of commercial vehicle drive axles is also a focus in the Class 1 through Class 3 truck segments. Driveline supplier American Axle & Manufacturing will soon enter production with a new drive-axle family, called Quantum, that offers up to a 30% mass reduction for 2500- and 3500-series live axles. For more information see feature in the May 2017 Automotive Engineering magazine, p. 28. Also http://video.sae.org/12271/

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