Powered Tag-Axle Traction Assembly
Copyright for this work is retained by the authors.
Project P’TATA encompassed the design of a selectively-powered tag axle for 6X2-configured Class-8 on-highway tractors, to aid the vehicle in regaining traction during reduced-friction events. For increased fuel savings, 6X2 (1 driving axle, two unpowered axles) configured tractors are preferred over their 6X4 (two driving axles) counterparts; however, the loss of a powered axel renders 6X2 axles more susceptible to slipping events, where the amount of torque required to maintain non-slipping contact with the road exceeds the abilities of the single drive axle. For this and other reasons, the 6X4-configured tractors are chosen despite the fuel efficiency benefits of the 6X2 configuration. Therefore, improvement of the 6X2 tractors’ ability to escape from slipping events should improve driver safety and enhance the tractors’ competitive edge in the market. The design process began with determination of the tag axle torque required to drive the truck out of a slipping event, and a comparison of different power transmission methods. After choosing a transmission method, the electronic control and mechanical systems were designed to so that a motor could engage the axle, transmit power, and then disengage. Using existing and custom-modelled parts, a 3-D model of the system was assembled in CAD software, and a scaled down prototype was constructed to test the control system. An electro-mechanical add-on system was designed to meet the criteria presented by Daimler Trucks of North America. This assembly, shown in Figure 1, employs one DC motor to power the tag axle. A Bendix drive allows for engagement between the motor and the 40:1 reduction worm gear box that multiplies the motor torque. Electricity is pulled from the tractor’s standard battery bank. The electronic control system monitors the wheel speeds at the tag and driving axles, and identifies slipping conditions based on a minimum difference between those speeds. Flowchart and block diagrams of the electronic control circuit were drawn up; parts were ordered and the prototype was built based on these drawings. Once the prototype control circuit was functioning properly, a circuit board was prepared and readied to accept the components of the control circuit. The circuit board was then tested, first for proper wiring and then to ensure proper operation. The final design provides 1120 ft-lbs of torque to the tag axle, remains lightweight at 99.5 lbs compared to the 380 lb differential used in a 6x4, and remains cost-effective with an estimated materials price of $798.42 out of the allotted $1500 (rough estimate not including manual labor). It is capable of driving the truck at 15 mph out of slip on a 4% gradient of sleek ice. The add-on also does not require significant alterations to the existing tag axle design.
The design meets the given criteria, but the industry advisor advanced some further design optimization requests that should be considered in future design iterations. The team recommends the implementation of a higher voltage 3-phase electric motor because they provide more horsepower and would require a lower gear reduction for the system. The Bendix-driven engagement mechanism may need to be redesigned to ensure reliable operation. Ultimately, the success of the system must be validated, both by computer stress simulations and by physical testing with full-scale prototypes, prior to large scale manufacturing and implementation.