Optimisation of Structural Performance of Trucks

The majority of truck and shovel operations in the Australian coal industry use Caterpillar 789, 793 or Komatsu 830, 930 trucks. These trucks are at various stages of their service life and many of them have experienced fatigue cracking at several locations after a relatively short duration of time in service (in some instances as low as 15,000 hrs). The life of a truck is generally limited by the life of its chassis.

The key objective of ACARP project C13043 was to optimise the structural performance of mining trucks in the Australian Coal industry. Industry monitors nominated the Caterpillar 793 truck type for the study. The main objectives of this research project were:

• Develop a thorough understanding of the structural performance of critical areas of the truck during normal mining operations and estimate service life.
• Outline the influence of mine operating conditions such as haul road conditions on the service life of the chassis.
• Recommend best practice procedures for the normal mining operations, including haul road condition, to maximise the life of the chassis.
• Recommend a cost effective repair procedure for 2-3 critical components on the chassis.
• Recommend an overall repair strategy to maximise the life of the chassis.
• Produce guidelines that can be used for preparation of risk management strategies for optimising the structural performance of the truck.

The study included a comprehensive instrumentation and test program of the RD1 – CAT 793C truck at Saraji mine, detailed FE analysis of the truck chassis, review and analysis of past failures of several CAT 793 trucks at BMA mines and a detailed fatigue assessment. The Saraji mine site provided a CAT793 truck for the instrumentation and test program.

Following the review and analysis of the history of failures, the industry monitors nominated the torque tube to main rail connection and the front strut beam connection areas for developing a modified repair procedure to improve the repair life at these locations, as their repair life was lower than 2,000 hours.

The following conclusions can be made from the study:

• Cracking was observed in several areas of the CAT 793 truck chassis (torque tube to main rail connections - top and bottom, front strut beam connection, vertical welding in main rail etc.) after 12,000 hours of service. Significant cracking in several areas was observed after approximately 25,000 hours of service.
• The service life of components after repairs was extremely low in most areas. At the torque tube to main rail area the service life after repairs was around 600 – 900 hours. In most areas, the service life after repairs was greater than 2,500 hours.
• Extremely high peak stresses, stress ranges and a significant number of stress cycles were observed in several locations of the truck chassis during normal operations. Stress ranges of up to 650MPa and peak tensile stresses of up to 434MPa were recorded during the test period which would be above the yield stress of the material. The highest stresses were observed due to racking/twisting, pitching/bouncing and rolling when driving out of the pit and dump area, and during dumping.
• The payload distribution during the test period was within the rated capacity of the truck.
• The stresses obtained from the FE analysis for the critical load cases were similar to those observed during the measurement.
• Fatigue assessments conducted based on BS7608 [1] for several critical locations provided results similar to the service hours to when the first crack was observed.
• The fracture mechanics approach for fatigue life estimation [2] clearly showed that initial flaw size has a major influence on the component life.
• For the torque tube to main rail connection, the Caterpillar repair manual [6] does not refer to a specification for the repair welds and no repair weld qualifying criteria is specified. This is the most highly stressed/critical area of the chassis and the repair welds should conform to a stringent welding standard.
• For all other areas, the manual specifies that weld repairs should conform to the AWS D14.3 [5] standard.
• Several repairs observed during the inspection of the trucks clearly showed poor weld repair quality which do not comply to the AWS D14.3 [5] standard.
• In order to improve the fatigue performance of the CAT 793 truck chassis the following should be addressed:
• The peak stresses, stress ranges and number of stress cycles experienced by the truck during operations should be reduced.
• Critical areas where repeat cracking has been observed should be modified/redesigned to reduce stresses to acceptable levels.
• Weld quality during repairs should be maintained to a very high standard as the initial flaw size has a major impact.

It is recommended that the following studies be performed to improve the structural performance of the CAT 793 trucks:

• Assess the impact of stress relieving on the torque tube area following weld repairs.
• Re-design specific areas such as the torque tube to main rail connection to reduce stresses to an acceptable level.