High mobility trailer: Diverse team surmounts design problems to produce a trailer capable of living up to its name - Army Equipment Modernization - United States Tank Automotive and Armaments Command

Program Manager, Nov-Dec, 2001 by Nancy A. Moulton, Eric R. Noyes

During testing, cracks and deformities in the surge brake's inner slide, outer housing, and lunette assemblies appeared. The IPT believed that the damage was caused by powerful up-and-down acceleration forces between the HMT and the HMMWV pintle as the truck and trailer negotiated the cross-country test courses at APG. Surge brakes on commercial trailers typically undergo very little vertical stress because they operate almost exclusively on paved or improved roads with little or no cross-country operation at all.

The performance of the HMT on primary and secondary roads was never an issue. Testing over cross-country terrain showed that the HMT could be safely towed at up to 12 miles per hour, with a full payload, without evidence of brake actuator wear or fatigue; however, this was still well below the HMT's required speed. An evaluation of the reliability, availability, maintainability, and maintenance ratio requirements - taking into consideration the fact that the brake actuator is a maintenance item - showed that in spite of the projected brake actuator replacement rate, the design would still meet all Required Operational Capability (ROC) requirements.

However, a careful safety assessment by the IPT determined that the location of the cracks could cause separation of the trailer from the HMMWV if not detected during Preventive Maintenance Checks and Services. Therefore, TACOM classified location of the cracks as a high-risk safety issue. In August 1999, PM-LTV asked the user community to waive the cross-country speed requirement in the ROC and accept a 10-mile-per-hour cross-country speed restriction, which was significantly better than the previous 6-mile-per-hour restriction for the M101. The user community refused to do so, and TACOM then charged PM-LTV with reducing the risk to "low" while achieving the 15-mile-per-hour average cross-country speed through a design change.

To better understand the nature of the problem, PM-LTV's IPT turned to computer modeling and simulation. IPT engineers from PM-LTV and Science Applications International Corporation (SAIC) created a three-dimensional computer model of the surge brake assembly using Computer Aided Design software. AM General Corporation, manufacturer of the HMMWV, also created computer models of the HMMWV's rear crossmember to study its stresses while towing the HMT.

Computer Solid Modeling

A computer "solid" model represents the actual item with features and properties that can be altered during the course of design and development. Modelers identify the solid model's characteristics - such as dimensions, volume, surface area, weight/density (based on the material selected), center of gravity, material properties, and natural frequency for vibration - as precisely as possible to accurately represent the physical item in the computer. This model then supports the process of Finite Element Analysis (FEA) in which the computer model is subjected to simulated physical forces to determine the location of stress points and the ability of the item or assembly to withstand the applied forces.