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

Surmounting a wave of unfavorable publicity and serious design flaws in the Army's High Mobility Trailer (HMT), an Integrated Product Team (IPT) at the U.S. Army Tank-automotive and Armaments Command (TACOM) successfully solved design problems in the HMT that were deadlining the fleet of trailers and delaying full delivery and fielding. The HMT, which will be fielded in three versions, is a new family of trailers designed to be towed by the Army's inimitable "Humvee" -- officially known as the HMMWV, or High Mobility Multipurpose Wheeled Vehicle.

Led by the Project Manager for Light Tactical Vehicles (PM-LTV), the IPT, which included acquisition managers, engineers, logisticians, and testers, developed a materiel solution through extensive use of modeling and simulation, tested the redesign and approved its application, and are currently fielding the HMT. In so doing, they are providing soldiers in the field with an outstanding trailer capable of living up to its name.

Process Improvement

In 1984, the Army began producing a 1 1/4-ton HMMWV to replace the venerable M151 series 1/4-ton Jeep and companion M416 1/4-ton trailer as the Army's primary light tactical vehicle. Each of the lightest HMMWVs would replace a set of two Jeeps and three trailers. As the HMMWV proliferated in the Army, units began using it to tow the M101 3/4-ton utility trailer.

The M101 was designed in 1952 to be towed by the M37 3/4-ton truck and has been paired with a variety of prime movers since then, such as the M880 series pickup truck and the Commercial Utility Cargo Vehicle. The HMMWV could tow the M101 without incident on roads, but it was not at all suitable for cross-country travel behind the highly mobile HMMWV. The M101 had a narrower track width than the HMMWV, causing stability problems; and its suspension did not provide adequate wheel travel and ride dynamics, causing a loss of mobility in the truck/trailer system.

When towing the M101 trailer cross-country, the HMMWV was forced to slow down to minimize trailer wear and tear and preclude the propensity for trailer rollovers. The Army needed a family of HMTs to match the HMMWV's mobility and to reduce the number of trucks and trailers needed to perform unit missions.

Army leadership directed that TACOM develop the HMT to meet this need. Two key requirements were that the HMT have the same tracking or tire spacing as its prime mover -- the HMMWV; and that it not degrade the mobility of the HMMWV by more than 10 percent. This translated to a requirement for a 15-mile-per-hour average and 20-mile-per-hour maximum cross-country speed while fully loaded the most demanding portion of the mission profile). To reduce acquisition time and development costs, PM-LTV procured the HMT as a commercial-off-the-shelf, nondevelopmental item. Following full and open competition, they subsequently awarded a five-year multiyear Firm Fixed Price contract.

The HMT tracks behind the HMMWV and uses two HMMWV runflat tires. The contract called for development of three HMT versions:

* A light cargo trailer with a 1,500-lb. payload

* A heavy cargo trailer with a 2,500-lb. payload

* A trailer chassis with a 2,700-lb. payload.

Following Production Qualification and First Article testing, production began in January 1997 to deliver 6,700 HMTs to the Army Although PM-LTV noted maintenance issues with the brakes and axles in these tests, they applied appropriate fixes, met all requirements, and obtained a conditional materiel release in July 1997.

Safety Problem

During initial performance testing, some HMMWV rear bumpers and cross-members were cracked by the forces exerted while being towed over rough terrain with full loads. In November 1997, during follow-on HMMWV bumper testing, an HMT drawbar completely failed when the aluminum drawbar broke apart, separating the HMT from the HMMWV. TACOM engineers analyzed the failures and determined that the aluminum drawbar design did not have an adequate safety margin.

In March 1998, TACOM issued a Safety of Use Message, deadlining the HMT fleet until a fix could be developed and tested. By this point, PM-LTV had already fielded over 1,700 HMTs to Army units. Leveraging the expertise of all key players, PM-LTV forged a full partnership between the TACOM Deputy for Systems Acquisition; the TACOM Research, Development and Engineering Center (TARDEC); and the Army Test and Evaluation Command. PM-LTV also fully involved all IPT members, including the U.S. Forces Command. Together, engineers from PM-LTV and TARDEC designed and developed a solution - a steel drawbar kit that would replace the previous aluminum drawbar design.

Using data collected from test courses at Aberdeen Proving Ground (APG), Md., engineers tested the solution on the TARDEC Pintle Motion Based Simulator. They also successfully conducted system-level testing at APG to validate the kit design.

During the accelerated cross-country Technical Feasibility Testing of the steel drawbar kit, damage occurred in the HMT's surge brake assembly A surge brake is a hydraulic brake activation system that uses the trailer's forward inertia to apply the trailer brakes. As the towing vehicle slows down, the trailer pushes forward against the vehicle. This compresses a hydraulic cylinder in the surge brake, which applies the trailer's brakes until the forward speed of the trailer matches that of the towing vehicle. When the towing vehicle and trailer speeds are equal, the pressure is removed from the surge brake and the trailer's brakes are released. This is a common braking system on trailers in the 6,000- to 8,000-lb. weight range that are towed by private vehicles and operated almost exclusively on improved roads, but it is not common on trailers used primarily for cross-country travel.