In 1951, this 70-ton Dart 75-TA was shown at its launching ceremony at the Bagdad copper mine in Arizona.
The massive tandem-drive truck launched by the Dart Truck Co. in 1951 was indeed a world-record beater for its size. Its rated capacity of 75 tons was unheard-of at that time and far exceeded the size of any previous off-highway hauler. Known as the model 75-TA, this giant earth hauler was designed by veteran truck builder Ralph Kress, then general manager of the company. Kress's colorful career later led him to design innovative off-road haulers for Wabco and Caterpillar, as well as for his son's company, Kress Corp., which he joined in 1969.
The Dart Truck Co. began building highway trucks upon its establishment in 1903 and expanded into the off-highway truck business with its first heavy-duty model in 1937. Over the next dozen years, Dart built off-highway haulers in ever-increasing sizes leading up to the launch of the 75-TA.
This monster truck sported two Buda diesel engines under the hood, each developing 300 horsepower, and each driving one of the two rear axles through torque converters. The engines, two of the largest ever installed in a truck, were mounted behind the cab to give the driver better visibility and keep him away from heat and fumes. The radiator fans up front, positioned 12 feet away from the engines, were driven by long belt-driven shafts from the transfer case. For simplicity and ruggedness, the front wheels were mounted on a steering axle that pivoted on a centrally mounted king pin on the truck frame. Instead of independent wheel steering, twin double-acting hydraulic cylinders moved the entire axle, providing smooth steering and effortless control by the driver.
The empty truck tipped the scales at just under 50 tons, and overall width and length measured 11 feet 9 inches and 32 feet, respectively. Dart intended to sell fleets of these trucks, but like many innovations, it was ahead of its time and only one was ever built.
Although the 75-TA did not live up to expectations, Dart did achieve a fair measure of success with its subsequent off-highway haulers. In fact, by the early 1950s, Dart began to phase out its highway truck line in favor of off-highway vehicles, several of which achieved industry firsts. These included a 95-ton capacity, rear-dumping tractor-trailer unit in 1960— the world's first mechanical-drive truck of regular two-axle configuration to beat the 100-ton barrier in 1966—and the world's largest tandem-axle mechanical-drive hauler at 85-ton capacity launched in 1980. Dart's complete truck line included both mechanical and diesel-electric drive units.
Dart was acquired in 1984 by Unit Rig & Equipment, Tulsa, Okla. Unit Rig itself became a division of Terex in 1988, and Dart products were phased out by the early 1990s.
Information for this article was taken from the author's illustrated book "The Earthmover Encyclopedia" in which you can read more about the evolution of construction equipment. Also, consider a membership in the Historical Construction Equipment Association, www.hcea.net.
