LeTourneau's 75-ton dump truck pioneered the electric wheel concept in 1959
Looking for the most economical way to haul large loads up long ramps in its massive Berkeley open pit copper mine at Butte, Mont., Anaconda Co. engineers decided that an electrically driven hauler might be more efficient than the existing mechanical trucks they operated. So, in 1958, they turned to R.G. LeTourneau, which had already built many types of off-road vehicles utilizing an electric-wheel design originally announced eight years earlier by its inventor, R.G. LeTourneau. Accepting the challenge, the manufacturer came up with a prototype off-road haul truck of 60-ton capacity and, after testing, shipped it to the Berkeley pit in 1960.
The LeTourneau electric-drive system mounted the main DC drive motors and gear reductions inside the wheel hubs, which received power from an on-board DC generator. Its efficiency was derived from doing away with gearboxes, transmissions, drive trains and final drives, replacing them with smooth, DC-electric power applied to all four wheels. The speed and direction of the high-torque motors was governed by a simple controller in the driver's cab.
R.G. took electric drive a step further. "Now that we have electric wheels," he said, "why not power them from some external source, so that the truck's own diesel engine can run at idling speed on the uphill grades, cutting down on fuel consumption."
The trolley-assist hauler was born, designed for big, open pits with long, uphill ramps. Here's how it operated: After receiving its load from the shovel, the hauler drove under its own power to the base of the ramp, where it contacted a semi-portable overhead electric wire system by means of a spring-loaded trolley. This boosted power to the electric wheels, allowing the hauler's engine to idle up the ramp. Near the dump area, the truck separated from the trolley system and its own engine took over for the dumping phase.
The prototype vehicle, known as the Pacemaker TR-60 Trolley-Dump, was a four-wheel articulated dump truck with pivot steering, initially rated at 60 tons and powered by a Cummins 335-hp diesel. Trials proved the truck was under-powered when disengaged from its overhead line, so a new front end was designed with a second engine of the same type, resulting in a combined rating of 670 horsepower.
The truck's rating increased to 75 tons, making it one of the largest haulers operating at that time. The TR-60 stayed at the Berkeley pit for a productive working life and is now preserved at the Butte Mining Museum in Butte.
Trolley-assist haulage was never popular because of pit layout limitations and inflexibility, but it was used successfully at a few mines around the world. Yet, the concept of electric wheels did become popular in large vehicles, continuing to be the drive of choice for the largest haul trucks in the 1960s and 1970s. In the past two decades, mechanical-drive technology has caught up with electric-drive technology, and the two compete head-to-head to drive the largest trucks in the industry, now reaching the 400-ton class.
You can read more about the evolution of construction equipment in Keith Haddock's illustrated book "The Earthmover Encyclopedia," available in most bookstores. Also, consider a membership in the Historical Construction Equipment Association, www.hcea.net.
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