Electric drive technology for trucks

Electric drives still provide worthy competition to mechanical drives in the rigid truck sector. ABE takes a look at the history

In the rigid chassis truck sector, there has long been a rivalry between the mechanical and electric driveline concepts. The origins of diesel-electric trucks go back many years and these were developed as a solution to the shortcomings of early mechanical drivelines, which suffered reliability issues.

Early failures of gearboxes, differentials, driveshafts and halfshafts resulted in many on-highway trucks grinding to a halt. Chain drives were also used on some on-highway trucks, although these could hardly be described as having offered a low maintenance alternative. As an alternative to mechanical transmissions, some firms opted to use DC wheel drives instead and these became a not uncommon sight from the late 1920s on.

Diesel electric trucks do offer benefits in certain applications. From rest they move off more progressively, which can help reduce tyre wear and driveline maintenance costs. On a downhill run the driver uses retardation from the electric drives to slow the machine instead of the service brakes, further cutting maintenance costs when compared with a mechanically-driven machine.

Given the tough working environment encountered by off-highway rigid trucks used in construction, quarrying and mining applications, it was a logical move to introduce electric rear drive to this market. US firm Dart built a diesel electric tractor unit that was used to haul coal in 1939 and RG LeTourneau developed the TR60 prototype electric drive hauler in the late 1950s and although this was put to work, its design was perhaps too radical to achieve market acceptance.

Shortly after, 4473 Unit Rig unveiled its M-64 Lectra-Haul prototype and the diesel electric concept received another boost when Ralph Kress designed a series of machines for 395 Caterpillar in the early 1960s. The Cat machines were not a success, so much so that the firm stated it would never build another diesel electric drive truck. Kress left to form his own business however and achieved some success, while LeTourneau introduced more conventional electric drive machines that attracted more customers. Other truck manufacturers such as 3816 Belaz, 4489 Euclid, 4492 Haulpak and 4494 Wiseda took note and also began offering diesel electric machines aimed at the larger end of the payload scale.

However, the development of mechanical drives continued at Cat. In the 135tonne payload class, Cat's 785 truck proved highly successful following its introduction in the mid 1980s and began to account for an increasingly large percentage of sales in this particular market segment. Other manufacturers took note of how market demand was shifting to mechanical drive. Only Belaz and 3652 Terex's Unit Rig persisted with diesel electric drive rigid trucks for the 135tonne payload class.

The development of the sophisticated diesel electric AC drive for 436 Komatsu's Haulpak 930E mine haul truck in the mid 1990s changed everything. The AC drives were brushless, which eliminated one of the major maintenance chores seen on DC drives. And the use of digital speed control allowed smooth acceleration and deceleration, as well as providing effective retardation, offering lower maintenance needs than conventional braking systems. Other firms followed Komatsu, with Euclid-6512 Hitachi, 439 Liebherr (which had bought out Wiseda) and Terex Unit Rig, and most recently Belaz, all developing AC drive machines for the large mine truck market.

Further down the size range though, Cat, Hitachi and Komatsu have continued with using mechanical drives for their 135tonne haulers class machines. However the Belaz 75131/75132 models and Unit Rig's MT3300AC are 136tonne payload trucks with AC drives and some customers still favour this configuration. Where large trucks are used on quarries with long descents, the benefits of electric retardation are likely to be apparent as these could offer lower maintenance needs in comparison with the braking systems on mechanical drive trucks. And some customers are likely to continue to buy machines such as the Belaz or Unit Rig units, feeling that these offer benefits over the more commonly seen mechanical drive trucks.