Caterpillar is the only manufacturer that offers large (182 t and above) mechanical drives. BELAZ is one of the OEMs which maintains that trucks from 130 tonnes payload class and above should be electrically-driven.
In the rigid chassis truck sector, there has long been a rivalry between the mechanical and electric driveline concepts, especially at the larger end of the payload scale. The pros and cons of either drive system are well-documented, but there is much more to that, as Munesu Shoko, editor of Capital Equipment News plods through the fundamentals.
Liebherr believes that with fuel efficiency, hauling productivity and operating costs high on their list of priorities, mine operators are particularly drawn to trucks with electrically driven wheels powered by diesel generators, instead of mechanical powertrains with gearboxes and driveshafts.
The mechanical versus electric drive debate has always taken centre stage as far as rigid haulers are concerned, particularly at the larger end of the payload scale. 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. Some original equipment manufacturers (OEMs) have found the diesel-electric option to offer many benefits than the conventional mechanical option, while some have maintained that mechanical options remain the best driveline option.
BELAZ is one of the OEMs which maintains that trucks from 130 tonnes (t) payload class and above should be electrical and machines in the 30-60 t class should be mechanically driven. However, the Belarusian OEM also successfully developed an electric truck of 90 t payload, and Rost Lavrenov of BELAZ Africa, tells Capital Equipment News that in this class size (90-100 t), both electric and mechanical drive options are quite viable depending on cost factors.
Caterpillar remains the only OEM to offer both drive options in its different rigid dump truck (RDT) offerings, even on the larger payload units. The OEM says its developments in the alternative drive systems have always been informed by proper monitoring and investigation to ensure that products bring the best value to customers. In the early years of exploring alternatives to mechanical transmissions, several firms opted to use DC wheel drives, and Caterpillar was one of the first manufacturers to develop DC electric drive trucks with the launch of its 75 t 779 model in the late 1960’s.
“After operating trucks at mines, we concluded, at the time, that a mechanical drive offered our customers the best haulage solution and Caterpillar has since steadily advanced mechanical drive technology,” comments John Ingle, Large Mining Trucks Marketing and Performance Manager at Caterpillar.
Today, Caterpillar is the only manufacturer that offers large (182 t) mechanical drives. “Over the years, mechanical drive has continued to move up in size as the preferred drive solution over electric drive as OEMs have been able to offer larger capable mechanical drive systems.”
Cat’s viewpoint
According to Ingle, Cat mechanical drive trucks have proven their value to miners and are favoured in most applications. In contrast to the notion that the electric drive option is the better choice on the larger end of the payload scale, Caterpillar offers the widest range of mechanical drive trucks from the 182 t payload and above class size.
The 136 t 785D starts what Caterpillar terms its Large Mining Trucks and is a mechanical drive truck. The 789D (186 t) and the 793D/F (227 t) are also mechanical drive trucks and are regarded the workhorses of the industry, according to Ingle. Cat’s mechanical drive goes as far as the 365 t 797F and Ingle argues that the mechanical drive option remains the most productive and lowest cost per ton drive system choice for mining customers.
However Ingle notes that the OEM also recognises that some customers prefer electric drive in some size classes, such as the 290 t and 330 t, which are established around AC electric drive fleets. Consequently, Caterpillar has invested into the development of AC drive trucks since the early 2000’s. “We have engaged key customers with electric drive experience throughout the development process to gain their input on machine design and performance,” says Ingle.
“Caterpillar is the only large mining truck OEM that designs, produces and integrates its own engines, powertrains, drive controls and software. This is true for both Cat mechanical and AC drive trucks and allows us to optimise overall machine performance and to provide product support for the entire truck,” adds Ingle.
“While there are pros and cons to either drive system, the bottom line is that Caterpillar mining trucks, either mechanical or electric drive, are designed to bring the most value to our customers,” says Ingle, adding that the choice depends on the size of truck required and specific customer preference.
BELAZ’s standpoint
According to Lavrenov, BELAZ is a true proponent of the electric drive system for the larger end of the payload scale. All its models from 90 t and above are powered by electric drivelines. “More specifically, these are the trucks of 90, 110-130, 180, 200-220, 240, 360 and 450 t payload capacity. The machines with 30, 45 and 55-60 t are powered by mechanical drive,” says Lavrenov.
“BELAZ believes that for 90 t and higher, electric drive, and AC drive in particular, provides better performance for customers than mechanical drive,” says Alexander Egorov, General Design Engineer at BELAZ. Already the company has made modifications to allow all its existing DC drive models to use AC drive, while all its new truck offerings have been designed with AC drive.
However, the company says some of its long-term customers have expressed their desire to stick to DC drive. This is partly due to the cost, and on the other hand, Egorov says it’s because DC drive technology has improved and customers want to stick to what they know in terms of maintenance. Current BELAZ shipments of large trucks with DC or AC depends on the size class, with some long established 130 t and 220 t models still seeing a lot of DC drive orders.
Why mechanical?
In terms of payload, Ingle argues that mechanically driven trucks in the 182 t to 220 t size class have a 10-20 t empty weight advantage over their electrically-driven counterparts. “The lower mechanical empty weight translates into more payload and higher efficiencies,” says Ingle. Lavrenov agrees, saying that less weight of the components in a mechanical drive system when compared with AC/DC drive systems, translates into less truck weight.
When it comes to drivetrain efficiency, Ingle also argues that for haulage trucks, mechanical is the most efficient drive for getting the most power from the engine to the ground. “Electric drive truck manufacturers talk about improved powertrain efficiency of AC over DC but the fact is mechanical drive is still more efficient than AC electric drive. This has been proven in field studies, and validated in the lab. The result is mechanical drive can do more with equal power or deliver the same performance with less power. This translates into better fuel savings,” says Ingle.
Meanwhile, mechanical drive trucks do not burn fuel when retarding downhill. Retarding forces drive need systems so fuel to the engine can be turned off. Ingle also says mechanical drive offers superior braking, retarding and traction control. “Mechanical drive trucks are recognised the world over by miners and operators as better to operate in poor underfoot conditions. Also, AC drive has a dynamic retarding capability limit that once exceeded, the truck starts to speed up. Mechanical drive brakes/retarding can bring a truck back into the desired operating envelope,” says Ingle.
Ingle also argues that mechanical drive trucks can generate and hold a higher peak rimpull (torque) than electric trucks. “This can be a bit confusing because electric trucks offer optional final drive reductions that improve peak rimpull at the cost of top end speed. In addition, electric drive systems can have a maximum electric current time limit for how long they can pull peak torque.”
In a nutshell, Ingle believes that mechanical drive offers lowest cost per ton. “Mechanical drive delivers the best balance of production and operating costs,” argues Ingle.
Why electric?
According to Egorov, application of electric transmission results in much smaller number of friction units (alternator, electric motors, reduction gear units and control cabinets), which saves up to 50% of oil compared with a conventional mechanically-driven RDT. “That simultaneously drives down operational costs by up to 38% during maintenance,” he says.
This view is shared by Lavrenov, who argues that mechanical drive requires more maintenance through oil changes, adding that service life of major components is shorter compared with electric drive.
Ingle says both drive options have engine oil, final drive oil, steering and hydraulic systems, but mechanical drive has a bit more fluids for the transmission and oil-cooled brakes. “But, an objective look at the actual impact on operating costs shows this only adds a very small amount of operating cost to mechanical drive trucks and is greatly offset by other mechanical drive operating cost advantages,” argues Ingle.
Lavrenov adds that electric drive has fewer major components than the mechanical drive option, and this translates into less potential failures. He also argues that fewer components mean less maintenance, which in turn translates into higher availability and ultimately higher productivity.
The trolley option is another key benefit of the electric drive system, and Ingle agrees to that fact. This is a major feature of Hitachi’s AC-3 drive system technology which it launched in 2014 in southern Africa. Hitachi pioneered the AC-3 technology to its RDT range in 2012 with the launch of the EH5000AC-3. The AC-3 technology was developed using advanced propulsion technology that also powers the OEM’s bullet trains in Japan. Hitachi Construction Machinery Southern Africa introduced the new EH4000AC in the local market in 2014. Its trolley assist is a fuel-saving method which is not available on mechanical drive trucks. It is provided by an overhead catenary power supply supported by poles mounted along main haul roads.
Another key benefit of Hitachi’s AC-3 drive system is that it comes with a slip/slide control system. If the system senses slipping or locking of rear wheels when travelling on slippery or frozen roads, it adjusts the torques of the wheel motors accordingly, giving the truck better stable travelling.
The AC-3 drive system also incorporates a pitch control system. If this system senses pitching when travelling on bumpy surfaces or an abrupt stop, it adjusts the torques of the wheel motors fittingly to reduce pitching on the truck, ensuring better operating comfort and less load spillage.
If the AC-3 Drive Control System’s side skid control function senses oversteer or understeer from the newly equipped sensors on the unit, it adjusts the torques of the wheel motors accordingly to stabilise the side skidding. This gives the gigantic truck smoother movement and more stable steering.
While dynamic retarding force is said to be another key benefit for electric trucks, Ingle argues that this is only applied to the rear wheels. “In many cases electric trucks can quote a high retarding power or fast ‘on paper’ retarding speed. In practical field applications, electric drive retarding speeds are mostly reduced to provide protection from exceeding the electric drive retarding capability envelope and because competitive electric drive trucks do not have braking force on their front wheels like Cat mechanical drive trucks (and the Cat 795F AC) which allows a more effective braking force and good machine control – resulting in the ability to better utilise potential retarding speeds,” he says.
Ideal conditions
According to Lavrenov, BELAZ’s argument that the larger end of the payload scale, starting from around 130 t, needs AC/AC systems is based on the fact that from this payload, trucks are involved in large-scale mining where machines need to get over steep gradients, and electric trucks are preferable.
He, however, notes that both drives need good haul roads with gentle sloping, quality maintenance and service, responsible and skilled operators and loading within the rated payload. “However, this is possible in an ideal world only,” says Lavrenov.
“At BELAZ we are of the opinion that there are more variants and configurations of operating conditions which suit electric trucks than mechanical. For instance, electric drive is an obvious choice for deep pits where trucks need to haul material from the bottom and get over steep slopes,” says Lavrenov. He adds that from a production point of view, mechanical trucks are notably slower on gradients when moving uphill loaded.
Meanwhile, Ingle argues that historically DC drive trucks were used in more flatter, moderate grade or easier applications, while mechanical drive was used in poor underfoot and more severe hauls. “However, AC drive has changed many of the past electric drive limitations. Today, mechanical drive is still recognised as best for many applications, but ultimately, it is more of a function from a mine’s point of view,” says Ingle.
Ingle, in conclusion, tells Capital Equipment News that when looking to the future of rigid haulers drive technology for large mining trucks, the industry tends to think of some sort of electric hybrid car approach. However, Caterpillar continues to research energy recovery for both mechanical and electric drive systems. Ingle further notes that at this time, energy recovery for either drive solutions are still not cost effective to implement.
