Take a load off
Many supporters of the concept of wheel loaders believe loaders represent a high level of efficiency – if not the highest level of efficiency for the work they do, while also representing economic advantages in a number of areas.
Overall, it is their ability to load haul trucks quickly. One example, observed recently at a Pilbara Iron site was an L-1350 loading 203 tonnes in 2.43 minutes, with an average cycle time of 29.2 seconds. They represent high levels of efficiency and at advantageous costs.
Mobility represents a ‘strategic’ advantage for loaders, with travel speeds of up to 16-19 km/h (10-12 mph). This can result in a reduction in the number of machines required for a mine that has multiple pits or the need for cost efficient methods of blending.
With the advantage of mobility, is the added ability to keep ground surfaces free of rocks where Haul Trucks are positioning, thus protecting the truck tyres.
A quick look at just a couple of surface mining sites revealed two of the nations biggest miners – the BHP Billiton Mitsubishi Alliance (BMA) in Queensland’s Bowen Basin and Rio Tinto in the NSW Hunter Valley – are operating a number of wheel loaders and personnel are quick to comment positively on their efficiency.
At BMA’s Saraji site in Central Queensland they are using two Cat 994s and one of LeTourneau’s big capacity L-1850 models.
LeTourneau’s loaders are generally recognised as the world’s most technologically advanced loaders. LeTourneau also build the world’s largest wheel loader, with a 40.5 cm bucket and operating capacity of 72,574 kg – it is rated for Haul Trucks from 240-400 tons.
Personnel at the BMA Saraji site speak proudly of their big boy, the L-1850 with its 38 cu m coal Digging bucket. It is regarded, by them, as an excellent machine for the job.
“It’s a perfect match for single side loading of our Haul Trucks, something we can’t do with some other loaders we have considered,” explained one of the senior on-site personnel.
“At 8,000 hours we are getting 90% availability every month and we use it 24 hours a day. We class it as absolutely reliable … and the operators love it,” he added.
With the constant movement and often rocky conditions, tyre life could be expected to be an issue but, in the case of the L-1850 just mentioned, the unit still has its original tyres at 8,000 hours.
Fuel consumption was the other factor covered with the L-1850. It was 25% less fuel usage over another competing make of wheel loader and is thus significantly down again on hydraulic excavators.
Mark Geerssen, mobile equipment maintenance superintendent at Rio Tinto’s Hunter Valley Operation, who has been with the company for over 20 years, briefly explained their position.
“The site is a 22km spread, mining 13 million tonnes of coal annually after moving 60 million tones of overburden each year. “To do that, truck loading and pre stripping is carried out using two Bucyrus Erie draglines (1570 and 1370), six P&H shovels (2*4100, 1*4100XPB, 2*2800XP shovels and 12800MKII), O&K RH170 excavator plus 60 240 ton Haul-Pak trucks, six LeTourneau loaders of varying capacity, including two big L-1850 … and three Cat 992 loaders.
“We have had LeTourneau wheel loaders on site for over ten years and they are worked hard. We have just removed one from service after 49,000 hours,” he said.
“Last night, in a 12 hour shift, we moved 13,500 BCM working in rock with a 21 m. bucket.”
With all the moving around the worksite, tyre life could be expected to be an issue.
“The tyre life, for example, on the L-1850 is over 8,000 hours. Generally we are using Michelins and getting between 4-5,000 hours on the front, before rotating them to the back as rear run outs, getting 8-10,000 hours all up,” explained Geerssen.
“The reason we look to wheel loaders is mobility and flexibility … mobility is the big issue. And, we find they are okay to service and maintain,” added Geerssen.
An engineer’s assessment points to a spread of factors which prompt many mine sites to look to wheel loaders.
Overall, machine designs point to a number of issues which result in a lower capital cost and lower maintenance costs.
Take the hydraulics, explained one engineer, pointing to the less extensive hydraulic system with loaders.
A loader only requires cylinders for hoisting, bucket control and steering control.
The alternative hydraulic excavator requires cylinders for hoisting, crowding, bucket control, dumping and multiple hydraulic motor and gear box drive systems for the swing function.
A large wheel loader, typically, may have seven hydraulic pumps and four valves, whereas an equivalent hydraulic excavator could be expected to have sixteen hydraulic pumps and eight valves.
The larger hydraulic system amounts to a significant addition to machine cost and can add significantly to on-going maintenance.
The popular LeTourneau loaders can deliver major fuel savings over hydraulic excavators (which have to power a bigger hydraulic system) and can even offer fuel savings of around 25% when compared with a competing, or even smaller loaders. Bucket geometry and breakout forces with loaders provide another point of interest.
There is a significant difference in the way breakout forces are applied for a loader on the one hand and a hydraulic excavator on the other. Large loader buckets have their pivot point attachment to the lift arm at the rear of the bucket. The hydraulic excavator has its bucket’s pivot point attachment to its support arm, up high vertically on the back and close to the centre of the bucket horizontally.
This increases the actual breakout force to a value greater than the SAE rating because the tooth to skid shoe is a much shorter dimension than from the tooth back to the lift arm pivot.
One of the key standout items with the LeTourneau loaders is the electric drive system. It is a unique and clever piece of engineering which separates LeTourneau from the others… a drive system which mine personnel identify as the single most outstanding aspect.
The system delivers independent four-wheel drive for shift-free operating simplicity, superior traction, resistance to wheel slip, regenerative braking and extended tyre life.
The motors have a very high torque at low speeds which gives the loader the ability to crowd into the bank with intense effort.
The high torque is produced by increasing current at low voltage. Only a small amount of horsepower, which is volts x amperes, is consumed even though the torque is high.
This provides a tremendous fuel saving advantage over hydraulic excavators which use high flow, high pressure hydraulic pumps, motors, and hydraulic cylinders for the propel, crowd and swing functions.
In addition, LeTourneau loaders have the only regenerative retarding system in the industry. Energy is recovered form the braking action of the drive system and delivered to the hydraulic system, further reducing the fuel consumption during the loading cycle.
12-Mar-2007