load limits
A truckmaker pushes the ratio of payload to frame.
Compared to the ants, a truck that hauls time
and a half its empty weight isn't serious competition. Plenty of ants
are good for at least 10 times their own weight. When it comes to mining,
though, ants are hard to train, and you'd need too many.
That's why trucks carry rubble and ore from mines, and some can handle
payloads as great as 1.4 to 1.6 times their empty weight. Now a manufacturer
in Newport News, Va., Liebherr Mining Equipment Co., is trying to push
the limit. Although it still isn't in the ants' league, a truck that will
bear almost twice its weight is aimed at saving mining companies money.
Liebherr argues that the more a vehicle hauls at a time, the fewer trips
it has to make, and if it travels lighter when it's empty, it saves a
little on the way back, too, when it carries no load.
Liebherr is working on a truck, called the TI 272, that weighs just over
165 short tons when it's empty and will carry 320 tons more—about
150 metric tons carrying another 290 or so. It's almost a million pounds
fully loaded.
The original idea started with BHP, a mining company based in Melbourne,
Australia. BHP connected with Liebherr Mining Equipment when it was still
known as Wiseda Ltd., before it was purchased by Liebherr Group, a Swiss
conglomerate with interests ranging from heavy equipment to hotel properties.
The companies developed a proof-of-concept prototype in the mid-1990s
that put in more than 6,000 hours of service at BHP's Saraji mine in Queensland.
Since then, Liebherr has bought the rights to the idea.
The new truck has two independent rear axles, each oscillating around
its own suspension pivot. A bump in the road that affects one pair of
wheels has little influence on the other. Distribution of load increases
the weight the tires can carry. The distance between the two dump-body
pivot suspensions equals 55 percent of the vehicle's width, making for
a more stable load distribution and less bending stress on the frame.
The dump body is carried by two hoist cylinders behind the cab and by
the dump-body pivots, which transfer weight through suspension struts
to the wheels and then to the ground. The design minimizes weight on the
frame.
Independent
rear axles, each with its own suspension pivot, are a key feature of the
TI 272, which can carry about twice its own weight.
Liebherr tested prototypes with strain gauges under typical working conditions
and found areas not up to design criteria and some fatigue problems. Loads
on the frame seemed to be different from those on the company's other
trucks in this category, called the ultra class.
Vladimir Pokras, senior mechanical structures engineer at Liebherr, said
differences were large enough so the design crew couldn't rely on prior
experience. No one really had any experience with this shape of truck
before.
The company brought in Ford Cook, an ASME member, as a consultant. He
had recently founded his company, Mechanical Simulation International,
in Newport News. Cook developed a model using DADS mechanical system simulation
software from LMS International of Leuven, Belgium. Guided by Liebherr's
data, Cook built a model from which he was able to calculate the direction
and magnitude of forces working on the system.
Given that information, Liebherr's engineers took the modeling in-house.
They conducted finite element analysis, using Pro/Mechanica software from
PTC of Waltham, Mass., to determine how the forces that Cook discovered
affect the truck design. The information represents a range of routine
and extreme cases: hauling, loading, dumping, hitting a bump, blowing
a tire, and so on.
James Whitfield, Liebherr's assistant R&D manager, said that the model
TI 272 is in an advanced development phase. A number of trucks are being
tested.
Pokras said the company is redesigning the frame to meet fatigue design
criteria without putting back too much weight. For instance, engineers
are considering adjustments in thicknesses and plate arrangements, and
the use of castings in place of some plates.
Pokras is developing what he calls "a virtual proving ground," combining various load cases to model an entire haul cycle. Using another LMS product, Falancs, engineers will simulate the truck in service over time to see how it holds up.
This article was prepared by staff writers in collaboration
with outside contributors.