Liebherr Mining Equipment is constantly challenged to optimize the ratio of the payload the truck can carry to the vehicle’s empty weight. Every pound that can be removed from the empty vehicle weight is added to the vehicle’s payload, and increases the amount of material that can be carried on each trip without overloading. The more the truck hauls at a time, the fewer trips it has to make. In addition, a lighter truck also saves on fuel and tires, when it drives empty on the way back. Liebherr engineers recently designed a new TI 272 ultra-class haul truck in an effort to improve on conventional trucks in this class that typically carry only up to 1.6 times their weight. LMS motion simulation technology played a critical role in the design of this innovative mining truck that carries 320 short tons, almost twice its own weight.
Liebherr Mining Equipment in Newport News, Virginia, specializes in building huge trucks used in the mining industry to carry overburden and minerals. The ultra-class comprises the largest trucks in the world and Liebherr is currently the only manufacturer with two products in this class. The ratio of payload to vehicle empty weight is critical to mining productivity and operating costs. The overall gross weight of the vehicle is limited by tire capacity. This means that every pound that can be removed from the empty vehicle weight is added to the vehicle’s payload, increasing the amount of material that can be carried on each trip without overloading the truck or violating tire load capacity limits. As has been noted, most vehicles spend almost half of their time operating empty, and the amount of fuel and tires consumed under these conditions is highly dependent upon the empty vehicle weight. This helps explain why a lighter weight haul truck carrying the highest payload provides the economic advantage that mine owners need in today’s economy.
An innovative design concept
Conventional ultra-class trucks use a single axle design and their frame carries the payload and dump-body weight in a way that makes them subject to substantial vertical bending loads. This design has been optimized and perfected over the years to the point that the most recent trucks offer a ratio of payload to empty vehicle weight of 1.4 to 1.6. Liebherr engineers designed the TI 272 ultra-class haul truck in an effort to improve on conventional trucks in this class that typically carry only up to 1.6 times their weight. The TI 272 weights just over 165 short tons when it’s empty and will carry 320 tons more, almost twice its own weight.
Liebherr Mining Equipment worked with BHP, a large Australian mining company, to develop the initial proof-of-concept of this innovative truck design in the mid 1990s. A few years ago, four 300-ton prototypes were built with two independent axles, each oscillating around its own suspension pivot. This means that road undulations and obstacles are primarily taken by just one wheel pair without great influence to the other pair. Even for the involved wheel pair, both tires keep continue carrying about the same load nearly all the time. This helps to increase the payload that can be carried by a given tire capacity. In addition, different tire diameters and air pressures do not automatically mean more wear on one tire. During curve travel, all four rear tires travel at different speeds, which improves traction in wet conditions.
A second important innovation in the new design is the considerably wider stance of the dump-body pivot suspensions. With 55% of the total vehicle width, they are almost three times as wide as on conventional trucks. This wider stance delivers a more stable load distribution and highly decreased wallowing on less-than-perfect haul roads. The weight from the payload is also spread further apart, reducing bending stress on the frame. The dump-body does not sit on and is not supported by the frame. Instead, the dump-body and the payload are carried in the front by two hoist cylinders right behind the cab. In the rear, they are carried by two dump-body pivots that, in turn, transfer the weight through the suspension struts and wheels directly to the ground. This allows the frame to be lighter, which is the primary reason why the TI 272 provides more payload capacity than other haul trucks that weigh the same.
Extensive prototype testing was not an option
The new design involved a major departure from existing trucks, so it was no surprise when the prototypes exhibited fatigue-life problems, particularly in the frame. Liebherr decided to engage in a major redesign in order to both address these problems and raise the payload capacity of the prototypes. The first step was to test prototypes with strain gauges under typical working conditions. These tests indicated that loading conditions on the frame were substantially different than on conventional trucks. However strain gauge testing is unable to measure the forces acting on truck components, which were needed to evaluate the performance of any given design in withstanding them using Finite Element (FE) analysis. One approach would have been to build and test a series of new prototypes, but the cost and time involved in building and testing new parts would have been enormous.
So, Liebherr engineers started looking for an analytical tool that would help them understand forces on the truck. Liebherr engineers brought in Ford Cook, from Mechanical Simulation International, in Newport News, to help model the truck using LMS DADS. Cook’s original LMS DADS model satisfactory correlated with the strain gauge testing and provided far more information than had been obtained with physical testing including forces and load case analyses on all components as well as the ability to change the design and operating conditions and determine the effects on design parameters. The success of this model convinced Liebherr engineers to purchase LMS DADS and to bring the development of the model inside the company.
Refining the multibody model
Liebherr engineers refined the DADS model of the truck to the point where it now contains over 30 rigid bodies representing all major moving and some welded or bolted components and assemblies. PTC Pro-Engineer production and FE models were used to determine inertia properties as well as produce enhanced graphics for animations. The frame is a flexible body represented by an FE beam model. Liebherr engineers created both a shell model and a simplified beam model of the frame, verified that the two models correlated with each other, then used the beam model as part of the multibody analysis.
The dump-body is modeled as a rigid body connected to other components by bushings with stiffness determined by FE analysis to simulate flexibility. The bodies are connected by appropriate DADS joint elements to model ball and pin joints, by bushing elements to model rubber pads, and by translational spring-damper-actuators to model damping of struts and hydraulic cylinders. Expression force elements are employed to simulate torque for brakes and electric motors, forces for steering and hoist cylinders as well as to model contacts between hoist cylinders and guides. The contact elements are utilized to model axle rotation stops that limit pivoting of the axles. Curve elements are widely used to specify characteristics of electric motors, hydraulic pumps, oleo-pneumatic struts, tires, etc.
Simulating complete haul cycles
Acceleration, retarding, braking, steering, backing, dumping and towing are controlled by proportional-integral-derivative (PID) controllers and may be executed separately or in combination with one another. Steering controls enable the truck to follow a specified travel path. A direct control of the idler arm angle is also allowed in order to execute some maneuvers such as steady-state cornering or lane changing. Engineers developed a series of routine and extreme load cases that simulate full propulsion, full retarding, loading, dumping, lane changing, running over a tall bump, front or rear tire blowout, and a wide range of other activities. They combined these load cases into continuous runs that simulate complete haul cycles.
Where the initial prototypes showed fatigue-life problems in the frame, multibody simulation with LMS DADS was used to calculate forces and determine load cases for FE analysis studies. Liebherr’s engineers used the insight from their simulation efforts to redesign the frame and several other structural components, without putting back too much weight. The production design of the TI 272 is now being completed and plans exist to carry the modeling process one step further by generating load histories for virtual durability analysis.
