The Unscramble Levels of Tribosimulation
For the purposes of this chapter, we shall consider four levels of tribosimulation. Figure 14.1 shows these levels, indicating the parallels between physical and computer tribosimulations. Generally, the more realistic the simulation, the more costly and complicated it becomes.
Level 1 tribosimulations use full-scale machines, such as entire automobiles, trucks, aircraft, ships, manufacturing machinery, and consumer products. These machines may be instrumented to measure loads, temperatures, strains, power demands on motors, and vibrations. In physical simulations, fullscale machines are operated under controlled conditions, such as on a test stand or inside an environmental chamber. For example, an entire communications satellite might be placed in a high-vacuum chamber and its antennas caused to rotate to determine whether the bearing lubricants will perform effectively in space. A truck might be rolled onto a chassis dynamometer and its wheels subjected to various braking loads and cycles. However, even the high-level simulations of Level 1 might omit certain factors present in the field. For example, the effects of zero-gravity on the aforementioned satellite cannot be completely simulated in a vacuum chamber on Earth. The effects of random potholes, loose gravel, and uneven road surfaces cannot easily be simulated on a test stand, but recent developments in computercontrolled test stands are making such simulations increasingly realistic.
Level 2 tribosimulations use subassemblies which are subjected to near-operating conditions. Examples include brake pads and rotor combinations on dynamometers, fluid pumps in closed-loop pump test rigs, and jet engines in engine test cells. Nearly as expensive as Level 1 tests, Level 2 tests offer additional control of the externally applied test parameters. At the same time, fewer of the field-associated effects on performance are faithfully simulated. For example, the effects of road-induced vibrations on piston motions, the vehicle-specific flow of air past brake components, and the introduction of environmental contaminants into wheel bearing grease may be omitted in subassembly tests. In dynamometer tests of break component materials it is common to apply a series of test stages in an attempt to simulate specific types of frictional phenomena, like fade effects at elevated temperatures. Even staged Level 2 tests as complex as these cannot totally simulate the full range of habits of individual drivers and driving conditions. In some cases, however, Level 2 tribosimulations can be very effective in screening materials or lubricants because the operating conditions of the system are more clearly known. For example, loopby-loop ballpoint pen testers can show how long the products will continue to write effectively and establish failure statistics for the entire pen, whose satisfactory performance depends on the ability of the point to deliver a clear, uniform line of writing fluid to the paper.
One tribosimulation area of particular medical interest is that of computerized hip and knee joint testing. Attention here is given to mimicking the forces, types of motion, and impact loads to which bioimplants are subjected. This subject area can make effective use of both physical and virtual component Level 2 simulations. The selection of the fluids to simulate synovial fluid and to correlate with clinical results is an important issue. Material swelling in situ, in the case of polymeric materials, and the role of debris particles as they interact with the soft tissues surrounding the joints, are also of interest. Level 3 tribosimulations involve test rigs designed to test specific components, like bearings and gears.
For example, bearing test rigs have been successful in developing empirical design and selection guidelines for rolling element bearings of many kinds. Multiple-station rigs, automated to take data or to ascertain critical failure conditions, like excessive heat or vibration, can be run unattended, enabling the compilation of lifetime statistics and related performance data for consumer products or machine components.
Level 4 tribosimulations involve test coupons of simple shapes. Examples include pin-on-disk tests, block-on-ring tests, four-ball lubricant tests, dry-sand-rubber-wheel abrasion tests, and vibratory cavitation tests. These tests are described elsewhere in this volume and in the wear testing literature [e.g.,ASM (1992, 1997)]. Their usefulness is based on their ability to simulate the key contact conditions of the components of interest. For example, a cam roller follower in the engine of a certain diesel engine might be simulated by two disks turning at different speeds to impart a desired degree of slip to the contact. Furthermore, the test disks could be supplied with a lubricant and heated to simulate engine conditions.
The linkage between tribosimulation levels can be important to establish the validation of Level 3 and 4 tests as effective screening methods. For example, if a set of Level 4 rankings agrees with relative rankings of the same set of materials or lubricants in Level 3 tests, and the validity of Level 3 tests in a certain application has been confirmed, then the usefulness of Level 4 tests will be greatly extended.
