1. Wear
   
   1. Wear of Mechanical Systems
      
   2. Wear of nanoComposites
      
2. Friction and Lubrication
   
   1. Granular Lubrication and Wear
      
   2. Gas Surface Interactions
      
3. Biomaterials Tribology
   
   1. Wear of Total Knee Replacements
      
   2. Hydrogels
      

Wearing Mechanisms, "Wear" are they going?
Kelvin I. Diaz


HE ABILITY TO PREDICT the useful lifetimes of mechanisms is of critical importance to designers and engineers. In 1997 Blanchet published a closed-form expression describing the coupled evolution of a "scotch yoke" mechanism's dynamic loading and the changing kinematics as a result of wear. A single parameter wear factor K(mm³/Nm) is used in the models to estimate the change in shape as a result of wear. A numerical simulation of the same dynamic system written by our lab is in excellent agreement with the analytical model of Blanchet.

These models, although complex and challenging mathematically, use only the most basic treatment for wear (K, mm³/Nm). The wear factor K is generally obtained from the slope of a linear region of volume loss versus sliding distance, while maintaining both the normal load and sliding speed constant. In mechanisms the loads and sliding speeds at the contact location are continuously changing. And furthermore, as the mechanism accommodates wear, the geometry and resulting kinematics change, which subsequently changes the loads and sliding speeds at the point of contact. The wear factors for the pair will be determined from laboratory tests using the averages of the mechanism's load and sliding speed at the point of contact. Thus the model predictions will be based on work that is independent of the experimental test-bed investigation. This will further facilitate unbiased comparisons between the model and the test-bed results.

During the summer term a scotch yoke mechanism test-bed (Figure 1) will be constructed and instrumented. The mechanism will be driven under steady rotational conditions allowing the kinematics and subsequent loading to evolve as the shaft slides against the yoke's slot. The changes in kinematics of the mechanism will be continuously monitored as the geometry changes due to wear of the specimens, the shaft and slot. After a certain number of cycles the surfaces of each specimen will be analyzed and the new shape as well as the amount of wear will be quantified. Both the shaft and yoke specimens can be changed to test varying material combinations.


Figure 1 The "Scotch Yoke" mechanism delivers pure harmonic motion. This motion is achieved when a shaft fastened to a disk, which is rotating at a constant angular velocity, slides within a vertical slot connected to the reciprocating mass.

The test methodology is to evaluate the analytical and numerical models to determine how well these models with single parameter wear factors perform on an ideal mechanism and steady operating conditions. These results can be used to further develop techniques at modeling wear in mechanisms. The test-bed mechanism can be naturally extended to examine wear under non-steady operating conditions. With new models and experimental techniques, startup and shutdown conditions can be taken into consideration when running the test-bed and analyzing the collected data.

During the design process and material selection, tribological impacts on both performance and longevity are frequently not considered. This is due to a lack of accurate tools available to the design engineer. The overarching goals of this research project are to develop general numerical techniques and software tools that include constitutive relations for wear and are capable of making performance and life predictions for other standard mechanisms such as cams, linkages, and gears.

Oxford English Dictionary

Simple Simulations in Java^TM

1. Java applet of a wearing scotch yoke mechanism

2. Java applet of a wearing cam-follower mechanism