20 Minutes With... Dr. Dong Zhu

Tribology & Lubrication Technology, Nov 2007 by Phipps, Karl M

2. How to model surface contact and hydrodynamic lubrication simultaneously. There are also two types of models. The first simulates contact and lubrication separately with different approaches. For contact, a dry contact model is used, while for lubrication the Reynolds equation is employed. This approach may work well in many cases. However, it may be difficult to determine borders and handle boundary conditions between contact and lubrication areas, especially when random or irregular surface roughness is involved. Concurrently, unified approaches are developed for solving both contact and lubrication with the same Reynolds equation system. Since dry contact is an extreme case of lubricated contact at ultralow viscosity and/or ultralow speed, theoretically dry contact can be simulated with lubrication models as long as the numerical solver is sufficiently robust to handle ultralow viscosity and ultralow speed.2

In the last 10 years, great efforts have been made by different researchers for developing 3-D deterministic mixed EHL models, which includes professors H.P. Evans and Ray Snidle's group at Cardiff University in the United Kingdom. Farshid Sadeghi's group at Purdue University in West Lafayette, Ind., Y.Z. Hu's group at Tsinghua University and lane Wang's group at Northwestern, among others. I have been closely collaborating with Northwestern and Tsinghua. Now our model is capable of simulating the entire transition from full film and mixed down to boundary lubrication with a unified approach under severe operating conditions. This appears to be a useful tool for in-depth study on gear tribology. Recently, the model has been validated with experimental data and results from other models and other researchers. Also, effects of differential scheme and mesh density have been investigated and the approach for handling contact studied.3 4

Most recently, two accomplishments have been made in collaboration with Northwestern: (1.) a full numerical solution for mixed EHL with coated surfaces, which may be useful for components with coatings or hardened layers and (2.) a threedimensional line contact mixed EHL model that can handle two-dimensional, macrocontact geometry together with threedimensional surface roughness. This is extremely important for gears, as most gears are typical line contact components but the surface topography is usually threedimensional.

What are some of the major factors that affect the gear train efficiency? How can we improve gear efficiency based on the current understanding of gear tribology?

Many factors may affect gear efficiency, including surface roughness, lubricant rheology, boundary film additives, operating temperature and churning. Other than churning, which is a practical issue more difficult to simulate, most factors can be modeled and predicted based on current understanding of mixed EHL. Basically, friction in gear tooth contact consists of hydrodynamic friction due to lubricant shearing and contact friction at local contact spots where boundary films may form.