(Dr. Richard Salant, advisor)

"Numerical Model of a Rock Bit Bearing Seal"

Abstract

The rock bit bearing seal is the mechanical face seal used in a rock drill bit.  Its function is to protect the bearings within the drill bit.  The seal is expected to work in the mixed lubrication regime.  Successful design of the seal requires thorough analysis of the mixed lubrication phenomenon at the seal interface.  The proposed research is to develop a numerical model to predict the tribological performance of the seal and provide a practical design tool.

Three sub-models are developed: axisymmetric model, non-axisymmetric model, and dynamic model. The axisymmetric model shows that the performance of the rock bit bearing seal depends on both the differential pressure across the seal and the ambient pressure, whereas the performance of the conventional mechanical seal depends only on the differential pressure.  The non-axisymmetric model shows that there is a range of equilibrium states of the seal, corresponding to a range of relative misalignment between the non-rotating face and the rotating face, and it is possible to have incursion of ambient fluid and negative leakage depending on the misalignment.  The dynamic model shows that when the seal is displaced from an equilibrium state, it returns to that state, but the leakage rate can be very large during that transient process.  It also shows that the seal can track an unlimited amount of runout of the rotating face.  However, a large runout will cause large leakage and incursion rates and high contact pressure, which are extremely detrimental to the life of the seal and the sealed bearings.

While the research is based on the rock bit bearing seal, the model, the method, and the computer code are applicable to other types of mechanical face seals, especially those in which friction from the static secondary seal is significant.