The Mechanism of Lubricant Trapping under Dynamic Compression

In the deformation processes in a pneumatic high speed hammer, friction is very low. This is due to the fact that much liquid lubricant is trapped between the tool and the specimen. In this paper, the mechanism of lubricant trapping under dynamic compression is analyzed with hydrodynamics and flow stress of the specimen material. The pressure distribution and the thickness of trapped lubricant film are calculated. The film thickness (h) is affected by compressive velocity (V), viscosity of the lubricant (η), diameter of the specimen (R), and flow stress of the material (p0). These are correlated as hocV1/3η1/3R2/3p0-1/3. Compression tests are carried out using a high speed hammer in order to ascertain the analysis. The experimental results agree well with the analytical results. It is found that most of the trapped lubricant remains entrapped even after a large deformation.