This paper describes that shear properties of polyisobutylene (PIB) lubricant under the starved condition using in-situ observation in order to understand the lubrication mechanism in the screw tightening process. Specifically, the study focused on the effect of operating condition (sliding speed and contact pressure) on the lubrication performance. In the test at various sliding speed, the film thickness decreased with expansions in the starved area, however, the thickness was thicker than the surface roughness of the specimens even in the full starvation condition. The shear stress under fully starved condition at 15 N did not depend on shear rate and was a constant value corresponded to almost 50 MPa in all the speeds. In the test at various contact pressure conditions, the film was formed in the contact area at the full starvation condition and the value was higher than the surface roughness. It was also found that shear stress at fully starved condition increased with increases in contact pressure. These results suggest that the solidification film was formed on the contact area in the starved condition and the film formation with high limiting shear stress inhibits direct contact and wear. This lubrication performance offers a stable value of friction coefficient in the tightening process and facilitates precise control of the clamping force.
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