Load enhancement effects due to polymer thickening in a short model journal bearing

Abstract

A model bearing is described which is 20.0 mm in diameter and 2.5 mm in length; a short bearing of diameter to length ratio eight. The clearance is large (500 μm) and the rotor may be run true or eccentric on its own shaft; in each case the mean load and frictional (tangential) force is measured as the centreline eccentricity is varied. Comparison is made between the lubricating performance of Newtonian and highly elastic liquids; the latter give load enhancement ratios of up to 300 and reductions in coefficient of friction by factors of the order 30. These effects are greatly in excess of those obtained when dealing with bearing of diameter to length ratio close to unity; possible reasons for this are discussed. A Newtonian oil and a polymer-thickened oil are tested in the same way, the latter oil is found to give load enhancement ratios of 1.4 (true rotor) and 3.5 (eccentric rotor) with corresponding reductions of coefficients of friction by factors of 1.5 (true rotor) and 3.0 (eccentric rotor). Such effects had not previously been observed when using oils in the internal cylinder geometry (journal bearing type) although somewhat similar effects have been found in the external cylinder and squeeze film geometries. The rheological properties of the polymer-thickened solutions are measured and the relevance of the results to friction and load bearing discussed.