Tribological Aspects of Operation of Hydraulic Fluids

This paper deals with the effect of a synthetic oil based on synthetic esters on a QHD 17R hydraulic gear pump with a geometric volume of 17 cm³, intended for use with mineral oil, during a 500-hour lifetime test. Changes in kinematic viscosity (a decrease of 1.9% at 40°C and 3.9% at 100°C), total acid number (an increase of 2.6%), ISO 4406 purity code (a change from 23/21/17 to 18/16/13), and the morphology of wear particles were observed and analysed to assess the influence of long-term workload on the hydraulic gear pump. Apart from changes of tribological properties, presence of unwanted particles in oil was evaluated. These particles occurred because of internal wear of the hydraulic gear pump and other hydraulic components. Amount of Copper in hydraulic oil increased by 308% to 0.49 mg·kg-1 and amount of Tin increased by 167% to 3.61 mg·kg-1. Metallic particles emitted this way can produce more wear, as they behave as additional abrasive particles flowing in the hydraulic medium. Although these particles did not immediately reduce the machine’s efficiency during the test, their long-term accumulation may accelerate internal wear and lead to reduced flow performance over time.