The origin, measurement and control of fine particles in non-aqueous hydraulic fluids and their effect on fluid and system performance

When assessing the cleanliness of industrial hydraulic fluids, current procedures focus on particles above 4 µm in size. However, the use of more sophisticated techniques has confirmed that substantial numbers of much smaller particles can be formed in use — mainly by high‐temperature degradation processes. The paper outlines the mechanisms for fine particle generation and procedures for their measurement and control. Significant quantities can adversely affect fluid surface‐active properties and hinder the operation of system components. The effect of flow electrification or applied electrical fields to assist filtration is also discussed. The paper suggests that residual charge could stabilise particle dispersions, assist the depletion of dispersed anti‐foam particles and affect foam stability. An extension of the ISO 4406 cleanliness code to quantify the presence of fine particulate is recommended as is the need for further work to investigate the limits that should be placed on their presence in different applications. Copyright © 2015 John Wiley & Sons, Ltd.