The general classes of filter materials are mechanical, absorbent inactive, and absorbent active.
• Mechanical filters contain closely woven metal screens or discs. They generally remove only fairly coarse particles.
• Absorbent inactive filters, such as cotton, wood pulp, yarn, cloth, or resin, remove much smaller particles; some remove water and water-soluble contaminants. The elements often are treated to make them sticky to attract the contaminants found in hydraulic oil.
• Absorbent active materials, such as charcoal and Fuller’s Earth (a claylike material of very fine particles used in the purification of mineral or vegetable-base oils), are not recommended for hydraulic systems.
The three basic types of filter elements are surface, edge, and depth.
• A surface-type element is made of closely woven fabric or treated paper. Oil flows through the pores of the filter material, and the contaminants are stopped.
• An edge-type filter is made up of paper or metal discs; oil flows through the spaces between the discs. The fineness of the filtration is determined by the closeness of the discs.
• A depth-type element is made up of thick layers of cotton, felt, or other fibers.