Abstract: Disclosed is a parallel filtering circuit comprising at least two filters of a depth layer filtering type connected in parallel to each other in conduits showing different fluid resistances respectively, whereby, even when both the filters are clogged with particles of contaminants, the total flow rate may not decrease sharply, so that a preventive measure can be immediately taken before a fatal situation occurs. An orifice may be connected at its inlet to the outlet of the conduit having a smaller fluid resistance and at its outlet to the outlet of the conduit having a having fluid resistance.

Abstract: A filter of depth layer type comprises a generally cylindrical filter body formed by laminating filter elements each in the form of a thin sheet of a filtering material having a central hole, and these central holes provide a conduit extending in the axial direction through the filter body. The filter body has a density distribution in which the density is lowest at the outer peripheral area sewing as an inlet for a fluid to be filtered and becomes progressively higher toward the conduit to serving as an outlet for the fluid to be filtered.

Abstract: A lubrication monitoring apparatus for a machine, in which an oil pressure gauge detects the pressure of lubricating oil of the filter, a first oil thermometer detects the temperature of lubricating oil before lubricating the mechanical section, a second oil thermometer detects the temperature of lubricating oil in the vicinity of the outlet of the lubricated mechanical section, and a warning device gives warning when any of the oil pressure, oil temperature, and oil temperature difference is out of a predetermined range.