Springs used in filters and valves

An adjustable element for use in a filter or valve comprises a coiled spring, of tension or compression type, which can be extended or contracted by suitable means to vary the degree of filtration or flow through the spring. The spring comprises wire of a generally square or rectangular cross-section, the surface of the wire being surface-hardened, eg. by making the wire by cold rolling round steel wire. The interengaging surfaces of the individual coils can be flat so that the coils engage squarely, or curved for tangential contact. In filter use, the smooth outer and inner surfaces of the spring reduce the rate of filter cake build-up, and together with the surface hardening, facilitate the accurate control of the adjustment of the element. When used as a valve, the spring can operate as a one-way valve or the spacing between coils can be adjusted by magnetic means or screw-threaded means. (author)