Various filters have been develped for filtering fluid, such as oil in hydraulic systems for example, and the desirability of providing bypass structure to permit the fluid to pass around the filter element when it becomes clogged is generally recognized. A popular approach has been to provide a bypass valve and movable filter element within a housing closed at one end by a cover. The filter element is normally spring-biased toward a position wherein the bypass valve is closed and filtration of the fluid can occur, but the filter element can move to an offset position wherein the bypass valve is open if the filter element becomes clogged and the pressure differential thereacross becomes too high so that fluid can flow around it and continue to circulate through the system without damage thereto. Various indicators have also been developed to signal such bypass conditions so that the filter elements can be changed, with U.S. Pat. Nos. 3,819,052 and 3,827,558 assigned to Donaldson Company, Inc. being representative of fluid filters having this feature.
The movable filter elements of such fluid filters, however, may under certain conditions experience oscillation which, if uncontrolled, may lead to problems. The possibility of filter element oscillation, which has been reported from the field and verified in the lab, increases as the filter element becomes progressively clogged, and is directly related to the sudden change in cross sectional flow area as it moves between the normal and bypass positions. As the filter element begins to move toward the bypass position in opposition to the retaining spring, the flow areea increases which results in decreased pressure drop across the filter element. This reduction in pressure enables the spring to force the filter element back toward the normal position, thereby decreasing the flow area and increasing the pressure drop, which in turn results in oscillation. Such oscillation may accelerate structural failure of the filter element and cause fluctuations in the electrical or visual indications of the filter conditions. Sudden displacements of the filter element by high pressure spikes such as during cold starts or other flow surges can also lead to such problems.
Fluid filters utilizing auxiliary mechanical means disposed between the filter element and the housing have been used heretofore for cushioning shock of the filter element during movement to the full bypass position; however, a need exists for an improved fluid filter incorporating an integral viscous damper to minimize and eliminate oscillation of the filter element.