Hydraulic partial-flow filters from STAUFF have the task of filtering the pressure fluid in the hydraulic tank in a separate circuit and relieving the main flow filters in order to increase the availability of a machine while reducing operating costs. Partial-flow filters operate independently of the actual system operation. The long contact time of filter material and fluid achieve a high level of filter efficiency. In contrast to filters in the main flow, the machine does not have to be switched off for replacing the filter.