Hydraulic system with unloading valve assembly

摘要

A hydraulic system having a reservoir fed charge pump supplying pressurized fluid through a filter to a supply pump for direction controlled fluid devices includes an unloading valve assembly on the return passage from the devices for limiting back pressure therefrom as long as the charge pump supplies sufficient pressurized fluid for the supply pump. The unloading valve assembly includes an unloading valve movable in response to the pressurized fluid at the connection between the supply pump and the filter to unload the return passage by connecting it to the reservoir and further to control the pressurized fluid going to the filter by connecting the charge pump to the reservoir at a predetermined maximum pressure of pressurized fluid. As the decvices require more fluid from the supply pump causing the pressure to the supply pump to drop, the unloading valve first blocks the charge pump from the reservoir and then the return passage. After the return passage has been blocked, the pressure therein rises to activate a makeup valve provided in the unloading valve assembly which connects the return passage to between the charge pump and the filter to cause return fluid from the devices to feed directly to the supply pump. The unloading valve assembly further includes a signal valve which responds to a predetermined pressure differential across the filter to provide a signal indicative of a clogged filter and to cause the unloading valve to connect both the return passage and the charge pump to the reservoir.