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This is a portion of an Arrow II landing gear hydraulic system diagram.
I noticed that there are two intake check valves, one located on each side of the pump, that would allow flow from the reservoir. Only one side indicates the existence of a filter.
Why do both sides not have a filter if they are both from the reservoir?
This design combines two advantages:
When the flight deck gear selection handle is put in the gear-down position, a switch is made that turns on the electric motor in the power pack. The motor turns in the direction to rotate the hydraulic gear pump so that it pumps fluid to the gear-down side of the actuating cylinders. Pump pressure moves the spring-loaded shuttle valve to the left to allow fluid to reach all three actuators. Restrictors are used in the nose wheel actuator inlet and outlet ports to slow down the motion of this lighter gear. While hydraulic fluid is pumped to extend the gear, fluid from the upside of the actuators returns to the reservoir through the gear-up check valve. When the gear reach the down and locked position, pressure builds in the gear-down line from the pump and the low-pressure control valve unseats to return the fluid to the reservoir. Electric limit switches turn off the pump when all three gears are down and locked.
To raise the gear, the flight deck gear handle is moved to the gear-up position. This sends current to the electric motor, which drives the hydraulic gear pump in the opposite direction causing fluid to be pumped to the gear-up side of the actuators. In this direction, pump inlet fluid flows through the filter. Fluid from the pump flows thought the gear-up check valve to the gear-up sides of the actuating cylinders. As the cylinders begin to move, the pistons release the mechanical down locks that hold the gear rigid for ground operations. Fluid from the gear-down side of the actuators returns to the reservoir through the shuttle valve. When the three gears are fully retracted, pressure builds in the system, and a pressure switch is opened that cuts power to the electric pump motor. The gear are held in the retracted position with hydraulic pressure. If pressure declines, the pressure switch closes to run the pump and raise the pressure until the pressure switch opens again.
Source: FAA
Ideally, a hydraulic system could be a closed circuit without a reservoir. The actuators have two cavities, one to push the piston out, and one two pull the piston in. The total volume is constant, except for the volume of the piston rods.
A practical hydraulic system has to cope with the fluid displacement of the piston rods, leakage, geometric imperfections, and the thermal expansion of both, the equipment and the hydraulic fluid. A reservoir of hydraulic fluid is needed to allow fluid to enter or leave the system. This compensation flow could be very minimal. The fluid also needs to get filtered, because of possible impurities in new oil as well as possible contamination from the hydraulic equipment and conducts.
The shuttle valve is switching the routing of hydraulic fluid:
Without the shuttle valve, there would be no point in the circuit where the main stream of fluid goes only one way. In such a design, there would be no place to put a filter out the contamination from within the system.
There really is no point in this case to install a second filter on the low pressure/return hydraulic line in this system. The point of the filter is to ensure the components of the system are protected. The incoming high pressure line filter will do that. If there are any contaminants in the system, they will return harmlessly to the reservoir and the filter will prevent them from being put back into the system. Also, as Bogl stated, you don't want anything interfering with the emergency extension system, which works by releasing pressure from the system. In that case, the hydraulic fluid returns to the reservoir. Anything blocking it on the return path would prevent that extension. Not good.
