Check valve for RAT re-stow pump

A pump unit comprising:

• a pump housing containing a tank of pressurised fluid and having an outlet port via which fluid from the tank exits the pump unit;
• and
• a check valve assembly between the tank and the outlet port to regulate flow of the fluid from the tank to the port;
• and
• a pump actuator which cooperates with the check valve to regulate the fluid flow;
• wherein the check valve assembly comprises:
• a valve body having a first end and a second end, the valve body extending along an axis (X) from the first end to the second end such that the second end is lower than the first end;
• a check valve outlet is located between the first and the second end;
• an actuator port provided in the check valve body at a location axially between the outlet and the second end, for fluid connection, in use, to the pump actuator;
• the valve body further comprising a fluid inlet via which the check valve is, in use, connected to a fluid tank,
• the fluid inlet being provided in the valve body towards the second end, axially between the actuator port and the second end;
• the assembly further comprising:
• check valve components located in the check valve body configured to control the flow of fluid entering the check valve from the tank via the fluid inlet, and leaving the check valve via the outlet, according to the first and second stages of operation, the check valve components comprising a first ball located in the valve body adjacent the first end, and a second ball located in the valve body adjacent the second end, and
• a check valve spring provided in spring force engagement with the second ball, the check valve spring being sized and positioned to bias the second ball towards the second end;
• wherein a first stage pressure differential between the actuator port and the fluid inlet creates a fluid flow path from the inlet to the actuator port by causing the second ball to press against the bias of the check valve spring to create a flow passage between the second ball and the second end, and causing the first ball to block the flow of fluid to the outlet, and wherein removal of the first stage pressure differential causes the check valve spring to return the second ball to its biased position to close the passage from the fluid inlet to the actuator port and to cause the first ball to move to open a fluid passage between the actuator port and the outlet;
• wherein pump unit includes one piston, disposed in the pump actuator, and a piston cylinder within which the piston moves between a retracted position and an extended position relative to the cylinder, and whereby, in a first stage of pump operation, the piston is retracted to draw fluid from the tank into the cylinder via the check valve and, in a second stage of operation, after the first stage of operation, the piston is extended relative to the cylinder to eject fluid from the cylinder to the outlet port via the check valve, and
• wherein locating the inlet port at a lower end of the check valve, and fluid flow from the tank to the check valve is assisted by gravity and is thereby independent on a level of fluid in the tank.