The return oil system (Figure 4-55) performs the following functions. 1.) A controlled flow through the housing maintains stable conditions for the internal parts. 2.) The fuel flow cools and lubricates the pump. 3.) It provides automatic air venting of the system.
The return oil vent passage is fed, from the transfer pressure annulus.
Fuel under transfer pump pressure is discharged from the transfer pressure annulus into a vent passage in the hydraulic head (Figure 4-56). Flow through the passage is restricted by a vent wire assembly to prevent excessive return oil and undue pressure loss.
The assembly is made of a hollow screw, into which a “ J” wire is installed.
The amount of return oil is controlled by the size of wire used in the vent wire assembly, i.e., the smaller the wire the greater the flow and vice versa. The vent wire assembly is available in several sizes in order to meet the return oil quantities called for on the specification.
Note that this assembly is accessible by removing only the governor cover. The vent passage is located behind the metering valve bore and connects with a short vertical passage containing the vent wire assembly and leads to the governor compartment.
Should a small quantity of air enter the transfer pump, it immediately passes to the vent passage as shown. Air and a small quantity of fuel then flow from the housing to the fuel tank and via the return line.
Housing pressure is maintained by a spring loaded ballcheck return fitting in the governor cover of the pump.
