THEORY OF OPERATION

of the pump are enclosed by bearing covers. The bearing covers prevent bearing grease from leaking out of the bearing cartridges. In addition, the bearing covers prevent dirt and water, or fuel from entering the bearing cartridges. The outside ends of the bearing cartridges are enclosed by bearing caps. A grease cup and a grease fitting are installed on both of the bearing caps to allow addition of grease to the bearings. Grease reliefs are also installed to release grease during heat expansion. Flexible Coupling. —The flexible coupling is designed to allow for misalignment between the motor shaft and the pump shaft. The coupling hubs are keyed to both the pump and motor shafts and are lubricated to reduce wear in the coupling. THEORY OF OPERATION. —The spinning impeller causes fuel to leave the discharge chamber of the pump. This creates a suction that causes a continuous flow of fuel to the pump. Fuel from the service tank simultaneously replenishes the fuel that leaves the suction chamber as long as the pump has a positive suction head. Centrifugal pumps WILL NOT draw a vacuum. Fuel in the suction chamber enters the center part of the impeller. The blades of the impeller propel the fuel toward the discharge chamber walls by centrifugal force. The expanding spiral shape of the discharge chamber slows the fuel which increases the pressure and creates a continuous flow through the pump. Flow is continuous as long as there is enough fuel at the suction side, air does not enter the pump, fuel discharge is not restricted, and the impeller rotates at the rated speed. MAINTENANCE. —Maintenance on the JP-5 centrifugal service pump is done in accordance with PMS and the applicable technical manuals. Typical maintenance is discussed in the following paragraphs. LUBRICATION. —The importance of proper lubrication of the ball bearings cannot be over- emphasized. But, it is possible to over-grease the bearings, which causes overheating and damage to the bearings. Additionally, the wearing rings and mechanical seals require JP-5 for lubrication. Running the pump dry will damage these parts. WEARING RINGS. —Wearing rings should be inspected when the pump does not discharge at the rated capacity. They are replaced when the radial clearance stated in the pump’s technical manual is reached. MECHANICAL SEALS. —Mechanical seals require no maintenance, but should be replaced whenever leakage occurs, or when the sealing surfaces have been disturbed. TROUBLESHOOTING. —Table 4-2 lists typical malfunctions, probable causes, and corrective action for the JP-5 service pump. Rotary Vane Blackmer is the most commonly used rotary vane pump in the JP-5 below decks system. These pumps come in different sizes with different operating capacities and are used as transfer pumps, auxiliary pumps, stripping pumps, and on the flight deck as defuel pumps. Each pump may vary slightly, but all are practically identical. The Blackmer (fig. 4-10) is a positive displacement, rotary vane type pump. The pumps used for stripping are designed to pump 50 gpm at 50 psi. The pumps used Figure 4-10.—Blackmer rotary vane pump: A. (End view); B. (Side view). 4-13