0 10 20 30 40 50 So 70 Bo 90 100 110 Percent Engine Speed

FIGURE 11 Positive displacement pump output characteristic versus engine flow requirements

FIGURE 12 Main fuel pump assembly (Courtesy Hamilton Sundstrand)

dry cavity that is connected to the engine overboard drain system. For reasons of safety, all fuel and oil passages separated by seals, static or dynamic, must have double seals with a drain port between them.

The type of external spur gear pump element used in main fuel pumps is the fully pressure-loaded type. Figure 14 depicts the basic details of the configuration used. The six parts shown in Figure 14 are assembled into a figure-eight bore arrangement in the pump housing. The four bearing blocks, which are a slip fit, are pressure-loaded towards the faces of the spur gears, and the assembly as a whole is pressure-loaded towards the inlet side of the housing bores. An initial axial sealing force is provided by springs. The axial

pressure blow-off and pressure loading forces are closely balanced. This provides a pressure-balanced seal between the end faces of the spur gears and the bearings. This minimizes leakage over a wide range of delivery pressures on low viscosity jet fuels. The overall arrangement provides the necessary flexibility to maintain the minimum possible leakage clearances between the discharge and inlet over the complete range of operating conditions for many thousands of hours of operating life. The inherent flexibility of the design provides it with the unique capability of automatically compensating for the differing rates of thermal expansion of the materials used in the construction of the pump and the wear that will inevitably occur in usage.

The gears are manufactured from highly alloyed tool steels and surface hardened to resist wear in the harsh environment of low viscosity and low lubricity aircraft gas turbine fuels. The gear profile, lead error, and tooth spacing are held to very low limits. The roundness of the gear journals is also held to a very close limit and a mirror-like finish is applied. These journal dimensional characteristics are required because of the very low film thickness (6 to 30 micro inches or 0.15 to 0.76 microns) encountered. This is attributed to the low viscosity of hot aircraft gas turbine fuel (1 to .5 centipoise). The bearing surfaces, radial and thrust, are a highly leaded bearing bronze alloy generally with a solid film lubricant coating to assist in the initial "bedding" in.

The bearings are designed to provide full film lubrication for all operating conditions with the exception of the starting condition. On some pumps, a hybrid bearing design that incorporates a high-pressure pad to augment the bearings load carrying capacity is used. The bearing thrust faces contain gear trapping relief cuts that control the gear mesh flow dynamics. This includes avoiding any fluid trapping conditions, controlling gear stage inlet and discharge pressure pulsations. Figure 15 is a photograph of a gear set with two of the bearings.

The design of the centrifugal boost stage is quite similar to that used for the airframe boost elements. The inlet must be sized to handle the total inlet flow rate consisting of the vapor phase and the liquid phase. The use of axial inducers has become common on all sizes of pumps because of the severity of the required inlet conditions. The selected overall pressure rise of the boost stage is primarily dependent upon the temperature and, therefore, vapor pressure of the fuel at the entrance to the high-pressure element and the various pressure losses between the boost stage discharge and the high-pressure element

FIGURE 14 Pressure-loaded spur gear pump

FIGURE 15 Spur gear pump gears and bearings (Courtesy Hamilton Sundstrand)

FIGURE 14 Pressure-loaded spur gear pump

FIGURE 15 Spur gear pump gears and bearings (Courtesy Hamilton Sundstrand)

FIGURE 16 Typical pump inlet pressure stack (bar = 0.06895 X psi; °C = (°F - 32) X 0.556; m =ft X 0.3048)

inlet. These pressure losses include filter pressure losses (clean filter and clogged filter), oil-to-fuel cooler pressure losses, and various coring and plumbing pressure losses. A pressure "stack" analysis between the boost stage inlet condition and the gear stage inlet pressure requirement must be made for all operating conditions across the pump input speed range —including the emergency conditions—to determine which operating condition is the critical sizing point for the pressure rise of the pump boost stage. A simplified example of a pump pressure stack is presented by Figure 16. Reference 5 discusses boost impeller sizing. Figure 17 is a photograph of various impellers and inducers that have been used in airframe and engine fuel pumps. The specific speeds in units of feet of fluid, gpm, and rpm range from 500 to 6000 (universal specific speed = 0.18 to 2.2) and suction-specific speeds in the same units range up to 40,000 (14.6).

FIGURE 17 Various impellers and inducers (Courtesy Hamilton Sundstrand)

With respect to boost stages, particular attention must be paid to minimize the conduction of heat from the hot high-pressure stage to the relatively cool boost stage. This is required to avoid the possibility of vapor lock through fuel "boiling" in low-pressure regions of the boost stage under conditions of low fuel burn flow rates and, therefore, low boost stage through flow rates. These conditions generally occur at engine idle and descent operating conditions.

The pump-splined drive couplings are key to the reliability and safety criteria of the pump. Similar to the pump gears, the splined couplings are fabricated from high alloy steels with a surface hardening treatment. The design of the involute splines in terms of profile wear must take into account the lubricant used for the splines and the misalignment imposed by the various drive line elements. A spline design that will satisfy the wear conditions at rated speed will generally meet all overload conditions, including the maximum shaft shear torque requirements. Spline lubricants that have been successfully used include engine oil, fuel, and specially blended greases. Specific spline design parameters must be applied for each lubricant.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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