Over 20 billion gallons of jet fuel and aviation gasoline fuel are consumed annually by American civil aircraft alone, according to a recent FAA Aerospace Forecast.¹
This number is expected to climb to just under 30 billion gallons by 2037. These volumes are astounding considering only a relatively small segment of the worldwide aerospace market is included in the forecast.
To handle the current volume as well as the anticipated growth, the infrastructure required to handle and transport jet fuel must be incredibly robust and carefully engineered.
Strict Regulations Fuel Innovation
Jet fuel pumping and metering is not only a massive industry, but it is also highly regulated and has stringent requirements. Fuel pumps and meters must meet the standards set forth by organizations such as the American Petroleum Institute (API) in order to be deemed fit for service. The API’s regulations require that fuel pumps must remain leak-free in various demanding scenarios, such as extended exposure to fire.
After all, the consequences of a spill or leakage at any level are very serious. This is why aviation fuel pump and meter manufacturers often opt to use carbon graphite as the material of choice for their pump bearings and seals.
Carbon Graphite Bearings Meet The Challenge
Inside jet fuel gear pumps, carbon graphite bearings are often used to support the drive and idler shafts. Carbon graphite is preferred over most other bearing materials in these pumps for a number of reasons. First and foremost is carbon graphite’s ability to use jet fuel as a lubricant.
Typically, jet fuel has very poor lubricating properties. It has a very low viscosity which results in the formation of an extremely thin hydrodynamic film between the bearing and the shaft. Most bearing materials require a thick film to prevent any atomic attraction that normally occurs when rubbing two metals together.
This attraction can cause galling and/or seizing in application. Carbon graphite, however, is not prone to these same atomic attractions and therefore can benefit from the protection offered by the thin hydrodynamic film formed by jet fuel.
As an added bonus, the self-lubricating properties of carbon graphite allow the bearings to run completely dry for certain periods of time, while most other bearing types can’t perform in these scenarios. This means that there is no need to inspect or replace these bearings if exposed to dry running conditions for short durations. This results in huge savings in maintenance and inspection.
How Carbon Graphite Bearings Fit Gear Pump Housings
Most carbon graphite bearings are press-fit into the housing of the gear pump. This allows for the bearing to exhibit approximately the same thermal growth as the housing. Thus, the thermal growth of the bearing will more closely mimic that of the shaft, as they will both have more metallic coefficients of thermal expansion. This is incredibly beneficial, allowing the running clearance between the shaft and the bearing to remain stable even at elevated temperatures.
Maintaining a tight running clearance (approximately 0.1 - 0.3% of the shaft diameter) throughout operation is critical for a prolonged bearing life and increased pump efficiencies. A closer running clearance between the shaft and the bearing allows for a more precise clearance between the pump housing and the gears.
This added precision means the gap between the gears and the pump housing can be decreased, resulting in less pressure loss and a more efficient pump overall.
In jet fuel pumps and meters, using carbon graphite is more cost-effective and efficient than most other materials. This is evident in the fact that an industry as large as jet fuel pumping consistently turns to carbon graphite for answers to complex problems. Carbon graphite development has made great strides in recent years to keep up with this demand and provide even more solutions to the problems fuel pump and meter manufacturers face as the industry evolves.
If you are facing a design challenge in your fuel pump application, carbon graphite may be the answer.
To get answers now, get in touch with a Metcar engineer. We’re ready to work with you to create a cost-effective and efficient solution to your design challenge.
¹FAA Aerospace Forecast, Page 76