The 5000-psi hydraulic systems developed by Parker Aerospace for the Boeing 787 and Airbus A350 are significant developments in their own right, but they arguably take a back seat to other hydraulic technologies being introduced on the aircraft. They include a hydraulic pump without bearings and a bleed valve that automatically releases air from the hydraulic system.
The 787 will be the first application for what Parker calls the “fluid film bearing pump.” Instead of employing roller bearings within the pump, which are bulky and cause wear, the pump uses the high-pressure, 5000-psi fluid to run its rotating parts. Through the process, the inner and outer rings of the hydraulic pump rotate around a “fluid film” instead of bearings.
“Finding innovative solutions to reduce the weight, cost, and envelope of our pumps by removing the bearings has been a major objective for years,” said Jon Jeffery, Director of Innovation for Parker’s Hydraulic Systems Division. “In a fluid film bearing pump, the outer ring is stationary and the inner ring is moving at a high speed. Instead of running on traditional roller bearings, it runs on fluid that's between these two simple parts.
"The key is keeping the metals from contacting each other so there's no wear. It runs on a cushion of fluid, basically.”
By eliminating roller bearings, Parker was able to design weight and cost out of the pump. Noise was also reduced because there are no bearings rotating at high speeds. The roller bearings were the longest lead item in the manufacture of the pump, according to Jeffery, so doing away with them lets the company deliver the pumps faster.
The second application for the fluid film bearing pump will be on the A350. It has the same design philosophy as the 787 system but will be a different size and have different components, according to Jeffery.
The second first on the 787’s hydraulic system (and one that will also show up on the A350) is an auto bleed valve based on a “bootstrap” reservoir, which is a closed system in which the fluid does not come into contact with air.
All Boeing and Airbus airplanes have traditionally used bleed air reservoirs that depend on bleed air off the engine. The bleed air is transferred to the hydraulic fluid reservoir, where it pushes and delivers the fluid to the pumps.
One disadvantage of bleed air coming off the engine is that it has to be conditioned. Water has to be taken out of it, and it has to be run through a heat exchanger to cool down the air.
Another disadvantage is that the bleed air from the engine pushes air molecules into the hydraulic fluid and makes it spongy. Because the fluid is super-saturated with air, the bulk modulus of the fluid (a measure of the fluid’s resistance to compression) is reduced, affecting the performance of the flight control actuators.
In addition, bleed-air hydraulic systems require a pre-pump to pressurize the fluid and remove air before it is transferred to the primary pump.
Parker’s bootstrap system for the 787 and A350 eliminates the pre-pump and the bleed-air conditioning systems, saving weight and reducing complexity, and prevents the hydraulic fluid from coming into contact with air.
“We supported Boeing and Airbus in their decision to go to bootstrap reservoirs that automatically remove air out of the hydraulic system,” said Jeffery, who is also Chairman of SAE’s Aerospace Actuation, Control, and Fluid Power Systems committee (SAE A-6). “In a bootstrap system, high pressure off the pump pushes on a small piston that is connected to a large piston, which pushes on the fluid to deliver it to the pump. It’s like pulling yourself up with your own bootstraps, hence the term ‘bootstrap reservoir.’”
Boeing and Airbus had one main concern with the technology. In the event that air somehow got into the system, how would it be expelled? Parker addressed that issue with what it calls the auto-bleed valve (ABV).
Through a series of prisms and a laser beam, the ABV determines whether the reservoir is devoid of air by the way the laser beam bounces. The presence of air in the system would deflect the laser beam in a different direction.
“If air gets up in there, a signal is sent that says ‘open a valve at the top and let that air out,’” said Jeffery.
Such an automatic sensing and air release function is vital because the bootstrap reservoirs are buried in an engine pylon on the wing of the airplane where access is very limited.