Navy Pilots Catch Their Breath with a
Fighter pilots routinely push both the aircraft’s performance envelope and their own physical limits. The CRU-103 oxygen regulator, with its low-spring-rate electrodeposited nickel bellows, helps give them an extra edge.
Paul Hazlitt, Servometer Corp.
Between the g-sensing air supply valve on the aircraft and the pressure valve in the regulator are highly flexible metal bellows (see Photo 2). As the bellows fill with air and expand, they close the regulator valve and increase the pressure of oxygen going to the pilot. According to Carleton design engineer Jim Talty, the thin-walled electrodeposited nickel bellows provided exceptionally low
Yank and Bank
Positive-pressure breathing to compensate for g-forces delays the onset of gray-out and G-LOC, and makes the anti-g training exercises commonly performed by maneuvering pilots less tiring. Increasing the partial pressure of oxygen in the blood compensates for the reduced blood pressure in the brain. Boosting intrathoracic pressure through breathing oxygen and a counter-pressure vest can hike pilots’ g-tolerance by ~2 g’s. The positive pressure, however, must be matched to the increasing g-load by an automatic regulator with a quick response.
Until recently, Navy pilots used three different oxygen regulators with their life support equipment. The least expensive cost only $300, but it varied breathing oxygen pressure only in response to altitude, not g-force. It could not provide enough oxygen to meet peak demand, and would cause breathing discomfort in pilots when they needed oxygen most. Despite its simplicity, the aneroid (containing no liquid) regulator also proved unreliable and costly to maintain. Two more sophisticated regulators could adjust their output pressures to both altitude and g-forces, but they cost around $4000 each.
The Navy sponsored Carleton’s development of the CRU-103 regulator as part of an Advanced Tactical Life Support System. To meet Navy requirements, engineers in Carleton’s Life Support Group looked for a reliable pressure-translating device that could reduce parts count.
Electrodeposited nickel bellows are widely used as the sensing elements in pneumatic regulators, switches, gauges, actuators, and pressure compensators. Compared with brass and other bellows materials, nickel combines high yield strength (110,000 psi min.) and high tensile strength (125,000 psi min.). The electrodeposition process maintains high chemical purity, and retains the mechanical properties of the metal. The bellows in the CRU-103 oxygen regulator have a projected service life >100,000 cycles.
Electrodeposition routinely produces walls one-quarter the thickness made by mechanical hydroforming. Electrodeposited nickel bellows typically provide one-fifth to one-tenth the spring rate of hydroformed brass bellows of the same size. When the nickel bellows expand, the amount of force lost in stretching the bellows is very low. The force also stays consistent from one regulator to the next.
The bellows assembly used in Carleton’s oxygen regulators is a 0.153-in.-long cylinder with walls 0.0015 in. thick and an o.d. of 0.375 in. The convolution pitch of the bellows is 0.032 in. with a depth of 0.025 in. The assembled regulator measures 3 in. high by 4 in. wide by 2 in. deep.
In operation, the bellows are typically triggered at 3.5 g’s, releasing oxygen under pressure at 0.1 psi. Supply pressure increases gradually to 1.0 psi at 9 g’s. The regulator’s goal is to give pilots exactly the oxygen they need without waste. Pressure to the mask and the compression vest is kept the same to maintain equilibrium inside and outside the pilot’s chest. The valve-actuating end cap soldered onto the bellows makes it easy for life support technicians to adjust the regulator. A maintenance kit includes a replacement bellows, diaphragm, and O-ring.
Paul Hazlitt is Director of Engineering, Servometer Corp., 501 Little Falls Rd., Cedar Grove, NJ 07009-1291; 973-785-4630 or 800-785-0756, fax 973-785-0756, email@example.com.