Soaring Switch Seals

Redesign of aviation switch extends lifecycle twenty-fold

When redesigning hermetic switches for signaling landing gear, flap and rudder positions in commercial and military aircraft, Haydon Switch, Waterbury, CT, sought to extend the life from 25,000 to 250,000 cycles in harsh environments. Among the challenges was the seal. To meet military standards, these switches have to maintain a near-perfect seal despite extreme temperatures and contact with common aviation fluids. 

Says Keith Kowalski, vice president of engineering at Haydon, “We make switches with elastomeric seals, but the really critical applications demand the lowest possible leak rate. You need a metallic membrane to isolate the switch from the outside air.” While metals stand up to fuel, de-icing fluid and other aviation chemicals, flexible metal diaphragms are limited by their fatigue life. The thin metal seals developed microcracks after 25,000 cycles. “Without a hermetic seal, contaminants come in,” explains Kowalski. “These switches typically carry a very low current, so any contamination changes contact resistance.” The longer-life 6600 series hermetic switch was designed to operate below 5 mA at 28 VDC and required a maximum leak rate of 1x10-8 atmosphere/cc/sec. Kowalski notes “At that leak rate, it would take approximately 25 years to evacuate the volume of the switch housing. The return action of the switch was specified to come exclusively from the spring action of the blade. Low spring forces in the seal were therefore important to maintain low actuation forces. “Preferably, we would have had no spring action in the seal at all.” Kowalski adds. Size played a critical role in the redesign, designated to be a scant 0.625- x 0.75- x 0.36-in.

Electrodeposited nickel bellows from Servometer, Cedar Grove, NJ, were selected to meet these stringent specifications. Electrodeposition makes highly flexible bellows down to 0.035 in. dia., and the dynamic properties of nickel can provide an essentially infinite life of 1x1016 cycles. The seamless, non-porous bellows are tested leak-tight to 1x10-9 cc of helium per second. Electrodeposition also makes bellows walls about ¼ the thickness of those produced by conventional mechanical hydroforming. ED bellows consequently provide 1/5 to 1/10 the spring rate or resistance of hydroformed brass bellows of the same size.

As seals, the nickel bellows will not corrode when in contact with most chemicals, nor do they oxidize in air. Haydon’s engineers chose to use them to isolate the interior of the switch housing from the outside atmosphere. Inside the bellows, an actuating pin travels up and down through the top of the housing surrounded by ambient air. Under the pin and between the bellows exterior and sealed housing interior, the switch actuates in an inert atmosphere. Pressure on the actuating pin extends the bellows and actuates the blade to complete the circuit. Releasing pressure on the pin allows the spring-action of the blades to lift the actuator and compress the bellows. The maximum movement differential is just 0.005-in. and minimum release force is 6-oz in the series 6600. 

Initially, designs called for a custom bellows to seal the actuator in the housing. However, Servometer engineers suggested a cost-saving standard nickel bellows. The flexible nickel seal has a 0.25-in. OD and 0.15-in ID, with 0.0015-in. thick walls. “That was our biggest challenge,” recalls Kowalski. “We had some work attaching the bellows to the housing due to the material and the wall thickness of the bellows.” The standard nickel composition lends itself to soldering at both ends of the bellows. With six convolutions, the 0.185-in. long bellows have a maximum extension of 0.024-in. and a maximum compression of 0.032-in. Spring rate is just 23.63 lb/in.

Life cycle testing at temperatures from –65ºF to 400°F yielded results exceeding 500,000 cycles—double the original design goals, and 20 times the life of diaphragm-sealed switches. The new switch withstood random vibration testing to 50.3G at 5-2,000 Hz, and shock testing to 65G minimum peak, 11 ms duration with a saw-tooth waveform. 

Service experience with this switch has been excellent and Haydon is seeking broader markets. “We’re looking at industrial applications in some really harsh environments where someone needs a switch,” says Kowalski.

—SG

For more information:
Haydon Switch,  connect directly to their website via the Online Reader Service Program at www.rsleads.com/204df-336

Servometer, connect directly at www.rsleads.com/204df-336