More than half of the 2 million air compressors now being manufactured yearly depend on two small, intricate pieces of Ryton® polyphenylene sulfide (PPS) engineering resin to operate.
The complex pieces, together as light as a large paperclip, hold the electrical contacts which start and stop the motor to compress air. If those contact carriers fail, the motor won''t start or, even worse, won''t stop when the compressor''s air reservoir is full.
"Some 2 million of these compressors are sold annually for about $1 billion worldwide," said Russ Morphis, Siemens manager of general purpose motor controls. "About 60% use our motor controls."
Siemens Energy & Automation''s Industrial Products Division makes controls for more than 1 million of the 5- to 15-horsepower compressors sold each year. And each of those Siemens controls relies on the Ryton PPS switches for thousands of cycles. Average retail price of a control switch is about $18, Morphis said, and Ryton PPS, a product of Chevron Phillips Chemical Co, accounts for less than 40 cents of that.
"Ryton PPS may cost about 8% more than some other engineering resins that we could have used, but the benefits of Ryton PPS far outweigh any additional cost," he said. The parts are molded by independent contractors, or at one of Siemens'' four manufacturing plants, in Reynosa, Mexico; Osceola, IA; Johnson City, TN, and West Chicago, IL.
Need for a "Snappy" Switch
John Couvreur, the Siemens engineer who designed the compressor motor control switch, explained that electrical switches must open and close nearly instantly to prevent serious equipment damage or dangerous operating conditions.
"If we used some other polymer without the mechanical strength and dimensional stability of Ryton PPS," Couvreur said, "the contact carriers could warp over time from heat generated by constant cycling over the switch''s operating life. That would result in a mushy, rubbery change state -- there would be no clean ''snap'' off and on.
"Switches that are slow to open or close can create a spark -- or arcing -- that could weld the contacts closed, so the motor couldn''t shut down." If over-pressure safety devices failed while the electrical contacts were welded shut, the air tank could explode or the motor burn out.
Each Siemens motor control switch is guaranteed for a minimum 1,000-hour mechanical life, whether it controls a compressor powering an air brush for delicate artwork, heavy-duty equipment to build homes and offices, or even provides air for divers'' SCUBA tanks or to help hospital patients breathe.
Small, Vital Parts
The Siemens motor control -- 3 in. high x 2 in. wide x 4 in. deep -- usually is dwarfed by the motor and air tank, yet it contains some 60 individual components, including the two electrical contact carriers. Although the contact carriers take up less than 1% of the room inside the switch, the entire control is useless if one of the carriers fails.
Using Ryton PPS lets the company make a single, cost-effective, compact, and relatively simple switch to reliably control air compressors ranging from tabletop models to large field compressors.
"That vital need for reliability and toughness under potentially very harsh conditions made Ryton PPS an obvious choice in the switches we make for major compressor manufacturers," Couvreur said. "We selected Ryton PPS for this demanding application because of its rigidity, dimensional stability, insulating properties, and resistance to a wide range of chemicals and temperatures that might be encountered on challenging work sites."
Easy To Mold
Ryton account manager Dean Stewart said Siemens designers also liked the way Ryton''s molding characteristics made it easy to manufacture the small parts.
"At its optimum stock temperatures -- 600°F (315°C) to 620°F (327°C) -- Ryton PPS can flow like water to fill complex molds," Stewart said. "When they cool, the Ryton pieces pop out of molds cleanly, with minimal flash or weld lines.
"Basically, a Ryton part can be installed right out of the mold," Stewart said. "We''ve shown this at our Plastics Technical Center in Bartlesville,OK, where we''ve molded very small parts to very tight tolerances time after time without the shrinkage variability and molding problems that some other polymers suffer."
Operation Determined Material Choice
Couvreur began working on the switch design early in 1994, and considered several materials for the contact carriers.
"I needed an insulating, non-metallic material with the stiffness and temperature-resistance of a die-cast metal," he said. The electrical contact carriers had to be stiff enough to avoid twisting, and still insulate the contacts.
Couvreur first tried glass-filled nylon for one of the contact carriers, but he worried about warping due to nylon''s lack of long-term dimensional stability and its temperature limits. "Nylon also had problems with high humidity," he said. "It could absorb water, affecting dimensional stability."
The Obvious Choice
Based on the switch parts'' stringent design needs, Couvreur called Ryton PPS "an obvious choice. Ryton is more stable than nylon, meets all UL (Underwriters'' Laboratories) and foreign electrical requirements, and provides exceptional thermal performance, so we made one of the two contact-carrying structures in Ryton R-4, a 40% glass-reinforced PPS," he said.
"To make a smoother-operating switch, and to reduce wear at spring and pivot points and other areas, we changed to PTFE-filled Ryton BR42C, for self-lubricated operation," Couvreur said.
Steward explained that Ryton BR42C is a drop-in replacement for R-4, from a shrinkage standpoint. And the tough, self-lubricating material was just what Couvreur needed. "It worked so well that we immediately specified the second part of the switch carrier in Ryton BR42C. We''re very pleased with the results."
Molding Complex Pieces
Once the design was completed, Couvreur and Clem Sitar, another Siemens engineer, began working to solve the engineering challenges.
"We needed to put 50-60 separate pieces within a 4-in. cube, then prove that the product met all of Siemens'' development and engineering standards," Couvreur said. "Without Ryton BR42C, the switch would have had to be bigger and have more parts."
He also praised the material''s moldability as another key advantage in keeping the part cost effective.
"Ryton produces very clean parts with minimal flash, even in the most complex molds," Couvreur said. "That alone reduces or eliminates the need for expensive, time- and labor-intensive secondary processing. Such an operation would have made the switch uneconomical for our customers who make the compressors."
Compressor Challenges
For such simple devices, air compressors are tough to design. They must safely maintain high air pressures within a narrow range, and deliver consistent bursts of air, so that air-driven tools, from art equipment to heavy duty industrial construction equipment, operate properly.
"Especially in the compressors operating between 125-175 psi (pounds per square inch), we need to maintain a pressure-delivery range that will, for example, place a nail at the same depth, shot after shot," said John Deeter, Siemens engineering manager.
"That might require that the compressor motor start at 90 psi and stop at 145 psi, or the operating range could be even narrower, making the motor contact switches open and close more often."
Demands of Different Users
"The capabilities of Ryton PPS make it possible for us to offer our customers a wide variety of options," he added.
Originally introduced in compressors rated to 150 psi, the Siemens motor control switches are being used to maintain higher air pressures to meet the demands of more powerful tools.
Home compressor users, said Deeter, usually demand more precise, constant operations at both ends of the air pressure range: these "hobby" models, operating up to the 140-150 psi range and with smaller air tanks than industrial models, need to keep air pressure more consistent so users can put small finishing nails into a piece of molding the same depth every time.
"A framing carpenter may just want a nail gun to penetrate a 2 x 4 or 4 x 4 and hold the wood together," Deeter added. "He doesn''t care that the nailhead depth into the wood changes by an eighth of an inch, depending on the pressure in his compressor. But the first time a home compressor punches a brad completely through prefinished molding, or leaves the nailhead sticking out a sixteenth of an inch, that homeowner is going to be upset. Our precise, reliable switches help OEMs prevent such complaints about their products."
Siemens Solutions
Siemens designs and manufactures products that help its customers solve their own problems.
"We work with OEMs that are using our switch to solve their manufacturing problems. In response to market demands and price pressures, OEMs are using smaller, more efficient motors. They are reducing component costs and looking for ways to use fewer parts and fewer assembly steps," said Deeter. "Ryton helps us to solve those challenges."
Marketplace Changes
While Siemens had long worked with industrial compressor OEMs and customers, the hobby compressor market has exploded, resulting in a dozen or so major hobby compressor makers with overlapping brand lines.
"We wanted to keep the switch performance the same and still help our customers reduce their costs to be competitive," Deeter said. "We will do whatever we need to do to compete in the switch market."
The marketplace also seems headed toward disposable compressors, he added.
"We have designed our switches to match planned compressor life, but we must make sure that our switches don''t fail before the compressors wear out. The long operating life of Ryton PPS helps us meet those operating-life demands with cost-effective switches.
"With Ryton® PPS, we can design small parts that can be molded in one piece -- parts which might have had to be two or more pieces in another material," Deeter said.
Morphis added, "We face price-minimization pressures along with our OEMs. It''s a competitive marketplace and we have cost-limited goals, so we need to be the value-added supplier. Ryton PPS makes that possible by letting us design cost-effective parts with long operating life."