Thursday, October 26, 2000
For patients suffering respiratory disorders such as asthma,
bronchitis, and emphysema, fast treatment means fast relief. Because in many
cases inhaling medicine provides more rapid relief than oral medication, the
number of patients using compressor-driven nebulizers that convert liquid
medication into a breathable aerosol is in the millions. And the fact that
government or insurance companies subsidize most nebulizer use creates an
environment where cost is key. So when medical-equipment manufacturer Invacare,
(www.invacare.com) asked Thomas Industries
(www.thomasind.com) for an air pump, it
was no surprise that Invacare wanted a unit with the same air flow performance
as existing designs, but for 25% less cost.
With Invacare's input at the concept stage, Thomas says that it
has achieved a cost-competitive alternative to imported nebulizer compressors
from Taiwan and Italy that have seen some success despite service life issues.
The softball-sized compressor, called Alpha, is the heart of Invacare's Envoy
Jr. Nebulizer Compressor. The design uses snap-fit assemblies, new lubrication
and relief-valve technologies, and engineering materials to contain costs.
Alpha uses a patent-pending connecting-rod lubrication system
instead of the typical oil-saturated wick for lubrication. An integral
pressure-relief valve in the head assembly, also patent pending, simplifies
installation and adjustment during assembly. Snap-fit assemblies in critical
locations result in a unit with only three screw fasteners, and most of the
components are injection molded using materials such as long glass-fiber nylon
from RTP Co. (www.rtpcompany.com) and
other engineering-grade thermoplastics.
The first question engineers had to tackle was how to get equal
airflow at the required pressure, in a substantially lower-cost package,
explains senior key project leader Roy Rozek. "Since pressure and flow more or
less dictate motor size, we had to reduce cost every other place we could."
For example, a connecting rod molded in a moly-lubricated nylon
eliminates a more expensive insert-molded ball bearing and a setscrew by using
the precision rod bore as the bearing surface for the small hardened-steel
eccentric pin. It also reduces costs by allowing use of a 1/4-inch diameter
shaft that's cut to length, rather than one ground to different diameters with a
machined flat for the setscrew.
To achieve the required life, Alpha's connecting-rod lubrication
system traps high-viscosity synthetic grease inside the bore during assembly. To
minimize forces on the eccentric pin and bearing area of the connecting rod,
Thomas chose a small-diameter piston. However, this requires a longer stroke to
maintain efficiency, which sets up the potential of generating relatively high
pressures (60-70 psi) if the outlet is plugged. To eliminate these potentially
damaging pressures, Thomas came up with a patent-pending pressure-relief valve
assembly consisting of an elastomer poppet, spring, and a die-cast zinc control
knob integrated into the head.
"While we didn't follow any formal design-for-assembly
procedures," Rozek notes, "we considered those concepts throughout the design,
and succeeded in producing a product that will be around for many years."