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Friction-Free Diaphragm Keeps the Pressure On.

Clippard Instrument Laboratory, Inc., Cincinnati, OH, a manufacturer of high quality miniature pneumatic products, was asked for assistance in developing a diaphragm to meet the demanding needs of a medical respirator application requiring maintainable output pressure within [plus or minus]1 psi. Clippard teamed up with Dia-Com Corporation, Amherst, NH, whose engineers applied a standard diaphragm seal made of a special tight-weave material to meet the needs of the design.

The pressure regulator in this application converts a high-pressure air source in the range of 80 to 100 psi to a precisely regulated 34 psi output. One end of the piston is exposed to output air pressure and is counterbalanced by a spring. When pressure rises, it forces a piston that opens a valve, releasing air to the atmosphere to reduce output pressure to the specified value. When output pressure is reduced, the piston moves in the opposite direction, opening a valve that increases the supply of pressurized air to the output stream.

The medical device supplier specified that when the unit was turned off and on, pressure would be maintained at the previous level within [plus or minus]1 psi. This high level of accuracy is required because the air is, after further processing, aspirated by the patient. A regulator used in the original design was not able to maintain this level of repeatability because the friction generated by the u-cup seal caused the piston to return to a slightly different position each time the unit was turned on. "The friction between the seal and the bore made it necessary for pressure to rise by a significant level before the piston would begin to move," said Clippard design engineer Rich Humason. "This created a dead band that prevented the piston from returning to exactly the same location."

Clippard considered a number of approaches before selecting a rolling elastomeric diaphragm. Rolling diaphragms are pressure vessels with a variable volume, flexible sidewalls and virtually zero friction because the seal between bore and piston is maintained by a rolling (rather than sliding) action. For this reason, they eliminate many limitations associated with other sealing methods like u-cups, O-rings, metal bellows and flat die-cut diaphragms. Molded diaphragms do not leak, are frictionless, have exceptional sensitivity and negligible hysteresis. They can withstand pressures up to 6,000 psi within a temperature range from -65[degrees]F to 600[degrees]F.

Diaphragm load paths

Almost the entire pressure load in a diaphragm application is supported by the piston head and only a small amount of the liquid or gas pressure is supported by the narrow convolution of the diaphragm. The lines of unit pressure, acting in horizontal planes because they must be normal to surfaces, force the diaphragm against the piston and cylinder sidewalls on that portion of the diaphragm in contact with the cylinder wall and piston skirt. Each line of unit pressure acts normal to the semicircular segment, thus any one of the pressure lines can be replaced by its horizontal and vertical component. The horizontal components, acting in opposition, cancel each other out. The sum of the vertical components of the unit pressures acting on this semicircular segment add up to the total pressure force and is equal to the normal pressure on the projection of this segment.

Buna N or nitrile is the most widely used elastomeric diaphragm material because it offers excellent resistance to fuels, petroleum-based oils, water, hydraulic fluids and alcohols. This material also exhibits low compression set, good abrasion resistance and high tensile strength. Fabric backing materials are available in tensile strengths greater than 7.8 lbs. per inch. The narrow convolution widths, with resulting low stress values in the fabric fibers, enable diaphragms to be used in applications involving high working pressures. As with other pressure vessels, the strength of the diaphragm should be considered with respect to safety factors. The maximum safe working pressure should normally be a fraction of the pressure that would cause failure in the convolution area.

Alternate diaphragm designs

Humason considered several alternate diaphragm designs, including the "top hat" approach which offsets the piston head from the plane in which the outer edges of the diaphragm are connected to the flange that seals like a gasket between the two flat surfaces of the bore and bonnet. This design provides the longest stroke-to-bore ratio, zero spring rate and no breakaway friction and constant effective pressure area. The main drawbacks of this approach are the additional assembly time required when inverting the top head corner radius during installation and an inability to withstand reverse pressure.

The alternate approach selected in this application was one in which the piston and flange are molded on the same plane while molded in convolutions to make it possible for the piston to reciprocate. The benefit of this style is that the handwork of forming the convolution is eliminated, which greatly reduces assembly time. This advantage is particularly important in high volume applications like the respirator. The drawbacks are that the built-in spring rate of the convolution must be considered during the design phase and a limited bore-stroke ratio.

Wide line eliminates tooling cost

Humason talked to several diaphragm suppliers. Many would have needed to build special tooling to produce Clippard's preferred design, thereby driving up the price Clippard would need to charge for its product. Only Dia-Com offered the desired style as part of its broad range of standard products. "Dia-Coin engineers assisted us by helping to engineer minor modifications in their standard product required to meet the precise needs of the application. By purchasing the diaphragm from Dia-Com, we saved a considerable amount of money by eliminating tooling costs and also took advantage of the firm's considerable experience in adapting diaphragms to a wide range of applications." Dutch Schwab, vice president of product engineering for Dia-Com, said that one of the keys to this application's success was use of a special fabric backing. "Elastomers have a tendency to stretch out when they are exposed to a repetitive duty cycle. To prevent this from happening, we selected a special FAO503 polyester fabric with an e xceptionally tight weave of 110 threads per inch. This fabric prevents the elastomer from stretching and helps maintain the repeatability of the application."

Humason reports that the manufacturer of the respirator, who had temporarily switched to a considerably more expensive regulator, was delighted when the new diaphragm-equipped version exceeded their requirements. "Our customer was very happy with the pressure stability provided by the new design. They told us it is considerably more stable than the more expensive design they used in the past." Clippard was happy with the performance as well, and plans to add the Dia-Com diaphragm-based regulator to its standard product line in the near future.
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Article Details
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Author:Boutell, Rich; Comstock, Bud
Publication:Medical Equipment Designer
Geographic Code:1USA
Date:May 1, 1999
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