DIAPHRAGM DESIGN
A diaphragm is a flexible barrier that spans the gap between a moving and stationary member. Its function is to prevent fluid interchange between two separated chambers. Diaphragms may be divided into the following two classifications. Static diaphragms separate two fluids and are subject to little or no motion and/or pressure differential. Dynamic diaphragms act as sealing devices between stationary and moving members; they usually transmit a force or pressure.
- Elastomeric Diaphragms:
- The elastomer or rubber diaphragm can perform many functions over a broad range of operating conditions—from the metering of fuels to the regulating of gas for heating systems.
- Diaphragms generally fall into two broad classifications; they are either solid elastomer or fabric supported. A fabric not only strengthens a diaphragm, it also allows a design engineer to develop a product with minimum thickness for maximum sensitivity.
- The type of diaphragm design and the elastomer and fabric materials specified are of the utmost importance. Equally important is the effective area directly responsible for successful operation of a diaphragm. These factors must be given serious consideration early in the design phase.
- Selection of Material:
- The wide variety of fabrics and elastomers available makes it possible to design a diaphragm for operation over a broad range of conditions. The selection of the correct fabric construction is dependent upon the design of the part and operating conditions. Critical factors include sensitivity, working pressures, distance and rate of diaphragm travel, temperature, smoothness of flange areas, flange pressures and working environment.
- Fabrics:
- The textile industry offers an almost endless supply of fabrics suitable for the manufacture of flexible diaphragms. Available fibers range from cotton and nylon to newer, more exotic materials such as Nomex and Kevlar*. In selecting a fabric, consideration must be given to fundamental requirements of a finished part. Environmental conditions must be well defined; assembly techniques, travel requirements and fabrication problems must be anticipated. A number of factors must be taken into account when selecting a fabric. Some of these are as follows:
- Differential pressure during normal operations determines the required sensitivity. Although a thin diaphragm is more sensitive than a thick diaphragm composed of the same materials, it is weaker. A compromise, therefore, must be sought between the fabric's thickness and strength.
- Flange sealing conditions affect the choice of fabric and coating. One of the most frequent causes for diaphragm failure is a tear at a flange bolt. In applications where flange pressure is low and uneven because of too few bolts or a thin flange cross-section, a stronger fabric must be selected. Flange surfaces may be corrugated or embossed to ensure good sealing properties with minimum pressure. In these reinforced diaphragms, a lighter material, which is more efficient and economical, can be used.
- Anti-wicking products must be used in applications where seepage can occur across a clampening area such as when a diaphragm edge (fabric) is exposed to internal high pressure. Wicking is prevented by subjecting the base fabric to a special process; however, such treatment may reduce the flexibility of the coated fabric.
- The rate of pulsation affects the bond between coating and fabric. If the diaphragm pulsates at high frequency, good adhesion to the base fabric and high-abrasion resistance of the coating are required. In such cases, relatively strong base fabrics are indicated. Also, higher temperature resistance may be required to extend life because of internal heat generation.
- The ratio of effective area to length of stroke indicates whether a flat or molded diaphragm is needed. Molded types are required in applications where a flat diaphragm will not give sufficient travel. For most molded diaphragms, synthetics are usually specified as the substrate because they are stronger for a given flexibility.
- Dimensional stability is vital in all applications, so it is essential for both the coating and base fabric to meet the highest standards in this area. Choose from the following tables for more information:
- - "Typical Fabric Characteristics"
- - "Flammability of Fibers"
- - "General Effects of Heat Exposure on the Strength of Various Fibers"
- Elastomers:
- Since the discovery of vulcanization, great strides have been made in elastomer technology. The most noticeable advancement is the development of new and better polymers. Initially, neoprene and nitriles were available. Then came the silicones, fluoroelastomers, urethanes, and epichlorohydrin and others. Fairprene® elastomer coated fabrics and composites utilize many elastomers. The "Comparative properties of Synthetic Elastomers" compares the basic physical properties of these elastomers.
- Fabric Coating Methods:
- There are several methods for combining an elastomer with a base fabric. The most common processes are: Impregnation, Spread Coating, Dip Coating, and Calender Coating.
- Impregnation: A combination of dip coating followed by a squeezing operation between rolls. This process offers greater penetration of the elastomer into the fabric substrate.
- Spread Coating: In this operation, the base fabric is passed beneath a stationary blade, then through a drying oven where solvent is evaporated. Several passes are usually required to build the coating to the specified thickness.
- Dip Coating: The fabric passes through the a tank containing the coating solution. After immersion, the excess compound is removed by a doctor blade before entering a drying oven.
- Calendering: Fairprene® uses a "Z" type calender, designed specifically for high quality, tight tolerance production. This is one of the most advanced and proficient pieces of coating equipment in the rubber industry. In this operation, a continuous mass of compounded elastomer is formed into two close-tolerance webs and applied to both sides of the fabric simultaneously. State-of-the-art computerized controls monitor and regulate roll gaps, coating weights and thickness to provide exceptional quality and consistency on an ongoing basis.
*Registered trademarks of Du Pont Company