Quite frequently, even well-experienced design engineers begin to feel doubtful when they have to determine type and dimensions of a specific diaphragm.

Mistakes made therein may rules for designing fabric reinforced diaphragms as presented on the opposite page follow the logical sequence of design procedure and provide useful guidance towards correct diaphragm design. Based on mathemati-cal derivations, these rules have been broadened into the least complicated for-mule of suffication precision for direct prac-tical application .A pattern basing on stroke –to- effecation diameter ratio allows correct determination of basic type of diaphragm with due consideration of functional and manufacturing aspects. Correct determination of the basic type of diaphragm is con-ditional for the correct determination of the resulting dimensions .The rules lead to a basically correct diaphragm design and to housing dimensions which, as a rule , will remain unchanged by the final optimization of the diaphragm itself . Such optimization requires the correct application of numerous differentiated criteria based on practical experience in diaphragm Manu-facture and performance, and should result form close cooperation between design engineers and EFFBE’s diaphragm specialists.

Remarks on Rules for Diaphragm Design:
At least 2 fundamental factors provided by the planned specific function are required to determine suitable type and dimensions of the respective diaphragm. All other fun- damental factors – unless provided by the planned specific function- should be determined in accordance with the rules.

By applying the rules:

An empirical factor is introduced that is to roughly provide for the influence of in- herent resistance on the force transmitted by a diaphragm under differential pressure and for the influence of tensions tensions in the working zone on the theoretically effective area. This factor leads to dimensions DG that will remain unchanged by the optimi-zation of the respective diaphragm. If only 2 fundamental factors are defined in the planned specific function, the coordi- nating factor

Will allow ongoing computation. This co-ordinating factor expresses the optimal relation Hmax: Dw for each basic shape of diaphragm, i.e. largest possible Hmaz depending on given DW or smallest pos-sible DW depending on given Hmax. Deviation form optimal relation Hmax: Dw may be required for functional reasons (larger DW for smaller Pmax or larger F, smaller Hmax given F etc.) and is, of course, allowable within the chosen basic shape of diaphragm.

Computation of diaphragm dimensions must go out form the actually given or chosen fundamental factors.

Diaphragms are hermetically sealing, flexible elements dividing two volumes of interchanging dimensions, interchanging pressures, and usually of different media. Outstanding feature of a diaphragm is its ability to flex in both directions vertical to the sealing flange plane, within predeter-mined stroke limits, flexing effected either by mechanical actuation or by reaction to differential pressure .Flexibility and stroke limits of a diaphragm depend on proper-ties of material used, its dimensions, and-in the larger stoke ranges- on the extent of preshaping provided in its manufacture. Important properties of diaphragm mate-rials are

• Flexibility( for dynamic function )
• Resilience( for sealing function )
• Mechanical strength( for pressure resistance)
• Chemical resistance ( for media exposure)
• Low permeability( for diffusion resistance)

These properties must be maintained for longest possible duration within the Tempe- rapture range, under the differential pressu-res, and in permanent contact with the media specified in accordance with appli-cation conditions.

Basic materials used under these aspects in the manufacture of diaphragms are

• Elastomers
• Elastomers coated fabrics and for additional reinforcements
• Fabrics
• Metals
• Thermo-and Duroplasts as well as
• PTFE (Teflon foil)

For surface protection against extremely aggressive media.

Dimensions and if necessary three dimen-sional shape of a diaphragm must be determined in accordance with specific criteria as to installation and function and with due consideration to the limits set by material characteristics and manufacture technology.

Advantages of Diaphragm Application

Basically, a diaphragm with all its compo- nents is the alternative to a piston equipped with a sliding seal and moving in a cylin- der in comparison, a diaphragm will offer the following advantages:

• Hermetical sealing
• No maintenance
• No lubrication
• No friction
• NO stick-slip-effect
• Low wear
• Long endurance
• Minimal requirements as to material, surface finish and measurement toleran-ces of housing parts.

Diaphragm application is, however, limited by maximum diameter - to- stroke ratio and/or maximum differential pressure.

Diaphragm applications in the field of power transmission are particularly affected there- by In the wide field of regulation perfor- mance basing on differential pressure, diaphragm application provides the import-tant advantage of optimal sensitivity and smooth stroke travel with generally suffi-cient stroke length and pressure resistance.