DIAPHRAGM ASSEMBLY
The assembly of a diaphragm unit involves attaching center plates, push rods, springs, and then sealing the diaphragm in the housing.
- Center Plate Attachment:
- Center plates may be attached to the diaphragm by cementing, spinning, riveting or bolting.
- Cement Method: By cementing the center plate to the diaphragm, the potential for a leak is eliminated as no center hole exists in the diaphragm. In some situations cementing is not practical.
- Spinning and Riveting Method: Spinning and riveting are essentially the same technique. Pressure is used to roll over a metal fastener and washer. Each of these methods is widely used and reasonably in expensive. Control of pressure is of prime importance.
- Bolted Method: Bolted center plates are used in units where breaking down and reassembling the finished part may be desirable. This method is more labor intensive and is rarely used.
- Push Rod Installation:
- Push rods are installed by the same method as center plates, except that they are rarely cemented.
- Spring Installation:
- Springs are not usually attached firmly in place; instead, they ride against the inside of the center plate and a predetermined area of the diaphragm housing. Provisions are often made for spring-tension adjustment by using a setscrew arrangement.
- Sealing the Flange Area:
- Bolting, crimping, or clamping techniques can be used to seal the flange area. The correct choice may save future problems.
- Bolting: Bolted flanges are widely used. With this method the diaphragm is bolted between two sealing areas. These areas may be flat, round, serrated, sandblasted or machined to insure adequate gripping surface.
- Torque should be the same on all bolts to create equal pressure over the sealing area. Points of high torque can cut the diaphragm and cause rubber to flow away from the sealing area. Either of these conditions results in leaky units.
- Crimping: Crimping is accomplished by rolling one flange area over the other to exert pressure on the diaphragm and, thereby, effect a seal. Even though crimping is fast and economical, this method can cause many problems which are difficult to correct. Inadequate crimp pressure results in diaphragm pullout; excessive crimp pressure damages the diaphragm.
- In either situation, the faulty part is not discovered until it is tested or placed in service. At this stage, the damage cannot be corrected as it is a sealed unit. Material selection is important when the design calls for the crimped flange.
- Clamping: Clamping is somewhat like crimping except that, instead of rolling the flange, a machine clamp is used. The danger of diaphragm damage with this method is not as great since clamp pressure is more easily controlled than crimp pressure. Damaged diaphragms may be replaced without destroying the entire unit. Clamping, however, is a more expensive sealing method.
- Assembly Problems—Their Causes & Solutions:
- Diaphragm assembly problems usually show up in the form of leaking diaphragms ("leakers"), cutting of the diaphragm edges (crimp cut), excessive diaphragm distortion (wrinkled diaphragm), eccentric diaphragm travel, calibration difficulties, and improperly attached center plates.
- Leakers: Diaphragm damage during assembly is probably one of the most common causes of leakers. Crimp cuts, burred flange and center plate surfaces are all responsible for this kind of problem. Leakers may result from improper sealing between flanges or center plates. Leaks also show up in the form of wicking through the base fabric. Wicking occurs if the rubber portion of the diaphragm is porous or if the hole provided for attaching a center plate and push rod is not sealed.
- Crimp Cuts: Another common source of damage, can result from excessive torque, excessive crimp pressure, non-parallel flange surfaces or lack of flange area to distribute pressure. The danger of torque can be minimized by controlled bolt torque or by specifically designed bolts that will strip at a specified load. Excessive crimp pressure must be controlled by adjusting the tool used to crimp the flange. Non-parallel flanges are more likely to develop when stampings, rather than machined parts, are used. In either case, periodic inspections on the parallel conditions of the flanging area should be conducted. Lack of flange area (or lands) may also cause cuts. This can be corrected by redesign.
- Center Plates: When the center plates are attached with cement, care should be taken with the preparation of the diaphragm surface as well as the cementing procedure. Permanent release agents, such as silicones, should be avoided since they will interfere with the bond between rubber and metal. Any cement used should be resistant to the media in contact with the diaphragm.
- Wrinkled Diaphragms: Wrinkles cause early failure because the diaphragm wears itself out during flexing. This problem is more common in non-molded dish diaphragms. If wrinkling becomes a problem, the designer should consider using a molded diaphragm.
- Eccentric Diaphragm Travel: Eccentric travel can be controlled by a guide system for the push rods or springs. Uneven flange area pressure can also produce eccentric travel.
- Calibration: If the unit does not calibrate properly from one run to another, one of the following could be causing the problem: improper springs, eccentric travel, or diaphragm damage from stretching.
