The Structure of a Diaphragm

 

The type of diaphragm determines to a great extent the performance of the pump.

The patented structured diaphragm makes it possible to significantly reduce the size for a given performance. Diaphragm pumps for compressing and evaluating gases can be classified by the type of diaphragm they employ. It is the diaphragm that determines the performance of the pump. KNF Neuberger’s development of the patented structured diaphragm can only be described as precision design.

 With a diaphragm separating the compression chamber from the mechanical parts and operating practically without friction, diaphragm pumps are completely oil-free. This makes them suitable for applications for which oil-lubricated pumps could not even be considered, for example:

Diaphragm Pump Advantages

Apart from the wide range of potential uses, the diaphragm pump offers further advantages over oil-lubricated pumps. First of all there is the low maintenance requirement. Not only are oil changes not required, but also the simple design and small number of parts, reduce the need for maintenance. At the same time the simple design ensures high reliability. In addition the hermetically sealed compression chamber prevents losses of the gases or liquids being handled, and makes compression and evacuation of expensive, toxic inflammable, radio-active, or otherwise dangerous gases possible. Currently diaphragms can be divided into three groups:

The Flat Diaphragm

The simplest type of diaphragm – the flat diaphragm - consists of a flat rubber plate. It is clamped art its edge between the crankcase and the pump head, and at its center to a rigid metal disc.

Top Surface of a KNF Flat Diaphragm PTFE-Coated with Clamping Disk

A connecting rod imparts movement to the diaphragm. Depending on the expected duty, the diaphragm hay have reinforcing fabric vulcanized into it to carry the forces arising from pressure created in the gas. The advantages of this type of diaphragm in a pump are:

The disadvantages of the flat diaphragm lie in the metal clamping disc and screw head, which are of necessity situated in the compression chamber. Applications with corrosive or aggressive gases demand a better solution.

Especially in the vacuum field a flat diaphragm is not the optimum solution. At   top dead center, the clamp disc and its fastening give rise to gaps and recesses that increase the dead volume, so reducing the volumetric efficiency of the pump. These limitations of the flat diaphragm led to the development of the molded diaphragm.

Molded Diaphragm

In contrast to the flat diaphragm, which is stamped from a flat sheet of rubber, the molded diaphragm is manufactured by pressing the finished part in an axially symmetrical mold. To provide good elastic deformability, the thickness of the diaphragm is reduced towards its circumference. The threaded stud that projects from the lower side of the diaphragm secures it to the connecting rod. The diaphragm presents an unbroken surface to the compression chamber. The contour of the pump here can be made to conform very precisely to the form of the diaphragm at full stroke.

Top view of a PTFE coated molded diaphragm

Top Surface of a
PTFE-Coated KNF Molded Diaphragm

This means that the dead volume can be reduced to a minimum, so that compared with the flat diaphragm a lower ultimate vacuum is achieved. The unbroken surface of the molded diaphragm increases the gas-tightness. Only the joint formed by clamping the diaphragm between the crankcase and the pump head, and the permeability of the elastomer from which it is made, set a limit to the possible gas-tightness. Because the metal parts of the diaphragm are protected by a layer of rubber, the molded diaphragm is particularly important in applications with corrosive or chemically aggressive media. The smooth surface of the diaphragm makes it practicable to coat it, for example, with PTFE, to provide protection against highly aggressive substances. Now, if the components of the pump head are made from PTFE, the resulting pump is reliable, and resistant to chemicals. The advantages of the molded diaphragm are:

Compared to the flat diaphragm, pump, a molded diaphragm pump of the same size and the same design life has a lower flow rate. Whereas, the central region of the flat diaphragm is rigid the pressure in the compression chamber deforms the molded diaphragm much more; this can result in up to 20% reduction in performance. If the stiff central part of the diaphragm were extended too far without reducing the size of the elastic part, the permissible stresses for the diaphragm material would be exceeded.

The KNF Structured Diaphragm

In the development of the structured diaphragm, the primary objective was to combine the advantages of the flat and molded diaphragms, and at the same time to eliminate the disadvantages. This was achieved by analyzing the stresses and led to the development of a structure for the underside of the diaphragm. By stiffening the diaphragm in the center it was possible to reduce the size of the vulcanized-in metal part significantly, compared with the molded diaphragm. These measures produce an average reduction of 15% in mechanical loading in the diaphragm, and reduce the effort needed to turn the pump.

Bottom Surface of aKNF Sturctured Diaphragm

The result is a diaphragm with:

For many applications the miniaturization of the pump that the structured diaphragm makes possible is particularly important. The optimized diaphragm structure makes it possible to design a smaller diaphragm, so that the successor model of a pump with a flow rate of 5 standard litres of air per minute could be smaller in all the main dimensions. This new pump occupies only 63% of the volume of its predecessor. The user can reduce the space for the pump in his product by more than two-thirds. Naturally, when the volume is smaller the weight is reduced as well.

By using the structured diaphragm the desire for increased performance without making the pump bigger can also be satisfied. Compared with a molded diaphragm pump, one fitted with a structured diaphragm can accommodate a longer stroke, with the same elastic deformation of the diaphragm material, and without compromising the mechanical data. At the same time less power is required to drive the pump. Combining as it does the advantages of flat and molded diaphragms, the new structured diaphragm represents a milestone in diaphragm pump technology. Pumps with structured diaphragms cannot only met the demands of existing fields of application better than was previously possible, but also fulfill the needs of completely new applications.

In Summary

 

 

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