Diaphragm Disc Springs

No other spring type can produce load-deflection characteristics with quite so wide a range as the Diaphragm Spring.

What makes Diaphragm Springs so Special?

The diaphragm disc springs' broad spectrum of performance makes it a very flexible element in machine design, and only with the diaphragm disc spring can we obtain horizontal and negative load curve characteristics. There are no standard diaphragm dimensions sets, instead as these disc springs are so versatile and adjustable, manuafacturing is more often done on a case by case basis mostly so that we can achieve extremely tight load tolerances, whilst also ensuring excellent fatigue life.

The Cone Height to Thickness Ratio

The value h₀/t determines the shape of the Load Characteristic as well as the range of the displacement or travel. You will have seen in our sections on conventional disc springs, we always provide the free cone height to thickness ratio/value and referenced the following graphic.

Fig 1.1 - Spring Characteritics for Disc Springs Compliant with DIN-EN 16983.

As you will now appreciate, this is a sub-set of a far greater range and this small set if only those Load Curves we expect when confining ourselves to the constrints of the DIN-EN16983 standard.

Fig 1.2 - The full Range of Spring Characteritics Driven by Unloaded Cone Height to thickness Ratio.

For h₀/t < 0.4, the characteristic is practically linear, as the value h₀/t increases, the curve becomes more regressive. At h₀/t = √2 the curve has a nearly horizontal segment (At h₀/t = √2 ,the load / force reaches maximum and then decreases again). This means that springs can be designed and developed with an almost horizontal characteristic, meaning there is little increase in load as deflection increases.
And now we have some insight into why the standrard defines the Dimensional Series as it does

For the A series, the h₀/t ratio is around 0.4,
for the B series, h₀/t is about 0.75, and
for the C series h₀/t is approximately 1.3.

h₀/t > 1.3 is a threshold beyond which the Standard conforming COnventional Disc Springs do not venture, because beyond here we can have the disc spring invert on itself and have negative travel/displacement.

Enhancing the Load Curve

Typically, Disc Springs are configured in Series to increase deflection, but this has limitiations. Diaphragm Springs overcome these limitations and simultaneoulsy can increase Deflection and reduce Spring Force/Load. This is done by providing the annular disc with fingers (also called tongues) of suitable length, and thus the support leverage and hence the deflection can be extended considerably. Likewise, for increasing Load / Force, Disc Springs are configured in parallel, (again with some limitations and contraining factors, friction being a big one, as well as fatigue), diaphragm springs help over come these limitations and contraints.

Key Dimensions of a Diaphragm Disc Spring

The basic spring element is still the annular portion of the diaphragm spring.

Fig 1.3 - A Conventional Diaphragm Disc Spring.

Some Terminology

>This is useful before we cover some calculation examples.

The Annular Disc is the section in the closed ring, bounded by D_e and D_t
The Fingers/Tongues reach from D_t to D_i, they are levers acting upon the Annular outer section
The Annular Section will be designed with a particular ratio of its own Cone Height (not to be confused with that of the entire spring - h₀) to Metal Thickness) in mind.
Note: If this diaphragm spring, were supported just by the edges of Outer Diameter, with the correct Cone Height to thicknesss ratio, it could be loaded beyond flat, meaning it would have a much larger deflection range with very little load variation - just what aclutch needs!
Sometimes this Diaphragm Spring is simply considered as a conventional disc spring, but with the load acting inside the edges as per the diagram below.

Fig 1.4 - Force Acting inside the edges of a Disc Spring.

In this instance the Force - F_L and Displacement - S_L are calculated as follows:

and

But what of the lever effect when bending the "fingers" and "tongues", so the approach above must be incomplete.

Load and Displacement Calculations

We must account for both the bending of the levers as well as the load acting on the inside of the edges.

where S₁ is the deflection of the Annular Section and S₂is the deflection of the fingers/tongues

where:

Z is the number of fingers/tongues
W₂ is the width of the finger at its base
T is the thickness of the material
E is Young's Modulus of Elasticity for this Material
μ is Poisson's Ratio
C is a Constant, derived from the ratio of W₁ to W₂
F is the Force derived from a function of Metal used, the annulus dimensions and S₁

where k₁ is again to be calculated and used, but of course the determining ratio to use is D_e / D_t

Base Material

We purposefully use Spring Steels in the manufacture of diaphragm springs because of these metals´ physical and mechanical properties of durability, pliability, toughness, and longevity (once heat treated properly!!). By far the most common Spring Steel is 50CrV4, but we also use 51CrMoV, both of whose properties relate to the spring attribute of reacting to loading by elastic deformation. There are instances where requirements such as non-magnetism, the ability to operate in elevated ambient temperatures or highly corrosive environments, require the use of alternative materials.