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One of the most popular flange designs in diaphragms
is the Beaded Type. This style of flange enables the
designer to control the amount of “squeeze”
applied to the diaphragm’s flange without concern
for the amount of force applied to the flange during
assembly.
Controlling squeeze avoids the three most common types
of premature failures:
- Not enough squeeze resulting
in flange leakage
- Over-squeezing the flange
and cutting the diaphragm
- Flowing the elastomer into
the working area of the diaphragm causing it to distort
and fail prematurely.
Remember, the benefits of a
sound bead and groove design can be lost if the bead
and the bead groove are not designed as a mating unit. |
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Understanding the deflection factor is key to success
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first consideration is how much to deflect the rubber
to effect a seal. This number may change for some compounds
but generally we recommend a minimum of 20% deflection
of the elastomer (B dim., Fig. 1). This number ensures
that the seal will be maintained even after the elastomer
takes its compression set. Since the flange thickness
and the hardware dimensions need tolerances, the design
should be calculated at 25%, with an over/under factor
of up to 5%. This is usually enough to allow for normal
tolerancing of the hardware to ensure a good seal. |
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Adjust accordingly
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are those situations where the variation in elastomer
thickness, or hardware dimensioning, is such that it is
impossible to keep everything in the range to maintain
the 20% to 30% deflection. In cases such as these we recommend
that the deflection exceeds the 30% recommended maximum
rather than go below the 20% recommended minimum. |
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Rubber does not compress, it flows
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| It must
be remembered, when designing a bead and bead groove,
that the elastomer is incompressible. When you deflect
it 25% to form a seal, the elastomer needs a place to
go. If you haven't provided that room in the groove area
of your hardware, then the elastomer will flow out of
the groove into the working area of the diaphragm. This
can cause cracking in the flange radius area of the diaphragm
or enough distortion in the diaphragm to cause the two
sidewalls to come together, resulting in failure. To avoid
these problems, simply design your bead groove so that
when the hardware is assembled, the volume of the groove
is such that it can contain the largest bead the spec.
will allow (A dim., Fig.1). |
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Designing parts that can be manufactured
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| Another
point to consider when designing beads on diaphragms is
to make it as “manufacturable” as possible.
This will ensure a better product, as well as help provide
better price stability. For example, the main problem
experienced in the manufacture of beaded diaphragms is
air entrapment. This “trapped air” displaces
elastomer in the bead resulting in a reduction of bead
volume. |
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Bead and fabric placement is critical
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of the time, air entrapment is not a problem on standard
“D-Bead” parts where the fabric is on the same
side of the part as the bead. The reason for this is that
the fabric acts as a leak path out of the mold for the
air, enabling the elastomer to completely fill the bead
area. However, when there is no fabric in the bead, as
in a homogeneous part, or one that the bead is designed
onto the elastomer side of the diaphragm, there is no
way of ensuring that all the air will be forced out of
the bead. This is due to the fact that the bead’s
geometry prevents the elastomer from moving in a straight
path (Fig.2) keeping the air in front of it. |
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Designing parting line to avoid air traps
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Solving
the placement problem is relatively simple. We recommend
moving the parting line to the opposite side of the bead
(Fig.3). This enables the elastomer to move in a straight
path keeping all the air in front of it and insuring that
the volume and height of your bead remain constant. There
are no special bead groove requirements for this because
there is no increase in volume of elastomer. It is important
to remember that you are obtaining the seal by deflecting
the bead from top to bottom, not side to side.
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