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ELECTROMAGNETIC
INTERFERENCE (EMI)
When electromagnetic energy from sources external or internal
to electrical or electronic equipment affects that equipment
adversely by causing it to have undesirable responses, such
as degraded performance or malfunctions, the electromagnetic
energy is called electromagnetic interference of EMI, and the
adversely affected equipment is said to be susceptible to EMI.
As explained below, EMI may leave a source or enter susceptible
equipment by conduction, coupling, or radiation. Interference
may occur between one part of the equipment and another, as
between a power supply and nearby circuitry.
EMI is conducted via
signal lines, antenna leads, power cables, and even ground connections, between
EMI sources and EMI-susceptible
equipment.
EMI is coupled between
components, circuits, or equipment having some mutual impedance through which
currents or
voltages
in one circuit can cause currents or voltages in the other
circuit. The mutual impedance may be conductive, capacitive,
inductive, or any combination of these. Conductive coupling
frequently manifests itself as common-mode interference
through a ground return used in common by two circuits. Capacitive
coupling may similarly cause common-mode interference between
two circuits that are not nominally connected. Inductive
coupling may exist between two circuits having self-inductive elements,
if mutual inductance exits between them.
EMI is radiated through
openings of any kind in equipment enclosures: ventilation, access, cable
or meter holes; around
the edges of doors, hatches, drawers, and panels; and
through imperfect joints in the enclosures. EMI may also be radiated
from leads and cables leaving a source, or picked up
by leads and cables entering a susceptible device. (Note that any
good radiator of electromagnetic energy is also a good absorber
of electromagnetic energy, and that an EMI source any
also be susceptible to EMI from another source.)
ELECTROMAGNETIC COMPATIBILITY (EMC)
Man-made EMI sources and nearby EMI-susceptible equipment
may be made electromagnetically compatible by reducing the
EMI from sources, by reducing the susceptibility of equipment,
and by introducing attenuation in all EMI paths between sources
and susceptible equipment.
Reducing EMI from Sources. Figure
1a symbolizes a source of conducted and radiated EMI. Ideally,
reduction of EMI should
begin by designing the source so that it generates less EMI.
The remaining EMI may then be contained within the
enclosure by filtering and shielding. Filters inserted in
each line at
the point where it enters or leaves the enclosure, as in
Figure 1b, will reduce conducted EMI. Radiated EMI may then
be reduced
by shielding, as in Figure 1c.
Reducing EMI Susceptibility.Figure
2a symbolizes an EMI-susceptible device. Total susceptibility may be reduced
by designing the
components and circuits so that the device is inherently
less sensitive to EMI. Conductive susceptibility may then be reduced
by inserting filters in each line at the point where it
enters or leaves the enclosure, as in Figure 2b. Radiative susceptibility
may then be reduced by shielding, as in Figure 2c.
SHIELDING PRINCIPLES
The purpose of electromagnetic shields is to attenuate EMI
between sources and susceptible equipment. One explanation
of how shields work is that EMI fields induce circulation currents
in the shields, and the fields set up by those circulating
currents oppose the EMI fields, so that the net fields on the
'shielded' side are reduced. Another explanation is that shields
attenuate EMI fields by a combination of reflection and absorption.
Regardless of which explanation is the more appealing, the
principles of application of electromagnetic shields are the
same. The law of reciprocity applies, whether the shield is
thought of as containing EMI from a source, or excluding EMI
from susceptible equipment. For a shielded source, the EMI
level outside the shield will be greatly reduced below the
level inside the shield, and all susceptible equipment will
be benefited; for shielded equipment, EMI from external sources
will be reduced to much lower levels inside the shield than
the level outside the shield, and only the particular shielded
equipment benefited.
When an electromagnetic wave impinges on
a shield some of its energy is reflected at the first surface
of the shield, some is absorbed by the shield, and some
is transmitted through the shield. (Some energy may also be reflected at the
second surface of the shield.)
EMI/RFI Gasket Design
INTRODUCTION
Gasket Design is a pragmatic science concerned with the sealing
of joints between mating surfaces to prevent a forcible penetration
of the joint or seam. EMI/RFI gasket design is a special application
of this science, in situations where the penetration of electromagnetic
or electrostatic energy, rather than of fluid, vapor, or gas,
must be prevented. However, since many gasket requirements
involve sealing against penetration by fluids of gasses and EMI/RFI energy simultaneously, a considerable variety of standard
gasket materials that combine both types of sealing has been
made available. All of the design questions and factors involved
in selecting the proper construction and materials should be
established as early as possible, whether for EMI/RFI or pressure
sealing or both. These pertinent factors and their related
materials and design have been assembled in the following procedures.
WHAT DOES A GASKET DO?
Generally speaking, the purpose of an enclosure is to isolate
the environment of the space that it encloses from the external
environment, in one or more ways. Ideally, for maximum isolation
the enclosure should be fabricated from a single, homogeneous
piece of material, without seams, joints, or openings. When
an opening must be provided for periodic inspection, maintenance,
repair, or other purpose, it can be equipped with a cover,
door, window, or panel of the same material as the enclosure,
with a tight, overlapping joint. But no matter how well made,
this joint represents an anomaly in the continuity of the enclosing
surface, and is subject to leakage and forcible penetration.
It is the function of a gasket to seal this kind of joint in
such a way as to restore the complete electrical an/or mechanical
integrity of the enclosure as an essentially on-piece element.
When
two relatively rigid surfaces are brought into firm contact,
slight surface irregularities prevent them from mating completely
at all points. The gaps may be minute, but they provide leakage
paths for gas or liquid under pressure, and for EMI energy,
even when very high closure force is applied. When a gasket
of resilient material is installed between the surfaces, and
closure pressure is applied, the gasket conforms itself to
all the irregularities in both mating surfaces, and accommodates
itself to all of the gradations in local compressions throughout
the joint, thus sealing it completely against penetration by
pneumatic or hydraulic pressure. In the same way, if the resilient
gasket incorporates metal distributed around or through its
volume in mesh or particle form, the joint can be sealed against
penetration by electromagnetic energy, thereby restoring the
conductivity and shielding integrity of the enclosure.
EMI/RFI + PRESSURE GASKETING
Some gasket applications require only restoration of the shielding
integrity of the enclosure, and can be satisfied by a simple
wire-mesh strip, perhaps including a soft elastomer core for
additional resiliency. Cored strips offer a limited amount
of environmental sealing, by positive blocking of dust and
rain. Exclusion of ventilating air or vapor requires some form
of combination strip incorporating a smooth finish, easily-compressed,
atmospheric sealing strip in parallel with the EMI/RFI shielding
strip. When an appreciable pressure differential must be maintained
between the interior and exterior of an enclosure, in addition
to EMI/RFI protection, gasket materials must be used. An elastomer
strip is used in parallel with the EMI/RFI shielding strip,
or the metal mesh is embedded within the elastomer, or the
elastomer is filled with suspended metal particles. These forms
are capable of sustaining high closure pressures, and of maintaining
a seal against high gas or liquid pressure differentials across
the joint, while the metallic element provides the necessary
shielding and conductivity.
PRELIMINARY CONSIDERATIONS
For the vast majority of requirements, it is much more practical,
reliable, and economical to utilize one of the many standard
gasket materials commercially available, than to design one
from scratch. The choice is wide, but each material differs
in its discrete combination of electrical, mechanical, and
chemical characteristics. The requirements of the installation
usually narrow the choice considerably, particularly if the
design of the enclosure is firm. In addition to choices of
size and shape dictated by the enclosing structure and the
joint itself, the following factors will govern the initial
eligibility of commonly available gasket materials:
Shielding Effectiveness
Gasket materials vary in their ability to exclude or confine
EMI/RFI. The intensities and frequencies of the interference
present, the predominance of electric, magnetic or plane
wave fields, and the attenuations requirements may exclude
certain of the available types of materials.
Service Life
Three fundamental consideration are involved here: (1) the
presence of chemical compounds, ozone, and temperature extremes;
(2) the number of times the joint will be opened and closed
during the proposed operating life of the equipment; and
(3) exposure to tools used by maintenance personnel. Resistance
to corrosion or structural deterioration due to environment,
the ability to maintain resiliency and integrity against
repeated compressions, and resistance to impact and abrasion,
are inherent properties of the materials composing the gasket,
and will restrict its applications.
Galvanic Compatibility
When dissimilar metals are placed in contact, in the presence
of moisture containing dissolved materials, they can form a "galvanic
couple", a chemical cell which generates a small local
electric current that will erode one of the metals. This can
be minimized by various means, but the simplest and most effective
is the judicious selection of a gasket material compatible
with the joint material - one that either employs a compatible
metal in its composition, or incorporates non-conductive barriers
that exclude moisture from the metallic region of the gasket.
Compressibility
Sponge elastomers have high resiliency, are easily compressed
at low closure pressures, and conform readily to provide
excellent sealing for rough and uneven joints. Solid elastomers
have lower resiliency, but stand up well to high closure
and environmental pressures and to repeated opening and closing
of the joint; however, they do not actually compress - they
accommodate pressures by changing shape, rather than volume.
This makes an important difference in joint design requirements
for the two compositions, since additional space must be
allowed for the spread of a solid elastomer under pressure.
Manufacturing Limitations
Whether extruded, die-cut, or molded, plastic gasket materials
are limited by the manufacturing process to certain minimum
and maximum cross-sectional dimensions, in commercial production.
The maximum over-all size of die-cut and molded gaskets is
also limited by the size of the presses maintained for these
purposes at the factory. Practical manufacturing tolerances,
of course, will also apply.
Cost
As in any procurement, quantity can have a considerable effect
on gasket cost. It can amortize the cost of special tooling,
and make molded or die-cut gaskets an economical solution
compared to the labor cost of splicing strip gaskets, for
example. But this is only one of many considerations in cost
analysis. Proper attention to gasketing early in the equipment
design phase can result in a much simpler, far less expensive
joint construction, by letting the gasket bear most of the
burden of assuring joint integrity, at a much lower investment
in tooling, materials, and labor. Metex Applications Engineers
are prepared to furnish all the information and advice you
need in evaluating and selecting optimum gasket and joint
expedients for R & D, prototype, or production designs.
The
above six factors are normally the principle considerations
that will govern the preliminary selection of gasket materials
qualified for a given application, from the many standard types
and sizes available. They are each discussed in detail, in
the material that follows, as well as the advisability and
feasibility of custom designs when no standard material appears
to be adequate, or when the advantages warrant the additional
cost.
ADDITIONAL FACTORS
Spliced, Die Cut, Molded, or Vulcanized?
Probably the most economical gasket to use on a one-shot basis
is one made from strip-gasket lengths, butted or spliced
together at corners during installation. For critical EMI/RFI
and pressure sealing requirements, the splices can be vulcanized,
or (for Conductive Elastomer gaskets) bonded with a conductive
adhesive.
Since there is appreciable labor in fabricating
and installing such a gasket, as the number of equipments
utilizing this gasket
increases, a point will be reached where it will pay, both
in man-hours saved, and in joint integrity and reliability,
to specify a pre-spliced gasket. This ensures essentially a
one-piece gasket, obtainable with corner radii at least equal
to the gasket profile width, as required for sound joint integrity.
An even better answer, within the limits of available factory
die equipment is a steel-rule die-cut gasket, cut from a single
piece of EMI/RFI sheet gasket material, to .015" (.38mm)
tolerance, or cut from an elastomer sheet, to which mesh shielding
is then bonded.
Molded gaskets are formed from conductive
elastomers, and feature both the high shielding performance
and stability
of
this material, and dimensional tolerances of *.005" (1.3mm)
in height, and *.010 to .030" (.25 to .76mm) in length
and diameter. Where the additional mold cost can be amortized,
the quality and performance of this gasket seal is unequalled
- except perhaps by a Vulcanized, Conductive Elastomer
gasket. Similar in composition and characteristics to the
molded gasket,
but bonded permanently to one of the mating surfaces, it
provides the finest electrical and mechanical characteristics
available
in material of this type. It must be borne in mind, however,
that vulcanized gaskets can be applied to panels or covers
only at the Metex factory.
Resistivity and Conductivity
As indicated earlier, the metal-to-metal contact, and the ability
of mesh to cut through surface films on the flange surfaces
reduces surface resistivity of wire-mesh gasket of the various
types to microhmcentimeter levels. The continuity of the
mesh within the gasket gives it high internal conductivity,
as well, Conductive elastomers, on the other hand, will exhibit
higher ohmic contact resistance at the interface, as well
as internally, since conduction takes place through adjacent
individual metallic particles. If the joint does not make
a tight seal with the gasket at all points, of course, the
gaps that exist will determine the amount of EMI/RFI and
pressure leakage that takes place, whether mesh or conductive
elastomer is being used. But assuming a full and uniform
seal in both cases, the uniform and contiguous contact of
the conductive elastomer will offer a far smaller maximum-slot
length than is possible with the discrete contact points
of wire mesh, offering higher over-all shielding performance,
and far higher effectiveness in the gigahertz frequency ranges.
Deflection
Deflection is the change in cross-sectional height of a gasket
under compressive load. In sponge elastomers, the cross-sectional
area shrinks, as the height decreases, with a slight increase
in dimension perpendicular to the applied force (in the absence
of lateral confinement), as the load is increased. In solid
elastomers, the cross-sectional area remains essentially
constant, as the height decreases, but the shape changes,
flowing laterally in the unconfined direction, as the load
is increased.
The compressive force required for a given deflection
depends on the hardness of the elastomer - the harder the
material,
the greater the force. Deflection characteristics are provided
in graph form, in the data sheets for each product. The pressure
applied should be sufficient to achieve the required electrical
and mechanical seal; it should not be arbitrarily increased,
and should never exceed the range specified for the material.
Compression Set
When some gasket materials are compressed for a period of time,
they do not return to their original height when the pressure
is removed. The difference in height is known as compression
set, and is indicated in % of gasket height, by a supplementary
curve on the deflection characteristic graph for the material.
The influence of compression set on the resealing capabilities
of the material depends on the joint design, and frequency
of access, and is discussed more fully in connection with
gasket design procedures.
Temperature Limitations
The specified operating temperature range for a given gasket
material should not be exceeded in either direction. At low
temperatures, elastomers become hard and brittle, and may
crumble under pressure. At high temperatures they soften,
and may foam or char. These effects are less drastic in the
absence of air, and longer life is possible under temperature
extremes if the joint design largely excludes air from the
seal region, but longest reliable life is obtained by operation
at intermediate temperatures.
Galvanic Corrosion
As previously explained, two dissimilar metals placed in contact
are capable of acting as an electrical battery in the presence
of moisture containing dissolved materials. A voltage appears
between them, and if a current path exists, a current will
flow and the less "noble" of the two metals (the
one lowest in the electrochemical scale) will erode. This
situation exists wherever a gasket containing one metal is
placed in contact with a flange made of another metal, in
the presence of air containing moisture.
The use of conductive-elastomer
gaskets, or gaskets incorporating elastomer sealing strips
at one or both edges, can exclude
moisture from the joint, or at least confine its influences
to the extreme edges of the seal. If the flange is made from
the less-noble of the two metals, it presents a large area
over which the erosion current is collected, slowing the erosion
rate per unit area. It must be remembered that some metals
such as aluminum and magnesium are subject to ordinary chemical
corrosion, in the absence of any galvanic couple, and that
a small degree of galvanic corrosion may not be separately
detectable, and can be entirely tolerable. Generally speaking,
galvanic corrosion can be eliminated or minimized by taking
appropriate means to remove and exclude moisture from the contact
regions of the dissimilar metals, including drain holes, desiccants,
protective paints and finishes, moisture barrier strips, edge
sealants, etc.
When it is feasible to select the metals used
in the gasket and/or flange, or to surface-coat the flange with another conductive
metal, the Galvanic Series, outlined in Figure 27, should be
used as a reference. Group I in the table is the most noble,
and Group VI the least noble.
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| Figure 27 |
Metals within any one group are fully compatible. There is
overlap between groups, thus metals from adjacent groups are
also compatible. If the two metals to be placed in contact
are not from adjacent groups, compatibility can be achieved
by inserting an intermediate metal. For example, if a Group
II gasket metal is to be used with a Group IV flange metal,
the Group II metal should be plated with a metal that appears
in both Group III and IV.
Outgassing
Elastomers, particularly the conductive silicones, normally
include small amounts of volatile materials, and absorbed
or occluded gases, representing less than 1% of the elastomer
by weight. These materials will be expelled under heat and
pressure. Conventional outgassing tests themselves will cause
this expulsion, with no subsequent occurrence of outgassing.
In situations sensitive to outgassing, the gasket material
can be processed at the factory to minimize the incidence
of outgassing; this may result in a slight change in physical
and electrical characteristics, in some cases.
Manufacturing Limitations
As indicated elsewhere, gasket dimensions are subject to the
limitations of the manufacturing process, and for die cut,
molded, and vulcanized gaskets, to the limits of the presses,
tools, and dies available at the factory.
EMI/RFI/Pressure Gasket Design
Procedure
THE THREE PRINCIPLE DESIGN FACTORS
Operating within the previously discussed design criteria,
three interlocking design factors must be accommodated in arriving
at an optimized gasket design; the joint unevenness, the required
compression pressure, and the gasket height:
Joint Unevenness
The degree of joint misfit, or joint unevenness, was previously
defined as the maximum separation ( H) that exists anywhere
between the two joint surfaces when they are just touching,
shown in Figure 31. Figure 32 shows
the same joint with a gasket installed and compressed; the
dashed lines indicate the full
height of the gasket before it was compressed. The minimum
compressed gasket height (Hmin.) occurs at the point
where the surfaces would touch in the absence of a gasket.
The maximum
compressed gasket height (Hmax.) is at the point
of maximum joint unevenness. Note that the joint unevenness
of the mating
surfaces is equal to this max./min. difference:
H = Hmax. - Hmin.
Here is a specific example: A covered box has a maximum gap
of .020" (.508mm) between the cover and the box lip when
they are just touching, without a gasket. H = .020 (.508mm).
The design engineer specifies a gasket that is .125" (3.175mm)
high (Hg = .125" or 3.175mm). It is compressed
so that Hmin. = .095" (2.413mm) and Hmax. =
.115" (2.921mm).
Then H = .115" - .095" = .020" = 2.921 - 2.413
= (.508mm). Obviously, Hmax. Must be less than Hg,
otherwise there would be a gap, and the gasket would not seal.
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| Figure 31 |
Figure 32 |
Required Compression Pressure
The required compression pressure on a gasket is determined
by three factors: the resiliency of the gasket, the minimum
pressure requited for a seal, and the average
pressure necessary
to fully compensate the joint unevenness:
Resiliency. This is the ratio of gasket compression to the
applied pressure, usually expressed
or
A soft gasket compresses more than a hard gasket with the
same applied pressure: conversely a soft gasket requires less
pressure than a hard gasket to compress the same percent of
gasket height. For example, a sponge neoprene gasket might
compress 10% under an applied pressure of a 6 psi (.422 kg/cm2),
whereas a solid neoprene gasket might require 50 psi (3.516
kg/cm2) to achieve the same 10% compression.
Minimum Pressure for Seal.
As previously indicated, a gasket must at least make full contact at the point
of maximum separation
between mating surfaces (Hg = Hmax.).
In most applications, a stated minimum pressure is necessary
at this point to assure
a seal. In a high-pressure hydraulic system, for example,
hydraulic fluid would blow-by between the flange and the
gasket at point
Hmax. in the absence of pressure across the joint.
The specified pressure at Hmax. must be sufficient
to prevent blow-by under all conditions. For EMI/RFI gaskets,
this minimum pressure
is determined by the pressure required to cause the metal
shielding mesh in the gasket to break through corrosion films
on the
mating surfaces and make a suitable low-resistance contact
with the flange metal. For most EMI gaskets this pressure
is 20 psi (1.406 kg/cm2), but it can be as little as 5 psi
(.351
kg/cm2).
Average Pressure. The
average pressure applied to the gasket must be large enough to compress it
so that the difference
between the maximum and minimum gasket heights is equal
to the joint unevenness (Hmax. - Hmin. = H),
as previously discussed. The maximum gasket height, Hmax.,
is determined by the Minimum Pressure for Seal mentioned
above, as will be seen in the following
discussion.
Gasket Height
The principles involved in meeting these criteria can be seen
by referring to Figure 33. This is a graph of the compression
characteristic for a .250" (6.35 mm) gasket material selected
for use in a joint. Assume that the minimum pressure (Pmin.)
for a reliable seal is 20 psi (1.406 kg/cm2), and the joint
unevenness (H) is .050" (1.27mm).
Taking a minimum pressure
(Pmin.) of 25 psi (1.758 kg/cm2) for safety (satisfying
the first criterion), it is seen that
the gasket height (Hmax.) at this pressure is
.212" -
.050" = .162" (5.385 - 1.26 = 4.125mm). This would
satisfy the second criterion. The pressure corresponding to
this height is seen to be 85 psi (5.977 kg/cm2). Average pressure
is calculated by:
(This assumes a linear compression curve, which is seldom
true. Average pressure computed in this way will be slightly
higher than actual - a good, conservative design procedure.)
The
average pressure is important in estimating the force required
to compress the whole gasket.
In gasket catalog pages, the
compression characteristics are ordinarily given in terms of percent of
gasket
height,
rather than in inches or millimeters, so that one curve
can be used for many actual gasket heights. Figure 34 shows the
curve of Figure 33 replotted in this manner. To distinguish
actual heights from percent height, the latter are designated
by lower-case h. In the example explained above, hmax. would
be 85%, hmin. would be 65%, and h
would be 20%.
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| Figure 33 |
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Figure 34 |
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| Figure 35A |
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Figure 35B |
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| Figure 35C |
|
Figure 35D |
TWO ADDITIONAL CONSIDERATIONS
In introducing practical gasket materials into ideal gasket
design, it is necessary to make allowance for two factors that
are encountered to some degree in all commercial gasket materials:
the manufacturing tolerance on gasket height, and the effect
of compression on the gasket, known as "compression set":
Effect of Gasket Height Tolerance
All gasketing materials required a manufacturing tolerance
on height. Whenever possible this tolerance for Metex products
is stated as nominal height with a plus tolerance, because
a higher gasket provides more actual compression. Figure
35 illustrates in generalized manner the effect of height
tolerance, and how best to allow for it. Figures 35A and
35B show the nominal (solid line) and maximum height (broken
line) compression curves for a relatively hard material.
The joint unevenness is also indicated. The pressures P'min.
and P'max. result if the gasket is compressed
to the same
actual height when a material with the maximum tolerance
on height is used. Notice the unacceptable increase in pressure
that results! However, if the gasket is compressed to the
same pressure, Figure 35B, then the actual compressed heights
will be increased, but the H
remains essentially the same.
Figures
35C and 35D illustrate the same principle for a relatively
soft gasket. This time the increase in required compression
pressure if the gasket is compressed to the same height is
not so pronounced, but may still be unacceptable. Again,
if compression pressures are kept the same, the resultant H
remains approximately the same.
Therefore, compressing a gasket material
with a specified compression force is generally preferred to compressing it
to a specified height.
Compression Set
Some gasket materials take a compression set; that is, they do not return to
the original gasket height after compression. This is illustrated in Figure
36. The compressed height is indicated by the solid colored line. When compression
pressure is removed, the gasket returns to the height indicated by the broken
colored line. This is compression set.
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| Figure 36 |
The importance of compression set depends on how the gasket
is used. Uses are categorized into three classes:
Class A. Permanently Closed - If the gasket is used under
a component that will, in all probability, never be removed,
the compression set is unimportant. It would show up only on
disassembly. This type of joint is called "permanently
closed" and identified as Class A.
Class B. Repeated Identical Open-Close Cycles - Many gaskets
are employed in joints that are frequently opened and closed,
but always in the same manner (e.g., hinged door, or asymmetrical
cover). In this case minimum compressed gasket height will
always occur at he same place. This will also be the point
of maximum compression set. In effect, original gasket height
ahs been reduced at his point, but the gasket is still being
compressed. For instance, Figure 36 shows compression and compression-set
characteristics for a gasket material (set is exaggerated for
emphasis). At hmin. = 70% of gasket height, the compression
is 90%. On repeat cycles the new gasket height (H'g) is .9Hg,
(the original gasket height). And on recompression, the gasket
will be compressed at the point if maximum compression to .7Hg,
but since H'g = .9Hg, the gasket will be compressed to .7 H'g
= .78H'g. In other words, with compression set the gasket is
now being compressed to 78% of its new height at the point
of maximum compression.
Class C. Completely Interchangeable - The problem of compression
set is a good deal more severe when there is a high compression
force, and complete freedom of positioning on repeat closures.
A specific example would be a round gasket in a waveguide that
might be removed and reused in almost any position relative
to points of minimum and maximum compression. The compression
set height at point of maximum compression may actually be
less than minimum compressed height! It would therefore be
possible to have no contact at all between gasket and mating
surfaces at this point.
Again, referring to Figure 36, if hmax. were 92% and Hmin.
70% on the first compression cycle, and if, when the gasket
is removed and replaced in the joint so that the point at which
hmin. (70% compression, 10% set) occurred was now positioned
at the point of hmax. (92%), the gasket would not even contact
the mating surfaces. The example clearly shows that for a Class
C joint with hmax. = 70%, hmin. MUST be less than 90% and preferably
less than 85%.
DESIGN PROCEDURES
Selection of the optimum material will depend on which design
parameters are open to adjustment. Minimum pressure, average
pressure, joint unevenness, and gasket height are intimately
related, and each one is fixed by the assigned or restricted
values of the others. For a given gasket material the (uncompressed)
gasket height is determined as follows:
- Choose a minimum pressure. This is
determined by minimum pressure needed for the seal specified,
plus a safety factor.
- Pick an average applied pressure. This is usually determined
by the total force that can be exerted by the closing
hardware (bolts, screws, latches, 1/2-turn fasteners,
etc.). Assume
that Pav is the mean of Pmin. And
Pmax.,
then Pmax. = Pav +
(Pav - Pmin.). For instance, if
Pmin. =
20 psi (1.406 kg/cm2), and Pav is chosen to
be 50 psi (3.516 kg/cm2), then Pmax. =
80 psi (5.626 kg/cm2).
- Determine the deflection range of the gasket, h,
from the compression curve in the data sheet. Pmax. determines
hmin. and Pmin. determines hmax.;
then h
= hmax. - hmin.
- Determine the required gasket height. The h
in percent of gasket height must be equal to joint
unevenness H
in actual inches or millimeters, which is also the
gasket deflection
range in actual inches or millimeters. By definition:
therefore:
If, for some reason, the derived gasket height is unsuitable,
the pressure factors can be varied accordingly, or a different
gasket material can be investigated.
The interrelationship can be entered in various ways. For instance, the designer
may begin with a fixed average pressure and a specific gasket height, and
investigate various values of joint unevenness to see what
the resultant minimum pressures
would be, or vice versa.
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