| Observing
prudent guidelines can greatly extend service cycles
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When
designing a diaphragm, always consider what can be done
to extend the life of the part. When you know the inherent
weaknesses of diaphragms, and then design accordingly,
the likelihood of successful applications soars.
The chief contributing factors to premature failure
of a diaphragm are:
- Burrs and sharp edges
- Abrasion
- Back pressure
- Circumferential compression
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Smooth the way with proper hardware
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| Clearly,
the first step is in the hardware design itself. The obvious
considerations are the elimination of burrs and sharp
edges that may come in contact with the diaphragm. Even
seemingly minor flaws can cut and tear at both fabric
and elastomer, resulting in premature failure. |
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The smoother the surface, the better
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| Not
so obvious is the relative smoothness of the hardware’s
finish. When pressure is constantly applied then relieved,
the diaphragm does rub against the supporting hardware.
If the surface of the hardware is rough, it can abrade
the fabric causing an earlier than expected failure. |
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How smooth is smooth enough?
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| It is
recommended that these surfaces be no rougher than 32
micro inches, and if necessary be finished to 16 micro
inches in higher-cycle applications. Although diaphragms
do not require lubrication, they may be coated with a
molybdenum disulfide prior to installation to help reduce
abrasion. The piston may also be coated with Teflon®
to reduce friction when the diaphragm shifts against it,
or with an elastomer coating, which helps prevent abrasion
by preventing the diaphragm from shifting. |
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Proper alignment and manageable back pressure
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The
quickest failure occurs when the sidewall of the diaphragm
comes in contact with itself. When this happens, the two
rubber surfaces lock together while the piston continues
to travel. This generally results in the sidewall of the
diaphragm being jammed between the piston and cylinder
wall, resulting in the elastomer and fabric being torn.
There are generally two causes for this.
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1. The
first is the alignment between the piston and cylinder.
There is usually no problem at high pressure where the
pressure itself equalizes on the diaphragm helping to
center the piston. However at low pressure, gravity can
take over and pull the piston to one side causing a problem.
This can be avoided with a bushing for the piston or some
other way of keeping the piston centered throughout its
stroke.
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| 2. The
second cause of this type of failure is back pressure.
Generally, a diaphragm can take a high differential in
only one direction. If the pressure, gets higher on the
low pressure side of the diaphragm, the sidewall collapses,
causing failure. The problems with back pressure
usually occur when the user is unaware that it even exists.
Since most diaphragm applications are in closed actuators,
there must be a means to adjust for the change in gas
or fluid volume above and below the diaphragm as it is
stroked up and down. |
Back pressure: Beware the one-way street
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| Stress
is usually not a problem on the high-pressure side of
the diaphgrgm, because changes in volume determine in
large part the function of the apparatus. Typically, back
pressure problems occur on the low-pressure side, where
the volume of gas or fluid must be removed and replaced
with each stroke of the diaphragm. |
Alleviating back pressure through ventilation
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| Vent
holes must be sized correctly to allow enough volume to
pass through in the precise amount of time it takes to
stroke the diaphragm. It is also important to remember
this when actuation sequences are increased during accelerated
testing, or whenever the device is under a higher load
of stress or more rapid cycling. |
Corner the dangers of sidewall compression
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| The
final cause of failure is circumferential compression.
This is a term used to describe the larger diameter sections
of the diaphragm sidewall being compressed around the
piston. This results in the sidewall forming an axial
fold in to sidewall to allow the diaphragm to conform
to the piston. Because the fabric used for support has
a square pattern, the folds occur at the four points that
the warp and fill threads are perpendicular to the convolution. |
Minimizing the chance of rupture
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| Circumferential
compression is most often referred to as “four cornering.”
It is not something that can be totally eliminated, but
rather controlled. The continuous folding at the same
location eventually leads to a break in cross thread,
leading to a rupture of the elastomer. |
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Ways to reduce circumferential compression
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| There
are several ways to reduce this circumferential compression.
Each has its advantages and drawbacks. Our engineers can
recommend what's best for you. |
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1.
Limit the stroke, limit the stress
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| The
first way is to use only the bottom half of the diaphragm's
stroke. Using the bottom half of the stroke limits the
section of the sidewall that must be compressed around
the piston to the top. This is the section of the sidewall
with the smallest circumference difference with the piston,
which means the folds will be smaller and not as sharp.
The result of this is longer diaphragm life. |
2.
Double tapering eases compression
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Another
way to reduce circumferential compression—and still
keep the total stroke capability of the diaphragm—is
the double tapered diaphragm.
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On a
standard “top hat” diaphragm, the sidewall of
the diaphragm is a straight line tangent to the flange
and piston radii.
On a double tapered top hat, the sidewall is a line tangent
to the cylinder radius running at a 45 to 60 degree angle
to a point approximately 60 percent of the way through
the convolution width.
At this point it wraps around a small radius then straight
to a point tangent to the piston radius. This makes the
sidewall at a much steeper angle for the usable length
of the sidewall, which in turn reduces the circumference. |
3.
“Pre-convolution” anticipates the problem
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| The
same effect can be obtained in a pre-convoluted diaphragm
by molding the diaphragm as an offset pre-convoluted diaphragm.
Simply put, this is a pre-convoluted diaghragm molded
in the full up position. The benefit is clear: It puts
the total amount of working sidewall at the piston circumference,
virtually eliminating circumferential compression. |
4.
Hardware tapering: A risky compromise
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The
final means of reducing the circumferential compression
is with a tapered piston. This simply increases the
piston circumference as the sidewall circumference increases.
Despite the benefit of adjusting the circumference,
this is probably the least dersirable means to solving
the compression problem for two obvious reasons:
- Circumference adjustment
resulting from tapering decreases the effective pressure
as the pressure rises. (Tapering decreases overall
effective pressure as pressures rise.)
- Tightening the convolution
increases the effective pressure as pressure decreases.
Both pressure-related effects
must be thoroughly considered—and tested—before
this solution is used.
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