how an o-ring functions
\ physical
properties of o-ring compounds \ types of seals \ general design and gland
dimensions \ standard
o-ring sizes and dimensions \ compound
information
Physical
Properties of O-Ring Compounds
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The Shore A durometer is the standard
instrument for measuring hardness of elastomeric Compounds. This instrument
measures indentation hardness, or resistance of the material to the
penetrating force exerted by a spring-loaded pin. Medium hard O-Rings of 70
Shore A are the most widely used. (80 in the case of oscillating or rotary
motion to avoid side motion and bunching in the groove). These give best wear and frictional
properties for running seals. Softer 50 and 60 Shore A rings stretch easier,
exhibit low breakout friction, seal better on rough surface, and need less
clamping pressure. Above 85 Shore A, leakage is greater because of less
effective wiping action, breakage in Assembling small rings can be greater,
and resistance to extrusion is better. Durometer hardness is specified in increments of 10 points with a tolerance of + 5 points, which is considered acceptable from batch-to-batch and allows for experimental error. |
Figure 13 shows the force necessary to compress.139 inch
(3.53mm) cross-section rings per inch of circumference. The force varies
slightly with the Compounds, with the hardness and the slope of the line
varies with the cross-section. The lower the hardness, the less force
required which may or may not be too important for face-to-face static seal
assemblies.
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Figure 14 indicates how the hardness of Compounds from
various polymers decreases on initial heating. Continuous service at
temperature will increase the hardness after initial heating. This may
indicate a need for "back-up" rings, or reduction in the clearance
gap for running seals at high temperatures, or to install harder Compounds.
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TENSILE
STRENGTH, TENSILE STRENGTH is a measure of the mechanical strength of the
material, and is the force required to break a specimen at ultimate
elongation. It is not ordinarily considered an important factor in O-Ring
designs if the Compound has over 1000 psi tensile. This property is useful
for quality control purposes. ULTIMATE ELONGATION - the percent increase in length at the breaking
point over the original length - is useful for knowing how much O-Rings of
various cross-sections and compounds can be stretched without breaking during
installation. Small piston grooves may require nearly 150% stretch to be
safe. For those, compounds exhibiting high elongation may be required.
Elongation also is useful for quality control purposes. MODULUS is an indication of the toughness or resistance to
extrusion of a Compound. High modulus usually indicates a tougher Compound.
Modulus usually is expressed as tensile strength at 100 percent elongation.
Modulus also is useful for quality control purposes. TEAR STRENGTH is another indicator of the toughness of a Compound. This test measures the force required to perpetuate a nick or cut. Materials exhibiting poor tear resistance are not usually recommended for dynamic service. |
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VOLUME CHANGE - When an elastomeric Compound is immersed in
fluid, normally it tends to absorb the fluid, and the accompanying increase
in volume modifies the hardness, resilience, and wear-resistance. This is
acceptable if there is no chemical attack, and excessive dimensional changes
do not produce erosion of physical properties. Compounds with free swell of up to 15 percent may be used safely for running seals; up to 50% for static seals (except crush seals). The actual linear cross-sectional change for a ring confined in a groove with one side in contact with the fluid is less than the free swell of a ring in a beaker of fluid. Figure 15 shows that a free swell of 15 percent results in a 7 percent increase for a confined ring or about .010 inch (.25mm) for a .139 inch (3.53mm) cross-section. |
Swell increases with fluid temperature and
immersion time until it reaches equilibrium. Swelling due to fluid absorption
has the same effect as adding plasticizer. It makes the seal more flexible at
low temperatures if the fluid has good low temperature properties. Excessive
swell can lead to extrusion, wear and short seal life. Some volatile fluids tend to extract the plasticizers, which make compounds shrink. Evaporation or "dryout" during idle periods may cause shrinkage. Shrinkage of more than 3 percent to 4 percent may cause running seals to leak. Any combination of shrinkage or low temperature contraction that causes the seal to lose squeeze will result in low-pressure leakage. |
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Compression Set Compression set is a very important
sealing factor. It is a measure of the expected loss of resiliency or
"memory" of a Compound. Compression set is determined by
compressing an O-Ring between two plates that are heated for a specified
time. When released, compression set is calculated as a ratio of
non-recovered thickness, expressed as percent. Figures 16 through 19 show the sets taken by Compounds of various polymers. The values vary from cross-section to cross-section. In the case of the .139 inch (3.53mm) section Nitrile O-Ring shown here, the set is 32% at 250°F, its top recommended temperature. Thus, the squeeze is reduced .139 x .075 or about .0105 inch (.26mm). The initial minimum squeeze was .009 inch (.23mm), but the swell caused by fluid is 7 percent or .010 inch (.25mm), so there still is a positive squeeze of about .0085 inch and the seal will not leak. Such calculations for Compounds of other polymers at other temperatures enable one to predict at what operating temperatures leakage may occur and suggest squeezes necessary for prevention. |
Low Temperature The combination of low temperature and low
operating pressure is hard to seal. A soft Compound should be used to provide
as much resilience as possible. At temperatures below -65°F soft Silicone
should be used. Compounds of other polymers are simply too stiff for colder
temperatures, particularly for air or gases. Some Neoprene Compounds tend to
crystallize at low temperatures, become rigid, lose their resilience, and go flat
in the groove. On heating to above the crystallizing temperature, they regain
their original properties, but each such exposure causes crystallization to
take place more rapidly. Neoprene seals, used in air conditioning systems to
resist Freon, may last through the first summer's exposure and fail later due
to lack of pressure in the system during the off-season. Low temperature flexibility may be
affected by extraction of the low temperature plasticizers used to obtain low
temperature resilience. If these are replaced by the system's low temperature
fluid, the seal may be more flexible than before. However, if the fluid is
allowed to dry during the off season, the seal may leak temporarily until it
re-absorbs the fluid at the start of the next season. Changes in physical properties at low temperature are not of a permanent nature. Unlike high temperature affects, low temperature changes can be reversed once the compound is warmed up. |
> FIGURES 16-19
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High Temperature Every Compound has a definite limit for its maximum operating temperature. The closer this approaches its original curing or vulcanizing temperature, the greater is the tendency to further cure the material, resulting in hardening, cracking, loss of resilience and ultimate leakage. In setting operating temperature limits, engineers are conservative on both the high and low temperature sides. Brief exposure to higher-than-recommended temperatures may not be harmful, but constant exposure is sure to result in short seal life. |
In general, Nitrile and Neoprene compounds
should not be used above 250°F maximum; and the EPRs may be used to 300°F;
the Polyacrylates to 350°F; the Silicones and the Fluorocarbons to 450°F for
a good life at continuous duty. Compounding Nitrile elastomers for maximum
low temperature results in a loss of about 25°F in the high temperature
resistance. The higher the operating temperature, the more some Compounds soften. Take this thermal effect into account when specifying radial clearance gaps. Any Compound that is used close to its maximum operating temperature should have a clearance gap compatible with its hardness as its actual operating temperature rather than at room temperature (see Figure 14). Fluorocarbon Compounds are an example of this, and explain why so many 90 hard Compounds are used to prevent extrusion at high temperatures. They initially soften to 60 or 70 hard at 400°F to 500°F before becoming harder in time. They have extremely good compression set and improved high temperature performance. |
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Steam and hot water are tough on
elastomeric Compounds. Most become hard, brittle, take a set, and lose their
resilience from further re-curing or become soft and putty-like for de-polymerization.
Severe degradation and hardening at over
180°F has prevented the use of O-Rings for steam and hot water valves and
fittings, railroad couplings, heat system equipment, coffee machines, and
many other applications. The Ethylene Propylene rubbers and Aflas* provide a
real breakthrough. Now O-Rings may be used to cheapen and simplify rotating
and static seals for such equipment. Permeability Gases diffuse into and through elastomeric Compounds, depending on the denseness of the base polymer molecule, the amount of fillers used, and the state of cure. |
Generally, harder Compounds which have
more carbon black added have lower diffusion rates. Of the popular Compounds,
Butyl has the lowest permeability followed by Fluorocarbons, polyurethanes, Hi Acrylonitrile
Buna N's, Neoprene, Low Acrylonitrile Buna N's, Polyacrylates, Buna S, and
Natural Rubbers. The Fluorosilicones and Silicones have such high rates that
they are not used to seal gases, particularly at high temperatures and
pressures. For any given Compound, the permeability
through the O-Ring depends on the amount of its compression or squeeze, the
area of the seal, and the pressure, temperature and molecular weight of the
gas being sealed. |
AGE CONTROL
SAE ARP5316, Storage of Aerospace Elastomeric
Seals prior to hardware assembly.
The Society of Automotive Engineers has a more accurate model to determine shelf life of elastomer-type seals. ARP 5316 extends the shelf life of elastomeric seals based on extensive studies. It addresses the needs of contractors and alleviates the confusion created by the cancellation of MIL-STD-1523A. Therefore, ARP5316 is being adopted.
This recommended practice discusses elastomeric seal components and offers a control document for those which require one. It includes recommended shelf life limits that are consistent with information from summative studies on age control.
American Seal Inc. has adopted ARP 5316 for a more realistic expectation for age control of elastomeric seals. Customers and o-ring seal manufacturers alike have contacted us and reported that they will move to the new industry criteria.
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5 to 10 Years
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Nitrile |
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Up to 20 Years
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Silicone |