Parker O-Ring Handbook.pdf

Percent Compression
40%
30%
20%
10%
5%
Figure 2-8: .275 Cross Section
Parker O-Ring Handbook
.275 Cross Section
60
50
70
.1 .2 .3 .4 .5 .6 .7.8.91 2 3 4 5 6 7 8910 2 3 4 5 6 7 89100 2 3 4 5 6 7 891000
Compression Load per Linear Inch of Seal — Pounds
2.4.7 Modulus
Modulus, as used in rubber terminology, refers to stress at a
predetermined elongation, usually 100%. It is expressed in
pounds per square inch (psi) or MPa (Mega Pascals). This is
actually the elastic modulus of the material.
The higher the modulus of a compound, the more apt it is to
recover from peak overload or localized force, and the better
its resistance to extrusion. Modulus normally increases
with an increase in hardness. It is probably the best overall
indicator of the toughness of a given compound, all other
factors being equal.
2.4.8 Tear Resistance
Tear strength is relatively low for most compounds.
However, if it is extremely low (less than 100 lbs./in.)
(17.5 kn/m) , there is increased danger of nicking or cutting the
O-ring during assembly, especially if it must pass over ports,
sharp edges or burrs. Compounds with poor tear resistance
will fail quickly under further fl exing or stress once a crack
is started. In dynamic seal applications, inferior tear strength
of a compound is also indicative of poor abrasion resistance
which may lead to premature wear and early failure of the
seal. Usually however, this property need not be considered
for static applications.
50
60
70
80
Shore A Hardness
90
80
50
60
60
50
70
90
80
70
50
80
90
60
70
90
80
2.4.9 Abrasion Resistance
Abrasion resistance is a general term that indicates the wear
resistance of a compound. Where “tear resistance” essentially
concerns cutting or otherwise rupturing the surface, “abrasion
resistance” concerns scraping or rubbing of the surface.
This is of major importance for dynamic seal materials. Only
certain elastomers are recommended for dynamic O-ring
service where moving parts actually contact the seal material.
Harder compounds, up to 90 durometer, are normally more
resistant to abrasion than softer compounds. Of course, as
with all sealing compromises, abrasion resistance must be
considered in conjunction with other physical and chemical
requirements.
2.4.10 Volume Change
Volume change is the increase or decrease of the volume of an
elastomer after it has been in contact with a fl uid, measured
in percent (%).
Swell or increase in volume is almost always accompanied by
a decrease in hardness. As might be surmised, excessive swell
will result in marked softening of the rubber. This condition
will lead to reduced abrasion and tear resistance, and may
permit extrusion of the seal under high pressure.
For static O-ring applications volume swell up to 30% can
usually be tolerated. For dynamic applications, 10 or 15%
swell is a reasonable maximum unless special provisions are
made in the gland design itself. This is a rule-of-thumb and
there will be occasional exceptions to the rule.
90
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com
Basic O-Ring Elastomers
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