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  Use (O-Rings)
   
  pic_o-ringsRubber compounds are highly susceptible to extreme temperature limits and harsh chemical applications.
   
 
  Fluids commonly found in contact with O-Rings, in addition to air (a gas), are:
  • water
  • lubricant oils
  • hydraulic oils
  • non flammable hydraulic fluids
  • fuels
  • cooling fluids
  • extinguishing fluids
  • food/beverages
  • medical fluids
  • physiological fluids
  • pure or mixed gases.
   
 
  Swell is a very common occurrence when an elastomer comes in contact with various fluids. High temperature and pressures magnify the phenomenon, which is why these factors must be considered at the design stage. A controlled amount of volume increase of the O-Rings can be beneficial in providing a good sealing effect within the system.
Swelling can often be a symptom of decay of the elastomer; that means that original characteristics of the O-Rings, in presence of a great deal of swelling, can cause rapid degradation. High temperature also has a significant influence.
Some elastomeric compounds experience extraction when brought in contact with particular fluids.
A decrease in volume is generated as a consequence of compound extraction and the original pre-compression is normally lost causing very dangerous result in the decay of original physical properties.
Elements within the atmospheric environment can cause harmful effects on rubber compound.
Most dangerous are: ozone, UV radiation, humidity (their influence shall be described more in detail in storage condition description).
   
 
  Low temperature produces a loss of elasticity and increases the hardness of elastomers. The point at which rubber becomes brittle is normally known as brittle point (ASTM D746, ASTM D2137).
Other common reported low temperature values are TR10 and TR50 (ASTM D1329) which represent temperature of partial recovery of elasticity at low temperature. There is a certain relation between TR10 and brittle point.

Continuous operation at high temperatures induces an increase of hardness on almost all types of elastomers and degradation of physical properties, which grows exponentially. High temperatures also cause irreversible surface damages in the form of cracking and subsequent loss of mechanical resistance. Ageing phenomenon can be delayed or kept at a minimum level by a proper choice of the elastomer and the use of suitable protecting agents. The temperature at which surface damage appear is important to foresee the life of rubber products (ref. GME 60 258 'Accelerated aging test for elastomers).

   
 
  These two physical conditions are antithetical, but have similar effects and limits for O-Ring sealing. An O-Ring used as a static seal can be submitted to continuous pressure of around 1000 bar (at room temperature), provided that gland dimensions and surface finish are suitable and Back-Up rings are utilized. In such applications, elastomer selection must be especially resistant to gaseous diffusion and have a recommended hardness of 90 shore A.
For dynamic applications, O-Rings without Back-Ups can work up to 100 bar. Higher pressures (up to 350 bar) require Back-Up rings of a suitable material for the application environment. Sharp pressure variations can seriously damage O-Rings, particularly elastomers with limited resistance to gaseous diffusion. At high pressure, gases and liquids tend to diffuse into the elastomer and surface during a quick decompression causing blisters and tears (Diesel effect) which destroy the O-Rings.

Proper sealing in a vacuum is quite difficult using O-Rings even if glands are very accurate in design and construction. Gaseous diffusion is a key consideration in such cases. For moderate vacuum conditions (10-4/10-5 torr), acceptable sealing is obtained with common elastomers (NBR, EPDM), for higher values of vacuum, fluoropolymers or HNBR are recommended for their resistance to gaseous diffusion.

   
 
  O-Rings in contact with food products require fundamental considerations:
  • every component of a rubber compound must be tested for potential toxicity and carcinogenity;
  • b) elastomers must not give bad taste or odor to substances with which they come in contact;
  • c) even during periods of prolonged contact, the rubber compound must not release any substance impairing food-stuff characteristics.

Listed are the most recognized organizations and their standards:

USA FDA (Food and Drug Adm.) USA Title 21 C.F.R. 177 2600
USA USDA (Un. States Dept.Agriculture) 3A Sanitary Standard
USA NSF International Standard 51, 61
D BGA Recommendations KTW 1.3.13
UE 82/711/CEE e 90/128/CEE
Italy DM 21/3/73, DL 25/1/92, DM 26/4/93

Organizations and acknowledged standards for drinking water are:

USA ANSI/NSF 61-92 Drinking water system components
G.B. BSI 6920 e WBS (Water Byelaws Scheme)
   
 
  O-Rings are often used in devices for the production of medicinal substances, and in aggregates for control and analysis. For example:
  • medicinal fluids (solutions, fluid mixtures, ect.)
  • physiological fluids (saliva, blood, urine, gastric juices, etc.).

In this sector there are no defined standards, but as a rule the standards and norms to be followed are those dealing with food contact. Exception is made for direct contact with the human body or for human implants, whereby, nomally reference is made to "ad hoc" norms based on medical doctrine and laboratory studies.

   
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