O-Ring Size Standards
Since O-rings can not be made exactly to their dimensions every time, manufactures are allowed to make them within range of their original dimensions. Variations in the rubber compound and in the manufacturing process cause slight variations in the shrink of the material and effect the finished size of the O-rings. Note that O-rings are made larger because they shrink in the mold and during the post curing stage. These variations make it difficult to mass produce O-rings to their exact dimensions so the need for tolerances comes in. Tolerances can be expressed in several different ways. The most common is a "±" figure like 1.239 ± .011. Others state a range that is acceptable for a dimension, like 1.228-1.250 inches. Therefore 1.239± .011 is the same as 1.228-1.250. Tolerances play an important part in seal design but we’ll discuss this in a future article.
O-ring seals are commonly made of rubber but they can also be made with plastic or metal. 36 different types of rubber compounds exist on the market today because of the different temperatures, chemical exposures and environments that O-rings are subjected too. For instance, Nitrile, also called Buna, resists oils and greases very well but will not last when exposed to sunlight or ozone. On the other hand ethylene propylene has good resistance to sunlight and ozone but is not good with hydrocarbon based oils and greases. Temperature range also plays a major role in material selection. Some applications require a material with a low temperature range. An air conditioning unit may see temperature as low as -40°F or more, some Nitrile works to -55°F, while other applications may go as high as 600°F or more. In this case silicone may be a good choice. For more information on material selection see the Material Characteristics Chart in this document.
Now that we went over types of elastomers and temperature ranges there is one more property of the rubber you have to consider when choosing an O-ring -- the hardness of the rubber. Rubber material can be made very soft, a low durometer reading, to very hard, a high durometer reading. The hardness is usually called out in increments of 5 durometer points, for example 60, 65, 70 and so on. The hardness of rubber also has a tolerance of ±5 points. This is due to the fact the hardness is hard to control because of all the variables involved in the compounding and the manufacturing process. You want to know some of the variables? Well, each of the ingredients of the rubber compound vary slightly from batch to batch not to mention the when you mix all the ingredients together to make the rubber compound will also vary from batch to batch. Add this to the variables in manufacturing like temperature of the mold and ovens, time in press and oven and so on can cause the hardness to vary. Therefore manufactures ask for a tolerance of ±5.
Durometer gauge with conveloader stand. The conveloader helps to make the durometer readings more consistent by controlling the force and rate which is applied to the gauge as well as keeping it perpendicular to the sample being tested.
Rubber is made in different hardnesses for several reasons. Some sealing surfaces may not be totally smooth. The little voids, pits and scratches allow a pathway for fluid or air to escape through. Softer materials tend to flow better into these voids and imperfections on the sealing surface creating a better seal. On the other hand, harder rubbers will not do this as well but they do resist extrusion cause by high pressures. Softer rubbers tend to extrude into the clearance between the two parts being sealed when exposed to high pressure causing a failure of the O-ring seal.
Coefficient of friction, either static, breakout or running friction is also effected by the hardness of the rubber. Softer rubber has a higher coefficient of friction, meaning if you take a piece of rubber and try to slide it across the surface of your desk. The higher the friction more force is needed to make it move and keep it moving. Coefficient of friction plays a factor when the O-rings are sealing a part that moves.
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