|
By Dale T. McGrosky
Introduction to O-rings.
An O-ring is a doughnut, or torus shaped seal typically
used to prevent the passing of air or fluid. In other words, O-rings are
used to keep fluid or air IN or OUT of a defined space. For example, My
underwater camera uses O-rings to keep water out and my SCUBA regulator
uses O-rings to prevent precious air from escaping. O-rings can be used
for more than preventing air or fluid from passing they are also used as
drive belt, decorative objects on furniture, cups and automotive parts and
also used as body jewelry.
How Does and O-ring Seal Anyway?
Before we go over types of materials, hardness, sizes
and tolerances of O-rings lets talk about how the O-ring actually seals.
Basically an O-ring is used to block a pathway that fluid or air may
escape through. O-rings are usually put into a groove to hold them in
place and then squeezed between two surfaces. When you squeeze the o-ring
between two surfaces you are taking up the clearance and blocking the
pathway that the fluid or air wants to escape through. When an O-ring is
squeezed the rubber has a memory. In other words it wants to go back to
its original shape. This memory is how the O-rings seal under low to no
pressure. When pressure is applied to the system this also helps the
O-ring seal by pushing the o-ring against the groove wall opposite the
direction of the pressure and forcing it expand perpendicular to the
direction it is being squeezed by the pressure. Wow, what a mouth full.
Lets see if we can explain that a little more simply. Take a water
balloon, for all practical purposes water is not compressible. When you
squeeze the water balloon between your hands it expand in the opposite
direction. Go ahead and get a water balloon and try it. The O-ring does
the same thing in the groove. Pressure squeezes the o-ring against the
wall of the groove forcing it to expand in the opposite direction helping
the o-ring to seal against the walls of the groove (See the diagram
below). Starting to get the picture?
Sizes
In most cases, O-rings are sized by the inside diameter
(I.D.) then by the cross section (C/S) or width (W). When you call out an
O-ring by its size you would give the I.D. first then the C/S second. Like
this: 1.239X.070 inches. This is also how metric O-rings are called out.
For example: 25.50X2.50. The reason for calling the I.D. instead of the
O.D. is the I.D. is what has the tolerance on it (We'll talk more about
tolerances later). The O.D. is usually given as a reference only.
You may have noticed that on the metric size we only
went to the hundredth decimal place, or 2 digits past the decimal point
(.01) and on the inch size we went to the thousandth decimal place, or 3
digits past the decimal point (.001). A thousandth of a millimeter
(.001mm) would equal .000039 inches which is really too small to be
concerned about for O-rings (See diagram). You can convert inches to mm
and vice versa by these formulas. Inches X 25.4 = MM and MM X
.03937 = INCHES.
O-Ring Size Standards
You probably have run across someone calling for a dash
number like -012 or –213 when they reference an O-ring. These "Dash
Numbers" are actually from Aerospace Standard AS568B that assigns a
dash number and tolerances to a size of o-ring, I.D X C/S. AS568B size
O-rings, commonly referred to as standard size O-rings, are probably the
most common used here in the US and pretty much found everywhere. But, it
doesn't stop here, there are several other size standards throughout the
world such as DIN3771, British Standard (BS), Japanese Industrial Sizes
(JIS-B-2401) and many others.
Tolerances
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. Therefore 1.239± .011 is the same as
1.228-1.250. (Go ahead, do the math, my boss made me double check this.)
Tolerances play an important part in seal design but we’ll discuss this
in a future article.
Type of Elastomers
O-ring seals are commonly made of rubber but they can
also be made with plastic or metal. Several different type 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 for 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, nitrile works to -65°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.
Rubber Hardness
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.
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.
Coefficient of friction, either 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.
Hardness of rubber is measured with a durometer gauge
(see picture above). There are many scales of durometer gauges like type
A, B, C, D, O, OO. Durometer is simply the measure of how far an indentor
penetrates a rubber sample. The softer the rubber the more the indentor
penetrates the rubber shows low durometer reading. The harder the rubber
the less the indentor penetrates the rubber causing a higher durometer
reading. The durometer scales are from 1-100. Glass would give a reading
of 100. Scale A is the durometer used for rubber. Small O-rings can not
accurately be tested using the type A gauge because of the small size and
curvature of the O-ring. So there is an IRHD durometer gauge. This
durometer gauge has a smaller indentor and is more accurate for checking
the hardness of the actual O-ring. Unfortunately you can not convert one
scale reading to another. There is no correlation between these scales.
There are many factors that effect durometer readings.
Temperature, humidity, how much force is applied to the gauge, how fast
the gauge is pushed down and when the reading is taken all play a part in
what type of reading you will get. Many durometer gauge manufactures have
a conveloader to reduce some of the variations by controlling the force
and rate at which is applied to the gauge. But note that it is not
uncommon for two people to get two different reading from the same rubber
sample and gauge. Durometer reading are only accurate to around ±2 so
don't get frazzled when the readings are not the same between you and your
boss.
O-rings come in many sizes, hardness, elastomers and
colors to suite the particular application. If you need further assistance
in choosing an O-ring seal please give one of our friendly customer
service representatives a cal at 800-322-8366 and we will be glad to
assist you.
|
