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Stan,
Here's more then you want to know about O-rings but
there have been a few questions about O-rings lately and I got on a role while
typing. ;-)
For sealing water oil or air, Buna is just
fine. It's cheap, works fine and lasts a long time! It's not really
great for weather exposure but most seals see little weather when in use
anyway. When your dealing with chemicals, high heat of very cold
conditions there are better choices. Anything you'll be diving a sub
in is ok with Buna.
There are several manufactures of O-rings but a
Buna ring is a Buna ring, no matter who makes it. "Parker" http://www.parker.com/ead/cm2.asp?cmid=983 is
a very large manufacture and has some good information available for gland size
and the squeeze required for different applications, but so do other
manufactures of O-rings. Here's another company with good groove design
information. http://www.allorings.com/ You need to
chose the proper application type for the proper groove design and ring
size. There are static seal applications (pipe flanges and gaskets).
There are dynamic applications, sliding and rotary. Only use O-rings for slow
rotary applications. Lip seals and face seals are better for rotary
applications such as motor shafts but hand levers and hand cranks are good
rotary applications for O-rings. Also, consider weather you want the ring
to snap in a shaft groove or pop in a housing groove. There are different
diagrams to follow for selecting each application groove and ring size.
O-rings can be purchased from just about any
bearing supplier. A distributor that handles ball bearings usually handles
bronze bearings, lip seals and O-rings. Another easy to use source is
"McMacter Carr" out of New Jersey. http://www.mcmaster.com/ Type in
O-rings and click FIND in the search they provide. They are a
major industrial supplier and handle almost any hardware, seals and
what-nots you'll need to build a sub. They are reliable and
fast. I order O-rings from .500 inch OD to 23 inch OD and get them
the next day UPS. The small ones come in packaged quantities but there
cheep and you'll have spares in case you rip one.
Keep in mind O-rings are stocked in standard
sizes. These sizes are referred to as standard "dash numbers." Each
ring is designated with a three digit dash number. You have to look at a
chart to find the number for the size you need. The dash number tells the
cross section, the first number, and the diameter of the ring, the second
and third number. Here's a chart. http://www.allorings.com/size_cross_reference_framed.htm When
ordering, you need to specify the material type also. A thicker cross
section O-ring is a better choice if you have the room for the larger gland it
requires. The thicker the ring the more forgiving the amount of squeeze
is and still get a good seal.
The clearance between the mating parts is a
factor also. If you have a big clearance between mating
parts your sealing, the O-ring seal can be squeezed out of its groove and
into the excessive clearance between a shaft and housing, by the pressure
your sealing. This type of failure is called extrusion and is more
common in hydraulics where pressures are in the 2000 to 5000 PSI
range. Most sub applications aren't nearly that high so extrusion of the
ring isn't much of a problem unless your sealing high pressure air.
Hope this helps someone,
Dan H.
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