At some point you may be called upon to make a gasket for something,
anything, and you won't have a pattern to trace it from. You
have to build one from scratch (correctly). Follow these few
steps and you'll look like a pro.
This formula below is done with a simple Texas Instrument
scientific calculator (TI 30), although any scientific calculator
will do.
Chord length = 360° ÷ number of holes =
÷ 2 = sine x diameter
The diameter in the above formula is referring to the bolt hole
centerline, not the O.D. or the I.D. of the gasket itself.
First we'll get our numbers. Let's say you have to build a
standard 8" flange gasket for a pipe. 13½" O.D with an 8⅝"
I.D. and 8 bolt holes ⅞" diameter. The centerline of the bolt holes
must be obtained.
13½ - 8⅝ =
13.5 - 8.625 = 4.875 (difference between O.D. and
I.D.)
4.875 ÷ 2 = 2.4375 (half of the difference)
13.5 - 2.4375 =
11.0625 (diameter of the bolt
hole circle)
Now we can plug some numbers into the formula:
Chord length = 360° ÷ number of holes = ÷ 2 = sine x diameter
Chord length = 360° ÷ 8 = ÷ 2 = sine x 11.0625 =
4.233
4.233" bolt hole center to bolt hole center
*Another thought regarding the formula above:
Make sure the calculator is set to DEG as shown below. The
numbers will not work if it is set to GRAD or RAD.
If you need to find the standard sizes for flange gaskets,
click here.
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