195
Technical Resources
Fittings Primer
Fittings
Fittings—typically comprised of a nut and ferrule—are designed
to connect and seal tubing. While simple in function, fttings can be
complex in description and use. General descriptive terms include:
the geometry of the receiving port (coned or fat-bottom); the tubing
size for which the ftting is designed; and a description of the threads
on the nut, e.g., 10-32, 1/4-28, etc. Fittings may also be classifed by
dimensionsandbythetypeofmaterial fromwhichtheyaremanufactured.
Additional information—such as tubing and port material, solvent(s) to
beused, andexpectedsystempressure—is requiredtodeterminewhich
fttings are best suited for a particular application
Threads
Several thread sizes are commonly used in analytical fuid transfer.
The most common sizes are 1/4-28, 10-32, and M6. The frst two are
U.S. Customary System measurements. The third, M6, is measured in
the Metric System.
U.S. Customary System
Two numbers are used to describe a thread
size. The frst number indicates the diameter of the threaded portion
of the nut. Thread diameter numbers range from gauge 1 (0.073”) to
gauge 12 (0.216”). Beyond 0.216” the thread diameter is given as the
actual diameter in fractions of an inch. The second number indicates
the threads-per-inch count. Thus, a 1/4-28 nut (Figure 1) has a 1/4”
(0.250”) diameter thread barrel and 28 threads-per-inch. A 10-32 male
nut (Figure 2) has a gauge 10 (0.190”) thread barrel with 32 threads-
per-inch.
Metric System
The Metric System also uses a two number system
to describe the threads. The frst number, preceded by the letter M
(for metric), indicates the diameter of the threads in millimeters. The
second number indicates howmany millimeters between each thread.
When the spacing between threads is 1 mm, the callout for the thread
often excludes that second number. Thus, an M6x1 thread is often
denoted by a simple M6 (Figure 3).
Please see theprevious page for a visual comparisonof common threads.
Stainless Steel Fittings
Although restrictive in use and application (see the Fittings Applications
table, bottom left), stainless steel fttings remain popular for many
analytical applications due to their chemical inertness and high
pressure-holding capabilities.
The dimensions and shapes of stainless steel fttings vary and can be
manufacturer specifc (Figures 4 and 5). Even so, the most commonly-
used stainless steel fttings for chromatography employ 10-32 threads,
allowing many stainless steel fttings to be paired with a variety of
receiving ports prior to being swaged onto a tube.
Tobeusedproperly stainless steel fttingsmust be swaged (permanently
attached) to the tubing they are connecting. To do this correctly, IDEX
Health & Science recommends the following procedure:
Place the nut and ferrule, in that order, on the tubing. Place this loose
assembly into a mating port and tighten the nut finger tight, while
ensuring the tubing is bottomed out inside the port. Now wrench
tighten the nut an additional 3/4 turn.
Please Note:
The ferrule is now
permanently attached to the tubing and should only be used in the port
into which it was swaged. Attempting to use a pre-swaged ferrule in a
receiving port that is different from the one into which it was initially
swaged may result in dead volume or leaks (see the Interchangeability
section, next page).
To properly tighten a pre-swaged stainless steel ftting, IDEX Health &
Science recommends wrench tightening only an additional 1/4 to 1/2
turn past fnger tight. Should any leaking occur, continue tightening the
ftting a little at a time until the leak stops. If the ftting requires more
than one complete revolution past fnger tight, we recommend it be
replaced, as excessive tightening typically indicates adamagedproduct.
Polymer-Based Fittings
Unlike their stainless steel counterparts, polymer fttings are nearly
universal in application (see the Fittings Applications table) and
are comparatively easy to use. Polymer fttings do not permanently
attach to tubing, and they usually do not require any tool (besides
your fngers!) to properly tighten. Additionally, these fttings come in
a variety of polymers, providing several cost, pressure and chemical-
resistance options.
Figure 2
10-32 Nut
Figure 1
1/4-28 Nut
Figure 3
M6 Nut
Fittings Primer
Figure 4
0.210”
(0.533 cm)
0.250”
(0.635 cm)
0.320”
(0.813 cm)
Figure 5
Fitting
Tubing
Port
Recommended?
Plastic
Plastic
Plastic
Yes
Plastic
Steel
Plastic
Yes
Plastic
Steel
Steel
Yes
Plastic
Plastic
Steel
Yes
Steel
Steel
Steel
Yes
Steel
Plastic
Steel
Sometimes
Steel
Plastic
Plastic
No
Steel
Steel
Plastic
No
Fittings Applications
Application Note
FOR ORDERING & DISTRIBUTOR INFORMATION +1 800 426 0191 • +1 360 679 2528 • www.idex-hs.com