Page 67 - British and Metric Catalog

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Sealing Methods Sealing Methods (cont.)

Tapered vs. Parallel Threads: 24° Metric Compression Fittings
A good first step towards thread identification is to determine whether the thread is parallel or tapered. Parallel means the Most metric threads available from Discount Hydraulic Hose are described as “dual
thread walls are straight; while tapered means the thread walls, if continued lengthwise, would eventually meet. A seal must purpose,” meaning they are capable of tube application as well as port. This also
be used for all parallel fittings in port application. includes Kobelco fittings which are essentially an L-series metric thread with a 1.5mm
thread pitch in all sizes. Most metric and Kobelco threads are tube application capable
This first step will help you eliminate a number of possible thread
types, as well as explain the method of seal. For a parallel thread in because of a 24° tapered throat angle that exists inside the male threads. To achieve
port application, the fitting will usually seal at the top of the thread a tube connection, a tube nut is tightened over the metric threads forcing a ferrule or
by means of an O-ring or seal. With a tapered thread, the fitting cutting ring to grip the tube. By having the nut securely fastened, the ferrule squeezes
typically seals via metal-to-metal wedging of the threads. It is also the tube which holds the tube itself in place.
possible for some tapered males to thread into certain parallel 24° fittings and accessories are made in three different series: LL series for low pressure,
female threads, however, there are many variables involved so L series for medium pressure, and S series for high pressure applications.
caution should be taken. Sometimes a thread sealant is applied
prior to installation to reduce instances of leakage on tapered
threads. 24° Metric Compression Fittings
Below are some examples of tapered and parallel threads that seal in the examples given:
Metric (tube)
Tapered Male X Tapered Female Parallel Male X Parallel Female Tapered Male X Parallel Female Kobelco (tube)
seal via thread wedging seal via O-ring/bonded seal seal via thread wedging Japan Steel Works (tube)

BSPT BSPP M-BSPT X F-BSPP (fixed female)
Metric Taper Metric M-NPTF X F-NPSM (fixed female) Face Seal Fittings
NPT SAE O-ring Boss Discount Hydraulic Hose also stocks Face Seal conversion fittings. This is a very
common fitting in North America known as O-ring Face Seal (ORFS), but is also used
with British threads (HIAB) as well as Metric threads (Metric Flat-Face). Essentially, face
seal fittings seal between an O-ring located in the face of the straight male thread, and
Flared Seat vs. Ball Seat: the flat face seat on the female thread.
The ability to properly identify seats, chamfers, and their corresponding angles
can also be a benefit to overall fitting identification. Flared seats refer to the
protruding “nose cone” on a male thread, or the internal flare on a female thread.
Ball seats however refer to the nose cone inside a female thread (swivel only) that
seals with the bevel / chamfer on the inside of a male thread. As opposed to other
methods of sealing previously mentioned, this form of seal is created between
the flared seats or between the chamfer and ball seat, as is the case in the latter
example.

Below are some examples of seat-to-seat sealing and ball seat-to-chamfer sealing threads:

Flared Seat-to-Flared Seat Ball Seat-to-Chamfer
Sealing Fittings Sealing Fittings
JIS M-BSPP X F-BSPP
Komatsu M-Metric X F-Metric
JIC M-NPTF X F-NPSM

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