direct thread fittings
Direct fittings are used in various industries including oil,
gas, petrochemical, shipbuilding and aerospace industries.
(UNC, UNF, BSP, fittings are well-known direct hydraulic connections)
The most common types of connections are UNC, UNF, BSP, and so on.
Because the thread is straight and the thread surface is not fully involved,
sealing is done with other mechanisms.
Gaskets, O-ring, or certain other materials are used to seal these connections.
For direct connections,
the use of sealants such as teflon strips is not required and is not recommended.
The use of lubrication is recommended based on the fineness and quality of the fabrication.
Below is the introduction of these threads.
4-1 Standard threads used in direct fittings
Worldwide, direct thread connections are very common, but all of these use 3 threads.
These three common thread types are: ASME B1.1 (Unified screw thread), ISO 228/1 and ISO 261.
Below is the introduction of these threads
4-1-1 Unified Screw threads
These threads, also known as the ASME B1.1
(American Society of Engineering Standard) thread,
Mostly used in North American countries.
Of course, such threads are used anywhere in the world.
The profile of this tutorial is as follows.
Straight threads (no cone angle or tipper during connection)
As shown in Fig. 3, the tip and root surface of the thread are flat.
The angle between the threads is 60 degrees.
The threaded pitch is measured in inches.
In the figure below, this thread has been shown.
fig 1-4 Unified Screw threads
The series of these threads are presented below.
1.UN thread: Male and Female thread
2.UNR: Only has a male thread
(the tip and root surface of the gear is a little conic)
3.UNC / UNRC: Threads head with low level filter
4.UNF / UNRF: Threads head with medium Level Filter
5.UNEF / UNREF: Threads head with high level filter
6.UNS / UNRS: Special Threads
2-1-4 ISO 228/1 Threads
These threads, also known as BSPP and JIS B0202, have the following characteristics.
Straight threads
The tip and root surface of the threads are round (curvature).
Threads have an angle of 55 degrees.
Threads are measured in inches per unit.
The following picture is an example of this thread.
fig 2-4 ISO 228/1 Threads
4-1-3 Metric direct threads
These threads are also known as threads ISO 261, JIS B0205 and ASME B1.13M.
The characteristics of these threads are as follows.
Direct threads
The tip and root surface of the thread are flat.
The angle between the threads is 60 degrees.
Threads are measured in millimeters.
In the following picture, this type of thread is shown.
fig 3-4 Metric direct threads
4.1.4 NPSM threads
This thread type, also known as the ASME B 1.20.1 thread, is very similar to the metric thread
With the difference that the threads are measured here with an inch unit.
As shown in the figure, the profile of this thread is very similar to that of metric threads.
This straight thread has a 60 ° angle between the ribs,
and the tip and roots of the thread are smooth.
As mentioned, the only difference of this thread
with metric thread is measurement of the threads per inch.
fig 4-4 View of NSPM thread
4-2 introducing direct thread fittings
These connections, which are very common in the industries, are also very diverse.
First, they are classified according to the type of thread, and then it will be dealt with.
Straight joints of the United System (Unified Screw Threads)
The variety of fittings used for this type of thread is high.
Below are the types of these connections.
1-2-4 straight gear fittings with American system thread
Variation of which profit of these types of thread are numerous.
Below has been introduced types of these fittings.
-SAE 37 ° (JIC) [1]
-SAE direct thread with O-ring
-Straight thread with O-seal
-SAE 45 °
-SAE J1453 with O-ring
-ISO 8434-3
-SAE 42 ° with reverse flange angle
The table below shows the size of the thread and its screw in terms of the nominal size of the fittings.
table 1-4 Dimensions of the gear and its screw are according to the nominal size of threaded fittings UST
4-2-1-1 JIS fittings (SAE 37°)
The ends of these connections are flare with a 37 degree angle,
which is most commonly used in the aerospace industry.
Of course, oil and gas and petrochemical industries are also widely used.
In the following, these connections are shown.
fig 5-4 JIS fittings (SAE 37°)
The mechanism of connection of these types of fitting to the tubes is shown in the figure below.
For these joints, a nut, an intermediate pipe, or tube itself, is used.
fig 6-4 SAE 37 ° Connecting Mechanism (JIS) with gland
fig 7-4 SAE 37 ° Connecting Mechanism (JIS) with Slide
You can also see the connection of the tube to these connections in the figure below.
fig 8-4 How to connect the tube to JIS fittings
In articles about tubes,
it will be discussed in detail on the ways in which tubes are flushed,
bended and connected to these fittings.
Also, in the articles on the types of fittings, to examine the material,
Their operation, the pressure and temperature of the fluid in them,
and more … will be addressed.
In these fittings:
The male fitting is threaded straight and has a flare angle of 37 degrees.
Female fitting, is a straight thread with a piece of sleeve with a 37 degree angle.
The sealing on these fittings is done using the flare surface.
4-2-1-2 Straight thread with O- ring
This type of fitting that uses an O-ring at the end of the thread for sealing is shown in the figure below.
fig 9-4 SAE fittings straight threaded with o- ring
table 2-4 SEA Hydraulic Fittings Standards Direct Threading with O-ring
In these direct fittings :
Male fitting, has a straight thread with an O-ring.
The female fitting, has a straight thread and the end of a conical part to push the o- ring.
Sealing with O – ring compression is done in the conical part of female.
This is shown in the figure below.
fig 4-10 SAE Hydraulic Fittings Straight thread with O – ring (female – male)
4-2-1-3 Straight thread with O- seal
These fittings have a section at the end of the thread that the O – ring is placed inside it.
fig 4-11 Straight thread fittings with O-seal
In the table below, the sealing mechanism and standards are mentioned.
table 4-4 SAE Hydraulic Fittings Standards Direct Threading with O-ring
In these direct fittings :
In these male fittings used straight thread and at the end of the threads on the end of the surface there is an O-ring.
The female part has a straight thread and its surface is smooth.
Sealing is done by compression of O – ring on male part via smooth end part of female.
SAE 45° 4-1-2-4
This is just like JIS connections, except that the flare angle is 45 degrees.
In the table, the sealing mechanism and their standard are listed.
table 4-5 SAE hydraulic fitting standards have a 45 ° flare angle
In these direct fittings :
In the male section, the end has a 45 ° graduation angle.
In the female part, a straight thread with a sleeve piece with a 45 ° flare angle is used.
The sealing mechanism here is just like JIS fittings. Surface with flare is used for sealing.
SAE Hydraulic Fittings have a 45 ° flange angle.
fig 4-12 SAE Hydraulic Fittings have a 45 ° flange angle
5-1-2-4 SAE J1453 with O- ring and ISO 8434-3
In this type of direct fittings , which uses the UST thread, is used to seal the O-ring.
O-ring according to the figure fits on the initial level of fitting.
fig 4-13 SAE J1453 fittings with O-ring and ISO 8434-3
In the table below, the sealing mechanism and standards are mentioned.
table 4-6 SAE hydraulic fitting standards have a 45 ° flare angle
In these direct fittings :
The male part has a straight thread and an o- ring that is embedded on its original surface.
The female part has a straight thread and uses a smooth surface sleeve for compressing the o- ring.
Here, sealing is done by compressing of O- ring.
fig 4-14 Schematic diagram of SAE J1453 fittings with O-ring and ISO 8434-3
6-1-2-4 SAE 42° with reverse flare angle
Unlike JIS fittings, these direct fittings have a reverse angle flange.
An example of a fitting with reverse flange angle.
In the following figure, the schematic diagram is illustrated for cases in which the tube has a head flange or not.
fig 4-15 SAE 42 ° fittings with reverse flange angle in tube with and without flare
In the table below, the sealing mechanism and the required standards are mentioned.
table 4-7 SAE hydraulic fitting standards have a 45 ° flare angle
In these direct fittings :
Male fitting is threaded straight and also at the begining is cone-shaped at an angle of 42 or 45 degrees to the inside. (Reverse flare)
The female has a straight thread and a sleeve with a reverse angle that fits with male.
Sealing is done with flare angle here.
[1] SAE: Society of Automotive Engineers