You can’t go around without talking about the thread when it comes to hydraulic fittings. There are a variety of thread types in the market, According to the thread profile, it can be divided into ordinary thread, pipe thread, trapezoidal thread, rectangular thread, serrated thread, etc. The first two are mainly used for connection, and the latter three are mainly used for transmission.
Thread parameters include the shape of the root, crest, the flanks angle, and the pitch per inch.
Screw Thread type of hydraulic fittings differs in different areas
Metric Thread
Metric Thread originates from DIN (Deutsche Industrial Norme), which is the national standard of German and is adopted by the ISO international standards organization.
The marking of ISO metric thread starts with the letter M, followed by the major diameter of the thread in mm. For example, M8 is a metric thread with a major diameter of 8 mm. If the thread does not use a generally large lead (for example, M8 screw uses 1.25 mm other than the lead), a [multiplication sign] will be added at the end, and the lead in mm will be added. For example, M8×1 has a major diameter of 8 mm. Rotating 360° will advance a metric thread of 1 mm.
BSPP vs BSPT
Sir Joseph Whitworth, the English mechanical engineer, and inventor, devised a uniform threading system in 1841 to address the incompatibility problem. The Whitworth thread form is based on a 55-degree thread angle with rounded roots and crests.
The Whitworth thread form was selected as a connecting thread for pipes, which was made self-sealing by cutting at least one of the threads on a taper. This became known as the British Standard Pipe thread (BSP Taper or BSP Parallel thread). The Whitworth thread is now used internationally as a standard thread for jointing low-carbon steel pipes.
NPT VS NPSM
In America, William Sellers set the standard for nuts, bolts, and screws which became the National Pipe Tapered Thread (NPT) in 1864. His 60-degree thread angle, in common use by early American clockmakers, enabled the American Industrial Revolution. These thread forms later became the American National Standard.
The best-known and most widely used connection where the pipe thread provides both the mechanical joint and the hydraulic seal is the American National Pipe Tapered Thread or NPT. NPT has a tapered male and female thread that seals with Teflon tape or jointing compound.
Pipe thread sizes are based on the inside diameter (ID) or flow size. For example, “1/2–14 NPT” identifies a pipe thread with a nominal inside diameter of 1/2 inch and 14 threads to the inch, made according to the NPT standard.
The word “tapered” in several of the above names points to the big difference between many pipe threads and those on bolts and screws. Many pipe threads must make not only a mechanical joint but also a leakproof hydraulic seal. This is accomplished by the tapered thread form of the male matching the thread form of the female tapered thread and the use of pipe sealant to fill any voids between the two threads which could cause a spiral leak. The bottoms of the threads aren’t on a cylinder, but a cone; they taper. The taper is 1⁄16 inch per inch in length, which is the same as 3/4 inch in a foot.
UNC vs UNF
UNC and UNF are symbols of the Unified Screw Thread Series.
The Unified Thread Series was agreed upon by the United States, Great Britain, and Canada in 1949 to obtain interchangeability, Recently, modifications have been made to the original 1949 specifications for ease of use.
The Unified Thread Series is classified into coarse threads, fine threads, and extra fine threads much like Metric screw threads are.
UNC: is the symbol for Unified coarse pitch threads.
UNF: is the symbol for Unified fine pitch threads.
The threads which are popularly used in the North American markets are adopted by most standards except NPT and NPSM systems. they are indicated in the form such as 1/2″-20, the fraction before the dash sign means thread diameter and the latter is the pitch per inch.
The thread type of commonly used hydraulic fittings
German Standard
Metric
- Both male and female have straight metric threads.
- The male has 60° seat. The female has a 24° or 60° seat.
- The seal takes place by contact between the 60° seat on the male end and the respective sealing area on the female end.
Thread Identification of Metric 60° cone
| Metric Thread | Female Thread ID (mm) | Male Thread OD (mm) | Tube OD 60°Cone （mm） |
|---|---|---|---|
| M12X1.5 | 10.5 | 12 | 6 |
| M14X1.5 | 12.5 | 14 | 8 |
| M16X1.5 | 14.5 | 16 | 10 |
| M18X1.5 | 16.5 | 18 | 12 |
| M22X1.5 | 20.5 | 22 | 15 |
| M26X1.5 | 24.5 | 26 | 18 |
| M30X2.0 | 28.5 | 30 | 22 |
| M38X2.0 | 36.5 | 38 | 28 |
| M45X2.0 | 43.5 | 45 | 35 |
| M52X2.0 | 50.5 | 52 | 42 |
British Standards
BSPT (British Standard Pipe Tapered)
- The British Standard Pipe Tapered has a similar thread type with NPT fitting.
- BSPT threads are well-known as Whitworth threads.
- Male fittings have a tapered thread.
- The BSPT male can be connected with a BSPT female or BSPP Female.
- The thread angle is 55°.
- They can’t be interchangeable with NPT because the thread angle of NPT is 60°, and the pitch per inch is different yet.
BSPP (British Standard Pipe Parallel)
- The British Standard Pipe Parallel threads have a straight thread, both male and female.
- The straight thread only functions in connection, It can be sealed with 30° cone anon the male and 30° chamber on the female.
- The BSPP thread angle is 55°.
- They are similar to the NPSM thread but differ in thread angle and thread pitch.
Thread identification of BSPT and BSPP
| Dash Size | Nominal Size(in.) | Thread Size | Female Thread ID (in.) | Male Thread OD (in.) |
|---|---|---|---|---|
| -2 | 1/8 | 1/8-28 | 11/32 | 3/8 |
| -4 | 1/4 | 1/4-19 | 15/32 | 17/32 |
| -6 | 3/8 | 3/8-19 | 19/32 | 21/32 |
| -8 | 1/2 | 1/2-14 | 3/4 | 13/16 |
| -10 | 5/8 | 5/8-14 | 13/16 | 29/32 |
| -12 | 3/4 | 3/4-14 | 31/32 | 1 1/32 |
| -16 | 1 | 1-11 | 1 7/32 | 1 11/32 |
| -20 | 1 1/4 | 1 1/4-11 | 1 17/32 | 1 21/32 |
| -24 | 1 1/2 | 1 1/2-11 | 1 25/32 | 1 7/8 |
| -32 | 2 | 2-11 | 1 7/32 | 2 11/32 |
American Standards
NPT
- National pipe tapered is abbreviated as NPT.
- Tapered Thread, Sealing takes place with the deformation of the threads
- NPT male has a 30° seat and will mate the NPT female with tapered threads.
- NPT male with 30° seat can mate with NPSM thread with 30° inverted seat. The sealing takes place on the 30° seat.
- Commonly used in fluid power systems but no not recommended for hydraulic applications.
NPSM
- NPSM has a full name with National Pipe Straight Mechanical Thread.
- Both male and female have a straight thread and 30° seat and chamber.
- The seal takes place by compression of the 30° seat on the chamfer.
- Used in fluid power systems.
Thread identification of NPT and NPSM
| Dash Size | Nominal Size (in.) | Threads per in. | Female Thread ID (in.) | Male Thread OD (in.) |
|---|---|---|---|---|
| -2 | 1/8 | 27 | 23/64 | 13/32 |
| -4 | 1/4 | 18 | 15/32 | 35/64 |
| -6 | 3/8 | 18 | 19/32 | 43/64 |
| -8 | 1/2 | 14 | 3/4 | 27/32 |
| -12 | 3/4 | 14 | 61/64 | 1 1/16 |
| -16 | 1 | 11 1/2 | 1 13/64 | 1 5/16 |
| -20 | 1 1/4 | 11 1/2 | 1 17/64 | 1 43/64 |
| -24 | 1 1/2 | 11 1/2 | 1 25/64 | 1 29/32 |
| -32 | 2 | 11 1/2 | 1 1/4 | 2 3/8 |
JIC 37° Flare
- JIC is the abbreviation of Joint Industrial Council.
- Both male and female have straight threads and 37 degree seat.
- The straight thread holds the connection and sealing takes place on the 37° flare of the male and 37° chamfer on the female.
- The threads are the same as SAE J512 45° in -2, -3, -4, -5,-8, and -10 sizes. but the sealing surface angles are different.
Thread identification of JIC 37° Flare
| Dash Size | Nominal Size (in.) | Thread Size | Female Thread ID (in.) | Male Thread OD (in.) |
|---|---|---|---|---|
| -2 | 1/8 | 5/16-24 | 17/64 | 5/16 |
| -3 | 3/16 | 3/8-24 | 21/64 | 3/8 |
| -4 | 1/4 | 7/16-20 | 25/64 | 7/16 |
| -5 | 5/16 | 1/2-20 | 29/64 | 1/2 |
| -6 | 3/8 | 9/16-18 | 1/2 | 9/16 |
| -8 | 1/2 | 3/4-16 | 11/16 | 3/4 |
| -10 | 5/8 | 7/8-14 | 13/16 | 7/8 |
| -12 | 3/4 | 1 1/16-12 | 31/32 | 1 1/16 |
| -14 | 7/8 | 1 3/16-12 | 1 7/8 | 1 3/16 |
| -16 | 1 | 1 5/16-12 | 1 15/64 | 1 5/16 |
| -20 | 1 1/4 | 1 5/8-12 | 1 35/64 | 1 5/8 |
| -24 | 1 1/2 | 1 7/8-12 | 1 51/64 | 1 7/8 |
| -32 | 2 | 2 1/2-12 | 2 27/64 | 2 1/2 |
SAE 45° Flare
- Both male and female have straight thread and 45° flare on the male and 45° chamfer on the female.
- The threads hold the connection mechanically.
- Commonly used on low-pressure occasions.
- The threads are the same as SAE J512 37°(JIC) in -2, -3, -4, -5, -8, and -10 sizes.
Thread Identification of SAE 45° Flare
| Dash Size | Nominal Size (in.) | Thread Size | Female Thread ID (in.) | Male thread OD (in.) |
|---|---|---|---|---|
| -2 | 1/8 | 5/16-24 | 17/64 | 5/16 |
| -3 | 3/16 | 3/8-24 | 21/64 | 3/8 |
| -4 | 1/4 | 7/16-20 | 25/64 | 7/16 |
| -5 | 5/16 | 1/2-20 | 29/64 | 1/2 |
| -6 | 3/8 | 5/8-18 | 9/16 | 5/8 |
| -7 | 7/16 | 1 1/16-16 | 5/8 | 11/16 |
| -8 | 1/2 | 3/4-16 | 11/16 | 3/4 |
| -10 | 5/8 | 7/8-14 | 13/16 | 7/8 |
| -12 | 3/4 | 1 1/16-14 | 63/64 | 1 1/16 |
ORFS
- Male fitting has a straight thread and an O-Ring. Female has a straight thread and a machined flat face.
- The seal takes place by compressing the O-Ring onto the flat face of the female.
- The threads hold the connection mechanically.
- Commonly used for hydraulic systems where elastomeric seals are accepted to overcome leakage.
Thread Identification of O-Ring Face Seal(SAE J1453)
| Dash Size | Nominal Size (in.） | Thread Size | Female Thread ID (in.) | Male Thread OD (in.) |
|---|---|---|---|---|
| -4 | 1/4 | 9/16-18 | 1/2 | 9/16 |
| -6 | 3/8 | 11/16-16 | 5/8 | 11/16 |
| -8 | 1/2 | 13/16-16 | 3/4 | 13/16 |
| -10 | 5/8 | 1-14 | 15/16 | 1 |
| -12 | 3/4 | 1 3/16-12 | 1 1/8 | 1 3/16 |
| -16 | 1 | 1 7/16-12 | 1 11/32 | 1 7/16 |
| -20 | 1 1/4 | 1 11/16-12 | 1 19/32 | 1 11/16 |
| -24 | 1 1/2 | 2-12 | 1 29/32 | 2 |
ORB (O-Ring Boss Seal)
- Male has a straight thread and an O-Ring. Female has a straight thread and a chamfer to accept the O-Ring.
- The seal takes place by compressing the O-Ring into the chamfer.
- Commonly used for optional leakage control in medium- and high-pressure hydraulic systems.
Thread Identification of ORB fittings
| Dash Size | Tube Size (in.) | Nominal Size (in.) | Thread Size | Female Thread ID (in.) | Male Thread OD (in.) |
|---|---|---|---|---|---|
| -2 | 1/8 | 5/16 | 5/16-24 | 17/64 | 5/16 |
| -3 | 3/16 | 3/8 | 3/8-24 | 21/64 | 3/8 |
| -4 | 1/4 | 7/16 | 7/16-20 | 25/64 | 7/16 |
| -5 | 5/16 | 1/2 | 1/2-20 | 29/64 | 1/2 |
| -6 | 3/8 | 9/16 | 9/16-18 | 1/2 | 9/16 |
| -8 | 1/2 | 3/4 | 3/4-16 | 11/16 | 3/4 |
| -10 | 5/8 | 7/8 | 7/8-14 | 13/16 | 7/8 |
| -12 | 3/4 | 1 1/16 | 1 1/16-12 | 31/32 | 1 1/16 |
| -14 | 7/8 | 1 3/16 | 1 3/16-12 | 1 7/64 | 1 3/16 |
| -16 | 1 | 1 5/16 | 1 5/16-12 | 1 15/64 | 1 5/16 |
| -20 | 1 1/4 | 1 5/8 | 1 5/8-12 | 1 35/64 | 1 5/8 |
| -24 | 1 1/2 | 1 7/8 | 1 7/8-12 | 1 51/64 | 1 7/8 |
| -32 | 2 | 2 1/2 | 2 1/2-12 | 2 27/64 | 2 1/2 |
Japanese standard
JIS (metric 60° cone)
| Metric Thread | Female Thread ID (mm) | Male Thread OD (mm) | Tube OD 60°Cone （mm） |
|---|---|---|---|
| M12X1.5 | 10.5 | 12 | 6 |
| M14X1.5 | 12.5 | 14 | 8 |
| M16X1.5 | 14.5 | 16 | 10 |
| M18X1.5 | 16.5 | 18 | 12 |
| M22X1.5 | 20.5 | 22 | 15 |
| M26X1.5 | 24.5 | 26 | 18 |
| M30X2.0 | 28.5 | 30 | 22 |
| M38X2.0 | 36.5 | 38 | 28 |
| M45X2.0 | 43.5 | 45 | 35 |
| M52X2.0 | 50.5 | 52 | 42 |
JIS GAS (BSPP thread) 60° Cone
| Dash Size | Nominal Size(in.) | Thread Size | Female Thread ID (in.) | Male Thread OD (in.) |
|---|---|---|---|---|
| -2 | 1/8 | 1/8-28 | 11/32 | 3/8 |
| -4 | 1/4 | 1/4-19 | 15/32 | 17/32 |
| -6 | 3/8 | 3/8-19 | 19/32 | 21/32 |
| -8 | 1/2 | 1/2-14 | 3/4 | 13/16 |
| -10 | 5/8 | 5/8-14 | 13/16 | 29/32 |
| -12 | 3/4 | 3/4-14 | 31/32 | 1 1/32 |
| -16 | 1 | 1-11 | 1 7/32 | 1 11/32 |
| -20 | 1 1/4 | 1 1/4-11 | 1 17/32 | 1 21/32 |
| -24 | 1 1/2 | 1 1/2-11 | 1 25/32 | 1 7/8 |
| -32 | 2 | 2-11 | 1 7/32 | 2 11/32 |
Conclusion
Straight threads mostly hold connections mechanically between different hydraulic components in the hydraulic fittings, Tapered threads function both connection and sealing, but these threads are only suitable for some low-pressure use.
Thread parameters include the shape of the root, crest, the flanks angle and the pitch per inch, common threads standards cover Metric, BSP, NPT NPSM, UNC, and UNF. they are adapted by different thread type fittings.
Different thread type fittings combine the specific thread and sealing type, it is an integration to shape the fitting and can’t be divided between the thread type and fitting type.
Do you agree with the above expression, Feel free to contact us to discuss.