Thread Identification & Measurement guide
Hydraulic Fluid Transfer, Piping, and Instrumentation Systems: Identification and Measurement Guide
The precise identification of fluid line connectors is critical before selecting and installing the correct fittings. In this guide, we will cover some of the most common hydraulic fluid transfer, piping, and instrumentation systems and how to measure them in the field.
How to Measure Threads
Step 1:
Use a combination O.D./I.D. caliper to measure the thread diameter.
Note: The threads of a used fitting can become worn and distorted, so the measurements may not be exact.
Step 2:
Use a thread pitch gauge to identify the number of threads per inch. For metric connections, measure the distance between threads. Place the gauge on the threads until it fits snugly, and match your measurements with the thread chart provided at the back of this guide.
Step 3:
If the port is angled, determine the seat angle by using a gauge on the sealing surface. The centerline of the fitting and the gauge must be parallel.
Measuring Tools
By using a combination of three tools, identifying connectors is easy to do.
- I.D./O.D. CALIPER
- THREAD PITCH GAUGE
- SEAT ANGLE GAUGE
These tools allow you to make accurate measurements of most connections. Many thread ID calipers provide both a caliper and a seat angle gauge in one tool.
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I.D./O.D. CALIPER: Used to measure the O.D. of a male thread and the I.D. of a female thread.
Important: When matching gauge measurements to thread charts, keep in mind that threads on connections that have been in-service may be worn and distorted from use, causing inexact comparisons to the thread tables. -
THREAD PITCH GAUGE: For English, British, and other European threads, the thread pitch gauge measures the threads per inch. For metric threads, the gauge identifies the distance between threads.
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SEAT ANGLE GAUGE: Used by placing the gauge angle on the sealing surface. The centerline of the fitting end and the gauge should be parallel.
Measuring Threads
Using the THREAD PITCH GAUGE, align the gauge on the threads and ensure it fits snugly. Match the measurement to the thread chart. Then measure the thread diameter with the I.D./O.D. CALIPER and match those measurements to the chart. A thread chart is provided at the back of this guide.
Measuring Sealing Surface Angles
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Female Connections: Insert the ID portion of the gauge into the connection on the sealing surface. Ensure the centerlines of the connection and the gauge are parallel to identify the correct angle.
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Male Flare-Type Connections: Place the gauge on the sealing surface to establish the measurement. Again, ensure the centerlines of the connection and the gauge are parallel to identify the correct angle. See the image on the far left for detail.
American Connections
National Pipe Tapered (NPT):
NPT style pipe threads have been widely used for over 100 years as a U.S. standard for tapered threads used on pipes and fittings. They effectively seal pipes for fluid and gas transfer. The nominal pipe size can be identified by physically measuring the thread diameter, then subtracting ¼”. Available in iron or brass for low-pressure applications and carbon steel and stainless steel for high-pressure applications.
NPTF (National Pipe Tapered Fuel):
These are commonly used in fluid power systems. They have a tapered thread that seals by deformation of the threads. Measured by subtracting ¼-inch from the thread diameter to establish the nominal pipe size.
NPSM (National Pipe Straight Mechanical):
Used in fluid power systems, the female component has a straight thread with an inverted 30° seat, while the male component has a straight thread with a 30° internal chamfer. A seal is created by compressing the 30° seat on the chamfer. NPTF male connections can also seal with NPSM female connections.
Society of Automotive Engineers (SAE) Connections
SAE J1926 Straight Thread O-ring Boss (ORB):
Recommended by the National Fire Protection Association (NFPA) for leak prevention in medium and high-pressure hydraulic systems. The male connection is a straight thread with an O-ring, while the female port has a straight thread and a machined surface. The seal is made when the O-ring is compressed into the chamfer during mating.
SAE J514 JIC/37˚ Hydraulic Connections:
Common in most fluid power systems, both male and female components have 37° seats. The seal is made by contact between the male flared and female coned seat.
SAE J512 45° Connections:
Used in automotive, refrigeration, and truck pipe systems. These connectors have 45° seats, with the seal occurring where the male flare and the female cone meet.
SAE J1453 (ORFS) O-ring Face Seal:
Considered the best for leak control, the male connector has a straight thread and an O-ring in the face, while the female has a straight thread and a machined flat face. The O-ring compresses against the flat face, providing a seal.
SAE J512 Inverted Connections:
Used in automotive systems, where the male connector is a 45° flare or 42° seat, and the female incorporates a straight thread with a 42° inverted flare. Sealing occurs at the flared surfaces.
SAE J518 4-Bolt Flange:
Two pressure ratings: Code 61 (standard) and Code 62 (6000 PSI). The female port is a smooth, un-threaded port with four bolt holes. The male fitting is a flanged head with a groove for an O-ring. The seal is made when the O-ring is compressed between the flanged head and flat surface. The connection is held by threaded bolts.
Measuring Four-Bolt Flanges:
Use a caliper to measure the port hole, then measure the flanged head diameter or the longest bolt hole spacing center-to-center.
British Connections
British Standard Pipe (BSP) and BSPT (Tapered):
These connections are similar to NPT but with different thread pitches. Sealing is achieved by thread distortion, so thread sealants are recommended.
BSPP (Parallel):
Comparable to NPSM male but with a different thread pitch, using metal-to-metal and O-ring seals. The female BSPP has a tapered nose flareless swivel where the seal occurs.
German Connections
DIN 7631 Series:
Common in hydraulic systems, the male connector has a straight metric thread with a 60° angle on a recessed cone. The female has a tapered nose, and the seal occurs when the cone and tapered nose meet.
DIN EN ISO 8434-1:
A male with three possible female halves, the straight metric thread male has a 24° angle and a recessed counterbore matching the tube O.D. The female may be compression-style, flareless swivel, or flareless with a DKO O-ring.
DIN 3852:
A male connector and female port controlled by German specifications.
Japanese Connections
JIS Tapered Pipe (PT):
JIS tapered threads are comparable to BSPT connections and are interchangeable.
JIS 30° Male Inverted Seat:
Parallel pipe threads, comparable to BSPP connections, with interchangeable threads.
JIS 30° Female (Cone) Seat:
Similar to American SAE 37° flare connections but with different dimensions.
JIS B 8363 4-Bolt Flange:
Used in fluid power systems, with two pressure ratings: Type I Code 61 and Type II Code 62. The male connector has a flanged head with an O-ring groove, and the female has a smooth, un-threaded port. The seal is made by compressing the O-ring.
JIS 210 Kgf/cm2 4-Bolt Square Flange:
Similar to SAE 4-bolt flange connections, except the flange is square.
ISO Connections
ISO/DIS 6162 4-Bolt Flange:
Two pressure ratings: Code 61 (PN 35/350 bar) and Code 62 (PN 415 bar). The design is the same, but the bolt hole spacing and flanged head diameters are larger on the high-pressure series. Seal is made when the O-ring compresses between the flanged head and flat surface.
ISO 6149 Port and Stud Ends:
Uses straight threads with an O-ring for sealing, similar to SAE J514 ORB. The female port has a machined surface and a chamfer for the O-ring to compress into.
Instrumentation Fittings
Double and Single Ferrule Instrumentation Fittings:
Used in fluid or gas transfer applications, these fittings feature ferrules that bite into tubing for a secure seal. The double-ferrule has an inner cone, while the single-ferrule has a larger front ferrule.
Conclusion
When selecting fitting connections, it is critical to follow the manufacturers' recommendations and specifications. Failure to do so can lead to leakage, premature failure of joints, and costly corrective maintenance. Ensure the materials, O-rings, and seal materials are compatible with the application’s fluid, temperature, and environmental conditions. The proper use of clamps can also reduce stress and prevent premature leakage.