Basic Knowledge of Thread Gauge

A thread gauge is a gauge used to check whether threads meet specifications. Thread plug gauges are used to check internal threads and thread ring gauges are used to check external threads.

Thread is an important and commonly used structural element. Threads are mainly used for structural connection, sealing connection, transmission, reading, and bearing. From general use conditions to harsh conditions (high temperature, high pressure, severe corrosion), from rough levels to very quiet, in short, a wide range of applications.

NPSM-American Standard Straight Pipe Thread for Mechanical Joints: These internal and external threads are used for free mechanical connection without internal pressure, and the product is inspected with a straight pipe through a stop ring plug gauge.

NPSL – American Standard Straight pipe threads for lock nuts: These internal and external threads are used for mechanical fit to prevent thread feed.

NH – American Standard Fire Hydrant Thread: These internal and external threads are used in fire hydrants, garden hoses, chemicals, and elevators.

NPSH – American Standard Thread for hose connection: These internal and external threads are used for steam, air, water, and other standard pipe thread connections.

NPSC – American Standard Straight pipe threads for pipe joints: The pipe joint has the same tooth shape as the inner straight pipe thread. When the outer cone thread NPT is tightened by a wrench when assembling the sealing packing, it can usually form a sealing connection and is used for a low-pressure pipeline system.

NPSF – American Standard Straight pipe internal thread for dry seal standard fuel oil: These internal threads are used on soft materials or ductile iron parts without sealing with NPTF external threads.

NPSI – American Standard Dry seal intermediate thread: These internal threads are used for assembly of hard or brittle materials with short PTF-SAE external threads, but can also be used for full-length assembly of male NPTF threads.

Thread gauges are classified according to their performance: Work gauge, acceptance gauge, proofreading rules, and benchmark rules.

Work Gauge: Thread gauges are used in the manufacture and inspection of workpiece threads.

Acceptance Gauge: The thread gauge used by the inspection department or the user’s representative to accept the thread of the workpiece.

Check Gauge: Thread gauges for manufacturing and testing work gauges. For cylindrical threads, there are usually only proofing plug gauges, which are used for the inspection of working ring gauges.

Benchmark Gauge: Certain tapered threads (such as oil drill pipe joint threads and NPT threads) require a datum gauge for checking calibration gauges.

When checking the thread with a cylindrical thread gauge, the thread gauge at the through end should be screwed in completely, and the thread gauge at the end should not be screwed in or be allowed to be partially screwed in, then the thread to be inspected is qualified.

When using tapered thread gauges to inspect threads, the following conditions are met: Use a limit gauge to check whether the position of the measuring surface of the tested thread (internal thread is the big end, and the external thread is the pipe end plane) is within the specified upper and lower limits; Use a standard gauge (such as a special thread gauge for petroleum) to check whether the measuring surface of the thread to be inspected is within the specified range (whether the tight distance of the special thread for petroleum is qualified). For petroleum-specific threads, it is also necessary to test whether the single element of the thread is qualified.

Code for Thread and Thread Gauge

The thread and thread gauge codes are shown in Table 1-1-3.

Code for Thread and Thread Gauge

  1. If the general gauge is flat, change T to TB
  2. Listed as a tolerance zone code
  3. When the gauge is a flat top, mark Rb
  4. GB/T 12716-1991 only NPT

 

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