Thread of Bolts – Basic Knowledge

When checking Bolt material coming, do you have feeling lose in the forest of BOLT & THEARD?

We will help you to identify them accordingly via this topic!

It has internal thread & external thread, can call male & female also.

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1)  Thread direction: The helix of a thread can twist in two possible directions.: left hand & right hand.

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2) Technical terms of thread:

  • Type of thread:

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  • Pitch / TPI ( thread per inch ):

– The pitch is the distance from the top of one thread to the next in mm.
– TPI (Threads per inch) is used by inch thread

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  • Diameter:

– The major diameter is determined by the thread tips.
– The minor diameter is determined by the groove of the thread.
– The pitch diameter is the distance of two opposite flanks or the distance of the centreline of the profle.

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  • Angle: 

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  • Crest / root:

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3) Type of thread:

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4a) M – ISO thread:

ISO Metric thread is a globally standardized thread. Compared to standard threads (coarse thread), a fne thread has a smaller pitch.

– Coarse Threadpitch can be displayed or omitted after thread size

– Fine Thread: pitch must be displayed after thread size

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b) NPT / NPTF / NPS / NPSM / NPSF – PIPE THREAD: 
The most common types of pipe thread are:

– NPT – American Taper Pipe Thread = National Pipe Taper
– NPTF – American Taper Pipe Thread for Dryseal joint without sealant compound = National Pipe Taper Fuel.
Note: NPT and NPTF appear to be identical. Both have the same pitch diameter at the top of the hole of the internal thread or end of the pipe on external threads and both have the same thread lengths or depths. However, there is a subtle difference in the root and crest diameters of the threads.

15.PNG– NPS: National Pipe Straight

– NPSM: National Pipe Straight Mechanical thread

– NPSF: National Pipe Straight Fuel

c) BSPP / BSPT: 

– BSPP (G) – British Standard Pipe Parallel
– BSPT (R/Rp) – British Standard Tapered Pipe, for pipes and tapered thread.
Note: An appropriate sealing compound can be used in the thread to ensure a leak-proof joint.

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d)  UNC/UNF – UNIFIED NATIONAL THREAD
The most common types of UN (Unifed National) thread are:
• UNC – Unifed National Coarse Thread, comparable with the ISO metric thread.
• UNF – Unifed National Fine Thread.
*Compared to standard threads (coarse thread), a fne thread has a smaller pitch.

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e) Other thread:

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4) How to identify thread?

1. Determine if the thread is male or female (visual inspection)?


2. Determine if the thread is tapered or straight/parallel (visual inspection)?
          • Check: Measure the thread with a calliper at the beginning and the end, if it is the same value the thread is straight/parallel.

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3. Measure the thread diameter (male: major diameter, female: minor diameter):
        • The diameter measurement obtained in this step may not be exactly the same as the listed nominal size for the given thread. The main reason for this variation is industry or manufacturing tolerances.


4. Determine the thread pitch:
          • Easiest with the use of a pitch gauge.

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5. Identify the end connection:
          • Most common angles: 30°, 37° and 45°.

5) Some questions need clarify:

  1. WHAT DOES NPTF STAND FOR: FEMALE, FINE OR FUEL?
    NPTF stands for National Pipe Taper Fuel. This could be male or female. NPTF is designed to provide a more leakfree seal without the use of teflon tape or other sealant compound. A common mistake is to assume that NPTF stands for NPT female or NPT fne. NPT fine does not exist.
  2. WHICH FEMALE THREADS ARE TAPERED/CONICAL?
    Most common used tapered internal threads:
    • NPT
    • BSPT: Rc
  3. WHAT IS THE DIFFERENCE BETWEEN G-THREAD (BSPP) AND R-THREAD (BSPT)?
    BSPT thread is tapered when we speak about male thread (R), but the female thread can be tapered (Rc) or parallel (Rp). BSPP thread is parallel, male and female (G).
  4. CAN I USE AN O-RING SEALING WITH NPT THREAD?
    Sealing NPT thread with an o-ring is impossible. NPT thread is tapered thread and should screw in only partway. Therefore, the sealing itself is realized directly on the thread, with PTFE tape or a liquid sealant.
  5. ARE NPT AND BSP PIPE THREADS COMPATIBLE?
    – While NPT threads are common in the United States, BSP threads are widely used in many other countries.
    – NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most diametrical sizes have a different pitch. NPT/NPS threads have a 60° included angle and have flattened peaks and valleys; BSP threads have a 55° included angle and have rounded peaks and valleys.(Sometimes ½” and ¾” NPT and BSP threads are combined, because they are very close in design, they have the same pitch) but not too much size can compatible.
  6. WHAT IS SCREW/NUT GALLING AND HOW CAN IT BE AVOIDED?
    – Galling occurs when two like metals rub against each other. As the metals heat up from friction, the molecules of each bind together, eventually causing failure as the surfaces weld together. The higher the speed and the higher the pressure, the more likely this is to occur. Often different kind of fasteners, such as those made of stainless steel, aluminium and titanium are most likely to be subject to galling when tightened. Galling can be avoided by good lubrication, by using PTFE tape and by using brass or plastic nuts instead of for example stainless steel nuts. Metal-to-metal contacts are appropriate for low duty cycles or when good lubrication can be supplied, otherwise, stainless steel nuts running on stainless steel screws should be avoided.

6) Thread of seal:

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Finish!!!

Thanks!!!

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