In mechanics, the term "thread" specifically refers to “screw thread”, a vital component integral to connectors, fasteners, and transmission elements. Screw threads are widely used in various equipment and structures, such as bolts, nuts, screws, and lead screws, all of which rely on them to fulfill their functions. Whether in manufacturing or daily life, the use of screw threads is ubiquitous. Without them, most machines would be unable to operate properly.
So, what exactly is a screw thread? What are the different types? This article provides detailed information about screw threads, helping you understand the various types and identify the most suitable thread for your project.
What Is Screw Thread?
Screw thread refers to a helical ridge or groove that wraps around the circumference of a cylindrical or tapered surface. This helical feature enables two or more components to be tightly connected together through rotational motion, or to convert rotational motion into linear motion.
The design of screw threads creates friction between the components, allowing for a secure fastening or locking. Additionally, screw threads possess a certain degree of self-locking ability, meaning that the connected components will not loosen spontaneously without external force.
There are numerous types of screw threads, categorized based on various standards. Before introducing types of threads, let’s have a general idea of the main geometric parameters of the thread.
Key Geometric Parameters of the Thread
Above picture offers an intuitive image of the important indicators. Below are the details about these parameters:
Major Diameter (Outer Diameter)
The major diameter is the largest diameter between the crests of the screw thread. This dimension is crucial for determining the size and strength of the thread, affecting its tensile and shear performance. Moreover,It ensures proper engagement and fit between internal and external threads.
Minor Diameter (Inner Diameter)
The minor diameter is the smallest diameter between the roots of the thread. It is an important parameter for thread strength and fit, determining the thread's shear strength and fatigue strength, and ensuring proper engagement between male and female threads.
Pitch Diameter (Effective Diameter)
The pitch diameter refers to the diameter at which the width of the thread crest and root are equal. It is located at the midpoint of the thread profile and is the main contact and load-bearing area for internal and external threads. The pitch diameter determines the tightness of fit and depth of engagement when a bolt and nut are screwed together, affecting the load distribution between the thread teeth. Additionally, an appropriate pitch diameter can enhance the thread's self-locking performance.
Pitch and TPI
Pitch refers to the axial distance between adjacent thread crests in millimeters (mm) and is commonly used for metric threads. It determines the density of the threads and the feed distance per rotation. Pitch is typically measured quickly using a Thread Pitch Gauge.
TPI (Threads Per Inch) refers to the number of thread crests per inch and is commonly used for imperial threads. The relationship between TPI and pitch is that they are reciprocals of each other.
Lead
Lead is the distance a screw advances along its axis in one complete revolution. It determines the efficiency of converting rotational motion to linear motion in threads, i.e., transmission efficiency. Additionally, lead affects the contact area and load distribution of the thread. Smaller lead values typically distribute the load over a longer contact area, thereby increasing load capacity and wear resistance.
Helix Angle
The helix angle is the angle between the helix of the thread and a plane perpendicular to the thread axis. It describes the angle at which the thread rises along the screw's axis, influencing both the transmission efficiency and the choice of manufacturing processes. A larger helix angle typically increases transmission efficiency but may also lead to higher friction and wear.
Thread Angle
The thread angle is the angle formed by the intersection of the thread's flanks with a plane perpendicular to the screw's axis. This angle is typically 60 degrees, which is common in standard thread designs, especially in ISO metric threads and Unified Thread Standard (UTS) threads. It mainly affects the geometric shape and mechanical properties of the thread, such as strength, load distribution, and self-locking performance.
Tooth Angle
In thread applications, the tooth angle and thread angle are usually the same, both referring to the angle between the flanks of two adjacent threads in the thread profile. However, in a broader mechanical design context, the tooth angle may specifically refer to the profile angle of gear teeth, while the thread angle is exclusively used for thread design.
How to Identify Thread Types?
Regarding thread types, the increasingly common classification is based on the subtle differences in key parameters. In the previous section, we introduced the key geometric parameters of the thread. In the following passage, as we learn how to identify thread types, we will gradually understand that the subtle differences in these parameters play a crucial role in determining whether a specific thread type can achieve optimal compatibility with your equipment.
Now, let’s combine visual inspection, basic measurements, and standard comparison to systematically identify and confirm the thread type.
Step 1: Male Thread vs. Female Thread
First, identify whether the thread is male or female through observing the overall shape and structure of the thread. A male thread is an external helical ridge found on components such as bolts or screws, while a female thread is an internal helical groove present within nuts or holes.
Step 2: Tapered Thread vs. Parallel Thread
Next, check whether the thread is tapered or parallel. The diameter of a tapered thread gradually decreases along its length to one end, while the diameter of a parallel thread remains consistent along the length of the thread. Sometimes this characteristic can be determined by visual inspection, but if not, a caliper will be helpful. Use the caliper to measure the first, fourth, and last full threads. If the measurements are the same, it is a parallel thread. If the measurements decrease, it is a tapered thread.
Step 3: Measuring Thread Pitch
The next step in identifying your thread type is to determine the pitch size。Position the pitch gauge onto the thread and adjust it until it snugly fits between consecutive threads. Subsequently, read the indicated pitch value, which represents the precise distance between the threads.
Step 4: Measuring Thread Diameter
After determining the pitch size, the next step is to measure the thread diameter. The diameter obtained in this step is not exactly the same as the specified nominal size of the specified thread. The main reason for this variation is industry or production tolerances.Different parameters of the diameter require different measuring tools. Here are some common tools used for specific parameters:
Major Diameter (Outer Diameter): This is the easiest parameter to measure directly. It is usually measured using external diameter tools such as vernier calipers or micrometers.
Minor Diameter (Inner Diameter): Measuring the minor diameter requires higher precision. Tools such as an inside micrometer, bore gauge, or small hole gauge are recommended .
Pitch Diameter (Effective Diameter): Measuring the pitch diameter is more complex and often involves specialized gauges or indirect methods. Standard tools include thread ring gauges and thread plug gauges. For higher precision, a thread micrometer or the three-wire method is used.
Step 5: Measuring Thread Angle
Use a thread angle gauge to measure the thread angle. Align the gauge with the thread flanks and ensure it fits snugly to obtain the angle between the flanks of two adjacent threads. Record the measured angle.
Step 6: Confirm Thread Type
Finally, based on the measured diameter, pitch, and angle, refer to standard thread charts or manuals provided in the latter part of this article to confirm the thread type.
Common Thread Standards and Thread Types
Thread classification is based on different standards and application requirements. For example, pipe threads are used for pipe connections (such as BSP and NPT), and machine threads are used for general mechanical and structural connections (such as ISO and UTS). Here are some common thread standards and types:
1. ISO Metric Thread Standard
Standard Number: ISO 68-1, ISO 261, ISO 965-1, ISO965-2
The ISO metric thread standard is a globally recognized thread standard set by the International Organization for Standardization (ISO). It uses the metric system for thread dimensions, and the thread angle is 60 degrees, with diameters and pitches measured in millimeters. Common thread types include Coarse Thread and Fine Thread.
Coarse Thread
Fine Thread
Thread Size (mm)
Pitch (mm)
Major Diameter (mm)
Minor Diameter (mm)
Thread Size (mm)
Pitch (mm)
Major Diameter (mm)
Minor Diameter (mm)
M3
0.5
2.980
2.459
M3*0.35
0.35
2.981
2.621
M4
0.7
3.978
3.342
M4*0.5
0.5
3.978
3.242
M5
0.8
4.976
4.134
M5*0.5
0.5
4.980
4.459
M6
1
5.974
4.917
M6*0.75
0.75
5.978
5.188
M8
1
7.974
6.917
M8*0.75
0.75
7.978
7.188
M10
1.5
9.968
8.376
M10*0.75
0.75
9.978
9.188
M10*1
1
9.974
8.917
M10*1.25
1.25
9.972
8.647
M12
1.75
11.97
10.106
M12*1
1
11.97
10.917
M12*1.25
1.25
11.97
10.674
M12*1.5
1.5
11.97
10.376
M16
2
15.96
13.835
M16*1
1
15.97
14.917
M16*1.5
1.5
15.97
14.376
M20
2.5
19.96
17.294
M20*1
1
19.97
18.917
M20*1.5
1.5
19.97
18.376
M20*2
2
19.96
17.835
M24
3
23.95
20.752
M24*1.0
1.0
23.97
22.917
M24*1.5
1.5
23.97
22.376
Coarse Thread is suitable for most industrial and mechanical applications, easy to manufacture and assemble, and ideal for general fastening applications. And Fine Thread is used for connections requiring higher strength and precision, providing better locking performance under circumstances with significant vibrations.
2. Unified Thread Standard (UTS)
Standard Number: ASME B1.1
The Unified Thread Standard is widely used in the United States and Canada. It also features a thread profile angle of 60 degrees, with diameters and pitches measured in inches. There are several series under UTS, including UNC (Unified National Coarse), UNF (Unified National Fine), and UNEF (Unified National Extra Fine).
UNC (2A)
UNF (2A)
Nominal Size
Major Diameter (mm)
Minor Diameter (mm)
TPI
Nominal Size
Major Diameter (mm)
Minor Diameter (mm)
TPI
1/4" x 20 UNC
6.322
4.978
20
1/4" x 28 UNF
6.325
5.360
28
5/16" x 18 UNC
7.907
6.401
18
5/16" x 24 UNF
7.910
6.782
24
3/8" x 16 UNC
9.491
7.798
16
3/8" x 24 UNF
9.497
8.382
24
7/16" x 14 UNC
11.076
9.144
14
7/16" x 20 UNF
11.079
9.728
20
1/2" x 13 UNC
12.661
10.592
13
1/2" x 20 UNF
12.667
11.328
20
5/8" x 11 UNC
15.834
13.386
11
5/8" x 18 UNF
15.839
14.351
18
3/4" x 10 UNC
19.004
16.307
10
3/4" x 16 UNF
19.012
17.323
16
7/8" x 9 UNC
22.176
19.177
9
7/8" x 14 UNF
22.184
20.269
14
1" x 8 UNC
25.349
21.971
8
1" x 12 UNF
25.354
23.114
12
2" x 4.5 UNC
50.726
44.679
4.5
UNC is suitable for most general mechanical and structural connections, particularly in applications with low strength requirements, such as building frameworks and general mechanical assembly. In contrast, UNF is used for mechanical connections that demand higher strength and precision, commonly found in the automotive and aerospace industries. For applications requiring even higher precision and tighter fits, such as high-precision instruments and small mechanical components, UNEF is the preferred choice.
3. British Thread Standard
Standard Number: BS 84, BS 2779 (ISO 228-1), BS 21(ISO 7-1), BS 93
British Standard Threads are primarily used in the UK and Commonwealth countries. They encompass several types, most of which have a thread angle of 55 degrees, and diameters and pitches measured in inches.common thread types are:
British Standard Whitworth Thread (BSW): Developed by Joseph Whitworth in 1841, BSW threads were the first standardized thread form and were once widely used in the UK and former British colonies. They have a 55-degree thread angle with rounded crests and roots. BSW threads are primarily used in general mechanical engineering and building structures, suitable for rougher machining environments.
British Standard Fine Thread (BSF): BSF threads are a variant of BSW, also featuring a 55-degree thread angle but with a finer pitch than BSW. They are used in applications requiring higher strength and tighter fits.
British Standard Pipe Thread (BSP): BSP threads are used for pipe connections and have a thread angle of 55 degrees. There are two main types: BSPP (parallel threads) and BSPT (tapered threads). BSPP threads are usually sealed using a sealing face or an O-ring and are commonly used in hydraulic and pneumatic systems. BSPT threads primarily rely on the thread itself for sealing; the interference fit between the threads forms the seal, making them suitable for high-pressure sealing in piping systems.
British Association Thread (BA): BA threads have a thread angle of 47.5 degrees and are primarily used for small screws and bolts. These threads are commonly found in the electronics and precision engineering fields.
4. National Pipe Thread (NPT)
Standard Number: ANSI/ASME B1.20.1
National Pipe Thread (NPT) is a tapered thread standard widely used in the United States and other countries. The thread angle is 60 degrees, with a taper of 1/16 inch per inch. NPT threads achieve sealing through thread interference fit,and are widely used in high-pressure liquid and gas piping systems, industrial equipment, and building plumbing.
Thread size
Major Diameter (mm)
TPI
1/16" - 27 NPT
7.938
27
1/8" - 27 NPT
10.287
27
1/4"- 18 NPT
13.716
18
3/8" - 18 NPT
17.145
18
1/2" - 14 NPT
21.336
14
3/4" - 14 NPT
26.670
14
1" - 11½ NPT
33.401
11.5
2" - 11½ NPT
60.325
11.5
5. Right-Hand Threads (RH) and Left-Hand Threads (LH)
A thread is called a right-hand thread if it tightens when turned clockwise. It is the most common thread direction used in most applications. Conversely, a left-hand thread tightens when turned counterclockwise. Left-hand threads are always used in situations where it is important to prevent the thread from loosening due to self-movement, such as with the left-hand bicycle pedal.
6. “V” Shape Threads
These threads have a triangular or V-shaped cross-section with a 60-degree included angle. This is the most common thread form and is used in various applications due to its versatility and ease of manufacture. “V” shape threads are suitable for both general-purpose fastening and load-bearing applications. Examples include the Unified Thread Standard (UTS) and ISO metric threads.
7. Square Threads
Unlike “V” shape threads, this type of thread has a square cross-section and is difficult to fabricate. Square threads offer less friction and wear due to their perpendicular thread flanks. This design also ensures a more even distribution of the load along the thread, reducing the risk of jamming or getting stuck under heavy loads. Consequently, square threads are ideal for applications that require high efficiency and the ability to transmit large forces, such as lead screws and jack screws.
8. Acme Threads
Acme threads can be considered a transformation of square threads but offer easier production . They have a trapezoidal profile and a 29-degree thread angle. Due to their broader and more stable cross-sectional design, Acme threads are stronger under load than square threads. Acme threads are widely used in applications requiring high strength and precision, such as brass valves, bench vices, and screw-cutting lathes.
9. Knuckle Threads
Knuckle threads have a rounded top and bottom, a unique design that provides high resistance to damage and wear by reducing sharp edges and minimizing wear and tear. The smooth, rounded profile also helps prevent the accumulation of dirt, debris, and other contaminants, making knuckle threads particularly suitable for applications where threads are frequently engaged and disengaged or exposed to harsh environments. As a result, knuckle threads are often used in industries such as railways and heavy machinery, where robustness and durability are critical. They are also commonly found in connections that need to be strong and resilient under tough conditions, such as couplings, lids, and covers that need to be screwed and unscrewed repeatedly.
10. Buttress Threads
Buttress threads are specialized screw threads designed to withstand high axial thrust in one direction. The thread profile has a nearly perpendicular load-bearing face and a slanted trailing face, typically at an angle of around 45 degrees. This design allows for efficient transmission of large forces along the axis of the screw while minimizing the risk of thread deformation. Buttress threads are commonly used in applications requiring significant load-bearing strength in one direction, such as hydraulic presses, vices, lifting equipment, and machinery that handles heavy loads or high pressure.
11. Worm Threads
Worm threads are used in worm gears, where they transmit motion and power between non-intersecting, perpendicular shafts. The worm, which has the worm threads, resembles a screw and meshes with a worm wheel. The thread profile of a worm thread is designed to efficiently engage with the teeth of the worm wheel, providing smooth and continuous motion. This type of gear system offers high reduction ratios, allowing for substantial speed reduction and torque multiplication. Worm threads are commonly used in applications such as conveyor systems, lifts, steering mechanisms, and machinery where precise control and high torque are required. The design also has a self-locking feature, preventing back-driving under certain conditions, which enhances safety and control in many applications.
12. Single and Multi-Threads
Single threads have a single helical ridge. Each full rotation of a single thread moves the screw forward by one pitch length. This design offers a balance of strength and ease of manufacture and is the most straightforward and commonly used type of thread.
Multi-threads have two or more helical ridges, allowing for faster linear advancement with each rotation. For instance, double-start threads have two helical ridges, meaning the screw advances by two pitch lengths per turn. This design reduces the time and effort needed to achieve the same linear movement compared to single threads. Multi-threads are particularly useful in applications where rapid movement is essential, such as in high-speed machinery, actuators, and precision instruments.
Conclusion
There are multiple thread types, each with its own design styles and levels of precision required. In product design, different thread types should be considered. We hope this article can help you identify the different thread types and choose the right threads for your mechanical and engineering needs.
At Chiggo, we pride ourselves on our expertise in CNC machined fasteners. For products of different materials, different thread types and thread processing methods are applied, and professional testing is adopted to control quality. Our engineering team, with over 10 years of industry experience, is ready to work closely with you to provide the suitable thread solutions for your project. Upload your CAD file now!
FAQ
What does the “F” in NPTF stand for: Female, fine or fuel?
The “F” in NPTF stands for Fuel. NPTF stands for National Pipe Taper Fuel, which is a dryseal thread used in applications where sealing is crucial, such as in fuel systems. NPTF threads are designed to create a seal without the need for additional sealant, unlike standard NPT (National Pipe Taper) threads.
Are NPT and BSP pipe threads interchangeable?
NPT and BSP pipe threads are not interchangeable due to their different standards, thread pitches, and thread angles. NPT follows the National Pipe Thread standard with a 60-degree thread angle, while BSP adheres to the British Standard Pipe Thread with a 55-degree thread angle. The thread pitch also varies with the thread size; for example, a 1/2" NPT thread has 14 threads per inch, and a 1/2" BSP thread also has 14 threads per inch, but their physical dimensions and thread profiles differ. These key differences in physical dimensions and structure make it impossible to directly interchange NPT and BSP pipe threads. If connection between these two different standards is required, special adapters must be used.
