For Unified inch screw threads there are six standard classes of fit: 1B, 2B, and 3B for internal threads; and 1A, 2A, and 3A for external threads. All are considered clearance fits. That is, they assemble without interference. The higher the class number, the tighter the fit.
Class 1A is a loose commercial fit for easy assembly and disassembly. Class 2A is a medium fit. Class 3A is a tight fit used where a closed fit between mating parts is required.
Classes 2A and 2B are by far the most popular thread classes specified for inch series mechanical fasteners. Close to 90 percent of all commercial and industrial fasteners produced in North America have this class of thread fit. Class 2A external threads have an allowance, Class 2B internal threads do not.
"G" designates Ground Thread. "H" designates the pitch diameter is on high side of basic. These two letters (GH) are followed by a numeral indicating the Tolerance of Pitch diameter oversize.
A 3/8"-16UNC 2B is defined as a 3/8" diameter Unified coarse pitch 2B class of fit inch screw thread. If the thread is a 3/8"-16NC2 and the letters U, A and B do not appear in the thread designation, the thread conforms to the outdated American National thread.
There are three standard thread series in the Unified screw thread system that are highly important for fasteners: UNC (coarse), UNF (fine), and 8-UN (8 thread).
Class 2B is the typical fit for commercial products and is used for fasteners and threaded parts. Class 3B is the tightest tolerance and used for close fit and high strength fastening applications where a close and snug fit are required.
BSPP thread is often referred to as “G” threads, the two types of thread are the same parallel straight, the threads are measured in threads per inch. Therefore, BSPP is the same as G.
m8 means metric thread 8mm of ISO standard and 6H refers to tolerance of threaded hole (know more about it in ISO 2768) 2 0.
The 5g indicates the tolerance class for the pitch diameter and 6g is the tolerance class for the major diameter. A fit between threaded parts is indicated by the nut thread tolerance designation followed by the bolt thread tolerance designation separated by a slash.
The term “Thread Fit” is defined as the measure of the looseness or tightness between mating threads when an externally threaded fastener is assembled into an internally threaded hole or nut. 2A/2B and 3A/3B mating parts are designed to fit together to allow free-running assembly with no interference.
The square threads are not so strong as V-threads but they offer less frictional resistance to motion than Whitworth threads.
A 40/2 is a 40 weight thread… 20/2 is 20 weight etc. A 20/3 is not the same size …. It is 20/3 = 6.67 which would make it about a 13 weight thread (2 x 6.67) in 2 ply terms. Fixed length systems……as the number gets bigger the thread is bigger.
Second AC or AC 2-tier (2A)
This is second AC Sleeper class by the Indian Railways. In terms of features it stands less than the first ac coaches but in terms of service and comfort this class is way above 3A class.
Classes 2A and 2B are the most common thread classes in use, offering a balance of ease of manufacture, cost, and performance. Classes 3A and 3B are best suited where close tolerances, safety, and strength are required, and thus are frequently specified for socket cap and socket set screws.
Metric thread is the most widely used today, and can otherwise be referred to as 'ISO Metric' or 'M'. Both coarse and fine thread fasteners are available, and can be supplied in a wide range of materials and sizes – with a number of popular DIN standards.
Here's an example metric bolt specification: M8-1.0 x 20. This bolt specification breaks down like this: The letter 'M' indicates that this bolt uses a metric thread. The number '8' refers to the nominal diameter of the bolt shaft in millimeters.
Tolerance Positions
Internal Threads (nuts): Large "G" = small allowance, Large "H" = no allowance. 15150 25th Avenue North/Suite 200. [email protected]. Plymouth, MN 55447. Fax: (763) 249-8151.
A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimetres and separated by the multiplication sign, × (e.g. M8×1.25).
BSP thread form stands for British Standard Pipe and is common in Australia and the commonwealth countries. It is based on trade size rather than actual diameter which can lead to some confusion when measuring ports.
The way to tell the difference between the two is the flank angle. The flank angle for NPT is 60° compared to the BSP tapered which is 55° (this can be verified with a thread gauge). BSP threads are measured in imperial measurements such as 1/8, 1/4, 1/2, 3/4 etc.
NPT feels a lot sharper than BSP. You can also measure the threads per inch and use the handy table on our website to identify your fitting. The Metal Company stocks both male and female NPT to BSP adapters, so you can easily work with both threads.
H and L limits are used for inch size ground thread taps. H (high) represents a pitch diameter that is larger than the basic pitch diameter, and L (low) represents a pitch diameter that is smaller. The number after the H or L, indicates the amount it is different than the basic pitch diameter.
Unified form thread designations are represented by abbreviations such as UNC (unified coarse), UNF (unified fine), UNEF (unified extra fine), UNS (unified special) and UN (unified constant pitch), with the thread major diameter preceding the designation for example, 9/16” UNC, 9/16” UNF.
It is commonly known that the minimum recommended thread engagement to make a strong connection for a component with a tapped hole is approximately 1 times the nominal diameter in steel and 2 times the nominal diameter in aluminum.