Tuesday, June 23, 2009

How to read Thread(Bolt) specification|Tolerance classes|Screw Thread Series chart|Fasteners Thread Gauges Table

Meaning of Threaded Fastener terms and How is a threaded fastener described:

A bolt is defined either in Inch or Metric. The machine screw or bolt is described by length, the type of head and the thread.

Generally, the convention for describing threads is to give the number of threads per inch, preceded by a gauge number if the bolt is smaller than a quarter-inch in diameter, otherwise by the diameter in fractions of an inch, say “2-inch quarter-twenty bolt,” which means

2 inch represents the bolt length,have a nominal diameter of a quarter of an inch, and the last term defines "pitch of a screw" ie., twenty threads in an inch.

A typical smaller fastener could be “a 1-inch 8-32,” “8” being a gauge number and “32” the number of threads per inch.

Based on the pitch of threads are classified as UNF,UNC & UNEF

 Thread UNC UNF UNEF chart

How to define tolerance class in bolts:

An engineer must provide a much more precise description of a fastener. Lets take

3/8-16 UNC 2B (21)

The “2B” is a tolerance class.

Tolerance may be defined in four class: Loose-fit (class 1), Free-fit (class 2), Medium-Fit (class 3), and Close-fit (class 4).

To find description of classes, go here for inch-based and here for metric.

The “21” is the gaging system number, as defined in ASME/ANSI B1.3M.

Right and Left Hand threaded:

eg: 3/8-16 UNC 2B LH (21)

Right Hand And Left Hand Threaded

How to define multiple threads in bolt specification:

To designate a multiple thread the word “DOUBLE” (or “TRIPLE”, and so on) is placed after the class of fit, like this:

3/8-16 UNC 2B DOUBLE (21)

Single and Double thread

Following are the Bolt chart which will explain screw thread series,thread classes for bolts sized in inches,grades of inch-sized bolts and many more..

Metric Screw Thread Series

Fig: Metric Screw Thread Series chart


Fig: UNC, UNF, UNEF, UNS screw thread gauges to one inch in diameter chart

grades of inch-sized bolts

Fig: SAE grades of inch-sized bolts chart

SAE grades

The best-known grades for inch-sized steel bolts are those defined by the SAE, a sequence of grades from 0 to 8, on the basis of the metal from which the bolt is made and the manner of manufacture. Available grades run from 2 to 8, with 8 the strongest. Higher grade numbers almost always mean increased strength (an exception is that some grade 6 bolts are stronger than grade 7).

Each grade is represented by number which corresponds to Bolt material (low- or medium-carbon steel,1541 steel,Medium-carbon alloy steel, quenched and tempered) with Tested Tensile,Yield strength with marking on bolt head.

External Threads

1A-The loosest fit. Only found on bolts ¼ inch in diameter and larger. Most people will never encounter a fastener with this class of fit. It is mainly used in military hardware where the ability to quickly assemble damaged threads under dirty conditions has the highest priority.

If a class 1B nut with the smallest allowable diameter were fitted to a class 1A bolt with the largest allowable diameter, there would still be a space between them. This is called the “allowance”.   Class 1A threads are not coated or plated.

2A-2A/2B is by far the most common class of fit.  Like class 1A, class 2A bolts have an allowance. They may be coated.

3A-The closest fit. No allowance. In fact, if the nut's minor diameter was at the minimum permitted, and the bolt's major diameter at the maximum permitted, they would be everywhere in contact, though this situation is highly unlikely.

Class 3A is used for high-strength bolts. 

Internal Threads

The tolerances for the internal threads are 30% greater than the tolerances for the external thread of the matching class, but there is no allowance.

1B-No allowance; that is only applicable to the 1A male thread. One reason for this is that it would be wasted, since it is very difficult to plate or coat the inside of a nut.

2B-No allowance; that is only applicable to the 2A male thread.

3B-The tightest fit; used for high-strength bolts.

Thanks to sizes.com for providing Bolt or Fastener thread systems terminology in simple manner.