Thread tolerance 6

[huiying]

Hi,
What is the advantage of having a class-3B tread compared to a normal thread?

Thanks & Regards.

 

[BillPSU]

First off a 3B thread is a normal thread. 3 is the class designation and B means its an internal thread. If a nut with a 1/2-13UNC-3B thread is mated to a bolt 1/2-13UNC-3A the clearance between the mating threads will fall between .0000 and .0085. Class 2 mating threads the clearance will be between .0015 and .013. Class 1 mating threads the clearance will be between .0015 and .0186.

The class of a thread determines the tolerance allowed when making the thread. All nuts and bolts no matter what class they are made to will screw onto each other. The difference will be the clearance between the mating parts.

 

[diamondjim]

There is also a difference on the tolerance
of the id or minor diameter of the internal
thread resulting in sometimes requiring
special tap drill diameters to maintain the
closer tolerance on the id for the 2B threads.
3B threads are easier to assemble which may
be the major reason for using them with 2A
screws or bolts.

 

[huiying]

thanks for replies.
Usually if the class is not specified, is there a default class to follow?
for eg, if it is just stated the thread as 1/2-13UNC instead of 1/2-13UNC-3B?

Thanks & Regards.

 

[MechEng13]

huiying,

For most normal circumstances I used Class 2 threads, e.g.1/2-13 2B

I believe most SHCS are made to the Class 2 tolerance, correct me if I'm wrong guys.



Alan M. Etzkorn
Project Develpment Engineer
Wabash National Corp.

http://www.wabashnational.com

 

[diamondjim]

huiying is right on
Most industrial applications call
for 2B threads.

 

[Kenneth]

diamondjim:

I think you mixed up your 2Bs and 3Bs in typing.

... the closer tolerance on the id for the 2B threads.

The minor diamter tolerance is closer (smaller) for 3B (not 2B) internal threads. The minimum minor diamter is the same; the maximum minor diameter is slightly larger on 2B threads (compared to 3B).

3B threads are easier to assemble which may be the major reason for using them with 2A screws or bolts.

The greater internal clearances afforded by 2B threads makes them easier to assemble...

Most commercial hardware threads are (understandably) Class 2; most aerospace hardware threads are Class 3.

 

[Billybaru]

Correction to the Socket Head Cap Screw thread class. Sizes 0 to 1 inch are Class 3A. Over 1 inch is Class 2A. Most commercial nuts, even in the higher strength Grade 8 are made to Class 2B.

 

[Screwman]

The default if unstated is Class 2 thread fit. The exception (as Billybaru said) is for Socket Head Cap Screws, where the standards are exactly what he said.

Metric is a whole 'nother can of worms depending on what 'standard' you want to follow.


Dick

 

[Kenneth]

Socket head cap screws made to ASME B18.3 (which specifies the thread classes noted by Billybaru) will also have a UNR thread form...