Thread Engagement 5

[IrwinL]

Hello,
I know both ASME Section VIII, Div. 1 and B31.3 require thread engagement for the full depth of the nut. But in reality, what does that mean? If a stud is flush with the nut, will that be considered sufficient engagement? Or there has to be a certain number of threads protruded / out of the nut to be considered sufficient?

Thanks in advance.

 

[Krausen]

Studs flush with nut is actually perfect. Anything more than that is actually just more area to corrode & requires more install clearance. Despite this, you'll see companies that require one or two threads to protrude. But you actually only need to engage stud threads to a length of 7/8 of the stud diameter to achieve full thread engagement. The reality is the bulk of your thread stress is taken up by the first two fully-engaged threads only.

 

[SwinnyGG]

Or there has to be a certain number of threads protruded / out of the nut to be considered sufficient?

This varies by company/designer - but requiring some amount of protruding thread is typical. This is done because the studs have a chamfer on the end, so the first thread (sometimes the first two threads) are not full depth because the chamfer cuts through them.

Stud flush with the top of the nut means you don't have full thread engagement.

 

[SnTMan]

Practically speaking, as per Krausen, with stud flush, you will have full capacity of the stud / nut.

However, I have seen a couple of really ugly scenes where the AI disagreed especially in cases where a client spec is makes a relevant requirement. I much prefer to have a couple of threads, even incomplete, outside the nut.

Edit in italic

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

Also consider the plus tolerance of the flange thickness.

Regards

 

[XL83NL]

Solid advice from Krausen. You may want to get a copy of ASME PCC-1 as well. THE best practice. In section 9(6) it refers to thread engagement and also how construction codes deal with it.

Huub
- You never get what you expect, you only get what you inspect.

 

[Krausen]

Stud flush with the top of the nut means you don't have full thread engagement

That is incorrect from a strict technical/code basis. Although you’re probably in the majority if you believe this common misconception.

 

[SwinnyGG]

That is incorrect from a strict technical/code basis

It's 100% correct from a reality basis. Stud ends are tapered. If the top of the stud is flush with the top of the nut, every thread on the nut is not fully engaged with a full height thread. Period.

HOWEVER - that is not the same thing as saying that the full ultimate strength of the nut is not available, which is not what I said.

 

[GBTorpenhow]

In ASME B31.3 'fully engaged" isn't well defined but ASME PCC-1 addresses this in a roundabout way using the term 'full engagement' to mean 'the end of the bolt is flush with the nut' when discussing excess thread protrusion (emphasis mine):

8.2.1 Excess thread protrusion can hinder joint disas­sembly due to corrosion, paint, or damage. A practice that facilitates joint disassembly (see section 15) is to fully engage the nut on one end (no bolt projection beyond the nut) so that all excess threads are located on the oppo­site end. Excess thread protrusion beyond the nut should be minimized.

My conclusion is that condition of 'bolt flush with the end of the nut' is acceptable per B31.3.

Practically speaking it takes less than one nut thickness of thread to develop full strength. ASME B31.3 2012 and older even allowed the bolt to be up to one thread short of flush with the nut.

 

[david339933]

Standard Designation: B31.3
Edition/Addenda:
Para./Fig./Table No:
Subject Description: ASME B31.3c 1998 Addenda Paragraph 335.2.3, Bolt Length
Date Issued: 07/01/1999
Record Number:
Interpretation Number : 17-20
Question(s) and Reply(ies):
Question: In accordance with ASME B31.3c 1998, Paragraph 335.2.3, if a bolt fails to extend beyond its nut and no more than one thread is visible inside the nut, is the bolt considered acceptably engaged?

Reply: Yes.

 

[GBTorpenhow]

Date Issued: 07/01/1999

This is an old interpretation and is no longer valid.

335.2.3 Bolt Length. Bolts should extend completely through their nuts. Any which fail to do so are considered acceptably engaged if the lack of complete engagement is not more than one thread.

335.2.3 Bolt Length. Bolts shall extend through their nuts such that there is complete thread engagement for the full depth of the nut.

 

[r6155]

No one mentions tolerances for flanges, studs, and nuts.
See ASME B16.5

Regards

 

[r6155]

If the stud elongation control is used for the assembly, I cannot measure it if the stud is flush with the nut.

Regards

 

[XL83NL]

Valid points r6155 but not really of interest or matter here.

Huub
- You never get what you expect, you only get what you inspect.

 

[BJI]

This clause has been updated in ASME PCC-1 2022 edition, with no projection and one less thread references removed. Under certain post-construction circumstances less engagement can be tolerated, but full depth of nut is the target. IMO this requires the full top thread of the nut to be engaged, i.e. requiring some extension beyond the nut face.

9 INSTALLATION OF BOLTS
(a) To support the assembly procedure, determine the
minimum adequate length of bolts.
(1) Bolt length should accommodate washers, nut
height, and the required thread protrusion.
(2) For assemblies involving bolt tensioning, the bolt
length should provide for the threaded portion of the bolt
to extend at least one bolt diameter beyond the outside nut
face on the tensioner side of the joint.
CAUTION: Avoid excessively long bolts. Excessive thread
protrusion can complicate joint disassembly due to corrosion,
paint, or damage on the exposed thread.
(b) The following instructions should be included in the
assembly procedure:
(1) Verify that the bolts, nuts, and washers conform
to required specifications [material grade, nominal
diameter, thread pitch, and nut thickness (heavy hex
versus regular hex)].
(2) Verify that the bolts are the specified length.
(3) Install the bolts such that the marked ends are on
the same side of the joint. Install nuts with the identification
marking facing outward. This practice facilitates
inspection.
(4) Install the nut on one end of the stud with
minimal thread protrusion such that any excess thread
length is located on the opposite end of the stud. This practice
facilitates joint disassembly (see section 14).
(5) Hand tighten the nuts. Then snug up the bolts to
15 N∙m to 30 N∙m (10 ft-lb to 20 ft-lb) but not to exceed
10% of the total target assembly bolt load (see
Nonmandatory Appendix O).
(6) Examine the bolts for adequate thread protrusion.
The criterion in the new construction codes2 is
thread engagement for the full depth of the nut.
However, it has been shown that the full strength in a
threaded fastener can be developed with less than
complete thread engagement, a consideration in
certain post-construction situations (e.g., see para.
15.13 and para. 15.15, refs. [4]–[6]).