Is it recommended to damage bolt threads after installing? 2

[Eng0410]

I am a graduate engineer and have been reviewing steel framing shop drawings for a project. I've noticed that the fabricator has noted on the connection details the following: "Fingertight & damage threads after installing". The members being connected are HSS, angles and WF with A325N bolts with long and short-slotted holes.

I wonder what is the idea behind damaging the threads after installation?

 

[desertfox]

Hi Eng0410

The idea behind damaging threads after assembly is to stop the nut coming loose on the thread due to vibration etc.



“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Eng0410]

I appreciate your response desertfox.

 

[kingnero]

The idea behind damaging threads after assembly is to stop the nut coming loose on the thread due to vibration etc.

The above answer is correct, but that doesn't mean it's a good idea.

 

[Eng0410]

Kingnero

What implications does this practice have?

 

[TugboatEng]

It sounds like a poorly designed joint.

The trouble with "damaging the thread" is how does one determine if the threads are sufficiently damaged to prevent loss of the fastener?

A pin or shouldered bolt would provide a more reliable result.

 

[dik]

Is the slotted hole for tolerance or to allow movement? If movement then finger tight is OK and either 'wrecking' the threads, or using Loctite Red is the way I go... else 'snug tight' (a lot more than finger tight).

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BrianE22]

Also, if only the lower part of the thread is damaged then you could have loss of preload without losing the nut.

 

[TugboatEng]

I see this type scenario with large diesel engines. The flywheel end of the engine gets torqued and pinned into place. The opposite end has enlarged bolt holes and is torqued to ~25%. Nylock or jam nuts are used. This allows the joint to slip some for thermal expansion. It's quite loud when it moves.

 

[CWB1]

I'm assuming the fabbies note is referring to staking the thread with a center punch and a BFH. Doing so has the same effect as thread-locker but isn't as controllable so the process isnt nearly as common as it once was.

 

[kingnero]

Finger tight => what the hell is that? Just throw on the nuts and hope for the best?
Staking the threads is very much dependable on the operator performing this operation (and of the location relative to the nut). Ideally you'll want to damage both nut and bolt threads simultaneously, but in reality this rarely gets done. So the nut can back off until it hits the bolt's damaged thread, resulting in complete loss of preload.

Currently, enough alternatives are widely available to stop doing this. I'm talking of thread locking compound (Loc-Tite being probably the most common name), washers (I'm thinking Nord-Lock if vibration is an issue), castle nuts, or even simply a high enough preload (which here might not be applicable, as it looks like a joint to accomodate (thermal) expansion.

Another downside here is wear and tear on the bolt. If the nut is only finger tight, it'll be moving all of the time. I wonder how long the connection will last.

 

[LittleInch]

This is filed in the "Don't ask stupid questions, we've always done it this way" box.

Usually means whoever wrote it doesn't really know why or isn't bothered to find a better way to do it.

The key issue is the wording is open to wide interpretation on site and has consequences if, e.g. you need to remove that particular bolt....

It's also filed in the "It's all right until something goes wrong" box....

Welcome to the world of imperfect work....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[r6155]

For A325 bolts WITH SLOTTED HOLES see:

Specification for Structural Joints Using High-Strength Bolts.

June 11, 2020
Supersedes the August 1, 2014
Specification for Structural Joints Using High-Strength Bolts
Prepared by RCSC Committee A.1—Specifications and
approved by the Research Council on Structural Connections.

Regards.

 

[IFRs]

It may be that they want to allow movement and don't want to use a clevis pin etc, or this is for shipping or alignment and the bolts will be torqued in the field when the assembly is made.

 

[dik]

I think, with hardened washers, you can use slotted holes for both 'snug tight' and 'slip critical' connections.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[gtaw]

This is structural framing, not machinery. The contractor has been given the direction that the fasteners are "finger tight". This is not the same as "snug tight". I imagine this type of connection is used for an expansion joint where the members can move relative to each other as the temperature changes over the course of the day. "Damaging the thread" must be adequate to prevent the nut, which isn't tight, from backing off. It isn't usually a case where there is significant vibration, so it doesn't take a lot of "damage" to keep the nut from backing off.

Best regards - Al

 

[dik]

With finger tight, you should try to keep the nut attached to the bolt... by 'wrecking' the threads or using a thread locker material. I use this all the time where you have finger tight (I specify a torque for that) and if adjustable vertically slotted connections, then also spec a thread locker...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Eng0410]

Thanks for your comments dik & gtaw.

The shop drawing is for the exterior girt framing of a building.