calc: min thread engagment - where find min/max thread dimensions

[electricpete]

Machinery Handbook 27th ed page 1510 provides method for calculating minimum length of thread engagement to "prevent" stripping of male and female threads. (Yes, I realize only the first few threads matter anyway, but I'd like to do the calc anyway, I won't trust it if it's too long).

The calc involves minimum and maximum values for internal/external thread diameters (pitch diameter, minor diameter, major diameter).

Page 1763 gives the basic (nominal) dimensions, but no min/max. I guess that depends on the tolerance selected. Where to go to find this info? I'm looking at 1.25" 7tpi A449 bolt and and A36 tapped female threads. (UNC).

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[desertfox]

Hi electricpete,

In the machinery handbook there is an error in the calculation where it comes to calculating the thread engagement. I know this from personal experience. A better reference to use is the one posted here:

http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 28.pdf

Thread tolerances (max/min) can be found in your Machinery's on Pg 1747.
Regards
desertfox

 

[electricpete]

Thanks very much. Do you recall what the error is? I don't see it from a quick comparison.

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[electricpete]

To give the whole problem statement: I am looking at holddown bolts for an 800hp vertical motor bolted to A36 steel stand. (The motor drives a centrifugal pump thru a rigid coupling... thrust bearing in the motor). We previously had A307 bolts torqued around 400 ft-lbf and I believe the motor moved, causing misalignment. There are no big thermal changes or unusual loads.

Bolts are 1 1/8", 7 tpi UNC

To get more holding power, we are going to put in A449 bolts.

On attached spreadsheet using nut factor 2, I calculated torque of 796 ft-lbf to attain 70% of A449 min yield = 0.7*81ksi = 57,700.

Also attached I calculated:

Min engagement to prevent external thread stripping = 0.8".

Min engagement to prevent internal thread stripping 1.27".

I don't remember exactly how much thread engagement we have, but let's say for now I have at least 1.27". Should I feel ok about the female threads?

Does anyone note any blatant errors in this approach or calculation results?

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[CoryPad]

I looked at the attachment (but not in depth) and there do not seem to be any blatant errors. I think the internal thread length calculation looks good - something approximately equal to the bolt major diameter is a good rule of thumb for a steel:steel fastener combination.

 

[desertfox]

Hi electricpete

The error was to do with the ratio's of yield stress and engagement lengths of the mating threads, I found it a while back when answering another thread, I'll try and find it later.
Can't see anything wrong with your calculation however you need to know the external load imposed on the fixings by the motor/pump combination.
Also using a torque setting can give about a 30% error in your clamping force, maybe this is why the motor moved, just a thought.
You could check the thread engagement using the reference I posted and see how it compares to the answer from the machinery's.

Regards

desertfox

regards

desertfox

 

[KENAT]

ASME B1.1?

Or if you want something free

http://www.boltplanet.com

as with anything free on the internet, use at your own risk. Also, don't blindly rely on the calculated values in the stress, I meant more for finding the max & min dimensions.

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[unclesyd]

Here are two sites, one that deals in generalities and the other in very detailed calculations for thread engagement length.

http://www.cncexpo.com/AThreadEngagement.aspx

http://www.hexagon.de/dose/form02e.pdf

 

[electricpete]

Kenat - Thanks. That bolt planet calculator is a good double-check.

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[Tmoose]

Hi Pete,

At the old torque of 400 lb-ft each bolt should provide slip resistance on the order of 4000 lbs +/- thickness of a brick. I'd be suspicious of torque loss due to embedment, etc.

Were lockwashers used?
How thick are the flatwashers between bolt head and motor "foot" ?
Do they show any evidence of "coning?"
How large are the holes in the motor "foot" ?
What kind of location is provided for the "stand" interface downstream of the motor?

If the new fasteners are commercial it would be fun to torque a few to yield or infinity and beyond.

Any way to convert to through bolts and nuts, or do you have a blind hole situation? Longer bolt grip length (MIL specs suggest at least 7 bolt diameters )

 

[electricpete]

Hi Danny. I think I know where you are coming from. I did not tell you one importance piece of information related to possible movement. When the motor was removed from the base, we saw nasty oily residue directly on the mounting surfaces. Attached is a calculation which shows the starting torque of this motor (800hp 1800 rpm) can be the same order of magnitude as the bolt friction force. By the way I had the bolt size wrong in my original post, it was really 1.25” (vs 1.125”), and still only torqued to 470 ft-lbf. I have to take the lowest coefficient of friction I could find for lubricated steel to the friction and torque forces in the same ballpark, but this proves to me that it is at least credible possible. Interested if you agree.

Another sidetrack we got to thinking about was the role of shims. We had shims under 2 of 4 bolts. That would seem to dramatically change the friction picture since contact area is dramatically different. But contact area doesn’t show up in the simple friction formula Ffriction = mu * Fnormal. I’m still trying to reconcile the math (contact area doesn’t make any difference) with my intuition (full contact should be much better than partial contact).

Don't ask me what the as-found alignment was... that information was not gathered due to circumstances to convoluted to explain. We have several reasons to believe the machine has moved and I'm interested if you agree it is plausible based on excel calculations attached.

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[electricpete]

I also forgot to say - the motor had been leaking oil for 2 years which is the very logical explanation for finding oil on those mounting surfaces. (This is the only one of 6 sister motors to do so). Then it developed erratic/increasing vibration including step changes after start. Likewise the only one of 6 to act like this. The role of oil in the vibratio problem provides an explanation to connect these two problems which otherwise are quite coincidental

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[electricpete]

Forgot direct answer.

Were lockwashers used?

Nope, never heard of that for mtoro holddown boltsw.

How thick are the flatwashers between bolt head and motor "foot" ?
Do they show any evidence of "coning?"
How large are the holes in the motor "foot" ?

We don't use washers. But I believe there are no anomalies llike this other that we were using a very low torque for thsi size bolt (470).

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[KENAT]

Pete, there are threads, or maybe a FAQ, around talking about the effects of washers on clamping loads etc, see if you can find it. May be over in "Mechanical engineering other topics" if not this forum.

As far as I've been taught/read friction force is pretty much independant of area, as counter intuitive as this may seem.

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[desertfox]

Hi electricpete

Looking at your last calculation I think your conclusion about the motor moving is valid but I'll study it a bit further after work.
Friction is normally considered independant of contact area however where high pressure between to surfaces is encountered friction is no longer independant of area, i haven't got my books to hand at present but I'll expand on that further later.
Having bolts with increased material between the bolt head and inner nut face alters the stiffness of the bolt and joint ie a longer bolt of the same diameter under the same load will stretch more than a shorter one.

regards

desertfox

 

[MechEng2005]

Friction is independent of contact area, but the friction factor (mu) will vary slightly just due to changes in the surface. A single value for the friction coefficient assumes that the contact surfaces are the same everywhere (lubrication, roughness, etc). Depending on the conditions, this may or may not be valid. I would expect less variance in surfaces 1" x 1" compared to a 36" x 36" contact area.

-- MechEng2005

 

[desertfox]

Hi pete,

Studied your calculations further now - and again your reasoning appears logical. However, there is one factor that has just occurred to me and that is, in one of your later posts, you stated that the motor had been leaking oil for the last two years. This seems to be an awful long time for the motor to slip out of line on the basis of reduced friction between mating faces.

Also, for the motor to move, this means that either none of the bolts are in contact with the bolt holes or only one bolt is in contact with the bolt hole to act as a hinge. So the amount of movement the motor could possibly make is only that clearance between the bolt and the bolt hole.

You are saying the motor has gone out of line, what problem has this caused? I would have thought the rigid coupling would have prevented the motor moving too far.

Regards,
desertfox

 

[paddingtongreen]

If the stand was made by a steel fabricator, the holes would normally be punched 1/16" oversize, but I have seen them punched a size larger (3/16") for machinery.
Questions:
How much movement is needed for misalignment?
Is the pump supported independently of the motor?
Is the stand braced with bracing members and rigid connections to keep its shape or could it have warped?
Has the motor been checked to see if it is an internal problem?

Timing has a lot to do with the outcome of a rain dance.