how to calculate max. load a nut can take? 2

[parind]

I have a 5" dia. nut with buttress thread on it that threaded to the shaft. shaft and nut materials are same (4130). can anyone guide me through the calculating the max load nut can take before threads stripping out.

Thank you

 

[desertfox]

hi parind

look at this site:-

http://www.utm.edu/departments/engin/lemaster/machine_design.htm

desertfox

 

[parind]

the load needs to be determined is axial.

 

[desertfox]

hi Again

another site with useful links:-

http://www.roymech.co.uk/Useful_Tables/Cams_Springs/Power_Screws_2.html

desertfox

 

[parind]

thanks desertfox, for the references.
is these fundamentals are good for buttress threads too?

 

[desertfox]

Hi parind

The references on the Roy Mech site are definetely for Buttress threads, the other site also as a power screw section but I haven't studied it closely.

desertfox

 

[Bribyk]

Machinery's Handbook has dimensions and tolerances for Buttress threads but not much on calculating the strength.

Keep in mind that these threads are stronger in one direction (push or pull) than in the other direction because the angle on the front and backsides of the threadform is different. Used in gun breeches, oilfield tools, aircraft propellers, etc...

 

[MJCronin]

parind..

See slide number 29 of this lecture:

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

It was always my understanding that, for properly designed ANSI inch-grade fastener designs of equal grades, the stud will fail before the nut.

My recollection is that the strength of the fastener is always governed by the stud.....never the nut stripping strength.

This is reflected in the testing methods for ASTM materials such as ASTM A194. A random sample of nuts are tested to destruction by threading onto a matching stud an applying a tensile force. Acceptable results are failure must always occuring in the stud....no stripping of the nut threads permitted.

I realize that your 5 inch example may be extreme, but I believe that your joint should follow this principal.

-MJC

 

[racookpe1978]

Be very, very careful of how the millwrights (mechanics) actually put together these nuts and studs.

if they are not absolutely clean threads, that the anti-sieze is uniformly and corretly applies, and if they are not carefully and accurately torques properly iin the peoper sequence EVERY time the joint is taken aprat and put together - all your "assumed" thread pre-loads and metal stresses might as well be thrown in the toilet and flushed.

A bad single thread or galled thread (with too little anti-sieze or too much rust or dirt) will easily mean that your carefully designed stud will only actually carry 1/2 the load that its two neighbors carry.

So its two neighbors stretch - then all three begin to pick up the load, which changes the stresses on their two neighbors outside the orignal three .....

 

[EdDanzer]

Racookpe1978,

You get a star.

I call them Butt-kiss threads, because you never know when you can kiss your ass good by installing them. The poorest thread choice possible for anything. Acme, or square if 60° is not applicable.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[GuyFromDenver]

typically when I am sizing fasteners I calculate the axial shear stress using the effective area along the pitch diameter. From there it's p/a. Once you have the load at yield apply tolerance for lubrication, surface finish and variations in material and torque application. As a rule of thumb I apply 20%, but all of your factors should be considered. It seems what you are looking for is "failure by thread pullout." This is fairly common and should never be neglected when sizing a fastener. Most people just look at the axial strength of the fastener, but as we all know, it's pretty easy to strip threads out! Something else you should look at is the strength of the bolt itself. There have been many times I was given a torque spec on a fastener but knew that the hex would shear before I got the torque I needed. Ford found that lesson out on the 85 Bronco II. Nobody could reach the torque spec because the hex stripped out before you could get there!