thread durability

[cervantes]

hello,

this seems to be a simple case however I have some doubts

there's a statement that thread coils (not sure if I used correct word, eng is not my mother language) are not carrying a load (screw core is always taken under consideration during calculations) - but when the thread of levelling feet will be destroyed? I mean, will there be a situation that nut with load will "Cut" thread and it will hit the floor?

shall I consider then in this particular case somehow load divided by total area surface of thread ?

If manufacturer of levelling feets gives in specification that steel version of M12 can handle some amount of Load, what exactly will happen with feet rod when this value will be significantly crossed?

 

[Tmoose]

I think the manufacturer could elaborate. I'm guessing the thread's load capacity is not setting the limit. Characteristics of the rubber pad on the foot, the lateral strength of the threaded rod (especially at maximum extension) and the capacity of the swiveling joint were probably considered too.

Are there any rotating components in the machine on the leveling feet? Those mounts' lateral stiffness is poor, so problems with resonant frequencies and resulting excessive vibration are not uncommon.

 

[cervantes]

thanks,

but imagine the foot is made of homogenouse material and let's focus only on behavior of the thread - I will make physical tests to check it, but I am trying to estimate a rough formula for thread damage in this situation

 

[3DDave]

It is very difficult to cause screw threads to fail in shear due to compression loads.

Most often the extended portion of the item will deflect and bend.

There is no general case solution.

 

[Tmoose]

http://www.amesweb.info/Screws/Metric_Bolt_Grades_Strength.aspx

If there are 2 diameter ( 24mm) of engagement even if the machine is made of aluminum the full bolt strength is likely available.

 

[desertfox]

Hi Cervantes

If the load on the machine foot or male thread exceeds the stated allowable from the manufacturer then the threads will fail in shear or as stated by yourself the thread will get cut and it will hit the floor.

The calculation is relatively easy in that you need to obtain the shear area's for the female and male M12 threads.
The relative shear area's will depend on the amount of thread engagement but in your case then the area's for minimum thread engagement should be calculated as these will be the worst case.
see this link for shear area male and female threads:-

http://www.roymech.co.uk/Useful_Tables/Screws/index_screws.htm

The link might be temporarily unavailable.Once armed with these shear area's divide the respective area into the load for each male screw and obtain the shear stress for the male and female threads, these shear stresses should be checked against the allowable shear stress for the respective material grades. Provided the shear stress is below the allowable then you're good to go.

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

 

[cervantes]

yes I was thinking about this kind of calculation but before I started I thought the results will totally not comply with experiment

I need to check it

 

[Tmoose]

https://www.grainger.com/product/S-...m/rp/s/is/image/Grainger/4TPY2_AS01?$smthumb$

The link is to 18 different kinds of leveling feet with M12 threads available from an industrial supply house. The Load ratings range from 250 lbs to 5000 lbs.

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here is a link to a bolt calculator. I did not see how to get it to accept metric units. I used a 1/2"13 bolt, which is a little larger than yours.
I note It does not take much more than 5 pitches ( about 0.4 inches, or less than 1X bolt diameter of engagement) to "protect" the nut or bolt threads from failure even when the tensile stress generated in the bolt is 30,000 psi.

http://www.boltplanet.com/Un/un.asp...r=xxx&txtseries_page=UNC&submit=Calculate+Now