Thread strength of casted structure

[Sjqlund]

Hi eng-tip readers

I have a large casted steel structure. I will be using high strength machine screws to attach another steel structure onto this casted structure.

Since it is not possible to use nuts, and hence the threads in the casted structure must carry all the bolt load, i am a bit worried about how much i can pretension the bolts.

Eurocode specifies a preload of 0.7*A_t*f_ub/1.1. I have M48 bolts, such that A_t is 1470 mm^2 and using high strength 10.9 bolts, f_ub=1000 MPa. This yields a pretension of more than 900kN.

I've found a rule of thumb that says the minimum thread engangement should be 1.5d, i.e 72 mm. The thread length is 65 mm though, so a bit shorter. However this rule of thumb does not take bolt quality into account, which makes me a bit worried. Of course i could just use less pretension, but at the same time i want as much as possible, since the friction is very benificial in the situation i'm looking at.

I've also tried calculating the average stress in the threads. However it is hard to know how many threads will carry the load.

The cast material has properties Tensile YS = 220MPa, Young's Modulus = 170GPa, Ulitmate YS = 310MPa.

What would you do in this situation?

Best regards, Jones

 

[desertfox]

Hi

You need to look at the internal thread area and check it for shear stress, go to this site it tables some internal thread shear area's.
I couldn't see the bolt size on your original post but I'd guess from your engaged length your talking 50mm diameter approx.

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

 

[Sjqlund]

Hi desertfox, thanks for helping out.

My problem with these thread stress calculations is basically the assumption of: "When the female and male threads are the same material."

I am connecting a material with ultimate strength of 310 MPa using a bolt with ultimate strength of 1000 MPa.

Of course i could just use a factor of 3, but then i end up with results being very conservative, i.e. far away from the rule of thumb of thread length>1.5*d

 

[TVP]

High strength bolts/screws are used in threaded holes in castings (cast iron, aluminum, etc.) everyday. The limiting condition is shear strength for an internal thread, while it is tensile strength for an external thread. This is why the external thread will fail before a softer internal thread as long as there is sufficient engaged length, i.e. generally greater than 1.5 x D. You can search the archives here for more information.

 

[desertfox]

Hi

Firstly before deciding how much to tension the bolts you need to know what the external forces are, then you can decide the size and number of bolts to use.
The limiting factor is the cast material and you need to know the allowable shear strength for that material(as an estimate say half the yield stress).
The preload on the bolt should then be divided by the shear area of the internal thread and the shear stress value should be around half the yield stress maximum as quoted above.
Don't fall into the trap of tapping the hole to twice or three times the bolt diameter because after the tapped hole depth exceeds one and half bolt diameters
There is little advantage.
How the load is divided across the bolt engagement is very complicated and there are different theories flying around

 

[SnTMan]

Sjqlund, Machinery's Handbook treats strength of threads in differing materials. If you have this book, see the section "Torque and Tension in Fasteners." If not, suggest you get one

Regards,

Mike

 

[Sjqlund]

Thank you for your answers


Would it be reasonable to find the shear stress as if half the threads are carrying the load? My reasoning is that some (allowable) yielding will occur which will help distribute the load + the tapped hole length is only ~1.4*D


I am aware of theory that says that the first thread carries 30-40% of the load. However if i were to use this for calculations, the allowable preload i'd find would be very low compared to the other approach.

 

[desertfox]

Hi Sjglund

I was say using half the thread depth is reasonable and its errs on the side of caution also.

Regards

Desertfox

 

[tbuelna]

Sjglund,

As you noted, the first (entry) thread of a tapped hole typically carries a disproportionate percentage of the initial fastener load. The result is that it yields, and then the remaining threads begin to assume more of the load. For some applications this type of material yield in a structure is acceptable, but for other applications it is not acceptable.

 

[Sjqlund]

I would maybe give it more consideration if i was designing for dynamic loading and fatigue, but right now it is just a static load and the structure is only to be used a 'few' times (few as in relative to fatigue).
And for this i think my assumptions are reasonable.

However this is just my own 'feel'. Let me know if you think i'm wrong.

 

[Tmoose]

For charts of what UNBRAKO felt in 1996 were reasonable tapped hole thread strength to develop full fastener strength of a quality socket head capscrew into brass, aluminum, cast iron and mild steel, look at page 66/68 here -

http://www.dalecompany.com/pdf/UNBRAKO.pdf

Note their comment on page 64/66 about using figgerin' and cypherin' to come up with tapped hole thread strength being "not entirely satisfactory". Thus what they chose to present in the technical section of their "engineering guide" were tables created using empirical data from tests done with threaded test pieces, real bolts, and torque wrenches of some type.

For an online discussion with a rule-of-thumb allegedly from a machine design handbook, look 2/3 of the way down here-

http://store.curiousinventor.com/guides/Metal_Working/Screws/

26th Edition of Machinery's Handbook has some formulas for calculating such stuff, too.