Bolt dimension 1

[KavaKovala]

Hi everyone,

I have a question about sizing a bolted joint. How to calculate the size of the screw? Knowing that the bolt material is 316 stainless steel and the thread material where the bolt will be bolted is 6061 aluminum. This screw will be pulled at 2000N.

Thanks!

 

[drawoh]

KavaKovala,

You need a bolt that holds 2000N plus a safety factor. This is basic machine design. Are you an engineer? Sizing and designing bolted joints is not absolutely simple.

--
JHG

 

[KavaKovala]

Yes, however I am Electronic Engineer.

I would like to attach a stainless steel part to the aluminum part by bolts. The length of the thread on the aluminum part must be 15mm. I know I must calculate the shear on the aluminum thread to choose the correct screw, but I don't know how to do it.

I will use two bolts.

Could you help with this?

Thank you for your reply.

Let me know if you need more details.

 

[desertfox]

Hi

Any chance you can provide a drawing or sketch of the situation, we can provide formula for the shear area’s of the threads but it’s not always that simple.

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

 

[KavaKovala]

Desertfox,

Please see the attachment. I changed it a litle bit. I will use only one bolt.

Please, let me know if it is enough.

 

[desertfox]

Hi

Have a look at this website:-

https://www.engineersedge.com/fastener_thread_menu.shtml

Now I wouldn't recommend using less than two bolts, one bolt on its own cannot stop the part you have bolted rotating.
Basically what you have to do is find the shear area's of the male / female threads, obtain the shear strength of the material for the male and female thread and then using a safety factor of say 2 calculate the shear stress in the threads, by dividing the shear area of each male or female thread into the 2000N load you mention above.
The shear stress calculated should be less then the allowed shear stress for the material icluding the safety factor.

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

 

[MJCronin]

While you are deciding both the number and size of the bolts, it would be wise to evaluate the method of bolt locking that you want to use...

Is this part subject to vibration or shock ??.... a single bolt may not be a good choice on this basis alone.

You have a mismatch in strength choosing Stainless bolt threaded into Aluminum alloy

Also, with an aluminum bolted joint and a SS bolt, you are asking for galling problems if the bolt is to be repeatedly loosened and tightened

A longer SS bolt, threaded into aluminum will not develop more tensile strength.... you may want to consider a threaded insert to develop more bolt strength

https://www.mscdirect.com/browse/tn...ad+locking+inserts+stainless&navid=4287924475

Tell us about more about your final design and the purpose of your device ..

MJCronin
Sr. Process Engineer

 

[rb1957]

any thermal effects ?

restraining a part with a single bolt is worse than bad practice. You need some way to prevent the thing being bolted to the base from rotating (as you tighten the bolt). Several ways to do this …

add a second fastener, maybe a small spigot designed to react load from tightening torque.

add a clip to the base plate, flush to one face, so the bolted member can't rotate.

add a counter bore, so the bolted member fits into the plate, and has a face bearing up to prevent rotation.

if this is a very cheap installation, restrain the bolted member during assembly, and accept that things will come lose and live with that.

why "15mm" plate thread ? Typical rule of thumb is that only three threads are effective. You'll be much better off using a Heli-coil insert than threading Al plate directly. You will want to make certain, absolutely certain, that your bolt will never ever bottom out in the tapped hole (easy to do, easy to overlook).

How much adjustment is needed in the overall installation ?


another day in paradise, or is paradise one day closer ?

 

[IRstuff]

The other side of the coin is that if the load introduces a torque, then you could possibly loosen the fastener over time.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[KavaKovala]

Thank you all for your reply,

rb1957,

There is a guide pin between both parts in order to avoid rotation. I did not add it in my drawing because it is only a draft. I will use helicoil on the thread. There is not any thermal effects (room temperature, about 70°F).

I do not know if it is correct, however I did the calculation below:

Thread shear area (Internal)
As = (pi*Le*dmin/pitch)*[(pitch/2)+0,57735*(dmin - D2max)]

I am calcutating to use M8x1,25 Hex bolt.

As = internal thread shear area
Le = bolt length engaged on the aluminum thread = 15mm
dmin = minimum major diameter of external thread = 7,760
D2max = maximum pitch diameter of internal thread = 7,348
Pitch = 1,25mm

As = 252,427 mm²

σ aluminum 6061 = 83 N/mm²
Safety factor = 2
Load = 2000 N

σ = P*2/As = 2000*2/252,427 = 15,84 N/mm²

Since the shear stress of aluminum is 83 N/mm² and I need a material with minimum shear stress of 15,84 N/mm², I conclude that the thread for M8x1,25 bolt is sufficient to support this load.

However I have a doubt, if only three thread is enough, why the formula above use bolt length engaged on the aluminum thread to calculate the Thread Shear Area?

Thanks in advance!

 

[desertfox]

Hi KavaKovala

The maths presented in your last post are correct, however there are a lot of things about this joint that we here still don't know, is the load you are applying static or dynamic? If the latter then fatigue might well have to be considered.
The theory about how a load is distributed over threads is very complex and its true that the first few threads so take most of the load however if those first few threads taking the load yield then the load is spread out more as more threads come into contact with the mating thread.
Considering using a helicoil is probably a good decision as this will improve the shear area in the aluminium but you also need to do a calculation on the bolt itself to see whether the bolt breaks or the male threads strip similar to the calculation you did for the feale thread.

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

 

[KavaKovala]

Hi desertfox

The load is static.

 

[desertfox]

Hi again

Thanks for the response, what causes the load of 2000N

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