Lude Generation

This bolt:


vs this bolt:


Imagine a force applied through the direction from the nut end to the bolt head. Not travelling through the bolt itself, but say the material the bolt is through.

I would presume the first one is the stronger option, but on light applications, is there any reason to avoid using the second option? And how crucial is the angle? I would've thought 90 degree is the best surely?

I realise I'm not using the proper terms and being a bit cryptic, but it's a pretty basic question (I hope).

Assume both the bolt and the material are made of the same stuff.

i would also use a bolt rather than the countersunk option, especially if it didnt matter if the head of the bolt protrudes through.

Short answer is that the flat head bolt is the strongest solution, however the reasons why are more complex...



Everything else being equal (bolt material, size and hardness, tightening torque, bolt head size etc. ) it isn't the bolt that's the most limiting factor, it's more the material that the bolt is fastened through.

In general, the flat head bolt will be able to transfer more load into the material, because the force is transfered in one direction only (tension or compression, depending on the direction in which the force is acting).

With the countersunk bolt however, you have the force being transferred laterally into the material too, so the forces act to 'spread' the hole rather than try to pull directly through it. The material has already been deformed by the countersinking, so is already 'affected' and not in its ideal state. There is also less material below the head of the countersunk head than the flat head to resist the forces. So basically the countersunk bolt is more likely to pull through the material than the flat head bolt.


Leaving aside the material and looking at the bolt itself, how the head has been formed will also affect its strength. If, for example, the flat head bolt has been forged but the countersunk head has been turned (cut) the flat head bolt would again be stronger (everything else being equal) because the structure of the metal hasn't been 'broken'. This means the head of the forged bolt is less likely to shear from the shank. With a large bolt, it's likely that a countersunk head will have been turned but a flat head forged.

Then there's the fact that on balance, a flat head bolt head has more material to it than a countersunk head, so it's better placed within itself to transfer the forces into the shank than a countersunk bolt, which will tend to collapse.


As for the angle of a countersunk head, yes that makes a difference also. The shallower the angle (e.g. 60 degrees v 90 degrees) the more likely the head of the bolt is to 'collapse' in on itself and the material deform outwards and thence allow the bolt to pull through the material. The corect angle to select for a particular application is based on the relative hardness of the 2 materials (and other limiting factors like thickness of the material/allowable amount of protrusion etc.)

wurlycorner wrote:So basically the countersunk bolt is more likely to pull through the material than the flat head bolt.

I thought as much. Deformity of the material and pulling through was my main concern but I hadn't given much consideration to the bolt itself. Should have known about messing with lattice structures all things considered.

Flat head it is then.


Just for fun:

M7 bolt, (no protrusion permitted on the countersunk bolt)
Material ~8mm thick and roughly 35mm x 35mm
140N applied evenly to the material in the direction of the bolt head

Is there a relative percentage of strength that can be easily calculated?

Again I'm being a bit vague and applied mat-- I mean engineering isn't my pair of chaps. And the 140N is a massive guestimation based on fag packet maths and laziness.

There are standard tables for the maximum strength (applied loads in specific directions) that different grade/size bolts can withstand. I think you should just be able to look them up on t'interweb.

You're being very vague, but that's probably because you want to solve the problem yourself... You also need to consider distance between centres (if more than one bolt holding your 2 mysterious things together).

You should be able to find worked examples by searching on t'interweb.

I would have to look up the equations myself cause I can't remember all that stuff any more (don't do it often enough, though I can still usually assess something put in front of me to work out whether they're talking BS or the calculation 'probably' stands up )

wurlycorner wrote:You're being very vague, but that's probably because you want to solve the problem yourself...

Yes


I suppose in a roundabout way what I was asking for was an equation so I could put the numbers in myself, but couldn't think of the right way to phrase a question to the oracle.

All I have in my head is extrusion but I don't think that's anywhere near what I need. It think it's the opposite?

@wurlycorner



That above, although the force is applied in the opposite direction to how it will be in my instance, because of Newton's third law it doesn't matter? I can just switch the position of the values for (I think it says) Aa and At? (well not switch but inverse function blah blah)

If I can actually find the equation that is relevant to that image and that it is the right equation to use, that is.

I no longer need to know as the decision is made, but I am curious

So you'd be pushing the die towards the left, instead of pushing the billet to the right. Yes it would be the same equation (normally just easier to move than material than the extruder though )

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And now, angry ticks fire out of my nipples.