How does a screw work? 1

[drodrig]

Hi everyone,

Today I come with a very basic question but I've been debating with some colleagues about this and we haven't agreed.

When a screw (or bolt) fixes together two plates that work in tensile mode (forces perpendicular to the screw axis). Is the screw working in shearing or is the contact surface avoiding the sheets to move (because of the screw pushing one to the other)?

Attached a sketch

http://files.engineering.com/getfil...8-f17a-4f1b-897d-ef2f5e2bbc87&file=sheets.png

Let's say we have a M10 screw which is threaded on the lower plate (it could be a bolt with a nut). The upper plate has a 11mm hole.

When tightening the screw the plates get together and the friction doesn't allow the force (F) to move them.

Or is the screw working in pure shear? The threads would leave marks on the hole inner wall.

Is it a mixture of both cases?

cheers,

 

[GregLocock]

In a conventional design for a bolted joint the axial force in the bolt clamps the two plates together, and the in-plane forces in the plates are transmitted by friction.

That's why bolts and pop rivets work OK even in clearance holes.

However, not every joint is a conventional one. I dare say your wheel studs don't see much shear, if they are done up correctly, but I dare say there's a few million wheels that are currently suspended by bolt tension/shear rather than friction.



Cheers

Greg Locock


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[JNieman]

If bolts only worked in shear, you'd never have more than two bolts doing the work in any bolt pattern. The assembly would rotate until 2 bolts were in contact with the clearance holes, and then you'd only benefit from the shear strength of two fasteners, regardless of how many are in the pattern. This of course does not consider deformation of one part to eventually wear into a position where more than one is in contact.

The answer to your question is "yes". Ideally you'll never have to rely on the shear resistance of your fasteners though. That's why automotive manuals come with suggested torque values to important fasteners.

For even more information on situations that negate the shear of the fasteners, look up some structural design guides for slip-critical connections. If you have any of the AISC steel construction manuals, there's plenty in there.

 

[drodrig]

Hi there,

First of all, thanks for the answers; now I understand the situation.

I still have another question:

In the case of the the sketch I sent, one has to calculate the screw in shear, right?

Why is that? it is only working in axial direction

cheers

 

[JNieman]

I have no idea what the parts in your sketch are doing, what their purpose are, the nature of forces they deal with (load reversal? static?) and thus; no clue as to whether or not it's worth the time evaluating shear failure. That's sort of a discretionary call, imo, as to whether or not you should. You should already know the answer, to be honest.

There are some pretty good fastener connection design reference guides out there. Some vendors have truncated fastener connection design manuals, some people like to use manuals from NASA (probably because it was easily found via google) or ASIC Steel Construction Manual (if relevant), or maybe there are industry-specific guidelines you are to follow. Context dictates direction, typically.

 

[drodrig]

My sketch was just a made up example for demonstration purposes only, but it can apply to many situations.

One case we have is a vertical plate where many parts are held in cantilever position

http://files.engineering.com/getfil...-4073-9e23-281e17edffa0&file=plate-weight.png

I'll check manuals about this
thanks

 

[BrianE22]

If the parts are clamped together tight enough that the friction can handle the load then you do not need to worry about shear in the bolt. The bolt/friction surfaces are very stiff compared to the shear stiffness of the bolt so there will not be much shear load in the bolt.

But if your clamp load isn't high enough....

 

[desertfox]

Hi Rodriguez

When a bolted joint is made the bolts are in tension and the components are in compression, now if a sideward load is applied as shown in your original post then the joint is in shear however if the bolt doesn't touch the side of the hole or holes it passes through then the shear is resisted only by the friction generated between the clamped plates.
If the bolt isn't preloaded sufficiently then the bolt will hit the side of the clearance hole and resist the shear force.

In your later post the offset weight hanging on the bolts generates tension in the bolts due to the offset of the load or moment, in addition the bolts are also in shear.



















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

 

[JStephen]

In structural connections, some bolts are designed for shear, some are designed to transmit forces through friction. The main difference is that the latter case requires some way to ensure adequate tension in the bolt.

In the case of wheel lug nuts, the nuts are tapered and the seat on the wheel is tapered to match, so it's not a pure friction situation anyway.

 

[GregLocock]

Jstephen, there is no taper in the wheel to hub joint. It is a classic bolted joint design. The taper is there for alignment purposes.

Cheers

Greg Locock


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[JStephen]

Huh?

 

[desertfox]

Thread68-351384

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

 

[GregLocock]

Take a wheel off your car. Hub has a flat flange. Wheel has a flat flange. The only taper is between the nut snd the wheel.

Cheers

Greg Locock


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[benta]

You both mean the same thing, but phrasing problems exist

Benta.

 

[drodrig]

desertfox, why in the second example the screws are in shear? because of the momentum? I would see more traction in the upper screw but not shear...

How do I know how much I have to tight a screw to make sure the surfaces work in shear? can anyone address me to a standard/formula?

thanks everybody!

 

[desertfox]

hi
~Look at this site particularly where its titled bolts in bending, notice each bolt shares the total value of the offset load in shear.

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

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

 

[drawoh]

drodrig,

There is another thing to consider about bolts in shear. I have evaluated some aircraft mounted structures for crash safety. I am not allowed to assume tight bolts because it is difficult to determine how tight the bolts are. For any load other than direct tensile pull, calculations are done with the assumption of shear loading. It is better to tighten the bolts down properly, but shear is assumed for analysis.

--
JHG

 

[3DDave]

Greg Locock - every car wheel I've taken off has a pilot diameter that takes the vehicle weight shear loads. With a little corrosion, it also takes the axle torque shear loads.

 

[GregLocock]

That's called a hub centric design. It is not the only common option, and it is not intended to take the weight of the csr. It provides alignment while you tighten the lug nuts.

Cheers

Greg Locock


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