How does a screw work?
How does a screw work?
(OP)
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/getfile.aspx?folder=6...
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,
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/getfile.aspx?folder=6...
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,





RE: How does a screw work?
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
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: How does a screw work?
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.
RE: How does a screw work?
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
RE: How does a screw work?
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.
RE: How does a screw work?
One case we have is a vertical plate where many parts are held in cantilever position
http://files.engineering.com/getfile.aspx?folder=f...
I'll check manuals about this
thanks
RE: How does a screw work?
But if your clamp load isn't high enough....
RE: How does a screw work?
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
RE: How does a screw work?
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.
RE: How does a screw work?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: How does a screw work?
RE: How does a screw work?
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: How does a screw work?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: How does a screw work?
Benta.
RE: How does a screw work?
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!
RE: How does a screw work?
~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/Bolt...
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: How does a screw work?
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
RE: How does a screw work?
RE: How does a screw work?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?