Is a countersunk flathead screw in and of itself a locking fastener?
Is a countersunk flathead screw in and of itself a locking fastener?
(OP)
A discussion has come up in our engineering department concerning countersunk flathead screws being a locking fastener. I disagree with this statement. I believe the additional surface of the conical head may reduce loosening, but don't believe it should be refered to as a locking fastener. What are my fellow engineer's thoughts?





RE: Is a countersunk flathead screw in and of itself a locking fastener?
John R. Baker, P.E.
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Siemens PLM Software Inc.
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RE: Is a countersunk flathead screw in and of itself a locking fastener?
Ted
RE: Is a countersunk flathead screw in and of itself a locking fastener?
RE: Is a countersunk flathead screw in and of itself a locking fastener?
RE: Is a countersunk flathead screw in and of itself a locking fastener?
RE: Is a countersunk flathead screw in and of itself a locking fastener?
RE: Is a countersunk flathead screw in and of itself a locking fastener?
I can't see how they are locking though. I don't think the area under the head is even detrimental to the friction / locking property. Rather, I think that prestraining the shank to a value at which rotation cannot occur, is the mechanism by which locking occurs (don't quote me on that though, I'm not 100%).
Also, F = u*R - Surface area doesn't come into it, and pondering further, wouldn't the reaction be grounded over a smaller radius anyway, for a conical head of the same surface area as a flat head?
RE: Is a countersunk flathead screw in and of itself a locking fastener?
RE: Is a countersunk flathead screw in and of itself a locking fastener?
The only forces acting are frictional, developed from the preload in the fastener.
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Like in McGyvr's example, there may be some "special" circumstances that could lead to locking behaviour, or the appearance of locking behaviour.
Post Hoc, Ergo Propter Hoc
Accepting something as standard practice or physical law based on a single event, or worse yet, extrapolating data to all designs can be looking for trouble. When I was 21 I had unprotected sex one time with Nancy, but I held my breath at "the moment", and she did not become pregnant. I would not suggest any useful technical conclusion can be drawn from that single data point, factual as it is.
RE: Is a countersunk flathead screw in and of itself a locking fastener?
In addition, a square shoulder bolt is constraining movement in one plane only, given the fact that the clearance hole for the bolt IS going to be slightly oversize. The FHCS effectively constrains in 3 planes, and, properly pre-loaded, will therefore reduce or eliminate the possibilities of minute movements, which I believe is the root cause of loosening in square shouldered (clamping perpendicular to the axis) hardware.
No, I still don't think you can rely on them to be a self-locking fastener in a critical application.
My 2 cents.
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Interesting thought.
True for a two-ply joint where ply one has the countersink and ply two is tapped.
Not true if there are more than two plies with the last ply tapped, or if used with a nut rather than tapped ply.
RE: Is a countersunk flathead screw in and of itself a locking fastener?
FAQ404-1257: Is a lock washer an effective torque retention device?
thread404-230741: Studies on the use of lock washers?
thread404-295664: Lock washer witth teeth useful?
thread725-85323: Lock washers with nylock type nuts?
thread725-188550: Double nutting?
thread725-89146: What value conventional lockwashers?
Mostly talking about lock washers in particular, but some of the comments might be useful/interesting. Somewhere in the vast reaches of the forum threads was a link to a video showing a fastener undoing itself under vibration; quite fascinating.
TTFN

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RE: Is a countersunk flathead screw in and of itself a locking fastener?
Now to throw the whole thing for a loop, check out http://www.spiralock.com/
You'd be ahead of the game with a flat head screw with that type of thread profile, not that they even exist or anything. It's just a cool setup.
James Spisich
Design Engineer, CSWP
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Ted
RE: Is a countersunk flathead screw in and of itself a locking fastener?
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Did I dream that up or has anyone else seen it?
PS: after tightening of course.
rmw
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Now, that's scary.
WRT staking the screw head, no, I haven't seen that, but I swear that I've seen the substrate extruded into the slot of a flat head screw by means of deep punch marks. Don't lose track of which screw goes in which hole...
Mike Halloran
Pembroke Pines, FL, USA
RE: Is a countersunk flathead screw in and of itself a locking fastener?
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Here is what Bickford has to say about conical surfaces and preload in Handbook of Bolts and Bolted Joints:
Caution is warranted.
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Ted
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Let me throw a few more dynamics into the mix. This fastening style will be used in a higher vibration application (commercial and military aircraft) and the joint is an electrical joint (not allowing the use of Loctite due to its insulative properties) that is allowed to see temperatures as high as 160 degrees centigrade. The flathead is used due to the need for a flat surface joint, plus the thinkness of the material does not lend itself to a counterbore with lockwasher and protruding head fastener (e.g. binder, pan, round).
We do use formed or cut locking threads, but also use Loctite as "suspenders" for a back-up in that fastening style. I am leaning toward a locking helical thread insert.
What is everyone's take on that idea?
RE: Is a countersunk flathead screw in and of itself a locking fastener?
If the joint is disassembled after cure, then reassembled without cleaning the cured resin, that's a different story.
I.e., you should test Loctite in your application before ruling it out.
Mike Halloran
Pembroke Pines, FL, USA
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Tap Spiralock threads:
http://www.spiralock.com/
Ted
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Regards,
Cockroach
RE: Is a countersunk flathead screw in and of itself a locking fastener?
even better, drive the screw 3/4 of the way home, then bend over/mangle the head of the screw into the panel being fastened. No way tha's coming loose, even with a screwdriver!
RE: Is a countersunk flathead screw in and of itself a locking fastener?
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If we learn from our mistakes I'm getting a great education!
RE: Is a countersunk flathead screw in and of itself a locking fastener?
ScottyUK, you are correct with your comment about the fastener not being a good conductor. The current carrying path is the material around the fastener. As far as Loctite's insulative properties, we have found if Loctite 290 (what we commonly use) gets between the surfaces of an electrical joint (bus to bus), it can greatly increase the resistance of that joint.
Thank you, Engineering Community, for all of your input.
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Countersinking will not accomplish this, so, my vote is no too.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Is a countersunk flathead screw in and of itself a locking fastener?
If your application is cabin only, and an electronic device, then proper seams & joint Design for EMC enclosures is required; and if so then proper contact pressure must be maintained to ensure conductance. Countersink flat head screws generally do not meet these requirements.
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Are the screws made from steel? also what are the two materials the screws are clamping together? The reason I ask is because if the clamped materials are copper or aluminium and the screws are steel there will be stresses generated within the joint when it gets to a temperature of 160 centigrade so you need to get the joint preload just right for the joints so it doesn't cause problems at the higher temperature.
I also agree with the others that the countersunk head screws are not self locking
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Mike Halloran
Pembroke Pines, FL, USA
RE: Is a countersunk flathead screw in and of itself a locking fastener?
I was being entirely facetious. As Mike points out, decent screws won't take much deformation before they crack.
If you have to maintain electrical and mechanical continuity, what about seperating the two functions - a grounding strap to connect the parts, with very little mechanical load being applied to the joint, and seperate mechanical fasteners with loctite to hold the heavier parts together.
RE: Is a countersunk flathead screw in and of itself a locking fastener?
If there is any question about why you should not use a split lock washer ... several references are summarized here:
http://hi
RE: Is a countersunk flathead screw in and of itself a locking fastener?
In my proffession, it has been used, with success, peening of the fastener to lock it in place. a punch is used and the very edge of the fastener is struck. the punch is actually placed on the intersection of the screws OD and the part it is screwed into. So when the punch is struck, both the screw head and the material around the screw are deformed. even with good screws, that part of the screw is rather thin and can be deformed slightly without cracking. staking the screw in 3 or 4 places will hold it in place without the need for loctite. the application was hot enough that most believed that loctite would not have good locking properties but would soften. loosing the screw was not an option as it was inside of turbomachinery.
removing a staked screw involved a little bit of grinding around the staked areas and the screw would be free to be removed.
RE: Is a countersunk flathead screw in and of itself a locking fastener?
Except they fall out when you *don't* want to remove them.
RE: Is a countersunk flathead screw in and of itself a locking fastener?