With the exception of &#;grub screws,&#; all screws have a head. It&#;s the top part of a screw that contains a recess for a tool bit. Screws are installed by placing a tool bit into the head&#;s recess and turning it. Although there are many types of screw heads, they can all be categorized as either countersunk or non-countersunk. Some screws have a countersunk head, whereas others have a non-countersunk head. What&#;s the difference between these two head styles exactly?
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A countersunk screw head is characterized by its ability to rest flush with the surface into which it&#;s installed. The head essentially &#;sinks&#; into the workpiece&#;s surface. If you inspect a countersunk screw head, you&#;ll notice that it tapers towards the shank. This tapered design allows the head to dig into the workpiece&#;s surface.
Screws with a countersunk head are typically used in woodworking applications. When building furniture, manufacturers often use countersunk screws so that the heads don&#;t protrude. If the head protrudes out of a piece of furniture, such as a recliner, it will likely be uncomfortable to consumers to use. Therefore, wooden furniture is typically manufactured using countersunk screws.
A non-countersunk screw head, on the other hand, protrudes out of the surface into which it&#;s installed. Rather than sinking into the workpiece&#;s surface, it protrudes out. Non-countersunk screw heads don&#;t taper towards the shank. Instead, they have a narrow shank that&#;s immediately proceeded by a wide head. Without tapering, non-countersunk screws can only be installed up to their head. When the head strikes the workpiece&#;s surface, the screw will stop. That&#;s as far as non-countersunk screws can be driven into a surface.
Non-countersunk screws are used in woodworking applications as well. They are oftentimes preferrable over countersunk screws because they place less stress on workpieces. The tapered area around countersunk screws is wider than their respective shank. As a result, installing countersunk screws into a workpiece can cause damage. Non-countersunk screws protect against such damage because they don&#;t have tapering. With non-countersunk screws, only the shank is driven into workpieces.
Countersunk and non-countersunk refers to the style of a screw&#;s head. Countersunk is a head style that features tapering, thereby allowing it to be driven deeper into a workpiece&#;s surface. Non-countersunk is a head style without tapering, resulting in the protrusion of the head.
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(Mechanical)
(OP)
5 Apr 12 16:10A 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?
(Mechanical)
5 Apr 12 16:16I agree with you. I would never trust something like that to be 'locking' without some additional 'intervention'.
John R. Baker, P.E.
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To an Engineer, the glass is twice as big as it needs to be.
(Mechanical)
5 Apr 12 16:17I would not consider it a locking fastener, the angle of the countersink is too wide. Also, other than the first screw in a pattern, the odds are against the remaining screws having 100% underhead contact.
Ted
(Aerospace)
5 Apr 12 17:06third vote for not locking ... sure there's higher friction in the CSK, but ...
(Mechanical)
5 Apr 12 17:12Maybe if the c'sink and the screw angles didn't match, and the part deformed to act like a lockwasher.
(Materials)
5 Apr 12 17:13Flat head (countersunk) fasteners do not provide locking nor extra loosening resistance. They do provide alignment and shear resistance in the absence of preload.
(Structural)
5 Apr 12 18:08Drive those mothers home, and then put a puddle weld over/around each head. That'll lock em down. Of course you gotta do the same thing at the nut, if one exists. And, you may want to weld the piece on the other side to the first piece unless there are several welded screws btwn. the two pieces. Or, you could use locktite or some such.
(Mechanical)
5 Apr 12 18:37I thought they were locating, rather than locking. We tend to avoid them in general, preferring counter-bored, if we need to hide the head.
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?
(Mechanical)
6 Apr 12 08:25Our UL inspector at one time said they accept flat head screws to fulfill any "no loosening" requirements on certain products/certain situations.
(Mechanical)
6 Apr 12 08:34Fundamentally, no difference from a bolt or screw were there under head surface is perpendicular to the thread axis.
The only forces acting are frictional, developed from the preload in the fastener.
(Mechanical)
6 Apr 12 11:21Flat head SHCS accept far less torque than regular SHCSs, so already are competing with one hand tied behind their backs.
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.
(Industrial)
6 Apr 12 13:41In actual practice and in absence of a seconday locking component, FHCS's require considerably more torque to loosen than do the typcial square shoulder configured bolts due to the conical geometry (wedge).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.
(Mechanical)
6 Apr 12 14:00Quote (Ornerynorsk):
The FHCS effectively constrains in 3 planes, and, properly pre-loaded, will therefore reduce or eliminate the possibilities of minute movements
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.
(Aerospace)
6 Apr 12 15:14All the above said, there have been a few threads here concluding that most accepted "locking" approaches are not, as well.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.
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(Mechanical)
6 Apr 12 16:34This is why loctite and nylon patches in the threads were devised.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
(Mechanical)
6 Apr 12 17:17I would suggest more torque is lost in friction due to the conical seat, therefore less torque is available for initial clamp load and the joint is loose from the start. The screw may be bound but the joint is not.
Ted
(Mechanical)
6 Apr 12 18:33I have been involved in several instances where fhcs' loosened. An additional issue seems to be the surface finish on the c'sink. With fluctuating fastener loads, high spots in the c'sink will burnish over time and tension is lost on the fastener.
(Mechanical)
6 Apr 12 21:48I've got a mental image of taking a flat headed countersunk screw, a punch and a hammer and staking the edges of the screw into the countersink in a couple or three spots.
Did I dream that up or has anyone else seen it?
PS: after tightening of course.
rmw
(Mechanical)
6 Apr 12 22:09>>>Our UL inspector at one time said they accept flat head screws to fulfill any "no loosening" requirements on certain products/certain situations. <<
Mike Halloran
Pembroke Pines, FL, USA
Goto yiyuan to know more.
(Automotive)
7 Apr 12 04:17I could be completely misunderstanding the question here, but isn't the reason car wheel nuts are designed with a taper to increase alignment and reduce the likelihood of loosening from vibration?
(Materials)
7 Apr 12 11:20Quote:
Surface irregularities exist on conical joint surface as well as flat ones. The effect on axial tension in the fastener, however, is magnified if the embedment occurs on a conical surface.
A given amount of relaxation perpendicular to the surface may mean substantially greater relaxation in the axial direction...
Vehicle wheel fasteners use contoured seating surfaces (conical or spherical) to provide alignment and shear resistance in the absence of preload.Here is what Bickford has to say about conical surfaces and preload inCaution is warranted.
(Mechanical)
7 Apr 12 11:47In wheel nuts, there may be some compression due to the seat which enhances thread binding/locking. Not the same as flat head capscrews.
Ted
(Mechanical)
(OP)
7 Apr 12 13:59Thanks folks for all of the great input.
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?
(Mechanical)
7 Apr 12 17:34Loctite only cures in the absence of air, and it ends up only in places where there ain't no metal anyway. I.e., it won't prevent or restrict the flow of current in a properly tightened threaded joint, because as a liquid, it can't keep the metal parts separated before it cures.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
(Mechanical)
7 Apr 12 18:27Locking Helicoils are stainless steel and may reduce conductivity of the threads. Test it.Tap Spiralock threads:
Ted
(Mechanical)
8 Apr 12 13:45Not self locking. I've had several experiences with them backing off with steel parts, even aluminum. Probably due to vibration, but not self locking.
Regards,
Cockroach
(Mechanical)
9 Apr 12 09:51Rmw,
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!
(Electrical)
9 Apr 12 10:53Your concern about the insulating properties of Loctite are interesting in itself. On heavy current joints I would normally expect that the joint is designed on the basis that fasteners used in the joint make no contribution to the current-carrying path, i.e. there is sufficient conductor material present around the fastener to carry the current in the circuit. In reality some current will probably flow through fasteners, but the joint is not designed with this in mind.
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(Mechanical)
(OP)
9 Apr 12 11:21btrueblood, we discussed your idea today and we are entertaining the idea of peening the edge of the head on the screw where it mates to the countersink. The only downside to that is rework no longer becomes an option.
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.
(Structural)
9 Apr 12 11:29From what I have seen, "locking" fasteners involve a connection where the bolt or screw is slowly engaged into a tension state as the lock washer, or other mechanism, is compressed, leaving the bolt or screw in constant tension.Countersinking will not accomplish this, so, my vote is no too.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
(Industrial)
9 Apr 12 12:35For aircraft application lockwire on screws / bolts is common, for engine assemblies.
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.
(Mechanical)
9 Apr 12 13:39Hi mecengtec
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
(Mechanical)
9 Apr 12 13:56If your screws are worth a crap, it will be difficult to peen the heads without cracking them, and it won't do the peening tool any good either. Better to peen the softer substrate over the edge of the screw head, or better, into the ends of any slots that reach the edge of the head.
Mike Halloran
Pembroke Pines, FL, USA
(Mechanical)
9 Apr 12 14:11"btrueblood, we discussed your idea today ..."
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.
(Mechanical)
11 Apr 12 17:54Based on the information you've provided, I would recommend a locking insert (helicoil or similar). In some extremely rare cases on military aircraft I've seen these fail, in which case Spiralok is a good alternative - but a bit pricy.If there is any question about why you should not use a split lock washer ... several references are summarized here:
(Mechanical)
12 Apr 12 11:45I have personal use with a bunch of flat head screws in high vibration application. if the fastener is not loctited, they will come loose.
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.
(Mechanical)
16 Apr 12 14:25Flatheads are locking in the sense that the hex usually strips out when you want to remove them.
Except they fall out when you *don't* want to remove them.
(Aerospace)
20 Apr 12 18:04Stake 'em with a blunted prick punch.
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