Is a countersunk flathead screw in and of itself a locking fastener? 1

[mecengtec]

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?

 

[CoryPad]

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 in Handbook of Bolts and Bolted Joints:

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...

Caution is warranted.

 

[hydtools]

In wheel nuts, there may be some compression due to the seat which enhances thread binding/locking. Not the same as flat head capscrews.

Ted

 

[mecengtec]

Thanks 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?

 

[MikeHalloran]

Loctite 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

 

[hydtools]

Locking Helicoils are stainless steel and may reduce conductivity of the threads. Test it.

Tap Spiralock threads:

http://www.spiralock.com/

Ted

 

[Cockroach]

Not 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

 

[btrueblood]

Rmw,

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!

 

[ScottyUK]

Your 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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If we learn from our mistakes I'm getting a great education!

 

[mecengtec]

btrueblood, 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.

 

[msquared48]

From 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

 

[pierdesign]

For 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.

 

[desertfox]

Hi 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

 

[MikeHalloran]

If 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

 

[btrueblood]

"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.

 

[rmedure]

Based 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:

http://hillcountryengineering.com/w...it-Lockwashers-Separating-Myth-from-Truth.pdf

 

[jbattershell]

I 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.

 

[imcjoek]

Flatheads 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.

 

[thruthefence]

Stake 'em with a blunted prick punch.