shear loads on countersunk screws

[tmschrader]

The shear (single) load capacity of a countersunk screws should consider the bending caused by part of the bearing surface being at a angle. The loads can really reduce thier capacity, when compared to non csk heads, from what I have seen. Are there any references on this topic so I can back up my calcs

 

[dooron]

My comments are general.
I believe that the effect of the countersink angle is secondary for the screw stress. The loads generated on the screw itself are still basically the same bending loads as with a screw that is not counter sunk if both are pretorqued and not loose in their attachment. there may be marginal additional tension loads and bending on the screw due to the vertical reaction force acting at an offset due to the countersink, but this may also occur in normal bolt heads (seperation forces). The benefit of the countersink is to increase the bearing area in the bolted material and therefore reduce bearing stresses (which we usually find to be the driving factor)compared with non counter sunk bolts.

 

[yates]

tmschrader,
I agree with dooron. You should suffer no disadvantage from using a countersunk head screw as compared to a similar, protruding head screw. However, you bring up a valid point which is often forgotten during bolted joint design, i.e. in shear applications with no preload, it is easy to have a situation in which the stress concentration in the underhead radius (due to bending stress cocking the bolt over onto one corner of its bearing surface)leads to failure, especially if fatigue is involved. Direct shear resistance of the bolt shank is not the only factor to take into consideration when designing a 'shear' application.

 

[gbent]

In this application, is the fastener being used as a screw or a bolt? Given the large, tapered area under the head of a countersunk screw, can the proper tension in the body be reached on a regular basis? My experience is not. The variable friction from forming the countersink in the workpiece, possibly combined with the misalignment of the countersink and the threads, makes accurate torquing of the screw impossible. When used as a bolt, and the nut can be torqued, the situation in very different.
My mechanical design professors always taught if the screw saw shear stress, the joint design was faulty. Ie, the preload in the fastener should cause sufficient friction in the clamped materials to resist movement, not the bolt acting as shear pin. If you want a shear pin, put one in.