Bolting 3

[cpercival]

Just wanted to know what are the considerations in changing from a regular head bolt to a C'sk head bolt in terms of the stress it can withstand & hence induced load. I am having to consider a C'sk due to reduced head height, and are using the same thread ( M8).

Thanks
Clive

 

[CoryPad]

Typically countersunk fasteners are designed so that they do not fracture at the head, so they should support the full force generated in the shank. The larger considerations are the mating parts and how they take the shear and compression stresses.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[cpercival]

Thanks for the information. Does this mean that generally that my C'sk bolt, as long as the grade of material is the same, will perform as well as the std. headed bolt I am replacing?

Thanks
Clive

 

[CoryPad]

GENERALLY speaking, yes.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dimjim]

Are you implying that you can buy these in grade 5 and
8 or 9 and use the standard clamping force. I do not think he is talking about socket head bolts in a counterbored hole. I thought c'sunk heads were only good
for small clamping forces.

 

[btrueblood]

The issue with c'sink heads is that in a multi-fastener group, if there is any real out-of-position error in the alignment of bolts to the c'sink, some of the fastener heads will see an added bending stress (the misaligned head is being forced to the center of the c'sink). Contrast this to a more standard head, where mis-alignment of the bolt and hole can be accomodated, with no added bending moment.

 

[KENAT]

btrue, I started to post the same but then started to think that as the OP says BOLT I'd guess he's using them with a nut, so they're floating fasteners.

If they are going into tapped holes in the mating part then that alignment is an issue. Increasing the csk will allow the heads to lie flush but doesn't solve the stress issue.

KENAT, probably the least qualified checker you'll ever meet...

 

[btrueblood]

Um. True, Kenat, provided there's enough clearance to allow the csk head to center itself. Another issue with csk heads is that the grip length of the bolt is reduced, which changes its tension-vs-turn characteristics; i.e. you are more likely to break the fastener when tightening, and getting the proper preload for the fastener is more difficult...which usually means you don't preload them as much, when means they can't carry as much load as a flat-faced fastener.

 

[Screwman]

On this one I am going to take an exception. You need to pay attention to the max load carrying capability of the head. Many flush head designs are head critical and will fail through the head and recess at a load that is lower than the maximum of the threads. In the case of socket flat heads this value is almost 20% less than the equiv. SHC part. 100 degree heads are very weak designs.
Unless proven by testing, I do not use a flush head design in a structural application that will demand full strength from the part.

 

[KENAT]

You're right btrue. Even if used with nuts by using CSK fasteners they are a 'fixed fastener' case with the impact on hole tolerances compared to 'floating fastener'.

KENAT, probably the least qualified checker you'll ever meet...

 

[cpercival]

Thanks for all the replies.They have been extremely useful.
I have now found out that the particular "taptite" fastener only comes in 8.8 quality. The bolts are bolting a steel ring to an aluminium substrate. Previously bolts were 10.9. I assume I am going to reduce my clamping load due to the lower toque I can apply to this bolt.

 

[Screwman]

Taptites are also available in an alloy steel version which will meet PC10.9 strength levels. This is a through hardened version that is designed just for tapping into aluminum.
I would consider that the likely failure mode of this assembly is going to be head failure of the bolt at a level below the theoretical max level that the bolt shank is capable of. Ask your fastener vendor if this is a head critical design.

 

[cpercival]

Thanks Screwman. I have unable to locate a vendor of 10.9 C'sk taptites with M8 thread. We need a good screwing recess like allen key hex or Torx.
If you know of anyone, I would be extremely grateful if you could let me know a contact.

Thanks

 

[Screwman]

Contact Acument at Taptite in Belvidere, IL.

 

[TVP]

Try the Camcar ecatalog (Camcar is an Acument brand):

http://camcar.smartcats.com/default.asp

 

[EffHeat]

Doesn't a bolt head fail due to the stress concentration at the transition from the shank to the head? If this is the case, then isn't it logical that a CS bolt would hold more load as the stress concentration factor is directly related to the radius of the transition. Smaller the radius = grater stress concentration factor. Because a CS does not have a full 90 at the transition I would think it's radius would be greater and therefore have a lower stress concentration factor to handle more load.

my $.02

 

[cpercival]

Thanks everyone. I looked at the Camcar catalogue & the thread size is limited to M^. I need M8.

Thanks again

 

[Screwman]

Effheat,
The issue with recessed heads is failure from the underhead radius into the recess, as a result of having either a large and/or deep recess in the head of the part. You end up on flush head parts with a catch 22 for high load applications where either you put in a nice big recess to generate plenty of torque and clamp (and then the head breaks off), or you make the recess small and you get a nice strong part (but you can't apply enough torque get it tight enough to generate much clamp without the recess failing). It turns out to be a compromise as to what your end goal is for the joint. FLush heads are always fun!

 

[cpercival]

I am back again, with a slightly different query. The Taptite C'sk screws I have chosen only seem to come zinc plated ( susceptable to hydrogen embritlement). These screws are to be used in an outside aggressive environment on land and in water ( including the sea). I am guessing that this finish is not entirely suitable.

Thanks in advance
cpercival

 

[CoryPad]

Correct, you should not use electroplated zinc for aggressive environments. A better coating is a zinc-flake based one - try Magni or Doerken.

Regards,

Cory

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