Fastener Torque Capacity

[mcguirepm]

Does anyone know where I can find a chart that shows fastener toqueing capacity based on the type of head (i.e. SHCS, Flat, button, Hex, etc.)?

Any help would be appreciated.

Pat

 

[NickE]

Google.

 

[drawoh]

mcguirepm,

What does the type of head have to do with fastener torque?

JHG

 

[ARenko]

I'm with drawoh. Proof strength and bolt dimensions (tensile area, tpi, nominal diameter) should be all that matter with respect to the bolt itself. Friction and preload relaxation are also important when considering the required torque for a specific clamping load.

I'm guessing not all head types are made in all grades. For example maybe you cannot find a Grade 8 button head capscrew, so in that sense you can find a difference.

 

[mcguirepm]

The type head dictates the size of hex socket and therefore the maximum torqus that can be imparted on a fastener. For instance, a 1/4" SHCS has a 3/16" hex while a 1/4" flat head has only a 9/64" hex. The difference in hex size would determine the maximum torque that can be conveyed through each size wrench.

 

[ARenko]

It may be true that the socket limits how much torque you can apply to a bolt, but I would think it is not the limiting factor in the torque capacity of the bolt. What good would the head be if it cannot take the torque required to put the proper tensile load on the bolt? If the head is a limiting factor, then maybe that bolt is designed for light fastening applications and not critical high strength applications. And in that case, do you really need to know torque capacity?

 

[swall]

With some high strength bolts, such as 12.9 metric, a torx internal bit is marginal. We used to break them all the time. But,they were Snap-On and they always were replaced free of charge.

 

[CoryPad]

There isn't a lot of published data on this. Here is one set of data:

http://www.textronfasteningsystems.com/products/torxplus/tp_eng_tools.html

Regards,

Cory

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

 

[Screwman]

There are two general groups of fasteners: structural and non-structural. The non-structural designs are those where the part will fail in a mode that is not through the threaded shank portion. Pretty much all external drive fasteners fall into the structual grouping. Virtually all of the low profile (truss, button, oval, flat) head designs in the industrial standards (aerospace excepted because of special head envelopes and requirements) fall into the non- structual group and will break from the underhead radius into the recess at a load lower than the UTS of the threaded portion.
The socket head cap screw design is an exception and it is structural, even under a wedge loading. The common pan head design is semi marginal. In almost all cases it will carry a full thread UTS in both straight and wedge tensile, but there are drive types and fastener sizes where it will not be sound under a wedge loading.
We had a long time engineer in our office who used to say that you can have a low head or a strong head, but not both.
One place to see this effect is in ISO 898-1, where they state in the scope of the spec that "Certain fasteners may not fulfill the tensile or torsional requirements of this part of ISO 898 because of the geometry of the head which reduces the shear area in the head as compared to the stress area in the thread such as countersunk, reased countersunk and cheese heads (see clause 6)."

 

[drawoh]

mcguirepm,

I have had problems in the past torquing 4-40UNC stainless steel hex socket flat head screws. I did not solve the problem by specifying the correct torque. I solved it by specifying phillips flat head machine screws (and changing the countersinks).

I do not think you should let the socket or other fastener engagement dictate the clamping force of your screw.

JHG