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Limited tightening torque of different bolt's head type

Limited tightening torque of different bolt's head type

Limited tightening torque of different bolt's head type

Hello everyone,
I have to design a bolt made of a material that isn't available in the market and in addition has to be a hex socket button head cap screw. I know that this type of screws has reduced tightening capability that are given in standard table for common materials. In addition I heard (I am not sure it's true) that in this case (low profile and minimized bolt head) the weak chain is the bolt head, and that's the torque is calculated with intention not to yield the bolt head and not the threaded shaft portion. In my case I have to calculate the allowed torque (derived from the axial tension and shear due to torque application) by myself. The problem is that in any design book the focus is on traditional hex head and in these case the strength of the threaded part and the thread teeth are taken into account for determining the allowed torque, and there is nothing about the bolt's head strength.
Is there any design factor taken when dealing with low profile head? Does anyone can refer me to a reference explaining it?

RE: Limited tightening torque of different bolt's head type

Calculate the shear surface of the material between the thread runout and the drive socket. Use the shear properties of your special material to calculate the force to cause shear. Equate that force to the capscrew preload. Calculate the amount of torque to create that preload. Apply your design margin to arrive at +- allowed torque.

Presuming failure mode in shear.


RE: Limited tightening torque of different bolt's head type

This question seems familiar and came up a couple of weeks ago. Same deal - button head recess strength question. I think the most reasonable answer is to proportion it to an existing fastener based on engagement depth and material yield strength. Also note that hex wrenches frequently have edge relief that can eliminate a large portion of the engagement and that grinding the ends of the hex wrench flat is needed to develop the full strength.

I cannot imagine much interest in designing a purposely weak fastener head, but this is the second recent request. Seems like a good research project to perform and write up. Most users of fasteners want them to not have the torque application tool interface feature fail before the fastener is tight. When they do, it's by careful creation of a shear feature, not by failure of the interface with the tool.

See https://www.eng-tips.com/viewthread.cfm?qid=477576

RE: Limited tightening torque of different bolt's head type

Thanks for replying.
Did you mean to take in account the peripheral area indicated in the pic? if not, can you draw it please?

RE: Limited tightening torque of different bolt's head type

hey again,
I checked it again and Bossard declare that such bolts indeed have a reduced strength:

They refer us to ISO 898-1.
It's written there that indeed such head design make the bolt fail not in the threaded part, but there is no instructions how to calculate the allowed tightening force or torque.

I think to take a design factor over a regular calculation.
I looked at a manufacturer recommendations and I see that the allowed torque of such weakened bolts is about half of a regular one.

What do you think?

RE: Limited tightening torque of different bolt's head type

Looks like what I posted on another buttonhead capscrew thread.


RE: Limited tightening torque of different bolt's head type

The difference in recommended torque could be due to the depth of the socket. The button head has 0.68 the depth of the hshcs.


RE: Limited tightening torque of different bolt's head type

The failure at socket-key interface is also to be considered. Socket may fail due to bearing stress at corners when key is loosely fitting in the socket. In addition the weak socket material can also fail in shear at cylindrical surface enclosing the socket hole.

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