Torque limit of a socket head hex drive 2

[geesamand]

Hello everyone,

Today I'm designing an assembly that must be 100% 316 S/S and I have good reason to use button head socket hex head screws.

It's been my experience on small assemblies that these hex driven heads can strip out. So I assume my usual torque specifications for 316 S/S are too much.

Is there a reference or simple calculation that will offer an appropriate torque for hex drives in softer materials?

* Yes, galling is a risk and is not part of my question.

Thanks,

David

 

[DAVIDSTECKER]

One of my old text books "Machine Design Fundamentals" by John Wiley & Sons, has a chapter on how to calculate thread engagement, stress's and torques.
The chapter is on "Detachable Fasteners".
I have used it many times for different materials.
Typically I treat SSTL as equal to or less than Grade 2.
Best regards, David

 

[Turn_of_the_Screw]

You should find what you are looking for in this table:

https://www.engineersedge.com/material_science/torque_values_stainless_steel_13353.htm

 

[IRstuff]

The question, it seems to me, isn't what torque your fasteners can handle, but what force your fasteners are intended to provide to meet your design requirements. After that, you choose the fastener that can sustain and provide that force, be it 6-32 or 1/4-20. When our MEs typically look at these applications, they look to vibration and thermal effects on fastener loosening due to insufficient preload; only then do they determine the amount of torque required.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[btrueblood]

Geesamand,

I don't know of any calculation for the torque strength (resistance to stripping) of the hex socket in a fastener, and, like you, have seen multiple failures by strip-out of the hex socket in a variety of stainless steel alloys. Two things that help:

1. use strain-hardened fasteners per ASTM F837 Condition CW or CW1. Avoid using annealed fasteners.
2. do not use ball-end type hex drivers to torque the fasteners, mandate the use of full-length (squared end) hex drivers only. The partial engagement of ball end drivers to the hex will cause the hex to strip at very low torque levels.

 

[geesamand]

btrueblood,

Good point about the ball ends. On a design level, I can specify that but I need to ensure there is room to torque each fastener using a straight driver.

DAVIDSTECKER, Turn_of_the_screw:
The point is that I do not trust a standard torque calculation by itself. If I must, I will procure samples and failure test them against my usual design torques for 316 S/S screws. Those torques are based on the usual limits for the threaded section and base metal compression.

IRstuff,
I'm selecting fastener sizes now. I will look into F837 instead of F594/F595 (I think that's the correct reference) but I'd rather design for the annealed strength.

 

[Turn_of_the_Screw]

David,

It sounds like your particular situation will, in-fact, require you to do the failure testing yourself, especially if you are going to use any thread-locker or anti-seize. The last thing you want to do is recommend a torque that will guarantee no stripping of the head, but not reaching the desired clamping force.

For more potential research: here is a helpful chart from Fastenal, including stainless fasteners, button head fasteners, but not both together. Perhaps you can extrapolate the information you need

[URL unfurl="true"]https://www.fastenal.com/content/merch_rules/images/fcom/content-library/Torque-Tension%20Reference%20Guide.pdf[/url]

Lastly, maybe contact the manufacturer or distributor. Fastenal appears to have an engineering team available, though I have never contacted them myself.

 

[mfgenggear]

geesamand

In addition I suggest specifying torque values on the drawing. and must use a torque wrench. I have used lots of socket head screws/bolts, and have never striped them out.
but that said I have years of experience turning wrenches. I have rarely seen socket head bolts stripped out. but that has been my experience. any thing can happened.

 

[DAVIDSTECKER]

The thread engagement required for the tension required can be calculated so they won't strip out.
If you want to use less tread depth then you can work backwards to find out what tension you can put on them.
Once you know your tension, then you can calculate torque.

 

[DAVIDSTECKER]

If you are stripping the treaded hole you probably need thicker material.
How does your material thickness compare to the standard nut?

 

[EdStainless]

BB has it right, use strain hardened hardware.
The other thing is to look at head profiles, to may need to specify height and depth of full formed socket.
The tools that we use for working with SS fasteners are all PH SS, we have warning labels that no steel tools are allowed (customers hate to see rust marks in the SS hardware).

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P.E. Metallurgy, consulting work welcomed

 

[DAVIDSTECKER]

OOPs, looks like I mis-read the initial post.
Something I used to do years ago when the Allen wrench wasn't holding is grind the end of the wrench to remove any rounded off corners.

 

[TugboatEng]

Companies like Wiha have stainless steel allen wrenches with a special hex shape that is designed to hold more torque without stripping. This could also improve your situation but may frustrate your customers when the strip everything using normal allen wrenches.

 

[3DDave]

Easy enough to make a test; it sounds like you are committed. The biggest problem in short engagement drives is that the conventional L-wrench puts a side-load that causes cam out. This can be avoided by using T-wrenches or screwdrivers, but that only applies on the factory floor if there are good controls. Going with short engagement button heads and soft materials makes things worse, so the amount of info about using them is limited.

 

[EdStainless]

I don't know if this is how Wiha does it but hollow ground tools engage better. Very sharp and full size at the end and then relieved slightly as you come up the shank. This gives enough 'play' so that you can engage it but the load transfer is at the bottom (similar to gunsmith screwdrivers). T handles are very helpful.

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P.E. Metallurgy, consulting work welcomed

 

[hydtools]

FWIW. Holo-Krome derates torque about 50% for buttonhead screws compared to sockethead screws for their alloy steel screws.

Ted

 

[geesamand]

FWIW. Holo-Krome derates torque about 50% for buttonhead screws compared to sockethead screws for their alloy steel screws.

Exactly the kind of information I was asking about, thank you.

David

 

[Strong]

In addition to the Holo-Krome information this might be helpful:

https://bondhus.com/

and see Resources tab and Bondhus 94205 Screw Grab Solution product. I have used Vibra-TITE 470 DriveGrip Anti-Cam Out Fluid, and it works well.

Walt