Stress in a Hex socket

[techwennie]

Hello everyone. Since I found this site I have been getting lots of great information and entertainment-thanks! I can't find the info I need so I get to make my first post. I am designing a custom fasterner with a hex socket for tightening. Are there formulas for designing hex sockets. I need to find the optimal hex depth vs width across the flats to prevent damage from the hex key torque.

thanks for the help.

JMT

 

[hydtools]

Find a hex socket head capscrew that fits your selected hex key and copy the socket.

Ted

 

[israelkk]

How are you going to manufacture the hex socket? In standard bolts the socket hex head is manufactured by cold upsetting (forging) of the head from the base rod of the bolt. This gives the desired grain directions that gives the strength to the hex. If you use anything else such as EDM machining your hex head will be much weaker.

If you can manufacture your custom fastener from an existing standard fastener then you will already have the original head.

The standard heads and bolts are designed for the tightening torque that the bolt can hold in order to develop a specified axial force in the bolt. A machined head bolt with the same dimensions as the standard bolt will not be able to tolerate the same torque and load.

 

[rb1957]

"I need to find the optimal hex depth vs width across the flats to prevent damage from the hex key torque."

design the socket to fit onto the head very closely !

it sounds like you're trying to minimise the head height. a "dumb" approach would be to machine down some bolt heads and see what works. if you want to "design" something, maybe FEA would help. I'm sure that the standard bolt head is not minimum height, but perhaps a good design proven by experience. You're thinking about the tightening torque; what about the applied loads (like tension) ?

 

[techwennie]

Thanks for the replies. I should have called it a threaded retainer, not a fastener, as it holds another part. It is rather thin and thus the need to determine the minimum required socket depth. I was hoping for a down & dirty equation that says: with this mat'l, this size hex and this torque you need this depth. I know --- nothing in engineering is ever this simple!
Making the part thicker would require changing other parts. I want to avoid that can of worms.
Anyway, the part is completely machined from 316 stainless and the hex formed by rotary broach.

JMT

 

[rb1957]

sounds like you have something drawn up already ... make one and test it

 

[eromlignod]

There is an ANSI standard for hexagonal sockets. The dimensions and limits are in the Machinery's Handbook. This is the standard for fasteners that are to be driven with Allen wrenches, so it should be exactly what you are looking for.

If you don't have the handbook, just tell me what size wrench you are designing for and I can give you the tolerance.

Don
Kansas City

 

[unclesyd]

You didn't state whether you require internal or external drive but I assumed internal.

Here are two types of low head fasteners that are off the shelf. These are metric but they are available in inch series. There is some torque limitations on this type head.
There is also a recessed socket flat head available.

You can buy or as stated above copy.

http://www.fullermetric.com/products/stainless/iso7380button_head.html

http://www.fullermetric.com/products/stainless/din7984low_head_soc

ket_cap_screw.html

 

[hydtools]

The downest and dirtiest I can think of is to copy. Look at stainless capscrews. Find the fastener size with the recommended torque you will be using in your application and copy the hex socket. If the socket depth is too deep for your application, go up a hex size and use a shallower depth. Keep the hex-to-depth ratio. Deforming force is proportional to hex size. Area over which the force is applied is porportional to socket depth. Roughly.

Don't forget to consider the wrench end round off when you look at alternate socket depths.

Ted

 

[IRstuff]

The aforemented flat-head socket:

http://www.fullermetric.com/products/socket/din7991-10.9flat_head_socket_countersunk.html

TTFN

FAQ731-376

 

[techwennie]

Thanks everyone.
Don - My Machinery's Handbook (24th ed) left the depth standards out. Guess I need to get an older version.

Anyway, I had not thought about looking at button heads for an example. So, I copied one and I'm having the shop make samples for testing.

JMT

 

[KENAT]

Could you extrapolate something from the formulas for keys/keyways? Section 14 of Roarks 7th edition has a small section on this.

I remember my chief stress engineer doing similar for a square drive, but can't remember details, sorry.

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