max tightening torque values for screw heads

[rmetzger]

I'm sure this question has been asked before but I'm unable to find a thread that addresses the question.

I'm looking for a chart or tabulated form that lists the max allowable torque for various materials and drive types for screws. With some of the smaller fasteners we use in products we run into limitations on the amount of torque that can be applied through the drive (typically female hex) long before we reach the yield strength of the fastener. typical materials are 18-8 stainless steel, 316 stainless steel, a-2 stainless, and grade 2 steel.

Does anyone have a good link or table for this type of info?

thanks

 

[MiketheEngineer]

Pick up that 2 ton phone and call the mfg??

 

[KENAT]

Oh come on Mike.

How many folk in the mechanical (machine design) world actually get to buy fasteners directly from a manufacturer or even reputable distributor?

Sure if it's going to fly or somehow safety critical etc. but much of the rest of industry is stuck with stuff from local hardware distributors or maybe places like McMaster-Carr which have little in the way of supporting data.

rmetzger - so you're OK with just information for the fastener itself, you don't need to take into account the actual joint/mating part?

http://www.thelenchannel.com/1torque.php

is a start but I don't think it's taking the head form limitations into account. This one has some of the same data but some extras too

http://www.mgtd.ca/Screw Torque.htm.

If you come across a good answer I'm sure many folk would be interested.

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[rmetzger]

Kenat nailed it - most of the time we don't even know who purchasing is buying fasteners from (usually some distributor that is getting stuff made in a dirt floor shop in China). The only time our parts fly is when they are thrown, and I can safely say that our products are not used as structural members of any great works of civil engineering. Its quite simple to call a domestic source of high grade fasteners and ask them what their tightening torque recommendations are for their high grade screws are, but one typically doesn't find much info on how much customer can tighten a 18-8 stainless socket cap screw before stripping out the head.

In many of our applications its important to have a fastener that is not easy damaged during product assembly or customer use. Joe Blow in the field using his tools to open a battery compartment isn't going to torque it back down to our spec, but he is going to complain if he damages the screw. Same goes for his brother in law's repair shop that is equally prone to damaging fastener drives. Thus we try to find the best fasteners we can (knowing we'll never know where they came from) that a customer can't easily damage when using their tools and their skill set. Hence the question regarding drive torques for the screw heads.

 

[Tmoose]

Sturtevant, the torque wrench people, published a magazine sized torque wrench reference that had a variety of tables, including one that had had one that spanned several pages and included LOTS of different materials. Nylon, brasses and aluminums, etc.

I'd feel better about running "Torque to yield, then bust" tests to confirm the spec, and then qualify new batches of screws received, too

 

[rmetzger]

I'll do some digging and see if I can turn up a copy - thanks.

We test head strength during design but it would be good to have a reference to get in the ballpark on paper prior to having to attain parts and begin analysis.

 

[drawoh]

rmetzger,

I created screw torque charts here using Microsoft Excel. We use stainless steel screws. One of our suppliers claims that his English screws have a yield stress of 55,000psi. All other other stress values were higher than this, so this is what I used.

Bickford's Handbook of Bolts and Bolted Joints has formulae for working out stress are on both English and metric screws.

My torque charts seem to work.

If you are stripping sockets...
[ol]
[li]...take a look at your drivers. Our assembly shop people have issues with people trying to use old, worn out Phillps drivers[/li]
[li]...do not under any circumstances use hex socket flat head cap screws, especially if they are under #10 or M5. The sockets strip. Phillips flat head screws are absolutely superior. It has been pointed out here that flat and button head cap screws are not as strong as the regular cap screws. [/li]
[/ol]

JHG

 

[btrueblood]

We tighten ss socket head cap screws (per ASTM F837, condition CW)* to about 80-90% of their proof load every day, and never (well, hardly ever) strip the heads. I took away every single ball-end hex driver from the shop guys, and stripped hexes are far less common. Also, the hex drivers get replaced when the black oxide coating is gone, indicating they have worn. Yes, it's easy to over-torque any small screw, so torque-limiting drivers, torque wrenches or similar tightening limits are helpful, at least as training aids.

*At least two national fastener chains in the USA stock SHCS to this standard, and are what you get when you order stainless SHCS from them.

 

[MiketheEngineer]

If you willing to buy "unknown" material from China or even the USA with no supporting data - GOOD LUCK. Only God knows what they are good fro!!

Your call.

They practically had to rebuild 2 or 3 of the Space Shuttles just because of this problem - no backup data. And I don't care if you are building F18s or kid's tricycles. - bad bolts, screw, nuts etc. can be disastrous..

I (we) just don't do it with out documentation!!

 

[KENAT]

Mike, who the hell said anything about us being willing?

For non structural/lightly loaded joints it's difficult to persuade purchasing/operations/management that it's necessary to have aerospace industry levels of traceability etc.

Hell, I can't even persuade myself.

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(probably not aimed specifically at you)
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[MiketheEngineer]

Like I said - Your Call