Larry Schramm

Bolt Size & Torque chart

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That's a good chart, but it doesn't specify if these are wet or dry torque values.  Given that they are showing 120 ft-lbs for a 1/2" fastener, I assume these are clean, dry numbers.  Lubricant will both reduce torque value for a given bolt tension and improve consistency in bolt tension.


The important factor in a bolted joint is actually bolt tension, not torque.  Unfortunately, measuring tension is very difficult to do, so torque is used a an easy to measure indicator.  In the aerospace industry, we've shown that something as simple as the friction under the head of the bolt can have a dramatic impact on the bolt tension for a given torque value.  I've worked on programs where highly critical fasteners have actually used embedded strain sensors inside the bolt to accurately measure bolt tension.  Most folks here are probably familiar with bolt stretch gauges used on rod bolts during engine assembly for the same purpose. 

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28 minutes ago, meteor said:

Joe, You must be the smartest person in the world!

 

Yeah, I'm a legend in my own mind...

 

Seriously, this is a topic near and dear to my heart as I've had to design a lot of critical bolted joints for aerospace applications.

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As Joe says, they don't consider the conditions under the bolt head and nut - spring or shakeproof or flat washer or none. They all affect the bolt tension achieved. They also assume new, fresh threads, not recut (e.g. to "clean") with die nut or tap of unknown accuracy and previous use, as well as smooth, non-rusty threads, not plated with something like cadmium (e.g. my Dodge 8) or zinc (with or without chromate, as in modern bolts).

 

I would like to know the equivalent grade of steel in our old bolts. In the absence of any information we have to assume Grade 2 for most of them. And just because you are using new high strength head studs doesn't mean you use the higher tightening torque. You are still screwing it into that old low grade cast iron.

 

And how accurate is your torque wrench?

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With regards to the effect of friction under the nut face, a good friend of mine, (apprentice mechanic), brought home some of that new-fangled Teflon, many years ago, and turned up some washers to fit under the head studs on his mad Morris Minor. Quite a shock when the studs snapped, one after another, attempting to reach the correct torque with the wrench. Seems the friction was no longer significant at the turning surfaces. That was a lesson neither of us has forgotten.

 

And Spinneyhill, you're probably as suspicious of torque wrench accuracy as I am. If I have reason to doubt them, I give them a test. Not sure of the accepted method for this, and my method might sound a bit rough, but owes something to common sense. I clamp a socket extension bar in the vice, horizontal, with the set torque wrench mounted on it, also horizontal, and measure out an accurate foot from the centre-line of pivot. Then a bucket hanging from the 'foot' line is slowly filled with sand, until the wrench 'clicks'. Then I weigh the sand and bucket. I'm always surprised how accurate my 45 year old torque wrench actually is! 

Mick

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29 minutes ago, Bush Mechanic said:

With regards to the effect of friction under the nut face, a good friend of mine, (apprentice mechanic), brought home some of that new-fangled Teflon, many years ago, and turned up some washers to fit under the head studs on his mad Morris Minor. Quite a shock when the studs snapped, one after another, attempting to reach the correct torque with the wrench. Seems the friction was no longer significant at the turning surfaces. That was a lesson neither of us has forgotten.

 

I learnt a lesson like that with my Honda motor bike in the late '70s. The barrel studs kept breaking when I torqued the nuts. Eventually someone made an off-the cuff remark about something and I twigged. They were cap nuts bottoming on the end of the studs. Another washer or two fixed the problem.

 

I have used a bucket of water at a measured distance on my socket drive in the absence of a torque wrench.

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Being a bit picky about my tools I always check when a borrowed tool comes back to make sure its clean, and unloaded. The kids always forget to unload the torque wrench when they are done.

I also leave the hydraulic valves open on the jacks when not in use.

If I have friends working out of my tool box I ask them to NOT put them away, I will do that. Seems when someone else puts tools away I don't know where they are.

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1 hour ago, JACK M said:

Seems when someone else puts tools away I don't know where they are.

 

Heck, half the time that happens even when I put my tools away... :D

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1 hour ago, joe_padavano said:

 

Heck, half the time that happens even when I put my tools away... :D

I often can't find them again when I put them down while working with them! Those blasted Borrowers....

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20 hours ago, joe_padavano said:

 

Heck, half the time that happens even when I put my tools away... :D

 

19 hours ago, Spinneyhill said:

I often can't find them again when I put them down while working with them! Those blasted Borrowers....

 

I think we all do that, multitasking.

I kick myself when I open a door of a parts car and find that long lost screwdriver.

 

I was working on an old Bentley several years ago and it ate an end wrench.

I all but took that car apart looking for it. That car went to Germany, I suspect my long lost end wrench is cruising the Autobon by now. (have a good trip)

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This thread got me thinking about how often I need to look up torque values when away from reference charts so I did a search for an app for my iPhone to see if anyone had written one. I found an app called iEngineer. It gives all kinds of specs for 93 different sizes, 9 head styles and 74 material grades. They give among a lot of other things dry and wet torque. Best part the US sizes are free, $1.99 if you want to add metric. Looks good, probably a similar app for androids.

I now have a bit more info in my pocket for when I need it (if I remember it is there).

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First one I looked at for Android was Mechanical Engineering One (free, but I don't know how many of the features available are free). Does lots of things too.

 

Edit: the only permission this one asks for is Accounts. I am not sure of the ramifications of that but it could be dangerous if it doesn't allow you to NOT log into something it asks you to log onto.

Edited by Spinneyhill (see edit history)

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