Nut and Bolt Torque

[Anonymous Poster]

All the discussion that started about a wheel breaking off has reminded me of something I don't think has been discussed on this board at least recently. That being you can't (I was taught) check the torque on a bolt. You can only check if it is "tight". The torque it takes to break a bolt loose is far more then the actual torque reading that would have been given with a torque wrench when it was torqued. You have to break the bolt loose and then in an even straight pull torque it. This may be nitpicking, as on most things the EXACT torque isn't as critical as it being tight. "Tight" means different things for different grade/size bolts as shown in the standard "torque tables". Those critical areas, cylinder heads, brake rotors, aluminum wheels etc seem to vary more by vehicle manufacturer then by the size of the bolt. Usually the damage occurs when it is more then half a turn loose. I find them now and then and I'm pretty careful to spend some time knocking the dust off all my sockets and wrenches looking for loose stuff.

If you catch them early and just get them back down it sure can save $$$. Replacing lock washers every time the bolt is loosened is a requirement. Loctite is neat stuff until you have to deal with it. The recent thread on BX tailights being stripped reminded me of something, on some auto applications I've seen loctite or something used on screws which is insane as they can be so unmovable that you tear the head off the screw long before you get them off, you can't take the impact-drive screwdriver to tailights for instance.

Wheel lugs might be oK as you've got plenty of room for the breaker bar. I get nervous when some mechanics start waving the propane flame on something to break the loctite seal. I can remember having to tell a muffler shop that the factory undercoating on my car was on fire many years ago. He wasn't too concerned.

Some of these torque readings are quite exciting, I was once talking to an old timer about a 220lb requirement as I didn't have an over 150 wrench and asked him how he did it. His reply..."Torque Wrench...? You tighten that as tight as you can get it and then a half turn more!"

Maybe not "correct" but he was one of those guys that could get your tractor out of the quag, fix the broken off bolts, and get you going with only a pair of tweezers and a wood hammer! (well almost)

Feel free to disagree on my "torque" beliefs!

 

[Mike H]

I've heard the application of ideal torque you're referring to as "pure torque". Yup, can't apply this with a torque wrench. This is applied by some type of T tool where you make certain that the force x lever arm is exactly equal and opposite (in other words, you need two hands - one pushing and one pulling). I only had to mess with this once in a lab test and when we put the particular device into production we couldn't be so particular. There is a slight difference between this "pure torque" and what we all do regularly because you can't use a normal torque wrench without imparting some type of side load through the tool.

 

[Mike H]

Specifying and measuring torque is just a convenient way of getting what we really need in the joint: preload. You're absolutely correct in that there's a substantial difference between installation and removal torque, breakaway and running torque, etc. What we want is for the threaded joint to remain together (i.e. not loose) under whatever the load is. When the preload is really critical, more direct measurements of it are used: strain devices to measure how much the bolt has stretched, stress devices to indicate how much load is in it, etc. I wouldn't be all that surprised to see Bird repairing a future generation of impact wrenches that has a built in tensionmeter - maybe an ultrasonic sensor that you calibrate to the fastening materials and then a light goes on when it's tight enough! Electronics in everything!

 

[Anonymous Poster]

Tight= turn till it breaks then back it off 1/2 turn.

 

[jdkid]

Hi ya's
well i hope this is a good place to post/ask?? on my new tractor i'm wideing the wheel base to do so i have to take out the center rim and swap wheel R-L and vice -vercer ok now some fool painted the bolts and got carryed away doing them up 1/2 of them have striped the nuts so i have to get new bolts any idea of loading for grade bolts??? the ones that come out are M16 (16mm )fine ((bit smaller than 5/8 i think 19mm=3/4)other thing fine or corse cut?? there is a setting of 310Nm bout 200Lbf (?) thats a rough work out .
any ideas
catch ya
JD Kid

 

[bgott]

I have a pocket reference book handy...It shows 16 MM coarse 2.00 pitch ( it doesn't show fine thread, sorry! ) grade 8.8 at 215 foot-pounds and grade 9.8 at 233 foot-pounds. I just noticed at the bottom of the page a footnote that says for fine pitch bolts increase the above ratings 9%. I did the math and it looks like with fine threads you are closer to 250 foot-pounds. In that case you would need grade 10.9. The standard dry torque of the 10.9 shows to be 269 foot-pounds for coarse. What is the marking on your bolt head? Most metrics I've seen have the number stamped or cast right on top.

 

[jdkid]

Hi ya
i found the marking it is 10.9 ,i talked to a nut and bolt guy and he thinks a mild nut may have been used making the nut strip
catch ya
JD Kid