Torque Values?

[jasona1700]

Well, here's a question for the engineers out there. I was reinstalling my front wheel after a tire change on my bike the other day.. I figured with something so important as the front axle, I'd torque the axle nut to the correct spec. So a trip to Sears and $78 later, I've got a torque wrench now. But it got me to thinking..anyone know how they determine torque values on things such as axle nuts, heads, wheel lugs on cars...etc?

 

[El Toro]

When you torque a threaded fastener, you are trying to achieve an elongation/stress level that will provide a clamping force large enough to hold things together, but not so large as to cause either the threaded fastener or some other part of the joint to fail.

When you torque a threaded fastener, part of your twisting moment goes into overcoming friction between the male and female threads in the joint, part of the twisting moment goes into overcoming friction "under the head," and part of the twisting moment goes into elongating one member and compressing the other. If there is a gasket involved, that affects the stiffness of the joint and it has to be considered to, preferably during the design stage.

The parts of the torque that go into overcoming friction don't do anything to hold your joint closed, although friction is a factor in whether or not the fastener will "remove itself" under vibration. If the friction coefficient is very low due to lubrication, you get more elongation and clamping force for a given amount of torque compared to "clean and dry" thread conditions. Oftentimes torque specifications will be given for "clean and dry" thread conditions, and when "clean and dry is specified, there is some implicit expectation that surface finish of the threads will be consistent (rolled threads are generally more consistent than cut threads, and would generally be called out for fasteners that were critical in the performance of a machine).

If the joint is statically loaded, you can generally torque the fastener to a fairly high percentage of the load that would cause plastic deformation of the threads.

If the joint is loaded with a time varying load, like a head bolt, it gets more complicated. You want to torque the fastener so that the clamping force is large enough to keep the joint closed, but you don't want it to be so large that the fastener will fail in fatigue.

There are a number of possible failure modes for threaded fasteners. You can strip out the female threads in the joint or you can break off the head (both more typical of static failure than fatigue failure modes). The male threaded fastener can fail at or near the shank/thread junction (more common in fatigue failure).

In a thoughtfully designed machine, torques for non-critical statically loaded connections will generally be taken from tried and true values that are published in handbooks. Critical statically loaded connections will be carefully considered, gasketing and other joint details will be specified, and the spec may be different than what one might find in the handbooks. Any joints with a time varying load will be carefully considered, including all joint details and the recommended torque.

 

[FJRBluesman]

 

[FJReady]

Yeah, what he said. Glad there are others here that get jollies from that kind of thing. Sometimes engineering is a lonely profession.

 

[Kevin Daly]

When you torque a threaded fastener, you are trying to achieve an elongation/stress level that will provide a clamping force large enough to hold things together, but not so large as to cause either the threaded fastener or some other part of the joint to fail...

El Toro,

You know too much about screwing... are you a porn star?

 

[madmike2]

FINALLY, an easy-to-read and understandable explanation. Thanks, El Toro!

 

[Hit the Road Jack]

El Toro explained it well!

And typically, engineers resort to a table of torque specs that call out a torque range based on the fastener's size, material and type.

 

[wfooshee]

Tighten it till it breaks, note the torque at snap-time, and don't go so far on the new part.

Ah attended enjineer collige at Aubrun Universitee.

So ah has credentchals!

 

[squeezer]

FINALLY, an easy-to-read and understandable explanation. Thanks, El Toro!

Agree. That was very well written. You have a future in textbooks, El Toro. Ole!

 

[jasona1700]

Whew..had to read it a couple of times..but I guess I get it. As the saying goes...I'll just press the "I believe button"..

 

[El Toro]

Tighten it till it breaks, note the torque at snap-time, and don't go so far on the new part.
Ah attended enjineer collige at Aubrun Universitee.

So ah has credentchals!

When I was young and turning wrenches back in the hay day of marques like Norton, Matchless, Triumph, BSA, Royal Enfield, Velocette, Ducati, Moto Guzzi, and, of course Harley Davidson, we were all eager to learn from the master mechanics. We hung on every word or subtle point. A few of these guys are now enshrined at the AMA Hall of Fame. But I remember one in particular who was famous for his race prep work and rebuilding prowess on Nortons. I asked him once to tell me how much I should torque the headbolts on an Atlas, and he answered "Just torque 'em down until they stretch, and then back off a little." Probably not the greatest advice, but there were enough other problems with these "great bikes" that specific torque for any reassembly was low on the list.

I was at D&H a week ago, and Gerald and I were talking about the large number of British and Italian bikes represented in Barber's collection. Gerald's theory was that the British and Italian bikes wouldn't run reliably, so there were lots of them left in good condition for acquisition in the collection. He thought that Barber couldn't find as many good quality Japanese bikes for curation because the bikes would run and run and run until they were used up.

I don't think Gerald is right on this point, but it was a fun discussion. My own belief is that Barber is partial to the charisma and almost mystical aura surrounding the old British and Italian stuff.

How's that for screwing up a thread? :dwarf:

 

[charismaticmegafauna]

When I was young and turning wrenches ... all eager to learn from the master mechanics. We hung on every word or subtle point.

My 'mentor' was a salty guy named "Mac" (whom I eventually replaced). I was young and inexperienced and was beginning to replace spark plugs in a bike with some 'Hitachi' brand plugs. Mac says, "You know what 'Hitachi' means don't you?" Me, "uh...no." Mac, "Horse-****." Me, "Oh...really?" Another time I was doing some basic repair/maintenence on an ordinary Honda 50 (C110) and Mac says, "When I do that, I turn 'em up-side-down and set 'em the handlebars and seat." Me, "Oh...really?"

Goes to show -- you really can't believe/trust those guys....

I was at D&H a week ago, and Gerald and I were talking about the large number of British and Italian bikes represented in Barber's collection. Gerald's theory was that the British and Italian bikes wouldn't run reliably, so there were lots of them left in good condition for acquisition in the collection.

While I don't think I can speak w/authority on old Italian iron?, I do think that old British stuff was poorly served by their lack of standardization of fasteners -- Whitworth, British Standard, some 'Motorcycle Only' thread sizes; and, later, American sizes -- all mixed together on the same bike. Neither were these fasteners -- sizes and corresponding threads -- well thought-out for their intended purpose (ex.: 20 T.P.I. regardless of diameter). The end result of poor fasteners on old British Iron was that if the tyro/tuner/owner ever took wrench in-hand -- the bike was fairly 'doomed' to never function quite properly again.

Hence, many fine examples still looking quite representative of the breed -- but, not actually 'functional'.

 

[El Toro]

While I don't think I can speak w/authority on old Italian iron?, I do think that old British stuff was poorly served by their lack of standardization of fasteners -- Whitworth, British Standard, some 'Motorcycle Only' thread sizes; and, later, American sizes -- all mixed together on the same bike. Neither were these fasteners -- sizes and corresponding threads -- well thought-out for their intended purpose (ex.: 20 T.P.I. regardless of diameter). The end result of poor fasteners on old British Iron was that if the tyro/tuner/owner ever took wrench in-hand -- the bike was fairly 'doomed' to never function quite properly again.Hence, many fine examples still looking quite representative of the breed -- but, not actually 'functional'.

I agree. There are so many different standards for fasteners on old British bikes, that it will make your head spin. Trying to do an authentic restoration can be aggravating.

A few other thoughts on this bolted joint torque stuff...

The first full time job I had as a real engineer was with one of the major US "fibers, chemicals, and plastics" companies. I was assigned to the Machine Design Group. Everything was secret. We were designing and building equipment for the various proprietary processes to make the fibers, chemicals, and plastics. No patents because to patent a process meant you had to describe it...and then someone might figure out how to get around your patent. Better to just keep it all secret. At least that was the thinking. When the Germans and the Japanese started figuring out how to make our stuff and eventually drove the products to commodity status, it might not have seemed like such a good idea.

Nevertheless, this company had generated an internal report called "The Bolt Document" and it was the bible as far as any threaded fastener joint design was concerned. In chemical processes you often have cyclic loading of pressure vessels. There are gaskets, making the joints complex. The failures are fatigue oriented, and they can be catastrophic.

The American auto companies have specialty groups devoted to fastening. I'm sure the Japanese companies do too. Rules develop are are documented within companies based on experience and experimentation. No one wants their "joint" to be among the TGW (things gone wrong).

I don't know if it still exists or not, but at one time the American Society of Mechanical Engineers had a special Technical Section devoted to fastening and it was located in Southeastern Michigan.

So I guess this long, convoluted point is that fastening is complex. Like so many other things, it appears to be simple. It also appears that you can ignore the technical issues and enjoy success, sometimes forever, but sometimes only for a period of time.

 

[Scab]

How's that for screwing up a thread?

Too much torque?

 

[v65]

FINALLY, an easy-to-read and understandable explanation. Thanks, El Toro!

Agree. That was very well written. You have a future in textbooks, El Toro. Ole!

Or maybe a nice addition to Wiki. Just a thought, their page on torque is a bit more physics based.

 

[ponyfool]

When I went to assemble the crankcase, I was surprised to see what was required for bolts 1-10 along the crank. It required lubed threads (which obviously effects the torque needed to secure them). The it called for 10 Newton Meters, loosen, then to 20 Newton Meters, and finally I needed to use a torque angle gauge for the final spinning of the bolts where I had to turn them each 115-125 degrees further. I suppose with a precision engineered bolt, they have it down to fit exactly what El Toro was describing. Just enough stretch for perfection. Of course, the bolts have to be replaced every time you split the cases.

 

[twowheelnut]

Engineers-schmengineers!

Monkeys. Dart board. Figger it out from there.

 

[OkieBikerDude]

El Toro said it best. But in the less than ideal world I use the blue loctite on all small blind fastners (8mm or less and no nut). This seems to lubricate the threads and help me relax with what feels like too little torque. Examples are where steel bolts thread into aluminum.

Okie

 

[slapnpop]

Engineers-schmengineers!
Monkeys. Dart board. Figger it out from there.

Hey, where do you think the side-stand came from!

They can't pay the smart engineers to do EVERYTHING can they?