Torque spec changes from nut to bolt head?

No, assuming the bolt can turn freely within the hole. But remember that torque is usually a poor way to obtain bolt preload because of all the friction under the bolt head and nut surfaces, and the threads.

High potential
I'm afraid the answer is yes. The 'rate' at which torque is converted into tension in a nut/bolt assembly is dependent upon the friction on mating surfaces. If the bolt head is held immobile while the nut is tightened (the ideal, design case)then the mating surface area on the substrate under the nut will be the determining area. If, on the other hand, bolt head is turned and nut is immobilised, the mating surface area under bolt head will determine installed tension in the bolt. If these two mating areas happen to be equal, then there is effectively no difference in recommended torquing values if torque is applied to bolt head or nut.
Another consideration is nut shape. Some nuts, particularly self-locking nuts, will deform when torque is applied. The torque applied to the nut when following torque recommendations by turning the nut, is not the same as the torque required to stop rotation of the nut when this same recommended torque is applied to bolt head. The resulting nut deformation will not be the same and this in turn will affect the friction coefficient of the thread mating surfaces. For regular hexagonal nuts however, nut deformation would be nil.

Yates,

OK, I'll compromise. In those cases where there is a difference in friction under each head, then you are correct. But the coef. of friction is relatively independent on surface area, especially for hard surfaces. Therefore while different materials could make the two methods give different preloads, surface area differences wouldn't.

Metalguy,

You are correct - surface area won't effect the results, but friction coefficient does. The question then becomes what affects friction coefficient? Material composition, surface roughness, coatings, etc.

HighPotential,

The answer to your question is maybe. Do you have different surface characteristics under the nut and bolt? If so, then the torque may need to change. If not, then the torque need not change.

Corypad
Even assuming constant friction coefficient, (metalguy - you are right surface area does not affect friction coefficient, but it does affect force to overcome friction.) the torque-tensile relationship in a nut/bolt assembly is determined by the surface area in contact. In most arrangements roughly one-half of this area is provided by the screw thread mating surfaces, and the remaining half by underhead (bolt-to-substrate) mating surface. All torque/tension fomulae that have been devised for nut/bolt assemblies include the two surface area terms mentioned above (thread and underhead bearing surface)

Thanks for the expert input everyone. I love this website. My specialty is Electrical Power Engineering and my question has to do with a power plant we are building in Los Angeles. The electrical mechanics find it necessary sometimes to torque the connections from the bolt head instead of the nut side due to clearance limitations and I was just wondering if there was any effect on the torque spec. They are using mainly hex head bolts and split lock washers and nuts. Sometimes they use "bell-crank" type lock washers but regardless, I now understand that for my application, it should not matter since the bolt is inserted through the connecting lugs and a nut is spun onto the other side. The nuts and the bolt heads are almost always the same size. Sometimes though, one lug is aluminum and the the other is brass. But regardless, my policy will be that it doesn't matter, at least for my application.

Thanks very much!

yates,

Good point - I was assuming constant bearing areas for nut and bolt, which can be a dangerous assumption.

HighPotential---If your building in LA then you must have everything inspected and the rules are pretty 'cut and dried'!If you are using split washers the grade will be A307 (ungraded?) and that only requires "torque" to be that of the abount "that an average man can pull with a wrench". If A325 (grade 5) or better torque can legally be applied to the head as long as the hardened washer is on the head side and the nut is restrained by some method. I have never had too much difficulty with bolting electrical trays or whatever with ungraded fasteners but the towers and walkways, typically A325/A490 are always inspected.
(With TS bolts the fashion these days, all this is moot.)

As to automotive (race) and aircraft applications, I move heaven and earth to avoid two things---lock washers of any kind and tightening fasteners from the head!!!

Rod

Looks like this subject is more involved than it first appears-which is why I made the goof in my first post.

Yates- "metalguy - you are right surface area does not affect friction coefficient, but it does affect force to overcome friction.) the torque-tensile relationship in a nut/bolt assembly is determined by the surface area in contact."

I don't see how the area can affect the "force" (torque?) if the COF remains the same-or am I missing something here?

HighPotential- "The nuts and the bolt heads are almost always the same size. Sometimes though, one lug is aluminum and the the other is brass."

Argh! That could cause a torque problem. IIRC, the COF between steel and alum. alloys can be pretty high-esp. if the alum. galls. But the COF of steel/brass is usually pretty low. You could perform a simple experiment-torque a connection from the alum. side, then see if it tightens more from the brass side. Then let us know!

CoryPad- Don't feel too bad about making assumptions-mine are usually wrong. Here's a money-making example for you. Just keep asking me if I'm buying or selling stock (S&P 500). Then YOU do the opposite. You'll be rich in no time!

Metalguy,

I am going to answer the question you pose to yates - the reaction torque under the nut or bolt head (bearing torque) is the product of preload, friction coefficient and mean bearing radius. The required input torque is the sum of pitch torque, thread torque, and bearing torque. If the mean radius changes (e.g. small bolt head OD, large nut OD), then the bearing torque changes, which changes the required input torque.

The discussion rages on I see. We are using predominantly Grade 5 fasteners. Sometimes stainless and sometimes non-graded bolts are used. Complicating the picture are the vendors torque ratings which sometimes do not jive with the standard material torquing tables. As a rule, the vendor spec'd torque is always the overriding factor but in most cases, we do not have a vendor spec'd torque.

CoryPad,

OK, I see it now (at last!). Forgot about the diameter differences. No wonder I dislike using torque so much.

HighPotential,

This is going from bad to worse, with the SS bolts. If they are 304/316 and annealed, they will gall (almost) just by looking at them hard!

HighPotential,
Now we're getting the full story, I think we can forget all the pompous theoretical stuff we gave you. It is only valid for sliding (hard) surfaces. If SS screws are involved, galling is a reality, as metalguy points out, and none of our theories holds up - we get into the realm of plastic deformation etc.

I kind of wonder if more twist in the bolt
is developed by turning the head rather than
the nut giving some slight difference in the
torque values creating the same tension. It
kind of makes sense in longer bolts that more
twist would develop by turning the head.
What are your thoughts?

diamondjim,

There is not more twist in a screw when turned by its head versus turning a mating nut. The shank torsion is the sum of the thread friction torque and pitch torque. These are unaffected by which part is turned.