Torque spec changes from nut to bolt head?
Torque spec changes from nut to bolt head?
(OP)
In a torque spec for a typical bolt, washer and nut arrangement, is the proper torque value affected whether it is applied to the bolt head as opposed to the nut?
Thanks!
Thanks!





RE: Torque spec changes from nut to bolt head?
RE: Torque spec changes from nut to bolt head?
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.
RE: Torque spec changes from nut to bolt head?
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.
RE: Torque spec changes from nut to bolt head?
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.
RE: Torque spec changes from nut to bolt head?
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)
RE: Torque spec changes from nut to bolt head?
Thanks very much!
RE: Torque spec changes from nut to bolt head?
Good point - I was assuming constant bearing areas for nut and bolt, which can be a dangerous assumption.
RE: Torque spec changes from nut to bolt head?
(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
RE: Torque spec changes from nut to bolt head?
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!
RE: Torque spec changes from nut to bolt head?
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.
RE: Torque spec changes from nut to bolt head?
RE: Torque spec changes from nut to bolt head?
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!
RE: Torque spec changes from nut to bolt head?
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.
RE: Torque spec changes from nut to bolt head?
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?
RE: Torque spec changes from nut to bolt head?
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.