Bolted Connection Fastener Torque Technique 6

[Mandalay]

When applying tightening torque to a bolted connection, I would typically hold the bolt and rotate the nut with the torque wrench. Is this the preferred technique? The other possible option being to hold the nut and rotate the bolt with the torque wrench.

 

[rb1957]

the only difference i can see between the two is that holding the nut means the bolt shank is rotating in the hole and possibly you might get scratches; if you hold the bolt you lessen the chance of this.

 

[CoryPad]

Usually there is little difference between the two.

Other factors to consider:

1) What are the surface conditions for the bearing surfaces of the bolt and nut, and the corresponding parts they contact? The friction coefficents may be different, which would affect the tightening procedure if it uses a torque-based method.

2) What is the allowable surface pressure of the joint material under the bolt and nut? The rotating member induces both compression and torsion stresses to the mating surface, so it may be possible to damage a coating layer or yield the base material.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SnTMan]

Mandalay, to further amplify on Corypad's 1st point, if the bolt does not turn freely in the hole due to fitup issues, you would see less preload than your torque number would indicate.

Mike

 

[dvd]

Bolts are not made to turn whereas hex head cap screws are. The vertices of the bolt head will have a tendency to dig in while the cap screw has a shoulder that keeps the hex off the surface.

 

[TheTick]

I prefer to turn the nut, especially if the bolt is long. Probably not a big deal, but I prefer not to have the bolt twisting around and wearing the surfaces of the bolt hole.

 

[Kenneth]

In aerospace applications, torquing of the nut (where possible) is the "preferred" method for many of the reasons noted above.

To expand on Cory's "Other Factor" Number 1:

Overcoming the bearing surface friction of the rotated/torqued component (nut bearing surface when the nut is torqued, or bolt underhead surface when the bolt is torqued) "consumes" a significant amount (typically about 50%) of the applied installation torque. The torque consumed in overcoming this frictional resistance does not generate clamping force. Self-locking (prevailing torque) aerospace nuts (the "norm" in the aerospace industry) are typically lubricated, where bolts (often "bare" passivated, high strength stainless materials) typically are not. In this case, the clamping force (preload) developed for a given installation torque will be significantly different for a given nut/bolt combination depending on whether it is torqued by the bolt or the nut "side".

Some aerospace installation specifications attempt to address the potential reduction in clamping force for joints tightened by the bolt side by specifying that joints torqued by the bolt side (when torquing by the nut side is impossible/impractical) are tightened to the high side of the specified installation torque range for a given nut/bolt combination, or other such "controls".

 

[CoryPad]

dvd,

What bolts are you using that cannot be turned? Vertices are part of drive features. These features are there to impart a moment. If there is a moment, there is a high likelihood that there will be rotational motion.

There are numerous product standards for both bolts and screws that have a rounded bearing surface to prevent damage to mating components.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[panduru]

Ok, folks, how about a compromise?

1. Turn the nut until the contact surface reaches the plane of the flange or whatever.

2. Now turn the bolt head, which has been previously lubricated, until the required torque is reached.

The idea being that the nut will merely reach its position without sliding against the mating surface, without pulling the bolt in its hole.

The bolt on its part will not move down in the hold any more than necessary to apply the required torque. The bolt head contact surface, being lubricated, will also not effect the torque reading significantly.

 

[Screwman]

I agree with CoryPad. I have read all of the various ideas of the effects of tightening from one side or the other and there are theoretical advantages to tightening the nut. What we have found in testing on commercial parts is that there is no statistical difference in the resulting tension between the two methods.
I don't want to imply that that will be the case in an aerospece environment where conditions are controled much more tightly, and I don't think that the idea should be blindly applied in all cases where you might have a nasty looking bearing surface or an oversized clearance hole, etc., etc.
What we have found is that the effects of which side you tighten from are lost in all the other noise that goes on when tightening a joint. The effect of switching from a power tool to a hand torque wrench is larger than the effect of which side of the joint you tighten from (by the way; power tools give better results than hand torque wrenches).

Dick

 

[Ralph2]

Consider a long... 20 foot bolt.. turning the head will apply a twisting motion that is restrained by the nut. Now consider the same thing when turning the nut.. I would speculate that there would be less rotational twisting when turning the nut.. The wrench is eliminating the resistance of the nut as opposed to adding to the twist when turning the bolt head. As a twisted bolt is not desireable it makes sense to turn the nut..
For what it is worth... my 2 cents

 

[CoryPad]

Ralph,

Even considering a long bolt, the torque in the bolt shank is the same whether you apply a torque to the bolt head or the nut. You can see this in calculation methods like VDI 2230, or in a book like Machinery's Handbook.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[diamondjim]

I'm with Ralph2 on the long bolt comment.
It would be nice to see some qualitative
information on the difference.

 

[Ralph2]

I use the example of a long bolt to show which method would have the higher twisting motion. To "torque" a bolt is actually a misrepresentation of what you really want to do and that is to induce a tensile pull... "only"... However because we typically use a wrench to turn a nut that rides on an inclined plane to achieve this tensile pull the "torque" or "twist" name has stuck.

 

[CoryPad]

Ralph,

I must have been unclear in my previous message. Your example of a long bolt does not prove a higher twisting motion. Of course bolt torsion is undesireable (and avoided using hydraulic tensioning), but it is an unavoidable consequence for most fasteners. However, whether the torsion is applied to a nut or its mating bolt, the shank torsion in the bolt is the same (excluding friction coefficient differences). The mechanics of an inclined thread have been measured, modeled, and calculated by many people, and they are published in many sources (like the two I mentioned in my previous message). I recommend you consult these for more information on this subject.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Ralph2]

just spent some time looking at a bolt... When one turns the head (on a normal RH thread) one puts clockwise twist into the bolt (viewed from the head).The tendancy to untwist would loosen the bolt. On the other hand turning the nut would impart an anti-clockwise twist (again viewed from the head)which would tend to keep the nut tight.

??

 

[Kenneth]

Ralph2:

Look again. The driection of bolt "twist" resulting from installation tigtening/torquing of a given nut/bolt combination is the same whether the bolt or the nut is torqued.