Bolt size and torque, question for mechanical engineers

[vavavroom]

In a recent thread, the opinion was voiced that wheel lugnut/bolt torque was not critical. I remember from a long time ago that the target torque for any critical bolt is calculated by a formula. Unless a bolt has been stretched a certain amount it can't act like a tension spring. It is this tension that keeps the bolt from coming loose. The target torque depends on the bolt grade, bolt diameter and the coefficient of friction. After the target torque has been calculated the specified torque is then established empirically. The least predictable metric is the coefficient of friction, which is why torque is usually specified for dry hardware.

Target Torque in inch pounds = coefficient of friction x bolt diameter in inches x bolt's desired tensile load in pounds.

Do I remember this correctly?

 

[CR94]

Semi-correct, but simplified. There are two relevant coefficients of friction: One at the threads, and another at the face of the bolt or nut, acting at two different radii. Thread pitch has some effect, especially if it's coarse relative to diameter.

Because the "desired tensile load" is proportional to diameter squared (for equal stress), your "torque target" is proportional to diameter cubed, for geometrically similar situations scaled to different sizes.

 

[vavavroom]

Thanks for the additional information.

 

[zeng]

Target torque is a function of bolt cross-sectional area (influenced by bolt diameter and coarse/fine pitch) and bolt material (as in ultimate tensile strength, yield stress and proof load)...... not forgeting a built-in factor of safety.
Torque is usually specified with dry bolt.

 

[Imp4]

Originally Posted by vavavroom
In a recent thread, the opinion was voiced that wheel lugnut/bolt torque was not critical.

Remember, all bolted connections are designed with a safety factor.
That way, when you get a chowderhead with an impact gun that likes to get happy installing wheels, the bolted connection doesn't fail unless he goes totally overboard.
Tldr: The 4 or 5 bolt connection can withstand the impact gun as long as the installer isn't a complete moron.

Now, when we design bolted connections for API 15,000 psi flanged valves for well head and christmas tree applications in the upstream oil and gas market (per API 6A), you bet your yaz that those connections are torqued per the design specification.

Because it's the right thing to do.

And it keeps people from being dead.

 

[Cujet]

If the bolt is in a shear application, torque may not matter at all. As long as there is some form of locking feature to prevent self disassembly.

 

[Imp4]

Originally Posted by Cujet
If the bolt is in a shear application, torque may not matter at all. As long as there is some form of locking feature to prevent self disassembly.

If a bolt is in shear or double shear, then it is treated as a pin.
The threaded element is inconsequential as long as the shear forces are acting on the non-threaded portion of the bolt.

Here's why calculating your bolted connections correctly is critical.
Hyatt Regency walkway collapse.

 

[Smoky14]

Originally Posted by Imp4
Originally Posted by Cujet
If the bolt is in a shear application, torque may not matter at all. As long as there is some form of locking feature to prevent self disassembly.

If a bolt is in shear or double shear, then it is treated as a pin.
The threaded element is inconsequential as long as the shear forces are acting on the non-threaded portion of the bolt.

Here's why calculating your bolted connections correctly is critical.
Hyatt Regency walkway collapse.

Normally a good idea on aircraft wing attach bolts also.
Smoky