What do Torque Wrenches Measure?
Torque is simply a measurement of the resistance to force when a nut is turned down the fastener's thread. Most of this resistance is friction. Therefore, torque cannot and should not be used in the context of a fastener's "tightness".
Friction consists of five parameters:
- Material at the bolt-to-joint interfaces
- Surface finish at the bolt-to-joint interfaces
- Lubricants at the bolt-to-joint interfaces
- Thread condition
- Lubricants at the thread
90% of the resistance you feel on your torque wrench is caused by friction.* Change the friction by 10% and you get a 100% change in bolt tension with the same setting on your torque wrench!
When a torque value is given, lets say 40 foot pounds on a bolt, an assumption is always made as to how much friction there is between the bolt, nut, and joint surfaces. This is why a torque specification must specify the Conditions of Torque.
Conditions of Torque include:
- Cleaning and surface preparation
- Type of lubricant
- Where to apply lubricant
- Whether to apply torque to the nut or the bolt
Different lubricants produce different amounts of friction. This is why one should always use the type of lubricant that the engineer specifies. Lubricants can be given a "K" factor in an attempt to assign values to their affect on fastener friction. In theory, different types of lubricants with the same K factor will have the same affect on friction.
Sometimes the plating itself can be assigned a K factor. For example, much aircraft hardware is cadmium plated. Cadmium is a natural lubricant. Most commercial hardware is zinc plated. Zinc has a higher K factor (more friction) then Cadmium. Based on the K factor, for the same size bolt, used in the same application, a cadmium plated bolt would have a lower torque value than a zinc plated bolt to achieve the same amount of clamping force.
Lets take an example: Using an AN5 bolt tightened to 75% of proof strength produces a clamping force of 4,000 pounds at the following torque values in foot pounds:
- 17 Dry, cadmium as received
- 12.5 Lubricated with oil
- 21 If our bolt were zinc plated instead of cad plate
Using a different lubricant and your joint might be loose or over-tightened even with the same torque setting on your wrench.
Where should you apply the lubricant? Notice from the pie chart that most of the friction is a result of the nut face having to rotate against the clamping surface. Many torque specifications say to apply lubricant but are not specific on where to apply it. This is no small matter. A fatal helicopter accident** resulted when thread lubricant was applied to the wrong area of the bolt!
If the torque specification states that you should apply a thread lubricant, do you apply it only to the threads? Or do you apply it to both the threads and under the bolt head?
We don't know unless the engineer who wrote the torque specification tells us. It's the engineer's responsibility to specify the conditions of torque including where the lubricant is to be applied. Without this specific information, the resulting joint tension can be almost anything. Many torque specifications published in general aviation maintenance manuals are deficient in this area. The mechanic cannot achieve proper joint tension with inadequate torque specifications. Joint failures are to be expected and are not caused by improper torque but by improper torque specifications.
Torque resources you can download from my web site at www.mechanicsupport.com
MIL-HDBK-60 "Threaded Fasteners - Tightening to Proper Tension"
* source "An Introduction to the Design and Behavior of Bolted Joints" by Bickford. Chart information from www.boltscience.com.
** For an interesting analysis of the above:
"Failure of bolts in helicopter main rotor drive plate assembly due to improper application of lubricant" by N. Eliaz, G. Gheorghiu, H. Sheinkopf, O. Levi, G. Shemesh, A. Mordecai, H. Artzi,
Published in Engineering Failure Analysis #10, 443-451, www.sciencedirect.com