A “nut factor” answer may be more than adequate for your application. However, depending on the degree of accuracy/certainty you need to establish your fastener tension values, approach "nut factor" usage with great caution. The reported ranges, and the ranges of those ranges, vary widely from publication to publication. Almost all of these lists (the responsible, enlightened ones) will emphasize their "reference" nature, and contain strong cautions regarding their use in a particular application without empirical verification (torque/tension testing).
A good discussion of nut factor, including some of its limitations and variables, can be found at
An excerpt:
"The K, or nut factor, not to be confused with the frictional coefficient, can be thought of as a combination of three factors: K1, a geometric factor, K2, a thread friction related factor, and K3, an underhead friction related factor. While there are published tables for K, these will usually vary from publication to publication. For a more detailed analysis it is desirable and often necessary to determine this value experimentally by using a specially designed torque-tension load cell."
The MIL-HDBK-60 torque/tension formulas are great (like "vonlueke" I recommend and use them) in that they attempt to take into account may of the variables that affect the torque/tension relationship that are completely ignored by more simplified "nut factor" tables (i.e. thread pitch/helix angle, pitch diameter, mean bearing area of the rotating fastener/structure bearing interface, the fact that underhead and thread friction coefficients can be -- and usually are -- different). Without that additional information (which I didn't see in this thread), I'm not quite sure how vonlueke got from the friction coefficient (not to be confused with nut factor) to the nut factor he mentions in his answer. In additon, the range of coefficient friction values themselves is quite large, and is further affected by even small amounts of supplemental lubrication.
Specific to silver plated nuts (from
"Silver Plating:
Since silver tarnishes from normal atmospheric exposure, the silver-plated nuts are commonly coated with clear wax to prevent tarnishing. Wax is a good room-temperature lubricant. Therefore, the normal "dry torque" values of the torque tables should be reduced by 50 percent to allow for this lubricant."
Although not noted by Barrett, nut manufacturer’s may also add supplemental lubricants to lots that may otherwise exceed a maximum locking torque requirement, or to help them meet multiple-cycle (reuse) locking toque requirements.
I have personally seen the variation noted by Barrett in silver plated nuts, and in testing performed on them. This might lead you to want to degrease your nuts to remove any such coating. However if your nut is of the all-metal, "self-locking" variety (the "norm" in the aerospace industry where silver plated nuts find the majority of their usage) and the "locking torque" was set by the nut manufacturer with the coating applied, you run the risk of developing excessive locking torques (reducing the tension "preload" for a given torque and increasing the potential for galling as well -- as affecting multiple cycle reuse capabilities). If the coating was applied after the locking torque was set, the effect is the opposite (potential lowering of the locking torque values to the point where vibration resistance, and/or multiple-cycle locking torque requirements, may be compromised, etc.). In former scenario you might generate non-conforming hardware by degreasing your parts; the latter scenario could result in the manufacturer shipping you nonconforming hardware. You can see why most users of aerospace fasteners users prohibit modification of fastener lubricants by anyone other than the fastener manufacturer, and require retesting after any such modification.
Bottom line: Even without the "wax" variable above, if I needed to know the torque/tension relationship for a given application with any degree of certainty, I would trade one torque/tension test for 100 calculations or "nut factor" tables. Most aerospace companies recognize this and maintain recommended "torques tables" (by diameter) for specific nut/bolt finish/lube combinations based on empirical testing. Note that even testing won't protect you from part-to-part and/or lot-to-lot variations in friction coefficients (the overriding factor in the torque/tension relationship), of which the "wax" vs. “no wax” example for silver plated parts above is just a single example.
of K = 0.185.