Thread strength
Nut threads are designed to support the rated clamp load of their respective bolts. If tapped threads are used instead of a nut, then their strength needs to be calculated. Steel hardware into tapped steel threads require a depth of 1.5× thread diameter to support the full clamp load.
If an appropriate depth of threads are not available, or they are in a weaker material than the cap screw, then the clamp load (and torque) needs to be de-rated appropriately.
Threads are usually created on a thread rolling machine. They may also be cut with a lathe, tap or die. Rolled threads are about 40% stronger than cut threads.
Setting the torque
Engineered joints require the torque to be accurately set. The clamp load produced during tightening is about 75% of the fastener’s proof load. Over tightening will damage threads and stretch the bolt, ruining the joint’s strength; see Hooke’s law.
If the hardware is Cadmium plated, or lubricated (or both) the torque is reduced by 15 – 25% to achieve the same clamp load. Specialty coatings exists that allow for a reduction of 50% in torque (compared to non-plated, non-lubricated hardware) to achieve the designed clamp load. Cadmium plated fasteners are no longer produced due to the toxicity of the metal.
Torquing the bolt is notoriously inaccurate. Even with a calibrated torque wrench large errors are caused by dirt, surface finish, lubrication, etc. The turn of the nut method is more accurate, but requires additional calculations and tests for each application.
There are more expensive tools for accurate torque setting, like ultrasonic meters, but they are out of reach of most shops.
Property class
There are many different property classes (grades) of bolts and nuts. The most common are listed below. Note that each nut property class listed can support the bolt proof strength load of the bolt it is listed beside without stripping. The first number in the bolt property class indicates the nominal tensile strength, and the second number the yield stress as a proportion of the tensile strength. In other words class 8.8 means tensile strength of 800 MPa and proof stress of 0.8 x 800 MPa = 640 MPa.