The critical element of a bolted joint is the clamping force on the gasket, and that is determined by applying the exact torque required to the fastener. Generally, in most joint designs, people achieve the clamp load by using a specified torque value with a torque wrench to generate bolt tension on the fastener. However, the real-world conditions surrounding the application of the bolt require adjusting the torque value. This is where the k-factor, also known as the nut factor, comes into play.

K-Factor is a critical value for calculating the target input torque for a fastener. An accurate k-factor can only be determined by doing experiments with the lubricant and fastener that you plan to use. It is essential to have an accurate k-factor to create a good torque-tension relationship when calculating the applied torque for threaded fasteners. This is necessary for creating a good seal around the gasket that keeps the contents inside the pipes.

There are various factors that can affect the mating surfaces and bolt thread conditions, such as relatively loose nut and bolt thread manufacturing tolerances, fastener thread condition issues, thread pitch, new versus reused fasteners, the presence of hardened washers versus nut rotation on the bearing surface, variations in nut dimensions, temperature, and the presence of coatings and lubricants.

To determine the k-factor, there are several different methods available, but there is no good ISO or ASTM standard for testing on fasteners. One of the methods involves predicting the installation torque on a fastener, placing it on a load cell, lubricating adequately, applying torque with a calibrated torque wrench, and measuring the clamping force on the flange to determine the preload of the fastener.

Once you determine your k-factor, you can use the equation T = K D F/12, where T is the target input torque, K is the nut factor, D is the nominal diameter of the fastener, and F is the target preload.

K-factor testing is critical for PTFE bolts because there is no manufacturing standard in the industry for the coating applied to these fasteners, and as a result, the k-factor needs to be tested for each manufacturer's methods. The k-factor for PTFE bolts is typically lower, as the nut has been overtapped to accommodate the coating, which means there will be less contact surface for the nut on a threaded fastener.

The coefficient of friction and friction factor are other terms used for calculating torque value. However, the coefficient of friction method only addresses the friction between two bodies and not bolt preload for a bolted joint. Friction factor methods are complex and require experimentation, which is why the nut factor method is recommended.

It is essential to understand that the k-factor chart from ASME PCC-1 (2019) is not for general use on bolted joints without understanding the variables at your site and the materials you are using. The purpose of the Target Torque Index is only to provide an example of how to calculate the k-factor and should not be used blindly.