Know Your Fastener K-Factor

Lubricant plays a significant role in determining the K-Factor of threaded fasteners.

By Brendan Schriber, DuPont

Photo is of a Skidmore Wilhelm bolt tension device, used to determine the clamping force of a nut and bolt threaded connection.

More than 200-billion fasteners are sold in the U.S. annually. To ensure optimal performance, it’s important to understand the fundamental function of fasteners, starting with torque. Each size and grade of fastener has a specific torque limit for operation. Applying too much torque can result in the fastener breaking. Conversely, not applying enough torque can lead to the fastener not fulfilling its intended function. To ensure proper torque, it’s important to understand K-Factor and how it’s affected by lubrication.

K-Factor represents the relationship between torque applied to a fastener and its resulting clamp force. Typically, K-Factor lies between 0.1 and 0.25. If K-Factor is too high (greater than 0.25), the fastener clamping force may not be sufficient relative to the applied torque. Adversely, if K-Factor is too low (less than 0.1), the fastener clamping force could be too high relative to the applied torque, causing the bolt to stretch and fail.

Controlling K-Factor helps optimize a threaded connection’s performance because a well-controlled K-Factor ensures the fastener provides predictable and repeatable clamping force.

To control K-Factor, lubricants are typically applied to the threaded connection. In general, most lubricants reduce K-Factor. Along with the lubricant, size and grade of a fastener can influence K-Factor. Therefore, it is best practice to test a lubricant and fastener together to obtain an accurate K-Factor value. One test method is to use a bolt-tension calibrator, which determines the clamping force of a nut and bolt threaded connection as it is tightened using a torque wrench.

Reference ISO 10647, “Fasteners—Torque/Clamp Force Testing,” before testing a fastener and accompanying lubricant as the standard outlines the mathematical method to determine K-Factor. As provided in the ISO standard, Equation 1 shows how K-Factor values rely on the applied torque (T), resultant clamping force (P_i), and the nominal shank diameter of the fastener (D).

Equation 1: K = T/P_iD

When determining K-Factor with a bolt-tension calibrator, choosing a torque range that represents the fastener’s operating range is crucial. Testing a fastener across its operating range provides a more comprehensive observation of K-Factor rather than testing at a single data point. A suggested method is to select five torque steps that progressively reach the fastener’s proof strength.

To do this, refer to a torque-tension reference guide and work backward from the maximum suggested torque for the bolt. For example, a 5/8-in., 11-TPI (threads/in.), Grade 8 bolt reaches its proof strength at 159 ft.•lb. (assuming a K-Factor of 0.15). Knowing this proof strength is 80% of ultimate tensile strength (UTS), the torque steps to reach 159 ft.•lb. were set as follows: 60 ft.•lb. at 30% UTS, 80 ft.•lb. at 40% UTS, 99 ft.•lb. at 50% UTS, 119 ft.•lb. at 60% UTS, and 159 ft.•lb. at 80% UTS.

After determining the appropriate torque range and selecting a lubricant, prepare five bolt samples with washers placed beneath their heads. Then, insert the first bolt into the bolt tension calibrator and tighten it to a snug fit using the corresponding nut on the other side. Set the torque wrench to the first step and apply torque to the fastener. The device will display the resultant clamp load, which can be recorded. Repeat the process for each succeeding torque step and determine the average clamping load at each step for the five bolts.

To determine K-Factor across the operating range of the fastener, Equation 1 is written as Equation 2 such that K-Factor is a linear regression line produced by the other variables.

Equation 2: 1/P_i = KD/t + 0

To illustrate, five bolt samples with lubricant and another five without were prepared for a 5/8-in., 11-TPI bolt. After data collection, the results were populated into an Excel spreadsheet where K-Factor could be determined through linear regression. The resulting K-Factor of an unlubricated 5/8-in., 11-TPI bolt was 0.25 and 0.16 in the presence of a MOLYKOTE lubricant. There was a strong correlation between the change in applied torque and increase clamping load as the coefficient of determination (R2) was greater than 99%.

The experiment highlights the effectiveness of a bolt tension calibrator in determining the K-Factor of a specific fastener in conjunction with its corresponding lubricant. It is important to perform this test as variations in bolt grade and geometry can influence the K-Factor value. If K-Factor is too high, the torqued bolt will not apply enough clamping force for the application.

Adversely, if K-Factor is too low, the bolt may be overtightened and cause a stress fracture. That’s why it’s important to lubricate a threaded connection. By applying a lubricant, the K-Factor of a fastener can be controlled to a predictable and repeatable value, achieving optimal performance from the threaded connection. EP

Brendan Schriber is an R&D Technical Associate—North America for MOLYKOTE Specialty Lubricants at DuPont, Midland, MI (dupont.com). He joined DuPont in 2022 as an R&D Technical Co-Op and has built an expertise in testing and test method development. In his current role, he is responsible for designing and conducting tribological tests to validate lubricants for unique applications and measuring and analyzing the noise, vibration, and harshness (NVH) of lubricants in tribology.