When using hydraulic bolt tensioners to induce a controlled preload into a bolted joint, several key factors including the load transfer factor require consideration and examination.
According to Nord-Lock Group, a provider of secure and customized bolting solutions, says the key to ensure the selected tooling is capable of providing a load greater than the required preload of the joint to overcome any potential losses.
Load can be lost within a joint due to a number of factors, including:
- Localized yielding
- Joint deformation
- An imbalance in geometry
It can, however, be observed that the difference in stiffness between the joint structure and the bolt can have an effect on the overall load loss in the joint.
Load loss in a joint can, theoretically, be calculated using varying formula primarily based on known data of the bolt diameter and clamp length. The value of these calculations is commonly known as the load transfer factor (LTF).
The LFT gives a theoretical ratio which can be multiplied against the required preload to generate an applied load, which overcomes the theoretical losses.
Applied load = Preload x LTF
The LTF is important in tool design and selection. The maximum output force of the tooling must at a minimum match the applied load, however, it often exceeds this value. Careful consideration of the stresses in the joint and bolt is required to ensure loading conditions do not exceed accepted design criteria.
In addition, performing the LTF calculation can highlight the need for changes in the loading conditions, joint design, and tensioner design solution.
It is important to remember that the load transfer factor is a theoretical value and cannot be used to determine the precise load lost within a bolted joint. However, for design and load application, LTF provides loading conditions, which allow for the physical losses within the joint.
Read the full article from Nord-Lock Group here.