IT TURNS OUT THAT THE TURN-OF-NUT METHOD IS SHORT ON TENSION
The Background
Care should be taken when selecting fasteners for Class 3 Structures, such as in bridges, power stations and grand stands; these are structures where the consequences of failure are severe. Fasteners play an important role in the longevity, structural integrity and design of structural steel. Fasteners need to be selected for the correct applications. The fastener assemblies used for such applications are classified as High Strength Fasteners for preloading, under standard EN14399, also known as construction bolts.
One of the requirements of construction bolts for preloading is the correct tensioning of the bolts and nut assemblies, particularly when the application is in the slip resistant (friction grip) mode. It should be noted that preloaded construction bolts have a wider across flats dimension because of the greater clamp load that they are tensioned to.
The Anomoly
The aim of this paper was to understand as to why a greater turn was needed to tension shorter bolts, a contradiction to what some standards state. Standard SANS 10094 requires 1200 turn of the nut for clamp lengths less than 4 times the diameter. CBC Fasteners has found in advising customers that 1200 is not adequate and this correlates to what has been mentioned by the Research Council on Structural Connections (RCSC).
According to RCSC, using the turn-of-nut method “short grip bolts may not produce the necessary preload (pretension) and if used in slip resistant (friction grip) joint, then a load indicting device should be used. Furthermore, it has been observed that some problems may arise with the turn-of-nut method, particularly with the Hot Dip Galvanized (HDG) bolts.”
Introduction
For the purpose of this research tension-torque testing was conducted on 3 sets of each HDG bolts as well as 3 sets of each on black bolts of varying lengths (M20 x 65 mm, M20 x 95 mm and M20 x 120 mm).
The clamp lengths for testing on each bolts were 41 mm, 57 mm and 76 mm, respectively. The use of clamp length in this research helps to accommodate for the variation in the bolts lengths, which in turn, creates an acceptable amount of thread length needed for the positioning of the nut for testing. This was a simulation of what would be found in the field. The load cell used is designed to numerically demonstrate how much force has been applied to the bolt under tension (preload or snug tight) and the equivalent amount of torque required. This load cell had been recently calibrated and so we were sure of its accuracy.
A torque wrench was initially taken to measure the NM at a preload of 50 KN Note 1 for snug tight for each bolt clamp length, then further continuing to torque the bolts in tension; measuring the degree of turn at 180 KN force.
The amount of preload required by SANS 10094 is 178 KN. For the purpose of this study, 180 KN was set as the target. An average of the 3 lengths were taken in order to obtain a sound conclusion, and captured in the table below:
These results are different from that expected from various standards. The shorter bolts (65 mm), for both HDG and black bolts, required a greater amount of degree of turn; 94O more for black and 145O more for HDG compared to longer lengths. For both types of bolts, the amount of degrees turn decreases with increased length on the 95 mm bolt, then increases slightly for the 120 mm bolt.
Co-efficient of friction
To further clear all possible anomalies that could have affected the results, the co-efficient of friction was conducted in order to establish a link between any external factors that might have an influence on the results obtained. The co-efficient of friction for HDG pre-lubricated nuts were found to be 0.13k and for the black were found to be 0.13k, which indicates that the co-efficient did not play or have any material influence in the testing.
The co-efficient of friction is documented to have a huge influence on the torque but not on the turn-of-nut method; because the turn-of-nut determines the amount of the bolt stretch.
The co-efficient of friction influences the amount of torque required to ultimately get to the force in the bolts nut assemblies, indicating why the turn-of-nut method is not an ideal method of tensioning shorter length bolts.
How to explain the anomoly
A greater turn is required for shorter bolts could not readily be found in our research in literature. Our own conclusion is that in shorter lengths there is a reduced amount of length to absorb torsional tension. Whilst in longer lengths, the torsional tension is absorbed through greater lengths.
Conclusion
Based on the results obtained a sound conclusion of the turn-of-nut method can be made. It was seen that for short bolts and HDG assemblies the method is not reliable. The clamping was found to be almost twice the size of the diameter and a 120O turn is simply not enough, where a turn of 260O for HDG and 216O for black bolts is needed.
Fabricators need to obtain guidance from the supplier of bolt and nut assemblies as to what is the recommended torque for an appropriate turn of nut rotation. CBC Fasteners has a well-equipped team to advise on this. (Contact 011 767 0000 Rocky, Ajith or Rob)
Note 1: It is acknowledged that snug is a subjective measure. We undertook simulations to determine if clamp length had any influence on this and our findings were that it did not. A stronger assembler attained snug tight around 80Kn for the average of the three clamp lengths. The explanation why clamp length does not materially affect this is because 90% of the friction in torquing is in the threads and between the nut and washer
Prepared By: Delphine Mulaba-Kapinga
Qualification: Baccalaureus Technologaie in Engineering Metallurgy