By R J Pietersma, CBC Fasteners (Pty) Ltd
The use of construction bolt assemblies in terms of the new standard EN14399 has been underway for the best part of nearly ten years now. It has been a steep learning curve, to say the least. This article is to share some insights into technical jargon and risks.
Are South African Manufacturers geared up?
Up until the new power station builds very few construction bolts were being installed, hence there was a general lack of awareness and capability. This was on many fronts and not just bolting. Medupi and Kusile changed this and it has clearly been established that design engineers, manufacturers and construction companies had a long way to go to catch up with developments internationally and best practice standards. This includes bolting amongst a whole range of other requirements.
Experience tells when choosing the construction bolt route, the first call is a manufacturer capability study with an audit of the production processes, quality system and compliance with ISO 898-1&2 as well as with all the requirements of EN 14399 and most importantly, EN14399-2. Unfortunately, in practice, bolt and nut assemblies still continue to be a last minute panic purchase.
SANS 10094, the standard dealing with Construction Bolting has recently been updated and approved. This standard does not recommend grade 10.9 Hot Dip Galvanised (HDG) bolts because of the risks of HE or HisCC. Nevertheless, in practice, there is still a call for this product. The risk can be controlled by the manufacturer avoiding acid contact and further controlling excessive hardness levels at the upper limit of grade 10.9. Further risks associated with undue stressing of grade 10.9 HDG bolts will be avoided if good installation practice is adopted.
EN14399-3 (grade 8.8 and 10.9) vs.EN14399-4 (grade 10.9 only)
Why a universal standard is not adopted is a puzzle. Clearly, there were principles that were not negotiable which has led to two possibilities. The historical position has largely been maintained in that the EN14399-4 nut (previously DIN 6915), has a lower height. The intended reason is that the nut threads should fail first (not guaranteed) in the event of over tightening, purposefully avoiding a sudden bolt fracture, with installer safety being compromised. Usual construction practice is that one would like to see the bolt fail in the event of over tightening because one would know it had occurred, whereas with thread failure, this may not present immediately and a future calamity may be lurking when the right conditions prevail. In South Africa, SAISC and SANS 10094 recommends the use of EN14399-3 in grade 8.8 and 10.9. Shear through threads is allowed whereas in EN14399-4 there is a shorter thread and the shear plane is through the shank of the bolt.
Whatever the bolt & nut assembly used, once pre-loaded and subsequently removed, they cannot be re-used. The reason is that the threads may have been subject to plastic strains during tightening.
The myth of Torque vs. Tension
The talk is always about torque, whereas the objective is clamp, a spring condition holding surfaces together. Torque (or the torsional rotation effort) is merely the means to getting to the correct clamping force. This whole process would be simple were it not for the introduction of friction. When tightening a bolt and nut assembly, 50% of the effort is as a result of friction between the nut and washer face, 40% is in the thread contact and a mere 10% of the effort is creating the clamping force. This friction can vary. In a rusted bolt and nut (B&N), coefficient of friction it is as much as 0.35, in a un-lubricated hot dipped galvanised B&N is starts at 0.19 and increases up to 0.27 as additional torquing takes place. With molybdenum disulphide lubrication (MoS2), coefficient of friction is between 0.10 from 0.16. So, by way of example, in the case of torquing an M20 bolt at 464 Nm with a coefficient of friction of 0.14, clamping force of 127kN is achieved; when the coefficient is 0.10, less torque of 363Nm will achieve an increased clamp load of 134kN.
This leads us to the next important point; The lubrication of nuts.
Pre-lubricated nuts with molybdenum disulphide (MOS2)
There may be a misconception since there has been so much talk and use of pre-lubricated nuts that this is a new standard requirement. Whilst we recommend pre-lubricated nuts for the reason there is a tested coefficient of friction that can be relied upon, this is by no means a general requirement. EN14399 specifically makes reference to surface finish as processed, meaning lightly oiled, or as agreed between purchasers and manufacturer. Nevertheless, appropriate lubrication is required during installation, particularly with HDG bolts. In the case of no lubrication, galling will take place and in laboratory testing, we have established the potential for failure due to torsional tension.
In the case of the turn of nut method of fastening in the B&N assembly with lubrication, up to 25% to 35% additional clamp can be obtained than required by the standard. Without lubrication, the likelihood of thread failure is almost 100%. All the torque value will be absorbed by the galling effect of the soft galvanised layer and if the bolt has not started to fail due to torsional tension, the correct tension will not have been achieved and a loose bolt left in place with future potential failure consequences.
We really do recommend pre-lubricated nuts that have been baked to a dry condition. The advantages; it avoids the wrong lubricant choice, the risk of attracting grit on nuts during installation due to sticky lubricant is reduced, the under or over application of lubricant is avoided and; of most importance, certification of the coefficient of friction is supplied, together with recommended torque values. This testing in terms of EN14399-2 also provides confirmation that the B&N assembly complies with the rigorous requirements of the standard.
Another question that has been raised, is it possible of paint over-lubricated nut. MoS2 is oilioscopic, which means it cannot tolerate detergents. So cleaning with an industrial degreaser would be the appropriate first step, then priming followed with a final overcoat. Under no circumstances should acid be used clean.
Many bolters rely on the torque wrenches having been recently calibrated. One of the overlooked checks that needs to be undertaken is the wrench verification. This should take place on the day the wrench will be used by testing at least 3 bolts of the diameter to be installed with that wrench on that day. The verification takes place using a static torque meter. The reason for this verification is that calibration can change if, for example, the wrench was dropped. We have observed that many installers do not do verify their equipment, nor have the required equipment to undertake the verification. However if one is using the turn of nut method (TON), recommended by SAISC, verification of equipment can be avoided. Provide the markings are correctly made and the tightening process is properly supervised, TON will result in a reliably tensioned assembly.
Experience in the field is that there is a huge amount of poor communication between original design through the manufacturers of Bolt and Nut manufacturers and the installer tightening the final bolt. Some of the examples include a request for Nylock nuts for EN 14399 construction bolts, failing this, Clevelock nuts or rejection of pre-lubricated nuts because the black colour gives the impression the nuts have not been HDG. Fortunately, many mistakes are covered by the tendency to “over design/deliver”; not only in bolt manufacture but also in structure design. As a result, problems get caught in a normal distribution curve of the applied margin of safety and no adverse outcome take place. Where outcomes are likely to be negative as in some of the above examples, responsible Bolt and Nut manufacturers make recommendations and institute appropriate training
The greatest adverse has been where design engineers have not been involved in the pre-qualification of manufacturers and audit of their quality systems and not have ensured complete certification is in place based on comprehensive testing. Thereafter they have not been on site verifying, compliance to their original specification (which is prescribed in regulations of the Occupation Health and Safety Act). Where all this has occurred timeously, we have seen trouble-free installation. Where this was deficient, particularly in the early stage of manufacturer prequalification, adverse outcomes have often prevailed. It is emphasised, the problems have not been the fastener manufacturer but the end users poor understanding of their requirements of a design engineer. Unfortunately, the B&N manufacturers have often been unfairly fingered in the process.