When it comes to bolted joints, utmost importance is given to joint reliability in applications such as pressure vessels & pipe flanges in oil & gas for leakage free joints; in automotive/machinery for guarding against vibration. But, when it comes to infra-structural joints of long-term nature such as bridges and towers, the Fit-it-Forget-it approach is commonly seen.

Joint reliability is determined by various factors, bolt load accuracy playing a predominantly critical role among these. In order to control the impact of external factors such as vibration and temperature on bolt load, it is time to talk about Controlled Bolting Mechanism which is a scientific method of bolting.

It is very unlikely that the bolts in infrastructure joints will be re-opened in its life time. Since bolting is more of permanent nature,the nuts are tightened or bolts are loaded to a higher degree than other type of bolts.The bolts usually used in infrastructure are of Grade / Property Class 8.8, 10.9, etc. Sometimes HSFG (High Strength Friction Grip) bolts are also used. One standard which specifies bolting for high strength structural bolting is EN14399. The bolt & nut assemblies are as per EN 14399-4 which normally consists of one bolt, one nut and two washers. All these bolts would be preloaded; the pre-load should be more than the load that would be acting on the bolts when the bolting is complete, and the structure loaded with the active loads like weight, etc.

Lubrication is a significant factor in achieving bolt load calculation through torqueing tools. In general, the bolts are pre-lubricated using coatings as specified to suit the environment/joint on which they will be used. The bolt manufacturer specifies the load that will be generated in the bolt when applying a specific torque value. This is done keeping in mind any deterioration of the lubricant during storage, and the time lapse between manufacture and the installation of the bolts. The load generated in bolts can be verified at sites by random sampling using load-cells or bolt tension measuring devices.

Due to time lapse between procurement, storage and actual installation of the bolts, the bolt load accuracy is impacted by the upkeep and maintenance of fasteners. Unfavourable site conditions do not allow for proper storage and upkeep, which may effect changes in fastener’s physical and mechanical properties, which causes difference in actual load calculation vs current load requirement. This calls for pre-inspection of bolts, right selection of tool for higher productivity and quality; proper storage, tools which can give part code wise data recording facility through the latest technology.

These bolts are tightened / preloaded using mechanised torqueing tools which come with the facility of pre-setting the torque value and achieving bolt load accuracy in a calculated manner. These tools record and notify torque values achieved while torqueing, which is not possible with manual tightening methods. The torqueing tools can be hydraulic, pneumatic, electric or cordless. However, one should take into account factors such as torque required, location of the joints, ease of use, etc. while selecting the right tool.

Cordless tools, the latest in controlled bolting, are quite compact, have good power to weight ratio and come with documentation feature that helps the quality department to download bolting data for records. In the latest technological advancement, the innovative pumps for the hydraulic torque tools come with data recording facility that gives the status of torqueing for every single bolt. They also provide audio & visual warning on successful completion of bolting and errors, if any. The routine inspection activities become easy due to data monitoring. Since historic data is available with the latest torqueing tools, inspection scheduling patterns become easier, which go a long way in cost control, data transparency, data identification and traceability.

While hydraulic torqueing tools offer the best power to weight ratio, they can be difficult for working on bolted joints at a height, due to requirement of a power pack, and are also relatively slower than cordless tools. The pneumatic tools need compressed air, so while working at height there could be loss in air pressure depending on the line length.The electric tools are quite heavy for a given torque compared to other type of tools.

Side loading on reaction points is another factor that affects bolt load accuracy and tool life. It is important to give attention to the reaction points of the torqueing tools and requirements of any customised reaction arms. The next-gen bolting is about usage of special washers and driversthat eliminate the requirement of reaction arms.

There are special washers that ascertain the achievement of the correct bolt load which may be through some jelly squirting or by flattening of the projections on the washers.

Documented procedures for bolting and skill of operators play a crucial role in achieving bolt load. ASME recommends deployment of certified bolting personnel for any bolting job*.Equally or more important attention should be paid to the safety of the operators by training them in safe operation of the tools and adherence to the systems and procedures without taking any shortcuts to achieve the objectives. To summarise, the achievement of required bolt load is a result of true combination of right tool and hardware selection, proper procedure, lubrication, operator skill on tool operation and safety.

Bolts are said to be pre-loaded as the loading (tightening) happens prior to the induction of active loads on the joint.
ASME offers Bolting Specialist Qualification program through its Authorised Training providers.

Advanced Bolting Solutions Pvt. Ltd. (ABS), established in 1998, is a leading multinational organization in providing consultative sales of the latest technology in torqueing, tensioning & on-site machining systems, in-situ industrial bolting & machining services and unique after sales customer support initiatives with offices located in India, UAE and Oman.

AUTHOR DETAILS:
Mathew Kurian is the Head – Technical Division at Advanced Bolting Solutions (ABS), with 25+ years of experience in the bolting industry. Being an ASME Authorized Training Provider, he is also actively involved in training in-house personnel and customers.

For any further information,
kindly visit www.absgroup.in call at 022 4171 4444.