The 2017 edition of the standard is considerably modified from the 2003 edition. The five Norbar articles are designed to explain the key differences and how the new standard is intended to be used. They are an overview, not a replacement for studying the standard. If you have questions we will be happy to try and help. Please follow the email link at the end of each article.
This fifth article of five has been written with industrial users in mind. Industrial applications for torque are still increasing as customers and manufacturers realise the importance of correct torque application to reduce customer complaints and warranty costs. It will help to first read article one on the general reason for change.
The following examples represent some types of users for whom this article may be useful.
Operations within assembly plants where hand torque tools are used to ensure the correct installation of threaded fasteners in the user’s products.
Maintenance and repair of equipment used for manufacture and assembly operations.
Installation of products by the user at their customer’s site.
Assessment of risk
The scales of assembly operations vary hugely and this is why the standard puts a responsibility on the user to assess and set their own procedures for control of torque tools. For example, some motorcycle and white goods factories produce thousands of subfamilies and finished products each day. Using the 5,000-cycle default interval in ISO 6789 would mean that each torque tool must be checked several times per week, or even several times per day. However, if the user buys high quality tools and looks after them, they are capable of maintaining their torque settings for 25,000 or more cycles. Therefore, the user needs to monitor the performance of tools and form an opinion on the interval between checks. The interval may vary according to the importance of the fastener being tightened. Safety critical fasteners and situations where excessive torque causes component damage may require a shorter interval. Training in the correct use of torque tools is important both for consistency between operators and for prolonging the life of the torque tool. At Norbar we have seen many production line wrenches returned for repair that have been used as hammers to drive the bolt through a tight clearance hole before tightening the nut. One enterprising customer realised that this was damaging the tool so they welded a hammer head to the ratchet! (Please do not follow this example. It reduces the ratchet strength.)
In maintenance the situation is quite different. Generally, the tool is being used by a maintenance technician who has mechanical sympathy with the torque tool. The number of cycles will be much reduced and the tool should therefore stay in tolerance for longer. The issue is more likely to be that maintenance tools are not regarded as important enough to require assessment and do not exist within the company measurement record system. This is clearly wrong, because they are used to maintain expensive and critical machinery. Incorrect reassembly can be expensive if the plant stops production and potentially dangerous to employees if it fails. Finally, although maintenance technician are generally cautious and follow procedures, a long torque wrench can be seen as a great tool to release a tight bolt. Torque Tools are for controlled application of torque and should not be used in a “pull till something breaks” approach. Norbar makes small and powerful torque multipliers for loosening stubborn bolts. The damage caused by one technician may only become apparent later, when a weakened tool is stressed again, perhaps within the normal operating range of the tool.
Site installation is similar to maintenance in some ways. The torque tools should be checked and their condition and performance noted, preferably before each project and during it if the project is long or involves thousands of fasteners. Installation technicians are generally trained and adopt safe working practices. However, in some countries and industries the skill level and experience is much lower and torque application training prior to starting work is recommended. There are increased risks when working at height. Tools should be tethered to the operator or the work platform according to local requirements.
Those industrial users holding accreditations to ISO9001, ISO/TS16949 or similar will almost certainly require a traceable certificate for their records. This means that a Part 2 calibration certificate is required. The user then has a number of ways to achieve this.
Calibration within the business, perhaps using an in-house metrology facility or quality control department. The condition of the tool should also be noted, including worn or slipping handles, missing components and ratchets that slip or have deformed square drives. One impact of the new standard is that the calibration system required and the complexity of the uncertainty budgets may lead in-house facilities to be upgraded or replaced by external service providers.
Specialist calibration service providers are increasing common. These organisations will often offer to look after the user’s entire range of metrology equipment, including torque tools. They are normally taken to the service provider’s laboratory off-site. As noted above, some service providers will not have compliant equipment and will need to either upgrade or stop offering torque tool calibrations.
Mobile “calibration” services are less relevant in the industrial market. In a ten-hour day, the operator can calibrate perhaps ten wrenches and their daily costs must therefore be divided by ten. An industrial user may have significant numbers of wrenches to calibrate and the economics need to be considered.
Those industrial users who do not specify traceable calibration in their quality management system can perform checks according to Part 1 and issue an in-house “Declaration of conformance”
To perform a visual inspection of a torque tool, check that the ratchet is in good condition and record readings using the procedure in ISO 6789-1:2017 takes about 20 minutes. To strip a tool; replace worn or defective items; lubricate and assemble; then perform all of the steps needed to issue a calibration certificate required in ISO 6789-2:2017 will take about 90 minutes.
Once again, the only way to produce a calibration certificate in accordance with ISO 6789 will be to follow Part 2. Part 1 only allows a declaration of conformance to be issued. The measurement method is the same, but other requirements are different.
If you have questions we are happy to help where we can. Please email ISO6789@norbar.com
Member of ISO working group on ISO678