Torque management ensures the security of threaded fasteners

Paul Boughton

Nobody wants a threaded fastener to fail. Paul Stevens reports on some of the latest developments for ensuring that threaded fasteners are tightened correctly and do not lose their tension over time.

While non-threaded fasteners are gaining in popularity for many applications, threaded fasteners are often a better option where close control is required over the joint characteristics. Consumer products from laptop computers to domestic washing machines feature threaded fasteners, as do high-value goods such as cars. On a larger scale, threaded fasteners are also used in, for example, wind turbine blades and flanged joints for pipelines. Depending on the application, failure of the joint might result in inconvenience for the user, warranty claims or incidents with catastrophic consequences for human life or the environment. To ensure the security of bolted joints, engineers today rely on both equipment that monitors the torque applied to the fastener, and torque-indicating fasteners that can show whether or not the fastener tension remains correct. Assuming that joints have been designed correctly, torque management can help to ensure that joints perform as intended over the lifetime of the application.[Page Break]

Torque wrenches apply a torque to a fastener in order to induce a tensile load that clamps the components together - and keeps them clamped under all anticipated service conditions, which might include temperature fluctuations, vibrations and static loads. Regular calibration to International Standards is vital to ensure torque equipment is operating within defined tolerances. Torqueleader, a manufacturer of torque tools, reports that its United Kingdom Accreditation Service (UKAS) laboratory is capable of recalibrating most hand torque tools, analysers and transducers in accordance with the standards. Furthermore, the UKAS laboratory can calibrate tools from many other manufacturers besides Torqueleader.

Crane Electronics (see panel page 43) offers some of the most advanced measurement technology with its latest product, the IQWrench Opta, which maximises the comfort of the operator.

Feedback on the fastening process is important if the operator is to make each joint correctly. Crane has improved the processing speed of its wrenches to improve the sampling rate, and operators have a choice of visual, audible and physical alarms to indicate the status of the fastening process. Visual indications are provided by multicoloured LEDs on both the front and back of the wrench. A real-time graph on the display gives a clear indication of the process characteristics such as target torque, lower and upper torque specification, or angle. In situations where the operator cannot look at the wrench, the tightening status can be signalled by a vibrating handle or audible alerts.[Page Break]

Tentec, a manufacturer of bolt tightening equipment, celebrated a record year in 2010, despite the recession. Newly appointed managing director Mike Gethings explained: "Our order book has been strong throughout the year. As well as pleasing sales for our standard bolt tensioning equipment, we have seen demand for our customised engineering skills also increase. Our in-house engineers continue to design innovative and cost-effective solutions for challenging applications and our manufacturing facility ensures that customers receive their products on time and within budget."

Hydraulic and pneumatic

In 2010, Tentec launched a series of hydraulic and pneumatic torque wrenches to complement its existing bolt tightening products. The SD, CD and PN ranges have been developed for engineers who require robust, easy-to-use and accurate tools for torque applications throughout industry.

Another UK company with global sales is James Walker Rotabolt, which reports that South East Asia has become one of its strongest growth areas. A recent success story relates petrochemical processing in Taiwan, where leak-free operations have been achieved and this has opened up a wealth of opportunities for the company's tension control technology. The first substantial order in the region resulted in Rotabolt technology being fitted to crane slewing bearings in Malaysia, Thailand, Vietnam and the Gulf oil fields. What made the fasteners an attractive proposition was their ability to achieve the correct design tension across the critical slewing bearing joint and then accurately maintain it over time.

Wind turbine manufacturer Vergnet is now fitting tension control fasteners on its GEV HP system. This French company uses thirty M90 Rotabolt fasteners on the main structural joint between the two halves of the turbine nacelle. The design allows the main turbine to be quickly disassembled into two halves so that the upwind nacelle section, complete with rotors, can be lowered to the ground and easily secured. This can be either as a protective measure in extreme weather conditions, or for quicker service and maintenance operations.

Traditional tightening practices using standard bolts only measure the force that is being applied to tighten the bolt; the clamping force, or tension across the joint, is unmeasured and unknown, yet it is critical to the joint's reliability.

In addition to achieving joint integrity during installation, the fasteners can be finger-tip checked during maintenance inspections to establish whether tension has been lost. This is because the Rotacap on the head of each bolt locks in position at the correct design tension but, as soon as any tension is lost, it spins freely, making it quick and easy to check without the need for tools or instrumentation (Fig.1).

Accu-Load, a manufacturer of specialist fasteners, offers bolts and studs that are capable of monitoring and indicating the tensile load during tightening and in use. Each fastener features a visual or digital readout option, which is built into the head of the device and indicates a percentage of the proof load. The bolts are individually machined and calibrated to suit the joint design and they can be used indefinitely (Fig.2). Accu-Load can also supply handheld mechanical or digital readout systems for remote monitoring. The technology is widely used on complex flange arrangements and critical environments on processing plants where even and accurate tensioning is required in the presence of vibration or thermal shock loads.[Page Break]

Combat fatigue

Another indicating fastener, the Ecostud from Arnold Wragg, is said to combat fastener fatigue failures and leaking joints by combining a quality fastener and an accurate tension controller within a single integral unit. The bolt allows easy identification of fasteners that are losing tension and enables them to be retensioned before a leak starts. As Ecostuds are a direct replacement for standard threaded fasteners, only the original size, material and required preload needs to be specified either for a new installation or a retrofit.

The Ecostud is fitted with a stainless-steel rotor that is locked when the stud is tight, but free to spin when the stud begins to lose load. A fingertip test on the rotor establishes whether the joint is still tight (Fig.3).

To illustrate the savings in maintenance times offered by products such as the Ecostud, consider a wind turbine. Each blade will typically have 60 retaining studs, equating to 180 studs per turbine - and there will be many other critical fasteners in addition to those retaining the blades. The blade studs are subject to high-vibration loading and must have the correct tension to avoid fatigue failure. Checking the Ecostud takes a fraction of the time required with the traditional method of engaging the torque wrench on each stud.

Finally, for mobile engineers, Norbar Torque Tools has developed a torque conversion calculator app available as a free download for iPhone and Android handsets. Similar to the online calculator on the company's website, the app offers instant conversion across SI, metric and imperial units.

Threaded fastening is a mature technology, but developments continue in the ways that fasteners are installed and tightened correctly. For applications where operational conditions could lead to the joint relaxing, engineers today have a wider choice of means by which fasteners can be checked for tightness both quickly and easily.[Page Break]

Torque wrenches and controllers used for engine assembly

For 10 years the Ford engine plant at Dagenham has relied extensively on torque control products from Crane Electronics. In the latest stage of this on-going relationship, Ford has invested in new TMAC Opta tool controllers for use in conjunction with its existing Crane Wrenchmaster heavy-duty torque wrenches (Fig.4). This combination is used on Ford's Puma Line, which produces diesel engines for Ford Transits, Jaguar (X-type) and the Land Rover Defender. The TMAC Opta replaced an original TMAC II torque controller.

Together the controller and wrench have to process data and withstand the harsh environment of a production line assembling up to 1500 diesel engines per day - representing some 6000 critical fastening operations on critical fuel injector assemblies. Crane's Wrenchmaster wrenches apply torque to 13mm nuts and communicate the data to the TMAC Opta for verification and communication with lineside PLC, confirming that the fastening status is within limits and allowing the engine to proceed to the next station on the production line.

TMAC Opta tool monitoring and control units have a number of features that help to ensure maintenance requirements are minimised and production is not interrupted. These include robust and highly intuitive software, isolation of internal circuits and a rugged housing. A second TMAC Opta is also to be used on the Ford's Lion line where EU5 Engine Compliance work is undertaken.

The relationship between Ford and Crane has also benefited from the ongoing support provided by Crane Electronics. This has included application advice and monthly site visits to ensure that any required repairs or recalibration work are carried out without delay or interruption to Ford's production schedules.

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