Fastening technologies ensure correct preload for critical applications

Paul Boughton

Countless millions of threaded fasteners have been installed around the world since this most useful of fastening concepts was invented. Nevertheless, the threaded fastener is not without its limitations, especially in critical applications. For instance, if a pipeline flange joint leaks because of a fastener failure, it could result in an extremely costly loss of product, plus downtime to carry out repairs and, depending on the fluid involved, risk to personnel in the vicinity and considerable environmental damage. Or consider the wheelnuts on a heavy truck; the loss of a wheel could lead directly to a fatal accident.

In either example, the fasteners may have been correctly tightened when they were installed, but threaded fasteners can become loose, especially in conditions where thermal cycling or vibration is present and the actual tension (or preload) in the fastener is not being monitored.

To address this problem, preventative maintenance regimes are often introduced to ensure that the fasteners are checked periodically, usually by means of a torque wrench. Even so, the use of a torque wrench does not necessarily guarantee that the correct tension is applied in the fastener, and both over-tension and under-tension can lead to a fastener coming loose or failing, with fatigue failures induced by under-tension being particularly dangerous due to their sudden and catastrophic nature.

When a torque wrench is used, a number of factors can influence the resultant tension in the fastener, such as thread lubrication, thread quality and wrench access (the use of offset or cranked extensions can give misleading results if used incorrectly). Not surprisingly, numerous alternative methods have been devised for assessing or indicating the tension in a threaded fastener.

Load-indicating washers

In the field of structural engineering, load-indicating washers are often used. These have small protrusions that deform when the fastener is tightened; feeler gauges are used as ‘go/no-go’ gauges to determine whether the fasteners are adequately tightened. Turnasure, a USA-based company, claims that an accuracy of ±1 per cent can be achieved with its 'Direct Tension Indicators’ (DTIs) that use this principle, and the tension in the fastener is independent of factors such as lubrication and corrosion.

A variation on this theme is the Squirter load-indicating washers from Applied Bolting Technology, also in the USA. These washers have additional small channels pressed into the underside of the washer that extend from the hollow underside of the protrusions to the outer edge of the washer. The voids beneath the protrusions are filled with a silicone paste; when the protrusions are flattened under load, the paste is squeezed out along the channels until it is visible at the edge of the washer – thereby indicating that the correct tension has been applied to the fastener.
An accuracy of ±2 per cent is claimed by Stress Indicators of the USA for its Precision DTI (direct tension indicating) Smartbolts that are available in sizes from M12 to M32 (and imperial equivalents) for use in temperatures from -20 to 75¢ªC. These reusable bolts are fitted with an optical microindicator element that enables the bolt's tension to be indicated via a small window in the bolt head. The microindicator is a miniature optical absorptance cell or variable density filter that reproducibly changes its spectral transmittance as the cell thickness is varied. A brightly coloured indicating area is placed at the rear wall of the cell and a small drop of optically dense fluid, which strongly absorbs the spectral colour of the indicator area, is contained in a hermetically sealed envelope between the indicator area and the clear window.

Ambient light passing through the window is selectively absorbed by the filter cell and selectively reflected by the indicator area to pass back through the cell and window to the observer. Extension of the bolt applies tension to the cell and varies its thickness, which therefore changes its spectral transmittance to indicate the tension in the bolt. Fully tensioned bolts show a dark colour in the window, whereas under-tensioned bolts have a brightly-coloured telltale visible, and the amount of under-tension can be estimated from the colour shown in the window.

Valley Forge of the USA has also utilised the concept of comparing the extension of the stressed bolt with an internal unstressed gauge pin. This company's Maxbolt load indicating fasteners display the bolt tension by means of a small needle that travels along a scale to read actual values or simply point to coloured zones. It is said that Maxbolts can enable uniform clamp loads to be achieved within 3 per cent of the design specification. Interestingly, because the load indicator is built as a cartridge, it can be removed for operation of the equipment in harsh environments, then replaced later to enable the bolt tension to be checked.

Another type of load indicating fastener from the same company, the SPC4, uses a gauge pin that is fixed into a small hole machined in the head of a bolt. A datum disc is also fitted to the bolt head in such a way that its surface is flush with that of the gauge pin when the bolt is unloaded. As the bolt is tightened, it becomes elongated and the gauge pin effectively retracts relative to the datum disc. By using a portable electronic displacement transducer to measure the distance between the two, a measure of the extension can be made that is translated into a tension reading by the handheld monitor, with a claimed accuracy of ±5 per cent. For critical applications, a monitor can be attached permanently to enable the bolt tension to be monitored continuously.

Ultrasonic measurement

Precision length measurement is not the only technology available for assessing bolt tension. Norbar, for example, measures preload with ultrasonic transducers that introduce a sonic pulse at one end of the fastener and accurately measure the time-of-flight required for the echo to be returned from the other end. When the bolt is tightened, the time-of-flight increases, enabling a value for the tension to be calculated that takes into account the material properties and operating temperature. High-temperature transducers can be used on fasteners at 175¢ªC, and the accuracy of the ultrasonic technique is said to rival that achieved with strain gauges – which are generally regarded as the most accurate way to monitor fastener tension (±1 per cent), though also the most expensive.

While the methods outline above have their advantages, they also have their limitations. A new technology, however, is said to be particularly useful for applications where accurate tightening is required with continuous remote in-service monitoring. It is also advantageous in applications where only periodic checking with a handheld instrument is required.

Developed originally by engineers from SJB Engineering, the technology utilises a precision capacitive transducer inserted in the bolt or stud. As the fastener is tightened, or if the tension alters during service, the transducer provides an accurate output to indicate the fastener's extension - and hence the preload (Fig. 1). Tests showed that tightening a four-bolt flange using a conventional torque wrench resulted in a load difference of up to 75 per cent between the bolts. Using the new technology on the same bolts enabled the variance in induced load to be reduced to less than 3 per cent – which would be adequate for almost any conceivable bolted joint (Fig. 2).

Truload fasteners are now being marketed by Scana UK, a company established by SJB Engineering to promote the technology (Fig. 3). While the company quotes an accuracy of ±5 per cent for fasteners tightened and monitored using the Truload technology, it is believed that ±1 per cent or 2 per cent is probably achievable, depending on the fastener parameters.

Two alternative monitoring systems are being offered. In the first, a handheld monitor is connected to each transducer in turn so that the tension in the respective fastener can be checked (Fig. 4). The other alternative, which offers potentially vast cost savings, is continuous monitoring. In this arrangement, numerous fasteners can be equipped with transducers, all of which are wired back to a central monitoring unit that can either be linked to a display for manual monitoring with or without automated alarm functions.

Remote monitoring

Furthermore, extended communications technologies enable the monitoring to be performed remotely. For example, on an unmanned offshore oil or gas platform, fasteners could be permanently monitored, with the data being sent to a remote onshore location anywhere in the world. This would avoid the need to send a maintenance operator to the platform to manually check and retighten the fasteners.

In addition, because the Truload system applies the correct tension to a fastener, the fastener is less likely to need retightening than one that has been either over-tightened or under-tightened, which itself helps to reduce the maintenance requirement.

Inevitably the Truload fasteners are more expensive to purchase that conventional fasteners, but the potential savings in lost product, plant downtime and damage to the environment – as well as the plant operator’s reputation – are far greater.

New developments for the Truload system include miniature Bluetooth-enabled transducers that will enable continuous monitoring on rotating machinery such as offshore windmills. High-temperature variants are also being investigated to enable the Truload's current 80¢ªC maximum operating temperature to be increased.

Meanwhile, other applications that have either benefited from the system already or that are being actively investigated include cranes, truck wheels, refineries and heat exchangers. For intrinsic safety applications, the handheld monitor is currently undergoing final trials and is expected to be certified soon. As such, the system will be very attractive for operators of refineries, petrochemical plants and similarly hazardous plant.

Continuous monitoring of fastener tension clearly has a small operating cost associated with it, and the initial installation cost is higher than for fasteners tightened by methods such as torque wrenches or 'turn-of-the-nut', but the risk of a fastener failing due to incorrect preload is significantly reduced, and the long-term costs could be substantially lower.

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