“There are two reasons why bolted joints normally fail – spontaneous bolt loosening, and slackening,” says Ryan Bassett, senior product manager, BMG’s fasteners, tools and equipment division. “Bolted joints in heavy industries are constantly subjected to external forces which results in spontaneous loosening, due to vibration and dynamic loads. With the development of new materials and techniques – such as corrosion resistance composites – slackening is a common problem that results from settlement and relaxation.
“Nord-Lock’s X-series washers have been designed for optimum bolt security for joints exposed to severe vibration and high dynamic loads, like those found in mining applications for instance. The wedge-effect design prevents spontaneous bolt loosening and an integrated spring effect of the system compensates for the loss of preload due to slackening.”
Each washer pair has cams on one side and radial teeth on the opposite side to secure the bolted joint with tension, rather than friction. The conical shape of these washers creates an elastic reserve in the bolted joint to compensate for the loss of preload and prevents slackening.
Advances in polymers and sandwich composite technology have the strength to weight ratio and corrosion resistant properties that make applications lighter, without losing strength. But these materials are softer than metal materials previously used and make slackening an additional challenge. The multifunctional wedge-locking concept of the X-series combats these problems.
Surface treatments, which are common for corrosion protection, can also lead to slackening as the bolt sinks into the treated surface after tightening. Even a thin millimetre surface would normally put bolts at risk, but with the compensating spring of X-series, slackening is prevented.
All bolted joints have a functional preload range in which the joint is safe. Serious problems start as soon as the preload falls outside the functional range. If the preload is too high, yielding occurs. If it is too low, there is separation or sliding between the parts.
The Nord-Lock X-series, which is quick and easy to install and remove with only standard tools, has been certified by TV, a leading international institute in quality and safety certification.
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