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Expandable bolts: Helping save time and money

13 November 2014

Superbolt Expansion Bolts offer a simple and quick bolting solution for large machine couplings and can help save both time and money, as Nord-Lock UK explains

Simply put, Superbolt Expansion Bolts are adjustable fitted bolts with an outer diameter that can be expanded to fit the actual bore size. They can replace fitted bolts where the parts to be connected must be precisely aligned and high shear forces act on the connection.

In most cases when fitted bolts are used, either the fitted bolts are machined to fit the bores or the bores are reworked to match the fitted bolts. Both methods can prove costly and time consuming; the dimensions must be measured accurately on site and then adapted.


An Expansion Bolt consists of five parts. The tapered sleeve is placed in the bore and centered by the spacer. The tapered stud is pulled into the sleeve by tightening the Superbolt tensioner on one side. The sleeve is split along the entire length so it can expand radially and adjust to the bore (yellow arrows, to the left), while a second Superbolt Tensioner on the other side is then tightened to clamp the joint together axially (red arrows, to the left).

Superbolt Expansion Bolts not only produce an optimal fit despite manufacturing tolerances, they can additionally preload the bore in a radial direction. The radial preload helps maintain the contact with the bore and prevents detrimental tilting. Expansion bolts can carry higher loads than fitted bolts. As a result, they can save both time and money. The savings are highest for new designs where the joint can be made smaller while requiring less bores with wider tolerances. The company's engineers will gladly support companies in optimising the joint. However retrofits also benefit from no on site machining, fast installation and full reusability.



Applications for Superbolt Expansion Bolts include shaft couplings, as seen on the Francis type pump/generator shaft coupling at a pumped-storage facility (pictured).


Couplings often require interference fit bolting elements for proper torque transfer. This makes manufacturing of the flange bores and mating bolts expensive. For proper functioning of these bolts, high demands are posed upon tolerances and surface finishes.


The coupling flanges often require further mechanical adjustments during assembly. Moreover, installation and removal of the bolts may require large expensive tooling. Bolts can become stuck during removal, causing unplanned man hours to remove and replace them. Also it is quite common that the flange holes end up being damaged in this process, requiring further rework prior to accepting new bolts.


Superbolt Expansion Bolts can be used to replace close fitted or interference fit bolts. Their design uses Mult-Jackbolt technology, along with a tapered stud and split sleeve, to eliminate problems associated with some other bolted methods. In this example, the bolting of the coupling was installed in several hours with simple hand tools. The customer was extremely pleased and reported that the unit had 'less vibrations' in operation compared to previous operations with more conventional bolting hardware.