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Bosworth Plastics launches Dowelock all-plastic locking dowel system for the tunnelling industry

Distrupol : 05 August, 2004  (Application Story)
Bosworth Plastics of Hinckley, Leicestershire, UK, has successfully launched a patented, all-plastic system for joining concrete segments for the construction of tunnels. The extensive development project has been completed in conjunction with DuPont and Distrupo
The Dowelock system, which requires no bolts or screws and suffers none of the corrosion problems associated with traditional metal dowels, is the result of nearly four years of technical development effort in conjunction with polymer supplier DuPont and its UK distributor Distrupol. The Dowelock is already being used in two UK tunnelling contracts, one of which is the new Terminal 5 at Heathrow Airport.

Plastics have gained the confidence of design engineers and end-users through their successful use in applications where metals have traditionally been used. In these applications it is absolutely vital that the specification of plastic components meet all relevant technical and performance criteria. Furthermore, plastics can bring additional benefits to the application, such as reduced weight and processing advantages, which can enhance a product’s commercially viability.

These factors have been prominent in the development of the self-aligning Dowelock system, which meets the requirements laid down in the British Tunnelling Society’s Specification for Tunnelling. The physical characteristics of two of DuPont’s engineering polymers, glass reinforced Zytel nylon resin and Delrin acetal homopolymer, are central to Dowelock’s performance.

Dowelock’s patented design sets new standards in the fixing of tunnel segments. The inherent tensile strengths of the two plastics used in the Dowelock system enable the tunnel segments to be self-aligning and held in place for accurate positioning. Use of these advanced materials limits the creep associated with existing plastic dowels and allows gasket loads of up to 2-3 tonnes per metre to be withstood, with minimal extension.

Dowelock is made from a 25% glass reinforced grade of black Zytel and is pushed by hand into the Delrin locking assemblies that are cast into the tunnel lining segments. The assemblies incorporate petal-shaped collets that flex to allow the dowel to enter with minimal force, but which lock onto moulded notches on the dowel - without deforming them - so that it cannot be withdrawn easily.

Dowelock can be used with all types of lining including trapezoidal rings, as the collets are retained in such a way as to allow the dowel to be entered off-line. Full locking capacity is maintained with up to a 15mm gap between adjacent tunnel units. Due to the efficiency of its design and the careful choice of materials, the Dowelock system can be produced in a range of sizes suitable for all segment geometries, without any reduction in performance.

The initial moulding trials for producing Dowelock at the Bosworth Plastics plant were carried out by Distrupol’s polymer engineer Derek Bean and Bosworth’s technical director Justin Robertshaw. Their knowledge has been particularly valuable in the elimination of internal voids in the nylon dowel, a part with a thick cross-section weighing some 300g.

“The combination of DuPont’s materials with Distrupol’s technical support has enabled us to maximise the effectiveness of both polymers,” says Peter Robertshaw, managing director of Bosworth Plastics and inventor of Dowelock. “The union of Zytel and Delrin has allowed us to offer a strong, durable and cost-effective alternative to metal fixings.”

“I had what was essentially a brainstorming session with Peter Robertshaw when the Dowelock project was in its infancy almost four years ago,” explains Distrupol sales manager, Richard Orme. “Peter developed the design concept from there.”

The 25% glass reinforced Zytel specified by Distrupol for the dowel provides the correct balance of elongation, toughness and tensile properties to achieve the right strength when the dowel is locked in position. Delrin acetal was the logical choice for the locking assemblies; the polymer’s high elongation characteristics make it ideal for this type of snap-fit connection.

Dowelock provides quick and clean installation without internal pockets, in contrast to bolted systems which require pockets to be incorporated in the concrete segments to allow the bolts to be tightened, and which also need to be filled with mortar. Approximately half the size of spear bolts, the logistics of delivering Dowelock to the tunnel location are also considerably reduced as only 12 fixing systems are required per ring for a 4-5metre diameter tunnel.
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