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Importance of Good Packing Rings (cushions) For Successful Projects The packing (cushion ) ring provided with all Denlok™ jacking pipe is of precision cut from finely ground particle board material. A packing ring (cushion) is typically used between all rigid jacking pipes used worldwide to minimize the point loading of the pipe ends when the joints are not in a perfectly parallel position in relation to each other. The packing ring (cushion) is also used to help compensate for some pipe materials that are manufactured to lesser standards which do not have well made, smooth or ends in a parallel plane. Click here to see EN 295-7 Diagram. Numerous studies in Europe have determine the suitability of the Denlok™ packing ring to achieve maximum performance from the pipe during the steering or joint defections experienced during all pipe jacking operations. This is true for microtunneling and pipe replacement ( pipe bursting ) installation methods. The studies of the packing ring materials include:
Additional studies have been made at the laboratories of Can Clay Corporation in Cannelton, Indiana, USA These studies confirmed that the use of high Poisson's ratio materials such as elastomeric rubbers result in unfavorable and non-uniform loadings on the pipe joint ends. See photos of unfavorable tests of rubber and chipboard. Rubber like materials and plastics used for packing rings (cushion rings) have been shown to induce tensile loadings on jacking pipe ends. Since most all rigid materials have less tensile capacity as compared to compression load capacity, it is important to load the jacking pipe ends in a manner that reduces or eliminates any tensile creating force vector. This can be illustrated by longitudinally compressing a cube of rubber material and observing that outward expansion in the lateral direction. This lateral expansion must be overcome by the jacking pipe ends, resulting in unfavorable tension. Microtunneling, D. Stein, K. Mollers, R. Bielecki, Ernst & Sohn Verlag,
Berlin, 1988, p. 270, discusses a
laminated steel and rubber cushion ring which has had success in limited use at
the jacking station only. The the high tensile capacity steel layer acts to
limit the transfer of tensile force vector loadings to the face of the pipe end.
"Directional corrections must be made gradually....". |
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