Skip to main content

A cam driven coupling for pressurised pipilines

Exploitable results

The coupling system for pressurized polyethylene pipelines of 125mm diameter has been developed. The system comprises of a grip ring, cap, seal and body. Coupling of the pipe joint is provided by a 50o turn of the cap which results in a cam driven connection of the grip ring teeth with the pipeline. The coupling system is an innovative and economic alternative to the inefficient current practice in pipe jointing such as fusion welding. The design, materials selection, computer modelling and stress analysis have been completed. Prototypes of all the coupling components have been manufactured and tested. Market analysis and user appraisal has been completed and commercialization of the system is being actively pursued. Work on field trials, standardized testing and final refinement of prototype components needs to be completed to reach the final commercialization stage. Negotiations with large end user organizations are underway with a view to licensing the technology for manufacture. The results of the research are comprehensively protected by international patents.
Prototyping was undertaken as an intermediate step between the detailed design and the final product. Rapid prototyping was undertaken of the most complex components of the coupling system (the cap and the grip ring). The stereo lithography technique was adopted with epoxy resin. Prototype caps, and body were manufactured in aluminium to enable full-scale testing of the full coupling assembly, by using CNC machining. Polyethylene spacer washers were produced on a lathe. The next stage was to develop rapid tooling for the grip ring and cap components by injection moulding prototype parts in steel moulds. A relatively soft steel was used for the tooling so that machining was not difficult. The existence of tooling enabled the manufacture of near-production prototypes of components (cap, grip ring) by injection moulding in large numbers for testing. Due to cost constraints, the full size prototype of the coupling body was machined from a billet of Luran 378 P instead of developing rapid tooling of the component for injection moulding. Rapid tooling was also manufactured for the seal component by the SME Bode Seal prototypes were produced in sufficient quantity for testing.
The design process of the grip ring was complex and required empirical data on the relationships of tooth indentation, normal (indentation) force, shear resistance and creep effects. This was achieved by laboratory tests on small elements which were then scaled up to the full grip ring. An indentation and shear test assembly was developed for testing the grip ring teeth elements to derive conclusions on optimum materials selection, teeth geometry and profiling and parameters for use in the design process. Full scale testing of the grip ring, cap and body was conducted after the production of the injection moulded components from prototype tooling. Testing equipment was designed and manufactured. The aluminium body and cap components were used when testing the plastic prototypes of the grip ring. The plastic cap prototype was tested using the aluminium body and a 'proven' plastic grip ring. The testing was in accordance with WIS 4-24-01: Appendix C. Testing on the prototype components verified that the grip ring provides a shear resistance significantly in excess of the design load; the cap and especially the lugs on it require some refinement to edge profiling for achieving the design capacity.
Computer models of the coupling components were developed using multimedia package. The aim was to provide an aid for understanding the mechanism of coupling component interaction and as a marketing tool. Special emphasis was placed on the cam mechanism and grip ring teeth. A detailed animation of the key mechanisms together with STL files of each component design were produced for the production of prototypes. The Finite Element Analysis was carried out using MSC Nastran for Windows package. Initially analysis of each individual component were conducted under the design load criteria set down by the WRC regulations. This was followed by detailed analysis of some complex aspects such as the localized stresses and deformations of grip ring teeth and the potential of buckling in the portion of the MDPE pipeline gripped by the coupling. Both elastic and plastic analysis were investigated. Final refinements to the component designs were made o the basis of the FEA analysis to provide optimal geometry and dimensions of the grip ring teeth (e.g. spacing, shape, depth, root and tip radii) and the cams on the cap. The advanced buckling analysis of the MPDE pipelines simplified the coupling design by alleviating the need for an internal sleeve to prevent pipeline buckling.
A large number of factors ranging from mechanical and hygienic properties to corrosion resistance, surface finish and cost were taken into account to select materials for the coupling system. The mechanical properties considered in the design included density, strength, elastic modulus, creep, ductility, hardness and toughness. Since in service the coupling system will be in contact with water, corrosion resistance and health criteria are important. As a starting point, the CMS database was employed to identify potential materials for the coupling system. Laboratory simulations of material performance in coupling component mechanisms together with assessment of creep effects etc. were fed into the materials selection process. The materials SAN 35% GFR (water dried) and PET 30% GFR (air dried) were finally identified for the coupling components. Based on cost considerations and their suitability for injection moulding, it was concluded that PET 30% GFR should be used for the grip ring component and SAN 35% GFR would be suitable for the Body and Cap Components. Material for the seal component was selected to conform to hygiene requirements of the current European practice by the seal manufacturer SME (Bode).