Electric heating – Heating devices – With power supply and voltage or current regulation or...
Patent
1979-12-28
1982-06-08
Reynolds, Bruce A.
Electric heating
Heating devices
With power supply and voltage or current regulation or...
219493, 219522, 219544, 219535, 156 86, 1562739, 1563797, 26427211, 285286, H05B 102
Patent
active
043341462
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for joining thermoplastic line elements with the aid of a sleeve having a winding of an electrical resistance wire, whereby after connecting to an electric power supply heat is produced in the winding and after supplying a given welding energy the sleeve is welded to the line element, and an apparatus for performing this method.
When joining thermoplastic line elements, i.e. pipe sections, shaped parts and switching means, e.g. valves it is known to supply the welding heat necessary for joining such elements by electrical resistance heating. In this connection it is known to arrange a conductor subject to current action in the vicinity of the connecting surfaces in the connections, e.g. in the sleeve ends or in separate sleeves and by it producing heat the junction is brought into a weldable state. When joining the ends of line elements by sleeves separate therefrom the current-charged conductor is generally a winding of a resistance heating wire, whose turns are spacedly arranged in the vicinity of the inner wall of the sleeve member. When current passes through the winding the inner wall portion of the sleeve and the adjacent parts of the line elements are brought into a fusable state and as a result are welded together, without there being any deformation of the parts, i.e. the ends of the line elements and the sleeve member at the junction. Since the initial use of the sleeves, called electric welding sleeves, their range of use has been constantly widened and nowadays covers all standard diameter ranges. However, it has been found that in the case of larger pipe diameters, e.g. nominal diameters of 160 to 400 millimeters problems occur which have no or little effect with smaller nominal diameters.
Electric welding sleeves for the above-indicated larger nominal diameters are made from a tubular sleeve member, which is initially widened accompanied by the heating thereof in order to give the sleeve member a shrinkage reserve which, triggered off by the welding process at the junction point, brings about the bridging of the clearances between the diameter differences and the pressing of the sleeve member onto the ends of the line elements. The generally monofilar winding is introduced when the sleeve member is in the widened state and is joined to the latter. However, even during storage, i.e. in the cold state there is a certain re-formation of the shrinkage reserve, which is taken into account by a correspondingly greater widening of the sleeve member. The resistance heating wire introduced into the sleeve member consequently undergoes compression stressing during the storage of electric welding sleeves due to the shrinkage of the sleeve member. Since, however, the winding is embedded in the thermoplastic material the individual turns are shortened in accordance with the effective compression stress. If in the making of a welded joint the winding of such a sleeve is charged with electric power the supporting action exerted by the sleeve member material is removed and metallic contact between two adjacent turns may occur.
The elongation of the resistance heating wire caused by the temperature rise during welding and which leads to a considerable elongation thereof has the same effect. The shrinkage reserve of the sleeve member triggered off by the welding heat and the simultaneous elongation of the resistance heating wire lead to the lateral moving apart of the individual turns, because between the turns the thermoplastic material is heated most and is therefore most flowable. As has already been stated the above two factors shrinkage reserve and wire elongation have a greater effect with large nominal diameters and are more likely to cause faults there than in the case of small nominal diameters. Even if the shrinkage occurring during storage could be eliminated by additional expenditure, e.g. by fitting the sleeve to a metallic cylinder which prevents shrinkage, the elongation of the resistance heating wire caused by the heating persists and consequently so does the p
REFERENCES:
patent: 3541429 (1970-11-01), Martin
patent: 3681569 (1972-08-01), Schwarz
patent: 3943334 (1976-03-01), Sturm
patent: 3944893 (1976-03-01), Hayden
patent: 4117311 (1978-09-01), Sturm
patent: 4176274 (1979-11-01), Lippera
Paschall Mark H.
Reynolds Bruce A.
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