Welding sleeve and method for its manufacture
FIELD OF THE INVENTION 
     The present invention is directed to a welding sleeve or socket and a method for manufacture of the welding sleeve. 
     BACKGROUND OF THE INVENTION 
     Commercially available pipes have, in part, wide tolerances and considerable ovalness. This requires the internal diameter of the sleeve to be designed to be very large. Also, in order to be easily installable, additional large thermal extensions or strains occur in electrically welded sleeves having a large diameter, resulting in pressure degradation in the welding zone. 
     A welding sleeve is disclosed in EP-A1 022 2287 in which reinforcement from a material having a lower coefficient of thermal expansion than the sleeve member of a polyolefin helps save welding sleeve material in the sleeve body. The welding pressure required for a satisfactory weld is obtained by a smaller elongation or extension of the reinforcement during the welding process. 
     The coefficients of thermal expansion, however, vary between the sleeve member and the reinforcement according to differing environmental temperatures during the course of welding, resulting in differing prestresses. For example, one obtains a different welding pressure at +45° C. than at -10° C. Constant operability therefore cannot be assured. The reinforcement configured as a pipe-shaped member or as a wire winding or coil is additionally not resistant against corrosion and does not therefore provide for the lasting reinforcement of the welding sleeve acting also against the medium pressure. 
     To overcome these and other disadvantages with previous welding sleeves, it is an object of the invention to provide an economical welding sleeve which permits a constant operability at varying environmental temperatures during welding. 
     Another object of the invention is to provide a welding sleeve usable for larger pipe diameters exceeding, for example, 160 mm and which is not prone to corrosion. 
     Yet another object of the invention is to provide a method of manufacturing a welding sleeve in a simple, automatic method. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention, which shall be apparent hereafter, are achieved by the disclosed welding sleeve and method for its manufacture, comprising a welding sleeve having a sleeve member made from a thermoplastic material. The internal surface of the sleeve member includes a resistor wire winding and the external circumference includes a reinforcement ring. The reinforcement ring is pressed onto the external circumference of the sleeve and has circumferential stresses because of the widening. These stresses act radially upon the sleeve member and increase the welding pressure required for a good welded connection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be better understood by the detailed description of its preferred embodiments with reference to the drawing of which the sole figure depicts a partial, longitudinal cross-sectional view through a welding sleeve. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The figure depicts a welding sleeve for rigidly pipe-shaped parts or pipes with a sleeve member 1 made preferably of a thermoplastic, weldable material. A resistor wire winding 3 is arranged at the inner surface 2 of the sleeve and comprises, for example, junction wires 4 or plug-in connections for connection to a current source. 
     A reinforcement ring 6, arranged at the external circumference 5 of the sleeve member 1, is connected to the sleeve member 1 in a frictionally adhesive manner by being shrunk-on or pressed on. The reinforcement ring 6 is also made of thermoplastic material, preferably from a polyolefin such as polyethylene. If the reinforcement ring 6 is heated prior to being pressed around the sleeve 1 and thereafter rapidly cooled, additional shrinkage stresses are frozen, which remain in existence up to the welding process with the pipe parts and are then available in their entirety for producing the welding pressure. The reinforcement ring 6 is pressed onto the sleeve member 2 in mounting the reinforcement ring 6 onto the sleeve member 1. The sleeve member 1 is already provided with a resistor wire winding 3. 
     It may be necessary or advantageous to heat the reinforcement ring 6 to a temperature above that of the sleeve member, depending upon the material used and the diameter. By using the thermoplastic material at the outer circumference of the sleeve member 1 by frictional connection, there arises an inwardly oriented pressure because of the existing stresses during welding. The inwardly oriented pressure acts upon the melted material, creating the welding pressure required for a good welded connection. Since the coefficient of thermal expansion of the reinforcement ring 6 corresponds largely to that of the sleeve member 1, equal welding pressure conditions are assured at all environmental temperatures. During the welding process the frozen stresses are released by the heating and act as radial pressure forces upon the sleeve member and the melted material. 
     The external diameter D of the sleeve member 1 is slightly greater than the internal diameter of the possibly heated reinforcement ring 6, whereupon the ring 6 is widened by the widening dimension. The widening process introduces stresses extending in a circumferential direction of the reinforcement ring 6, which, as a radial pressure force, increases the welding pressure during the welding process. By selected heating temperature of the reinforcement ring 6, it is possible to utilize the dimensional change of the reinforcement ring 6 when cooling for the stress buildup, wherein a combined pressing-shrinkage fit is produced. 
     Because of the melted material in the welding zone, the stiffness of the sleeve member 1 is additionally reduced whereby the radial pressure forces are also increased. This increases the welding pressure and additionally assures a solid, welded connection with larger pipe tolerances. 
     By selecting the widening dimension, the chosen wall thicknesses of the reinforcement ring and possibly its heating temperature, the desired stresses or the required welding pressure can also be achieved whereby an optimum layout of the welding sleeve is achievable. 
     The wall thickness S2 of the reinforcement ring 6 should advantageously be smaller or slightly larger than the wall thickness S1 of the sleeve member 1. Small wall thicknesses are advantageous for the saving of material. 
     While the preferred embodiment of the present invention has been presented in detail, modifications and adaptions thereof may be undertaken without departing from the spirit and scope of the invention as defined in the following claims: