Thickness Measurement and Control in the Manufacture of Polyethylene Cable Sheath

The manufacture of multiple sheath for Alpeth and Stalpeth cables requires the application of a sheath of polyethylene over a sheath of corrugated metal which is flooded with a rubber asphaltic compound. For high quality and minimum cost, this outer sheath must be of uniform thickness throughout its length. One of the problems in cable sheath manufacture is to maintain the concentricity and average thickness of the extruded polyethylene sheath to close limits during manufacture. This article reports on: (1) The application of a capacitance sensitive bridge to the measurement of the eccentricity and average thickness of the sheath on cables moving at speeds of 20 to 100 feet per minute; (2) The method of thickness calibration; and (S) The use of the thickness measurements in maintaining the sheath concentricity and average thickness within close limits during the sheathing operation. HISTORY In the manufacture of multiple sheath for Alpeth and Stalpeth cables, an outer sheath of polyethylene is applied. It is desirable for high quality and low cost to make this outer sheath of a uniform thickness throughout. The construction of these cables is shown in Fig. 1. In both designs, the outer sheath is polyethylene extruded onto a corrugated metal undersheath which has been flooded with a rubber asphaltic compound. The extrusion art had been unable to obtain a high degree of control, primarily because measurements of the thickness could not be obtained until after the sheath was applied to the cable core.