Electric cables with metallic protective sheaths

An electric cable protected by a corrugated hollow metallic sheath composed of an aluminum alloy type 3004.

BACKGROUND OF THE INVENTION
 Electric cables often are installed in hazardous or corrosive environments.
 Such cables employ a core of circular diameter which is covered by an
 external metallic sheath used as a protective member. These sheaths
 provide an impervious metallic envelope capable of preventing entry of
 humidity and resisting corrosion caused for example by moisture, acids,
 gases and the like. Conventionally, such sheaths are formed from aluminum
 or aluminum-manganese alloys. These sheaths form a protective envelope.
 The thickness of the sheath depends in part upon the size of the core
 diameter and increases with increasing size.
 The installation of the sheath about the core utilizes strips of aluminum
 metals or alloys which are formed around outer surface of the core of the
 cable and then are longitudinally welded and corrugated.
 However, cables utilizing metallic sheaths, as for example heavy power
 cables, are normally manufactured in long lengths and are wound on reels.
 For cores of larger diameter employing thicker sheaths, the weight of the
 upper layers of sheathed cables causes the upper layers to crush downward
 against the lower cable layers, often causing some deformation of the
 sheath and core in the lower cable layers. This deformation creates
 problems in subsequent manufacturing steps such as jacket extrusion, cable
 termination and also in final installation in cable ducts. In addition,
 the cost of the sheath material increases substantially as the sheath
 thickness is increased.
 The present invention is directed toward a metallic sheath which retains
 sufficient crush resistance to overcome the deformation problem while at
 the same time exhibiting a reduction in the sheath thickness which
 provides substantial reduction in the amount and cost of sheath material.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a known metallic alloy
 never previously used as a protective sheath for the cores of electric
 cables which when used as a sheath has sufficient strength to prevent
 deformation during processing and sufficient reduction in sheath thickness
 to produce a substantial reduction in cost of the sheath.
 These and other objects and advantages of the invention will either be
 explained or will become apparent hereinafter.
 The mechanical strength of a cable sheath is determined by a so-called
 "crush test" in which an empty metallic sheath in the form of a hollow
 tube is mounted between two platens and subjected to compression until the
 sheath collapses. It has been determined experimentally that in order to
 avoid undesirable deformation of the sheath when a sheathed cable is wound
 on a reel, the maximum acceptable deformation of the sheath should be
 limited to about 5% of the outer diameter under a compression force of
 1000 pounds applied to a sheath specimen of 150 millimeters.
 The deformation characteristics of aluminum alloys commonly used as cable
 sheaths and identified in the art as type 3003 are marginally acceptable.
 However when these characteristics were compared with the deformation
 characteristics of an aluminum alloy identified in the art as type 3004
 and never before used as a cable sheath, it was found surprisingly that
 alloy 3004 displayed much improved deformation characteristics using the
 crush test. Moreover, it was found that for any given diameter of cable
 core, the increased deformation characteristics of alloy 3004 enabled an
 acceptable sheath of alloy 3004 to be much thinner than a corresponding
 sheath of alloy 3003.
 In particular the D/S which is the ratio of the outer diameter D of a
 sheath to its thickness S provides a means of comparison of alloy 3004 and
 alloy 3003.
 For example, when the D/S ratio of alloy 3003 is compared to the D/S ratio
 of alloy 3004, over a range of diameters D between 10 and 100 millimeters,
 and the sheath thickness of these two alloys are computed using these
 ratios for the same diameter value, the sheath thickness of the newly used
 alloy will always be at least 20% smaller than that of the commonly used
 alloy.
 Consequently, the sheath thickness can be reduced substantially as for
 example by at least 20% while maintaining the desired crush resistance
 when a type 3004 metallic alloy is substituted for a type 3003 metallic
 alloy. Both these alloys have the same unit weight and same unit cost, so
 that this reduction in sheath thickness results in substantial cost
 reduction. Alloy types 5052 and 5454 have even higher D/S ratios and
 permit further reduction in sheath thickness, but are more expensive.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Referring first to FIG. 4 illustrates a conventional cable 10 and
 corrugated metallic sheath 12. Referring now to the graph of FIG. 1, the
 D/S ratios, where D is the outer diameter of the corrugated metallic
 sheath and S is the thickness of the sheath, were measure and plotted
 against D for the alloy 3003 and for the alloy 3004. This graph
 demonstrates the advantages of alloy 3004 as compared to alloy 3003 in
 reduction of sheath thickness over a diameter range of 10 mm to 100 mm and
 a corresponding ratio range from 20 to 100.
 For every value of the diameter D, the D/S ratio of the alloy 3004 is
 substantially higher than that of alloy 3003, whereby as explained
 previously, the sheath thickness of alloy 3004 is substantially reduced as
 compared to the sheath thickness of alloy 3003.
 Referring now to FIG. 2, surprisingly, the percentage of retention of the
 radial diameter of a specimen of alloy 3004 having a length of 150 mm when
 subjected increasing deformation force from 1000 to 1600 pounds decreased
 only from 95% to 65% while the percentage of retention of a like specimen
 of alloy decreased from an already unacceptable 89% to about 52% over a
 smaller range of 1000 to 1300 pounds.
 FIG. 3 is a chart comparing the properties of alloy 3003, alloy 3004, and
 two additional alloys 5052 and 5454. These additional newly usable alloys
 are characterized by even higher crush resistance and even greater
 reduction of sheath thickness than alloy 3004, but are more expensive in
 application.
 While the original tests of the newly used alloy did not enable the
 selection of a minimum value of sheath thickness for a given value of D,
 further tests resulted in the empirical discovery that the D/S ratio of
 alloy 3004 for a minimum value of S is approximately defined by the
 equation
EQU D/S=1.1[D]+10
 where D and S are defined in millimeters or by the equivalent equation
EQU D/S=2.6[D]+10
 where D and S are defined in inches.
 Since the sheath fits snuggly over the core, the inner diameter of the
 sheath is approximately the same as the core diameter. The inner sheath
 diameter can be calculated once the thickness and outer diameter of the
 sheath can be determined using these equations. This enables the cable
 manufacturer to select the appropriate sheath thickness to be used for a
 given core diameter.
 While the invention has been described with particular references to the
 drawings and detailed embodiments, the protection solicited is to be
 limited only by the terms of the claims which follow.