US3051774A

US3051774A - Strain relief

Info

Publication number: US3051774A
Application number: US697584A
Authority: US
Inventors: Robert K Schelke
Original assignee: Belden Manufacturing Co
Current assignee: Belden Manufacturing Co
Priority date: 1957-11-20
Filing date: 1957-11-20
Publication date: 1962-08-28
Anticipated expiration: 1979-08-28

Description

MLV

R. K. SCHELKE STRAN RELIEF Filed NOV. 20, 1957 Aug. 28, 1962 United States Patent Office 3,051,774 Patented Aug. 28, 1952 3,051,774 STRAIN RELIEF Robert K. Schelke, Richmond, Ind., assignor to Belden Manufacturing Company, Chicago, lll., a corporation of Illinois Filed Nov. 20, 1957, Ser. No. 697,584 3 Claims. (Cl. 174-135) The present invention relates to strain relief means and more particularly relates to strain reliefs for use with electrical conductor cords and the like.

Strain reliefs of various types have been utilized to minimize strains in mechanical connections between electrical conductor cords and electrical connectors. Strain reliefs have also been utilized to minimize wear in electrical conductor cords where such cords are directly interconnected with electrical apparatus, such as appliances or the like, rather than through a connector, and also at various strain points along the length of conductor cords.

It is of primary importance that a strain relief be durable in use and that it be effective for its intended purposes While avoiding imparting to the conductor cord or similar component with which it is associated undesired properties such as undue rigidity or relative intlexibility. Furthermore, the stran relief should be relatively simple and inexpensive to fabricate from readily available materials. Fabrication of the strain relief directly around an electrical conductor cord or the like is, of course, advantageous in view of the simplicity of fabrication and absence of subsequent assembly steps. Also, the strain relief should be of pleasing and harmonious appearance so as to enhance the acceptability of the product with which it is to be used.

Certain types of strain reliefs have been found to be relatively difficult to fabricate directly around conductor cords and the like. When such fabrication, as by molding or the like, is attempted, the conductor cord has a tendency to be displaced transversely and/or axially during the process. Consequently, in the finished product the conductor cord frequently is not properly centered within the strain relief throughout its entire length so that the side wall of the strain relief is of uneven thickness. In some extreme cases, a considerable area of the conductor cord is exposed through the side wall of the strain relief. The uneven thickness of the side wall of the strain relief not only results in a product of inferior appearance but also impairs the performance of the strain relief function. When this occurs, the strain relief does not uniformly reduce strain on all portions of the conductor cord disposed therein and in some instances sets up additional strains due to a tendency of the conductor cord to bend more readily in the direction of the thicker portion of the side wall of the strain relief.

In order to minimize the chances of the conductor cord protruding from the side wall of the strain relief, certain strain reliefs which are fabricated directly around conductor cords are purposely made relatively thick walled. Such thick walled strain reliefs, however, usually have reduced exibility and, moreover, are more expensive t fabricate than thin walled strain reliefs.

`In some instances, strain reliefs are fabricated separately and thereafter assembled around the conductor cord. An example of this is the conventional cone-shaped wire type of strain relief which has been used with conductor cords for some appliances. This procedure, of course, is somewhat expensive. Moreover, a tight t between the strain relief and the conductor cord, which increases the efficiency of the strain relief, may be diicult to obtain by such means.

Accordingly, it is a primary object of the present invention to provide an improved strain relief. It is a further object of the present invention to provide an inexpensive strain relief construction having controlled wall thickness and pleasing appearance. It is another object of the present invention to provide a strain relief which is disposed tightly around an electrical conductor cord accurately aligned in a predetermined position within the strain relief. Other objects and advantages of the present invention will be apparent from a study of the following detailed description and of the accompaying drawings in which:

FIGURE l is a side elevation showing one embodiment of a strain relief which includes various of the features of the present invention in operative position around an electrical conductor cord;

FIGURE 2 is a view of the strain relief of FIGURE l taken from the right hand side of FIGURE l;

FIGURE 3 is an end View of the strain relief of FIG- URE l;

FIGURE 4 is a plan view of a mold partially broken away to show the relative positioning of the strain relief of FIGURE 1 therein;

FIGURE 5 is a side elevation of the mold shown in FIGURE 4;

FIGURE 6 is an enlarged fragmentary cross section taken along the section line 6-6 of FIGURE 4 showing an electrical conductor cord positioned within the mold before the strain relief is formed therearound;

FIGURE 7 is a view similar to FIGURE 6 but shows the strain relief formed about the conductor cord;

FIGURE 8 is a side elevation of another embodiment of a strain relief including various of the features of the invention, portions being broken away to show the internal construction; and

FIGURE 9 is an enlarged cross section taken along the line `9--9 of FIGURE 8.

The present invention generally comprises an improved strain relief construction fabricated in accordance with the method disclosed and claimed in the divisional application of the common assignee, Serial No. 78,076, which was led on December 23, 1960. Generally, the method disclosed and claimed in the aforementioned divisional application includes disposing an electrical conductor cord within a cavity, as in a mold, and holding the cord in predetermined alignment within the cavity by contacting the periphery of the cord with a plurality of suitable holding means, such as pins, which are disposed at a plurality of points in aligned relation within the cavity and which are secured to the cavity wall. A strain relief is then formed within the cavity around the conductor cord while the conductor cord is maintained in its desired position by the holding means. Thereafter, the formed strain relief and conductor cord are withdrawn from the cavity as a unit. The strain relief so produced has a controlled wall thickness and shape and is provided with a plurality of decorative apertures which extend into the side wall thereof. These apertures improve both the appearance and the performance of the strain relief.

Referring more particularly to the accompanying drawings, FIGURE l shows one embodiment of a strain relief 12 having various of the features of this invention. In FIGURE l, strain relief 12 is shown disposed in tightly abutting relation around the periphery of a conventional electrical conductor cord 14.

Conductor cord 14, illustrated, is generally circular in cross section and comprises, as shown more particularly in FIGURES 6 and 7, a pair of electrically conductive wires 16, each of which is enclosed in an insulating sheath 18. The sheathed wires `16 are encased in an insulating jacket 20 and the space between the sheathed wires 16 and the jacket 20V may be filled with an insulating and cushioning medium 22, such as fiber or the like, as illustrated. The specic construction of conductor cord 14 does not form a part of the present invention and, ac-

.0 cordingly, will not be described in further detail. It will be understood that elongated conductors of other design could be utilized in conjunction with the strain relief of the present invention.

The strain relief 12 (FIG. l) comprises a body portion or sleeve 24 which is formed about the cord and which at one end is provided with a head 26 which is adapted to be connected to an appliance or other equipment to which the cord is to be attached.

One manner of connecting a strain relief into an electrical appliance frame or the like is generally set forth in United States Patent No. 2,494,814, to Huth, issued January 17, 1950, and a method of interconnecting a strain relief with an electrical connector which may be disposed ou the end of a conductor cord is shown in U.S. Patent No. 2,788,502, to Schelke et al., April 9, 1957.

It will be noted that in certain instances it may be desired to modify the design of head 26 to approximate the appearance of sleeve 24, for example when the strain relief is to be applied to some intermediate portion of a conductor cord rather than adjacent the end of the conductor cord, and is not intended to be interconnected with an appliance, connector or the like.

Now referring more particularly to sleeve 24, the sleeve shown in FIGURES 1, 3 and 4, is formed of resilient exible material. Sleeve 24 is generally oval in cross section and is generally arcuate in shape. Moreover, it tapers downwardly from the end 28 adjacent the head 26 to an end wall 30. The end 28 of sleeve 24 which is opposite the end wall 30 and which is integrally connected to the head 26 is of substantially greater diameter and wall thickness than the opposite end 32 adjacent end wall Sti. The cord 14 is shown in dotted outline in FIGURE l and in `this figure the relative wall thickness of sleeve 24 and of head 26 can be readily observed. This construction produces a relatively greater exibility at end 32 than at end 2% of sleeve 24.

The outer surface which delines the sleeve 24 is provided with beaded portions 34 running along the length of the inner and outer curvatures of sleeve 24, as shown in FIGURE 1, primarily for decorative purposes.

The outer surface of sleeve 24 is also provided with a plurality of apertures 36. These apertures extend through the side wall 38 of sleeve 24 radially of the cord 14 (FIG. 7) and communicate with cord 14. In the embodiment of FIGURES l to 7 inclusive, the apertures are uniformly spaced and generally circular in cross section. However, the apertures need not be uniformly spaced and furthermore, may be of any other suitable shape and shapes, such as rectangular, square, wedge shaped or the like.

Apertures 36 correspond to the location of a plurality of holding means, such as pins, more fully described hereinafter, for holding conductor cord 14 in alignment within a cavity, as in a mold, at the time strain relief 12 is formed therearound. Apertures 36 are decorative and, furthermore, expose portions of jacket of conductor cord 14 to view. Accordingly, if jacket 20 is in a contrasting color to that of strain relief 12, the appearance of the combination of the conductor cord and strain relief is further improved. Apertures 36 have the further advantage of imparting improved flexibility to sleeve 24. In this connection, the apertures act as foci for stretching and compressing of sleeve 24 during flexing thereof and tend to decrease cracking of side wall 38 of the sleeve.

Head 26 is integrally connected with end 28 of sleeve 24, as described, and is formed of the same resilient, flexible, durable material as is utilized for sleeve 24.

Head 26 is generally oval in cross section and comprises an enlarged abutment portion 40 and a connecting portion 42 integrally connected thereto and tapering at its outer end. Head 26 is disposed around cord 14 in the manner set forth regarding sleeve 24. Connecting portion 42 is somewhat flattened along two opposite sides,

as shown in FIGURES 1 and 2, and is provided with means for interconnecting head 26 with an electrical connector or an electrical appliance frame or the like, generally as shown in United States Patent No. 2,494,814 to Carl M. Huth. More particularly, connecting portion 42 has space 44 in the form of an interrupted annular groove around about three quarters of the circumference of portion 42.

The attachment of strain relife 12 to the frame (not shown) of an electrical 'appliance or the like can, for example, be carried out by first passing head 26 into the appliance through an aperture in the frame lt-hereof until space 44 is centered over the frame. The frame aperture should be slightly larger and of the same general shape as a cross section of portion 42 of head 26. Moreover, the thickness of the frame should be at least the Width of space 44. Strain relief 12 can then be rotated 90 degrees to strongly engage the opposing Walls of space 44 with the opposite sides of the frame adjacent the frame aperture and achieve the desired clamping action. Strain relief 12 can thereby be locked in an operative position in the appliance. In such position, abutment portion 40 of the strain relief 1'2 shields those portions of the frame aperture not covered by connection portion 42 of head 26.

The specific manner and procedure of assembly of the strain relief of the present invention will necessarily depend upon the specific design of the head of the strain relief and of the appliance or connector, if any, to which the strain relief is to be attached. Such procedure does not form a part of the present invention.

Strain relief 12 may be fabricated from any easily shaped, resilient, flexible, durable material, preferably having electrical insulating properties, such as natural or synthetic rubber, plastic, or the like. The material may be identical or different from that utilized for outer sheath 20 of conductor cord 14. The material utilized to fabricate strain relief 12 should be capable of being readily shaped at a temperature below that which would deleteriously affect conductor cord 14, since the strain relief is directly formed around the conductor cord. Moreover, the material for strain relief 12 may be colored with suitable pigments to enhance the pleasing appearance of the strain relief.

The strain relief of the present invention can be fabricated in accordance with the method disclosed and claimed in the aforementioned divisional application by first disposing an electrical conductor cord within a suitably shaped cavity, as in a mold or the like, and holding the cord in predetermined alignment within the cavity by contacting the periphery of the conductor cord with a plurality of suitably aligned holding means which extend into the cavity, and then forming the strain relief directly around the conductor cord in the cavity. The conductor cord with the formed strain relief is then withdrawn from the cavity and out of contact with the holding means. Such procedure can, for example, be carried out while utilizing apparatus as illustrated in FIGURES 4 to 7, inclusive, of the accompanying drawings.

In this connection, a mold 46 is provided which cornprises a top section 43 and a complementary bottom section 50. Sections 4S and Sil are provided with complementary spaces so that when the sections are properly joined together, as by a plurality of aligning pins 52, shown in FIGURE 4, these spaces define a cavity 54, the intermediate portion 56 of Which has the size and shape of the outer surface of `strain relief 12, and the end portions 58 of which have the size and shape of conductor cord 14. Mold 46 is also provided with a molding compound inlet 60 formed by cooperating grooves in

sections

48 and 50. Inlet 60 narrows at its inner end within the mold, as shown in FIGURE 4, to a small neck portion 62 which communicates with cavity 54. A plate 64 may be provided, if desired, which plate has an aperture 66 therein aligned with and of the same size as the outer end of inlet 60. Plate 64 may be secured to sections 48 and Sil, as by pins 68 running through suitable apertures in plate 64 and sections 4S and 50, so as to aid in maintaining

sections

48 and 50 in rigid alignment during use of mold 46.

Sections

48 and 50 are also provided with conductor cord aligning and holding means which, as shown particularly in FIGURES 6 and 7, may comprise a plurality of pins 70 seated in suitable channels 72 in mold 46. Pins 70 preferably extend at small angles from the vertical, about to 45 degrees, preferably about 30, into the intermediate portion 56 of cavity 54. It has been found that angling pins 7 0 into mold 46 allows the formed strain relief 12 and conductor cord 14 -to be more easily disengaged from the mold after fabrication. The portion 74 of each pin 70 which extends into cavity 54 may be reduced in diameter and the end 76 thereof which contacts the periphery of conductor cord 14 may have a curved surface corresponding to the curve of jacket 20 of `the conductor cord. Pins 70 are aligned within mold 46 so that when conductor cord 14 is disposed within cavity S4 with its periphery in contact with pins 70, cord 14 is held in a desired position and is prevented from being displaced during formation of strain relief 12 therearound.

It should be understood that the holding means may comprise, instead of the pins 70, any other suitable plurality of spaced means which extend into the cavity containing the conductor and which Ihold it in place while the strain relief is fabricated therearound. In this connection, the holding means may be, for example, a plurality of spikes, barbs, wedges or the teeth of a comblike or blade-like implement which extend to and contact jacket 20 at a plurality of points. In any case, the holding means should be arranged so that the side wall of the strain relief will be essentially continuous when formed around such holding means.

In accordance with the method disclosed and claimed in the aforementioned divisional application, the conductor cord 14 is first disposed within the cavity 54 in mold 46. In this regard, cord 14 is placed Within the lower section 56 of mold 46 after pins 70 have been inserted in that portion of the mold. The top section 48 of mold 46 with its pins 70 in place, as shown in FIG- URE 6, is placed on the lower section of the mold and the two sections are secured together, as by means of aligning pins 52, plate 64 and associated pins 68. When sections 4S and 50 of mold 46 are thus secured together, cord 14 is held in predetermined alignment within mold 46, since pins 70 contact the periphery of cord 14 at a plurality of points Within the intermediate portion 56 of cavity 54, and center cord 14 within that portion of the cavity. End portions 58 of cavity 54 are just large enough and of a suitable shape to snugly hold the parts of cord 14 disposed within these portions. A cross sectional View of mold 46 in the assembled position with the conductor cord 14 properly centered within the mold before strain relief 12 is molded therearound as shown in FIGURE 6.

The number, size and shape of the holding means may be varied in accordance with the requirements for the particular strain relief-forming process. In this regard, if the conductor cord is relatively thin and very flexible a greater number of pins 70 may be desired than the 12 utilized in fabricating strain relief 12, in order to prevent the conductor cord from becoming transversely displaced during the subsequent strain relief-forming step. The number and size of the holding means utilized may also somewhat depend upon the degree of exibility it is desired to impart to sleeve 24 of strain relief 12. In this regard, the greater the number or diameter of the holding means, the greater the number or diameter of the apertures in sleeve 24 and, accordingly, the greater the flexibility thereof.

If it is desired to prevent axial displacement of the conductor cord as well as transverse displacement during the strain relief forming step, -pins 70 may be disposed within portion 56 of cavity 54 so as to define a cylinder slightly smaller in diameter than that of cord 14, so that when the cord is placed within cavity 54 and the sections of mold 46 are secured together, pins 70 will press into and slightly indent jacket 20 of cord 14 without impairing the electrical properties thereof. In any event, cord `14 should not be pierced by pins 70 to an extent which would ,substantially interfere with the durability or electrical properties of the cord.

In accordance with the method disclosed and claimed in the aforementioned divisional application, after conductor cord 14 is centered within cavity 54 strain reliefy 12 is formed around conductor cord 14 while the cord is maintained in the desired position in portion 56 of cavity 54 by pins 70. The step of forming the strain relief can be carried out under any Well-known conventional molding or casting conditions utilizing the previously described readily shaped resilient material. The particular conditions utilized will depend upon the particular material selected for use in fabricating the strain relief. For example, strain relief 12 can be pressure molded around conductor cord 14, utilizing polyethylene plastic molding compound, the compound being introduced into intermediate portion 56 of cavity 54 under high pressure in fluid or plastic form through inlet 60 until the intermediate portion is filled, as shown in FIGURES 4 and 7, thereby forming strain relief 12. After cavity 54 is filled, the molding material is set to a resilient, flexible solid. Thereafter,

sections

48 and 50 of mold 46 are separated, after removing plate 64 and pins 68, as well as pins 52. The unit comprising conductor cord 14 with strain relief 12 disposed therearound is stripped from the mold and is then ready for use, after assembly with an electrical apparatus, connector or the like, in the previously described manner.

If the strain relief of the present invention is formed of the same material as that constituting jacket 20 of conductor cord 14 or of another material compatible therewith and of substantially the same melting or softening point, the inner surfaces of the strain relief may be bonded to the outer surface of jacket 20 during processing so that the strain relief is integrally connected to the conductor cord. In some instances, this may be a preferred construction.

If desired, the strain relief of the present invention can be molded around the conductor cord utilizing a different molding technique than high pressure molding or can be cast. In any event, the die or mold should contain a cavity with a plurality of holding means extending thereinto so that the conductor cord can be held in predetermined fixed position during fabrication of the strain relief therearound.

Various commercial embodiments of the strain relief of the present invention have been successfully fabricated around various types of conductors. A preferred strain relief is one of the general shape and proportions Set forth in FIGURES l to 7, inclusive. This strain relief is constructed of rubber and is disposed around a centrally located, 1A inch diameter and longitudinally extending two wire conductor cord. The strain relief has an overall length of about 2% inches, with a short straight head about 5/8 inch long and generally oval in cross section, and a curved and tapered sleeve, also generally oval in cross section.

The sleeve in cross section is about 5%; inch wide along its major axis and about 7/6 inch wide along its minor axis at the end integrally connected to the head, while uniformly tapering to a width of 7/16 inch along its major axis and 3A inch along its minor axis at its opposite end. Maximum wall thickness of the head at the end of the sleeve adjacent the head is about -9/16 inch while that at the opposite small tapered end of the sleeve is about %2 inch. Accordingly, the flexibility of the strain relief sleeve increases in the direction of the smaller tapered end.

The sleeve is curved substantially uniformly along its major axis so that its small tapered end is at an angle of about 30 degrees from the longitudinal axis of the strain relief. Twelve apertures of about 2/32 inch diameter are symmetrically disposed in the sleeve wall and communicate with the surface of the conductor cord. The conductor cord has a black outer sheath while the strain relief is white so that a decorative contrast is afforded, which is enhanced by exposure of the conductor cord to View through the described apertures.

Another embodiment of the strain relief of the present invention is set forth in FIGURES 8 and 9 of the accompanying drawings. As shown, strain relief 78 is straight and essentially circular in cross section, rather than curved throughout a portion of its length and oval in cross section, as strain relief 12. Strain relief 78 is fabricated from resilient flexible material, as previously described for strain relief 12. Strain relief 78 comprises a head 80 and a body portion or sleeve 82 integrally connected thereto at one end thereof. The strain relief is tightly disposed in abutting relation around a conductor cord 14a centered within and longitudinally extending through the strain relief.

Sleeve `82 is tapered downwardly throughout substantially its entire length, that is, from the point of connection with head 80 .to its opposite end which is of smaller diameter and which terminates in a raised annular rim 84. The side wall of sleeve 82 is provided with a plurality of uniformly spaced apertures 86, generally rectangular in cross section, which communicate with conductor cord 14a disposed within the sleeve.

Head 80 includes an expanded abutment portion 88 adjacent sleeve 82 and a connection portion 90, the latter terminating in an expanded portion 92 for connection of strain relief 78, gener-ally in the manner previously described for strain relief 12, with a connector or electrical apparatus.

Strain relief 78 can be fabricated in accordance with the method disclosed and claimed in the aforementioned divisional application, for example with apparatus similar in construction to that previously described for strain relief 12. However, the plurality of holding means for the conductor cord necessarily are of generally rectangular `cross section, at least for lthose portions thereof extending into the strain relief-forming cavity.

From the foregoing, it will be apparent that a strain relief of improved characteristics and adaptability can be readily fabricated in an inexpensive and efficient manner directly around an electrical conductor. Such modifications in the design and construction of the strain relief of the present invention and in the steps and apparatus for carrying out the method yof fabricating the same, as will be apparent to those skilled in the art are contemplated as being within the scope of the present invention.

What is claimed isy the following:

1. In combination, a flexible electrical conductor cord and a strain relief tightly molded about said oord, said strain relief comprising a head for attachment of said strain relief and said cord to equipment with which said CII strain relief `and said cord are to be used, and a molded sleeve of flexible resilient material integrally connected to said head at `one end thereof, `said sleeve being unidirectionally tapered along substantially the entire length thereof so that the widest portion of said sleeve is adjacent said head, said sleeve having walls of uniformly Varying thickness and provided with a plurality yof apertures disposed in spaced apart relationship along the entire tapered length thereof and extending therethrough to the surface of said cord so that inspection of said cord is permitted and flexing of said sleeve is facilitated.

2. In combination, a flexible electrical conductor cord and a strain relief tightly molded `about said cord, said strain relief comprising a head for attachment of said strain relief and said cord to equipment with which said strain relief and said cord are to be used, and a molded sleeve of flexible resilient material integrally connected -to said head at one end thereof, said sleeve being unidirectionally tapered along substantially the entire length thereof so that the widest portion `of said sleeve is adjacent said head, `said sleeve having a wall of uniformly varying thickness and provided with a plurality of apertures disposed in spaced apart relationship along the entire tapered length thereof and extending therethrough to the surface of said cord so that inspection of said cord is permitted said apertures being arranged in spaced apart aligned groups that dene a plurality of transverse weakened planes along said sleeve thereby facilitating flexing thereof.

3. The combination lof a flexible electrical conductor cord and a strain relief molded tightly around the cord, which strain relief comprises a headed reinforcing sleeve of flexible resilient material molded with a unidirectional taper along substantially the entire length thereof away from the headed portion of said sleeve, said headed reinforcing sleeve being provided with a plurality of apertures that extend through said sleeve to the surface of said cord so that inspection yof said cord is permitted, the apertures being formed by support means utilized for locating said cord during the molding of said headed sleeve around said cord and arranged along said sleeve in spaced apart aligned groups that dene a plurality of transverse weakened planes along said sleeve thereby facilitating exing thereof.

References Cited in the tile of this patent UNITED STATES PATENTS 1,607,432 Bryce Nov. 16, 1926 2,171,331 Folson Aug. A29, 1939 2,494,814 Huth Ian. 17, 1950 2,530,258 Marsan Nov. 14, 1950 2,573,600 Pruehs Oct. 30, 1951 2,713,700 Fisher July 26, 1955 2,724,869 Merrill Nov. 29, 1955 2,727,088 La Wall Dec. 13, 1955 2,973,501 Mapelsden et al Feb. 29, 1961 FOREIGN PATENTS 439,642 Great Britain Dec. 11, 1935