FI126887B

FI126887B - Strain relief

Info

Publication number: FI126887B
Application number: FI20165399A
Authority: FI
Inventors: Henri Juslén
Original assignee: Helvar Oy Ab
Priority date: 2016-05-10
Filing date: 2016-05-10
Publication date: 2017-07-14
Also published as: FI20165399A

Description

Strain relief

FIELD OF THE INVENTION

The invention relates to strain reliefs that help to keep electric wires or cables from coming loose from their connectors if accidentally subjected to a pulling force.

BACKGROUND OF THE INVENTION

Many electric devices must have a strain relief for electric wires or cables. As an example an operating device for lighting is considered. The operating device is an electronic circuit, the task of which is to convert mains power into suitable voltage and current for driving a light source such as a LED module. It may be located inside the mechanical structure of a luminaire or it may be installed independently, for example within the structures of a false ceiling that has openings for the light sources. Two wires connect the operating device to the mains power network, and a third one may be added for a protective earth connection. Two or more wires are needed to connect the operating device to the light source(s), and two wires more may be provided if there is a control connection, like a DALI bus (Digital Addressable Lighting Interface) for example.

National and international standards define the cases in which an operating device for lighting must include a strain relief. The purpose of a strain relief is to absorb any accidental pulling forces so that they do not load the electric connectors at which the external wires and cables connect to the electronic circuit. The strain relief must therefore make physical contact with the wires and cables at one or more points along their length, relatively close to the electric connectors. The manufacturer of the operating device and its strain relief can seldom influence the thickness of the wires and cables that will be used, for which reason the strain relief must be designed so that it can make sufficiently solid physical contact with wires and cables of various thicknesses and surface materials. In many cases a threaded joint of some kind is employed, in which a screw or bolt can be turned tight enough to pinch the wire or cable between serrated or otherwise friction-enhancing surfaces.

Foisting a wire through a narrow slot between serrated surfaces that can be relatively close to each other already before tightening the screws, and turning a screw that may be relatively small and thus necessitate using two hands to aim the tool accurately, may be difficult and time-consuming and even physically hazardous if the person installing the operating device must simultaneously stand on a ladder high up near the ceiling. It would be highly desirable to provide a strain relief that would enable finalizing the wiring of a lighting system (or any other system that must be assembled at an awkward location and/or in a tight time schedule) quickly and easily, preferably with one hand only or even automatically with a robot arm or the like, and would adapt to various cable thicknesses and surface materials.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a strain relief structure that enables finalizing the wiring of a lighting system or other system quickly and easily, with only modest requirements of accuracy of movements and access to the devices that are being wired. Another objective of the invention is to provide a strain relief structure that has only a limited number of parts so that it can be effectively produced in mass production and places only modest requirements to component logistics. Yet another objective of the invention is to provide a strain relief structure that can easily accommodate wires or cables of different thicknesses.

The objects of the invention are achieved with a strain relief structure as defined by the respective independent claim.

According to an aspect of the invention there is provided a strain relief for an electric apparatus, comprising: a frame with a base and a cover, a pair of mutually aligned elongated holder means in said cover, with the distance between the holder means and said base decreasing towards that end of the holder means that is directed out of the strain relief, a wedge part with a pair of edges arranged to mate with said holder means so that the wedge part is movable in the longitudinal direction of said holder means, a resilient member on a bottom side of the wedge part that is against said base when said wedge part is in place in said holder means, and an opening in said cover at one end of said holder means.

The exemplifying embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" and its derivatives are used in this patent application as an open limitation that does not exclude the existence of also features that are not recited. The features described hereinafter are mutually freely combinable unless explicitly stated otherwise.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following detailed description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a base and a cover, figure 2 illustrates a strain relief structure, figure 3 illustrates a wedge part according to an embodiment, figure 4 illustrates a wedge part according to an embodiment, figure 5 illustrates a wedge part according to an embodiment, figure 6 illustrates a wedge part according to an embodiment, figure 7 illustrates a wedge part according to an embodiment, figure 8 illustrates a wedge part according to an embodiment, and figure 9 illustrates a wedge part according to an embodiment.

DETAILED DESCRIPTION

Fig. 1 illustrates a part of a strain relief structure according to an embodiment of the invention. The strain relief structure comprises a base 101 and a cover 102, which can be the base and cover of the enclosure or casing of an electronic apparatus, but which can also be specific to the strain relief itself. The last-mentioned alternative may be practical for example in a case in which the strain relief is an add-on part that can be attached to the enclosure or casing of an electronic apparatus that can be installed also without any strain relief in some applications. Operating devices for lighting are an example of such a case, because official regulations may require that an operating device that is installed inside the closed cover of a luminaire does not need a strain relief while exactly the same operating device must be equipped with one if it is installed independently. According to yet another example the base 101 may be part of the enclosure or casing of the electronic apparatus while the cover 102 may be specific to the strain relief structure, so that the same base is used in all installations while a cover is only added if a strain relief is needed.

The designations “base” and “cover” are not limiting in the sense that the “base” part should be flat while the “cover” part should be concave. These designations are used only to enable referring to the two parts in an unambiguous manner. Both of them may be concave, or the “base” part may be concave while the “cover” part could be essentially flat. The base and cover do not even need to be physically separate parts, but they can be portions of a common mechanical entity. Together they define a hollow space through which at least one wire or cable is meant to pass on its way between an electronic device and some external connection point. One or both of the base and cover may themselves be comprised of two or more parts.

Fig. 2 is a partial cross section that shows the base 101 and the cover 102, of which the latter is shown with dashed lines. A cable 201 passes through the hollow space between the base 101 and the cover 102. One end of the cable 201 is attached to a connector 202 located on the surface of a printed circuit board 203 that is enclosed in a housing, parts of which are the base 101 and the cover 102.

The cover 102 defines a pair of mutually aligned elongated holder means. In the embodiment of figs. 1 and 2 these are two grooves, the upper ends of which are seen in fig. 1 with reference designators 104 and 105. In fig. 2 one of the grooves can be seen as a lane delimited by two dashed lines, between which is an edge of a wedge part 204, which will be described in more detail later. The distance between the holder means and the base 101 decreases towards that end of the holder means that is directed out of the strain relief, that is towards the right-hand end in fig. 2. A wedge part 204 is provided, with a pair of edges arranged to mate with the elongated holder means in the cover 102. Together the holder means and the edges of the wedge part 204 constitute a sliding joint, so that the wedge part 204 is movable in the longitudinal direction of the holder means. A resilient member 205 is provided on the bottom side of the wedge part 204. Here the designation “bottom side” refers to that side of the wedge part 204 that is against the base 101 when the wedge part 204 is in place in the holder means. If the right-hand end of the wedge part that is arranged to be directed towards that end of the holder means that is directed out of the strain relief is called the first end, the resilient member 205 is a wing part directed to the direction opposite of the first end.

The cover 102 defines an opening at at least one end of the holder means. In the example of figs. 1 and 2 particularly the opening 106 at the upper end of the holder means is meant. There is another opening at the end of the cover 102, but in figs. 1 and 2 said other opening is only for the wires and cables to go through. In the embodiment of figs. 1 and 2 the opening 106 in the cover 102 is large enough for the whole wedge part 204 to slide through the opening in the longitudinal direction of holder means.

The structure of figs. 1 and 2 can be assembled for example as follows. The printed circuit board 203 is attached to the base 101, and another cover part (not shown) can be added to cover the printed circuit board leaving only the connector 202 visible and accessible. The electronic apparatus may be installed at a desired location. The end of the cable 201 (and the ends of any other similar cables or wires) is attached to the connector 202. At this stage the cover 102 is not there, so the person doing the assembling has easy access to the connector 202 and does not need to persuade the end of the cable through any narrow passages.

The cover 102 is attached to the base 101, for example by pressing the cover 102 and base 101 against each other so that a snap-on mechanism locks them together. The wedge part 204 is pushed into place through the opening 106 by sliding it down along the elongated holder means until the resilient member 205 touches the cable(s) 201. When the pushing is continued, the resilient member 205 undergoes an elastic deformation that causes the cable(s) 201 to become tightly pinched between the resilient member 205 and the base 201. Simultaneously the elastic deformation causes a force that pushes the wedge part 204 directly upwards, i.e. not backwards along the direction of the holder means but against the cover 102. Together with the coefficient of friction between the wedge part 204 and the cover 102 this pushing force causes a static friction force between the wedge part 204 and the cover 102 that keeps the wedge part 204 in place.

On the other hand, friction between the resilient member 205 and the cable 201 causes a phenomenon where any pulling force on the cable 201 makes the wedge part 204 move along with the cable 201. This only squeezes the wedge part 204 more tightly into the narrowing space between the holder means and the base, increasing both the force that pinches the cable 201 in place and the force that pushes the wedge part 204 against the cover 102. The bottom side of the wedge part 204 may comprise a biting edge, serrations, bumps, teeth, a friction surface and/or other means that enhance the friction between it and the cable(s) and make it more difficult for the cable to slide past the wedge part 204 under the influence of a pulling force.

If both the base and the cover are specific to the strain relief structure, the assembling may be performed so that first the base is attached as an extension to the base of the electronic apparatus, then the wires are connected (or: the wires are connected first and the base is attached thereafter), then the cover is snapped into place, and finally the wedge part is pushed or pulled against the wires or cables.

Fig. 3 is a perspective view of a wedge part 204 with its pair of edges 301 and 302 that are arranged to mate with the holder means. The resilient member 205 on the bottom side of the wedge part 204 is slightly narrower than the main body of the wedge part 204, so that the holder means may support the edges 301 and 302 on their top and bottom sides. This is not obligatory, because for the operation of the strain relief support on the top side is more important; fig. 4 shows a wedge part 204 in which the resilient member 205 is as wide as the main body.

Static friction between the wedge part and the cover may be enough to keep the wedge part in place, but an alternative is to provide mating lock-in means in the wedge part and at least one of the base and the cover for locking the wedge part in place at a selected point along its movement in the longitudinal direction of the holder means. Such mating lock-in means may comprise at least one ridge in the wedge part (as ridge 501 in fig. 5) and at least matching groove in the cover, or at least one ridge in the cover and at least one matching groove in the wedge part. Such ridge(s) and groove(s) are preferably perpendicular to the longitudinal direction of the holder means. Additionally or alternatively the mating lock-in means may comprise pairs of teeth and slots, at least one of which are integrated with the holder means.

Fig. 6 illustrates an alternative embodiment in which the resilient member 205 is not a wing of uniform thickness that extends out of the bottom of the main body of the wedge part 204 as in figs. 3 to 5. In fig. 6 the resilient member is a solid portion of the wedge part and limited by the bottom surface that extends from the lower end (right-hand end) of the wedge part to the biting edge 601 at the end of the resilient member 205, which biting edge 601 as such is similar and similarly located as in the embodiments of figs. 3 to 5. Fig. 7 illustrates an alternative embodiment in which the bottom side of the wedge part 204 comprises serrations 701 for increasing its capacity to hold in place cables or wires squeezed between the bottom side of the wedge part 204 and the base.

Fig. 8 illustrates an embodiment in which the whole wedge part 204 is made by cutting and bending a single piece of a sheet-like material. Fig. 9 illustrates an alternative embodiment in which the wing part is divided into two or more longitudinal sections 901 that are elastically bendable independently of each other. Dividing the wing part (or other kind of resilient member) into sections enables using parallel wires or cables of different thicknesses, because each section only presses against the wire or cable that is directly under it and allows the neighbouring section(s) to bend less or more.

The advantages of the invention involve the extremely low count of separate parts or components: especially if the base and cover are parts that the enclosure or casing of the electronic apparatus would have anyway, there is only one additional part (the wedge part). The count of components that are additional to an electronic apparatus without a strain relief may be two, if the base is a portion of the base that the enclosure or casing would have anyway so that only the cover and wedge part must be added to provide a strain relief. The components of the strain relief structure may be exactly the same regardless of the thickness of the wires or cables that are used, because the structure adapts to any thickness of the wires or cables by only pushing the wedge part as far as it goes with the currently used thicknesses. Some embodiments of the invention even accept wires or cables of different thickness in the same apparatus, as has been described above.

The features and embodiments of the invention that have been described above are presented as examples, and numerous modifications and combinations are possible. For example, the slightly curved form of the main body of the wedge part is not obligatory, but the wedge part and the holder means may be straight. The opening shown in the cover does not need to be large enough for the whole wedge part to slide through the opening in the longitudinal direction of the holder means; it is possible to pre-install the wedge part inside the cover and only utilize the opening to press it against the cables or wires with a finger or with a tool. The opening may even be provided with a lid, which may be elastically deformable so that it can be used to press a preinstalled wedge part under it after which it comes back to its original position. Alternatively the lid can be a separate part that is pushed in place to cover the opening after the wedge part has been pushed into its final position. A yet another alternative is that a top portion of the wedge part acts as a lid that essentially closes the opening when the wedge part has been pushed into place against wires or cables of a default thickness. In this embodiment if wires or cables of different thickness are used the top portion of the wedge part may remain slightly above or below the level of the outer surface of the cover, but this seldom matters as long as the opening is closed in the sense of disabling unauthorized access to live electric conductors.

Another class of possible variations concern the holder means. Above the holder means have been described as grooves that mate with edges in the wedge part, but the holder means may comprise grooves in the wedge part and edges in the cover, or a combination of bumps, indents, notches, recesses, ridges, teeth, or any other means that mechanically allow to move the wedge part in the longitudinal direction of the holder means. In such cases the characterisation of the holder means as being elongated and having a longitudinal direction may be interpreted so that a group of individual holder means, like a group of bumps or notches, constitutes an elongated formation.

According to yet another alternative of the invention the opening is the same as or adjacent to the opening through which the cables come, so that the opening is arranged to allow pulling (and not pressing) the wedge part to make it slide in the longitudinal direction of said holder means. Such an embodiment naturally necessitates that the wedge part extends sufficiently far outside the opening to be grasped with a tool or with fingers. As an example the first end (the right-hand end in the drawings) of the wedge part may be equipped with a thin pulling strip that extends through the opening similarly to the cables or wires, so that the person installing the strain relief may grab the strip and use it to pull the wedge part against the cables or wires.

Claims

An electrical strain relief device comprising: a housing having a base (101) and a cover (102), said cover (102) having two elongate elongate clips parallel to each other such that the distance between these holders and said base (101) decreases toward that end of the holders. extending outward from the traction sheave, a wedge portion (204) having two edges (301, 302) arranged to fit said clamps such that the wedge portion (204) is movable in the longitudinal direction of the clips, a resilient member (205) is against said base (101) with the wedge portion (204) in place in said clips, and an opening (106) in said cover (102) at the ends of the clips.

A strain relief device according to claim 1, having matching locking members in the wedge portion (204) and in said base (101) and / or said cover (102) for locking the wedge portion (204) in place at a selected position along its longitudinal movement.

The strain relief according to claim 2, wherein said locking means comprises: at least one ridge (501) in the wedge portion (204) and at least one corresponding groove in the cover (102) or at least one ridge in the cover (102) and at least one corresponding ridge in the wedge portion (204) ), said at least one ridge and at least one groove being perpendicular to the longitudinal direction of the holders.

The strain relief according to claim 2 or 3, wherein said mutually compatible locking means comprises pair of teeth and recesses, at least one of which is of a single structure with said holders.

The strain relief according to any one of the preceding claims, wherein said opening (106) in said cover (102) is large enough for the entire wedge portion (204) to slide through the opening (106) in the longitudinal direction of said holders.

The strain relief according to any one of the preceding claims, wherein the opening (106) is arranged to allow the wedge portion (204) to be pressed or pulled to dissolve it in the longitudinal direction of said holders.

The strain relief according to any one of the preceding claims, wherein: the wedge portion (204) has a first end disposed in the direction of the end of the brackets extending out of the strain relief, and said resilient member (205) being a wing portion disposed in an opposite direction to the first end.

The strain relief of claim 7, wherein said wing portion is divided into two or more longitudinal portions (901) which are resiliently deflected independently of one another.

The strain relief according to any one of the preceding claims, wherein at the bottom of the wedge portion at least one of the following: biting edge (601), groove (701), protrusions, teeth, friction surface; which is or which increase its ability to hold in place the wires or cables (201), which are pressed between a wedge member (204) and Poh-half of said base (101).

The strain relief according to any one of the preceding claims, wherein: the base (101) and the cover (102) are parts which would be included in the housing or housing of the electrical appliance even without the strain relief.

The strain relief according to any one of claims 1 to 9, wherein: the base (101) is a part which would fit into the housing or housing of the electrical appliance even without the strain relief, and the strain relief has a snap fastening between the base (101) and cover (102).

The strain relief according to any one of claims 1 to 9, wherein: the strain relief has a snap-on attachment between the base (101) and a part of a housing or housing of an electrical device without a strain relief; ).