GB2147653A

GB2147653A - Two part snap fastening

Info

Publication number: GB2147653A
Application number: GB08327024A
Authority: GB
Inventors: John James Steel; John Arthur Howe
Original assignee: TRW Connectors Ltd
Current assignee: TRW Connectors Ltd
Priority date: 1983-10-10
Filing date: 1983-10-10
Publication date: 1985-05-15
Also published as: GB8327024D0

Abstract

In order to convert a rocker switch to foot use, a cover 30 is added and held in a way that is resistant to continual pivoting. The cover 30 is provided with two or more flexible barbs 36 that pass through holes 19 in knob 10 and engage under the rims by snap action. The snap action is assisted by resilient tongues 20 which partially close the hole and urge the barbs away from each other. Both tongues 20 and the barbs 35 may have chamfered edges. <IMAGE>

Description

SPECIFICATION Two part snap fastening The present invention relates to a two part snap fastening e.g. for joining two plastics mouldings together. A particular application of the fastening is in attaching a cover or sleeve of large surface area to a rocker switch knob for adapting the switch for foot operation and to ensure water or other fluid ingress is kept away from the internal switching mechanism and contacts.

Two part snap fastenings involving projecting lugs which extend through holes in another part are known. In general these are vulnerable to continual flexing of the parts involved in a rocking motion and which can tend to snap the parts apart possibly exposing vulnerable or dangerous components, and can also be forced apart by small tools.

The invention is particularly concerned with a solution to this problem.

According to the invention there is provided a two part snap fastening comprising a first part having a wall formed with a plurality of through holes and a second part having a bottom surface from which project a similar plurality of lugs adapted to snap fit through the holes in the first part to secure the parts together, wherein the lugs each have a stem and a tapered head which is thicker than the stem on one side so as to define a barb, the stem and head presenting a generally flat continuous surface opposite to the tapered side, the barbs of at least two lugs being generally opposed, and the space between the barbs and the said bottom surface conforming to the thickness of the wall, the tapered surfaces of the lugs being arranged to engage with the side edges of the holes at entry whereby entry is achieved by resilient flexing of the lugs and/or the material of the second part, and wherein each hole is partially barred by a tongue which extends from the side opposite to the side edge engaged by the barb, the tongues being resiliently flexible to permit entry of the lugs but thereafter presenting a rigid abutment in a direction perpendicular to the flat stem surface of each lug, thereby preventing removal of the second part by pivotting motion.

Preferably the tongues are of lesser thickness than the wall and conveniently they have an undersurface flush with the undersurface of the wall.

They may be joined to the remainder of the wall along one edge and the opposite or leading edge may be chamfered to assist the entry of the lugs.

For this purpose also the opposite edge of the hole may be rounded.

In the preferred case the second part has two lugs arranged with their barbs facing outwardly from each other.

An embodiment of the invention is hereafter described with reference to the accompanying drawings, in which: Figure 1 is a sectional view through a switch knob and extended cover plate joined by means of the invention, Figure 2 is an enlarged plan view of part of the knob showing a hole and tongue, Figure 3 is an enlarged sectional view illustrating the entry of a lug into a hole, and Figure 4 is a sectional view of a rocker switch fitted with the knob and cover of Figure 1.

In Figure 1 there is shown at 10 the knob of a rocker switch other components of which are shown in Figure 4. The knob 10 is formed as a one-piece moulding from plastics material e.g. stiff polyethylene or polypropylene and has a generally dish-shaped main portion 11 with a generally flat upper wall 12 having a planar upper surface 13 and a planar lower surface 14 bounded by planar side walls one of which is shown at 15 and curved end walls 16. Between the side walls extends one, or a number of parallel rocker arms, one being indicated at 17. Extending outwardly from each endmost rocker arm is a spigot 18 about which the member rocks in operation.

At positions equidistant from the rocker arm 17, the wall 12 is formed with a pair of holes 19 more clearly shown in Figure 2. The holes are of square outline and a tongue 20 extends from the side edge adjacent the rocker arm 17 and protrudes towards the opposite edge leaving a gap 21 between the leading edge 22 of the tongue and the opposite edge 23 of the hole 19. As shown in Figure 2 the tongue does not extend the full width of the hole 19, leaving a space 24 on each side to increase the resilient flexibility of the tongue.

To further increase the resilient flexibility, the tongue 20 as shown in Figure 1 does not extend for the full thickness of the wall 12, which is recessed from the upper surface 13, the bottom surface of the tongue 20 and wall 12 being flush or coplanar. To assist entry of lugs to be described the leading edge of each tongue is chamfered at 25.

It will be observed that the tongues extend in opposite directions from positions adjacent the central rocker arms 17.

Fitted to the knob or first part is a second part in the form of an extended cover or sleeve 30 which extends the effective surface area of the knob to allow it to be operated by the foot.

The cover is generally dish shaped and of an outline which generally conforms to that of the knob 10. It has a top wall portion 31 having an upper surface 32 which is concave as viewed in Figure 1 bounded by generally flat perpendicular side walls one of which is shown at 33 and curved end walls 34.

In the area of the knob 10 the lower surface of the wall 32 is planar and thickened at 35 to match the upper surface of the wall 12. From the surface 35 extend a pair of lugs 36 positioned and adapted to snap fit through the holes 19 to secure the parts 10 and 30 together. The lugs each have a stem 37 and a tapered head 38 which is thicker than the stem on the side opposite to the rocker arm 17 to define a barb, the stem 37 and head 38 presenting a generally flat surface 39 opposite to the barb.

Thus the barbs of the two lugs are generally opposed and the space between the barbs and the surface 35 from which the lugs extend conforms to the thickness of the wall 12 so that the barbs clamp the parts together snugly. To assist this the upper surface 40 of each barb may be upwardly chamfered to grip the bottom surface 14 of the wall 12.

The cover 30 may be snap-fitted by a direct engagement against the knob 10. As indicated in Figures 1 and 3, the lugs are so positioned that the end of each lug engages within a gap 21 with the side edge of the hole 19 opposite to the tongue engaging against a tapered surface of the lug forcing the lug to flex inwardly towards the rocker arm 17 as shown in Figure 3. To assist this the engaged edge 41 is rounded. As the lug 36 flexes, the bottom edge 42 of the head opposite to the barb engages against the chamfered surface 40 of the tongue 20 flexing this downwardly sufficiently to enable entry of the lug.

Once the lugs are fully inserted, the tongue 20 snaps back and presents a rigid abutment extending perpendicularly to the flat stem surface 39. This effectively prevents any relative pivotting motion of the parts 10 and 30 which if allowed to take place could cause the cover 30 to snap out of engagement with the knob 10. As can be recognised, considerable pivotting force can be exerted by foot operation of a rocker switch.

The completed rocker switch is indicated in Figure 4 where it can be seen that the rocker arm 17 engages against a spring 50 and, when the knob and cover are rocked, the rocker arm causes engagement and disengagement of contacts 51. The housing for the switch is provided with resilient wing members 52 which resiliently engage against the interior surfaces of the curved end walls 34 of the cover. The ends 53 of the wings 52 are bent backwardly and knurled to frictionally engage with the cover to assist in holding this in each end position.

Although the invention has been described in connection with a pair of oppositely facing lugs, other embodiments are possible, e.g. with three lugs at the corners of a triangle. In such a case it would be preferable for the barbs to extend outwardly from the centre as viewed in plan so that any two of the lugs remain generally opposed. Likewise the end view of Figure 1 could represent a row of lugs 36 instead of simply a pair of lugs. In this case also there would be pairs of generally opposed lugs. Although it is preferable for the barbs to point outwardly, they could alternatively point inwardly, in all cases, with the tongues projecting from the opposite side to that shown.

Claims

1. A two part snap fastening comprising a first part having a wall formed with a plurality of through holes and a second part having a bottom surface from which project a similar plurality of lugs adapted to snap fit through the holes in the first part to secure the parts together, wherein the lugs each have a stem and a tapered head which is thicker than the stem on one side so as to define a barb, the stem and head presenting a generally flat continuous surface opposite to the tapered side, the barbs of at least two lugs being generally opposed, and the space between the barbs and the said bottom surface conforming to the thickness of the wall, the tapered surfaces of the lugs being arranged to engage with the side edges of the holes at entry whereby entry is achieved by resilient flexing of the lugs and/or the material of the second part, and wherein each hole is partially barred by a tongue which extends from the side opposite to the side edge engaged by the barb, the tongues being resiliently flexible to permit entry of the lugs but thereafter presenting a rigid abutment in a direction perpendicular to the flat stem surface of each lug, thereby preventing removal of the second part by pivotting motion.

2. A fastening according to claim 1 wherein the tongues are of lesser thickness than the wall.

3. A fastening according to claim 2 wherein the tongues have an undersurface flush with the undersurface of the wall.

4. A fastening according to any preceding claim wherein the tongues are joined to the remainder of the wall along one edge.

5. A fastening according to any preceding claim wherein the leading edges of the tongues are chamfered in a direction to assist entry of the lugs.

6. A fastening according to any preceding claim wherein the two parts have mating surface portions in the vicinity of the lugs and holes.

7. A fastening accordiing to any preceding claim wherein each part is a one piece moulding of plastics material.

8. A fastening according to any preceding claim wherein the second part has two lugs arranged with their barbs facing outwardly from each other.

9. A fastening according to any preceding claim wherein the first part is a rocker switch knob and the second part is a cover adapting the switch for foot operation.

10. A two part fastening substantially as described herein with reference to Figures 1 to 3 of the accompanying drawings.

11. A rocker switch substantially as described herein with reference to the accompanying drawings.