Control element for fastening to a front panel

The control element according to the invention which for example can be in the form of a pushbutton switch or display element, is intended to be fastened to a front panel having at least one circular opening. It has a housing with an essentially cylindrical section and, at its front, a collar projecting radially outward and beyond the cylindrical section. The cylindrical section is provided with at least one all-around groove in which a ring of an elastically deformable material is present. When the control element is inserted into the front panel, it is fixed into the opening by virtue of the fact that a support surface of the collar rests on the front plate on the one hand and by the ring being pressed against the lateral surface of the groove by the lower edge of the opening on the other hand.

BACKGROUND OF THE INVENTION
 Conventional control panels having control elements, for example having
 pushbutton switches or display elements in house doorbells, transport
 means, elevators, etc., envisage that one or more control elements are
 fastened to a front panel or introduced into said panel and the latter in
 turn can be fastened in the desired place. In the installation of such
 control panels, the control elements are fastened to the front panel in a
 first step, which can be effected, for example, by means of a screw nut
 which is screwed onto a thread provided on the control element. The front
 panel as a whole is then fastened. However, such a procedure does of
 course require that the front panel be prefabricated as a separate
 workpiece. The installation, too, can be inconvenient and tedious
 depending on the cabling of the control elements and depending on
 accessibility of the control panel. Moreover, the front panel must be
 detached as a whole if a control element is faulty.
 It would therefore be desirable to be able to fasten a control element
 directly to an already installed front panel. Such a front panel then need
 not be a separate, prefabricated workpiece. It would also be entirely
 possible for the casing provided with suitable openings, the housing or
 the like of the article on which the control panel is to be mounted also
 to serve directly as the front panel. In this way, high costs both for the
 production of the control panel and for its installation would of course
 be saved. A known means for permitting the fastening of a control element
 to a front panel installed in a fixed position consists in providing the
 front panel with additional holes into which, for example, plastics arms
 provided with a sawtooth profile and mounted on the control element can be
 introduced, which arms then fix the control element to the front panel. An
 other solution is disclosed in the two U.S. Pat. Nos. 4,070,559 and
 4,016,359. These two publications each disclose a control element which
 can be formed, for example, as a switch or lever or as an electrical
 insulator and can be fastened to a front panel with a circular opening. In
 these known cases, the control element has a housing with a cylindrical
 section and an annular groove running all round the cylindrical section.
 The cylindrical section is dimensioned so that it fits virtually without
 play into the circular opening. These known control elements furthermore
 each have an annular spiral spring which fits into the annular groove and
 serves for fastening the control element without additional fastening
 means, i.e. in the manner of a snap fastener, to the front panel. On
 insertion of the control element into said front panel, the spiral spring
 is pressed into the annular groove. It relaxes again in the inserted state
 and then rests against the inside of the front panel, with the result that
 the control element is fixed in the opening.
 This known method of fastening has the disadvantage that the cylindrical
 section has to be dimensioned so exactly that it can be inserted into the
 opening of the front panel so that it fits tightly, i.e. without play, and
 that the control element is relatively expensive because the spiral spring
 has to be specially made for this purpose.
 SUMMARY OF THE INVENTION
 It is the object of the present invention to provide a control element
 which does not have the disadvantages of the known control elements, can
 be fastened in a simple manner to a front panel and furthermore is
 economical to produce. This object is achieved by a control element having
 at least one circular opening with diameter d.sub.0 and a thickness T
 constant in the vicinity of this opening. The control element has a
 housing which has a front, a back and an essentially cylindrical section
 located in between and having a diameter d.sub.T, wherein
 d.sub.T is slightly less than d.sub.0,
 the housing has, at the front, a collar projecting laterally beyond and
 radially away from the cylindrical section and having a support surface
 making an angle of 90.degree. with the cylinder surface and intended for
 supporting the front panel,
 the cylindrical section has at least one groove running all around in a
 plane perpendicular to the cylinder axis and having a width d measured at
 its broadest point,
 for the distance a from the middle of a groove to the support surface,
 measured parallel to the cylinder axis, t-b/2 &lt;a&lt;t+b/2,
 a ring which comprises an elastically deformable material and whose
 external diameter is greater than d.sub.0 is present in the groove, and
 the ring is dimensioned and arranged in such a way that, in the state when
 inserted in the front panel, the control element is fixed in the opening
 by virtue of the ring being pressed against the lateral surface of the
 groove by the lower edge of the opening.
 The present invention has substantial advantages compared with the prior
 art. Thus, it has the advantage that the housing of the control element
 can be produced in a very simple and economical manner. For example, if it
 is metallic, it can be produced by turning in a single operation. The
 installation of a control panel provided with control elements according
 to the invention is also very simple, and the control elements need only
 be provided with the necessary electrical connections and then inserted by
 pressing into the opening provided for them. For example, the installation
 in poorly accessible places is thus simplified and also permitted or
 simplified especially when the back of the control panel is not accessible
 or scarcely accessible. However, because of the control element according
 to the invention, the front panel in particular need not be a separately
 produced adapter. It can readily be formed by the casing, the housing or
 the like of the article on which the control panel is to be mounted. Thus,
 depending on circumstances, the production of a control panel provided
 with control elements according to the invention can simply consist in
 drilling at least one circular opening of a standard size in a casing
 formed, for example, by a metal sheet and then fastening the control
 element by manipulation. Compared with the customary prior art, this
 procedure does of course constitute a substantial simplification and is
 thus associated with considerable cost savings.

The control element in the form of a pushbutton switch has a housing 1 with
 a front 1a and a back 1b. It is inserted into a circular opening having a
 diameter d.sub.0 in the front panel 2 of a control panel, which front
 panel is, for example, part of the casing of the article on which the
 control panel is mounted. In the example shown, the thickness of the front
 panel in the vicinity of the control element is t=2 mm. A pushbutton
 switch actuation member not shown in the drawing is present in the
 interior of the housing 1. Said member can be actuated from the front 1a
 of the housing and has an actuation mechanism known per se; it may be in
 the form of, for example, a touch-sensitive sensor or a mechanically
 actuatable electric switch. The actuating member can transmit electrical
 switching pulses through leads 5 shown schematically here to a controlling
 device.
 The housing 1 has an essentially cylindrical section 11 with an axis 7 and
 a diameter d.sub.T which is only slightly less than d.sub.0 and is thus
 dimensioned so that the cylindrical section 11 can be pushed through the
 opening having the diameter d.sub.0 ; for example d.sub.T is 0.3 mm less
 than d.sub.0. A radially projecting collar 12 is adjacent to the
 cylindrical section 11 at the front of the housing 1a. This collar 12 has
 a support surface 12a which is at right angles to the lateral surface of
 the cylinder and is intended for resting on the front panel 2. In the
 example shown, the collar 12 is rotationally symmetrical with respect to
 the axis 7, but it may readily also be of another shape, for example
 rectangular. Furthermore, in the cylindrical section 11 in the example
 shown in the drawing, the housing 1 has three grooves 13 which run all
 around and are each in a plane perpendicular to the cylinder axis 7. The
 width b, measured at its broadest point, of each groove is, for example, 1
 mm and its depth is, for example, 0.6 mm. In FIG. 3, the distances from
 the grooves to the support surface 12a, measured parallel to the cylinder
 axis 7, are denoted by a.sub.1, a.sub.2 and a.sub.3 ; they are, for
 example, a.sub.1 =1.2 mm, a.sub.2 =2.2 mm and a.sub.3 =3.2 mm. The grooves
 are preferably shaped in such a way that, in a longitudinal section
 through the cylinder as shown, for example, in FIG. 3, they have two
 lateral surfaces 13a and 13b facing away from one another and a transition
 surface 13c connecting the lateral surfaces 13a, 13b to one another. The
 lateral surface 13a facing the front 1a of the control element is at right
 angles to the cylinder axis 7, whereas the lateral surface 13b facing the
 back 1b makes an angle of, for example, 40.degree. to 80.degree. with the
 cylinder axis 7. Because, in the embodiment shown, the grooves are a
 distance apart which corresponds to their width, the rear 13b and front
 13a lateral surfaces of adjacent grooves 13 are contiguous with one
 another, an edge 14 being formed between them, so that, in the
 longitudinal section through the cylinder, a sawtooth profile results.
 In one of the grooves, in the middle groove of the example shown, an
 essentially toroidal ring 8 of an elastically deformable plastic, for
 example a synthetic rubber or of silicone, is present. This ring 8 may
 readily be a commercially available gasket of a standard size. The
 diameter of the circular disk which forms if the ring 8 is cut at any
 point along a surface through the torus axis is preferably equal to the
 width of the groove. If the control element is now inserted into the front
 panel 2, the lower edge 2a of the circular opening in the front panel 2
 presses on the ring 8 and deforms it. Because the ring rests against the
 lateral surface 13b and cannot escape from pressure by this edge 2a, it
 prevents the control element in FIGS. 1 and 3 from being pulled out upward
 and out of the opening in the front panel 2. In this way, the control
 element is firmly fixed in the opening as a result of the support surface
 12a resting on the panel 2 on the one hand and said resting of ring 8
 against the lower edge 2a on the other hand.
 The distances from the middle of the grooves 13 to the support surface 12a
 are chosen so that they are less than half the groove width b larger or
 possibly smaller than the thicknesses of the front panels usually used. If
 the front panel in which the control element is to be inserted is clear
 from the outset, i.e. if, for example, control element and front panel
 form a unit as a result of a standard design or in another manner, it is
 of course sufficient to provide only a single groove 13 in the cylindrical
 section 11 of the housing 1.
 Owing to the design according to the invention, the fastening of a control
 element to a front panel is very simple. In a first step, the control
 element must be provided with the necessary electrical connections. It is
 then pressed into the opening in the front panel 2 by pressure on the
 front 1a, the ring 8 being inserted into the groove 13 whose distance from
 the support surface 12a is only slightly greater than or possibly less
 than the thickness t of the front panel. A bevel 2b at the upper edge of
 the opening in the front panel 2 makes it possible for the ring to be
 deflected on insertion of the control element, with the result that this
 insertion can be effected with relatively little pressure. However, once
 the control element is present in the opening provided for it, it is
 firmly fixed and can be removed again only with application of
 considerable pressure and possibly with destruction of the ring 8. The
 difference between the force applied for insertion of the control element
 into the opening and that applied for removing said control element
 therefrom is particularly large if the groove 13 has the special shape
 described above and comprising the two asymmetric lateral surfaces 13a and
 13b, since the ring can deviate from a force applied to it only in the
 direction of the housing front 1a but not in the direction of the housing
 back 1b.
 At this point, it should also be mentioned that the control element
 described above is by no means the only possible embodiment for the
 invention but can also be modified in many respects. Thus, it is readily
 possible, for example, to dimension the sizes of the grooves 13 and of the
 ring 8 completely differently from those proposed above; for example, both
 may readily be twice as large as in the embodiment described above.
 Of course, it is furthermore by no means necessary for the control element
 according to the invention to be in the form of a pushbutton switch. It
 can equally well be, for example, a display element or have another
 function.
 The front panel, too, can be of a form different from that in the example
 shown. Thus, it need not necessarily be a part of the casing or of the
 housing of the article on which the control panel is to be fastened but,
 as in the case of customary control panels, may be an adapter which, for
 example, can be screwed on. Furthermore, it can be produced, for example,
 so that it has a countersink with the dimensions of the collar 12 in the
 vicinity of the opening, so that the control element is countersunk and
 its surface is, for example, flush with the surface of the front panel.