1. A potentiometer comprising,
2. A device according to claim 1 wherein the spring urges the slide against the spindle.
3. A device according to claim 1 wherein the spring is a wire formed in a U-shape whose end portion engages the slide and said screw thread, and whose limbs contact the resistance track.
4. A device according to claim 3 wherein the slide includes two longitudinal slots which are engaged by limbs of said spring.
5. A potentiometer comprising a contact member adapted to be displaced linearly by means of a transport screw spindle in a trough-shaped casing, and comprising a contact spring having a portion engaging the screw thread of the spindle and operating as a nut and as a contact and whose second portion forms a sliding contact adapted to move along a resistance track, characterized in that the contact member is located between the spindle and the bottom of the trough and comprises an elongated sledge extending parallel to the transport spindle having a gutter-shaped profile slidingly fitting to the spindle, said sledge being laterally guided by the inner walls of the trough so as to prevent rotation and being urged by the contact spring against the spindle, said contact spring having the form of a metal wire bent to the shape of a U whose closed end portion is slightly bent over and is taken through a fitting transverse slot near one end of the sledge, the web portion of said end portion engaging the screwhead of the spindle, whereas the limbs of the U-shaped spring have a further bent portion or knee projecting from the sledge and contacting under inherent spring pressure a resistance track on the bottom of the trough whereas the ends of the limbs of the contact spring engage the surface of the sledge facing the resistance track.
6. A potentiometer as claimed in claim 5, characterized in that the limbs of the U-shaped contact spring are located for the major part in two fitting longitudinal slots in the side of the sledge facing the resistance track from which slots said further bent portions of the contact spring protrude.
The invention relates to a potentiometer with a contact member adapted to be displaced linearly by means of a transport screw spindle in a trough-shaped casing, and comprising a contact spring, a portion of which operates as a nut and as a contact and a further portion of which forms a sliding contact displaceable along a resistance track.
An object of the invention is to provide a potentiometer of this kind of very slender structure, which is important when a plurality of potentiometers have to be arranged side by side in one row, for example, in a television tuning device comprising pushbuttons and capacitance diodes.
The device according to the invention is characterized in that the contact member is located between the spindle and the bottom of the trough and comprises an elongated sledge or slide extending parallel to the transport spindle and having a gutter-shaped or grooved profile slidably fitting on the spindle. The slide is laterally guided by the inner walls of the trough so as to prevent rotation and is urged against the spindle by the contact spring. This spring has the shape of a metal wire U-shape bent to the closed end portion of the U being slightly bent over and passed through a fitting transverse slot near one end of the sledge, the web portion of said end portion engaging the screw thread of the spindle, while the limbs of the U-shaped spring have a further bent portion projecting from the slide and contacting a resistance track on the bottom of the trough, whereas the distal ends of the limbs of the contact spring engage the surface of the sledge facing the resistance track.
The invention will be described more fully with reference to the drawing, in which
FIGS. 1 and 2 are a longitudinal and a cross-sectional view respectively,
FIGS. 3, 4 and 5 are a bottom view, a longitudinal and a cross-sectional view respectively of a part of the device shown in FIGS. 1 and 2 and
FIG. 6 shows a further part in a perspective view.
The potentiometer shown in FIGS. 1 and 2 comprises a trough-shaped insulating casing 1 of a synthetic resin, in which a transport screw spindle 3, which may be provided at one end with a driving gear wheel 5, is rotatably arranged. The screw spindle 3 has a screw thread with teeth cut off as viewed in an axial section and is journaled at one end, here the left-hand end, in the left-hand end wall of the trough-shaped casing 1; the other end is journaled by means of an L-shaped metal bracket 7, which establishes in addition, together with a known U-shaped contact spring 9 the electric connection to a contact pin 11 in the bottom of the trough 1. The trough 1 may be closed by a lid 13.
The transport screw spindle 3 serves for the linear displacement of a contact member comprising a contact spring 15 (see FIG. 6) having roughly the shape of a metal wire folded to U-shape, for example, of phosphor bronze. The closed end containing the web 17, of the U-shaped contact spring is bent over with respect to the further spring portion over an obtuse angle in the manner illustrated in FIG. 6 and is inserted into a transverse slot 19 in an elongated, parallelopiped-shaped sledge or slide 21 of a synthetic resin, for example nylon. The slide is shown separately on a slightly enlarged scale in FIGS. 3, 4 and 5, which sledge forms together with the contact spring 15 the complete displaceable contact member. The web 17 of the contact spring 15 is through the slot 19 in electrical and in mechanical contact with the screw spindle 3, whereas another spring portion projects from the contours of the slide 21 and has a further bend 23. This further portion is resiliently urged into contacting engagement with a resistance track on the bottom of the trough 1, for example a carbon track 25. The free distal ends 27 of the legs of the contact spring 15, as is shown in FIG. 6, are slightly rounded and engage under spring pressure the surface of the sledge 21 facing the resistance track 25 so that the slide-- which is located between the spindle 3 and the resistance track 25-- is urged against the spindle. The side of the slide 21 which is urged against the spindle 3, in this case the upper side, has a gutter-shaped profile which slidingly fits the spindle 3, since it is smooth and is not provided with screw thread. The web member 17 at the closed end of the spring 15 operates as a "nut"; as stated above, said web engages the screw thread of the spindle 3 through the fitting slot 19 of the slide 21 under spring pressure. By turning the spindle 3 the contact member 15, 21 is displaced linearly, so that the U-shaped wire spring 15 slides with the kinked portions 23 of the two limbs, that is to say at two places along the resistance track 25. The slide 21 is guided during the movement by the spindle 3 and, in addition, by the inner walls of the trough 1, the distance between which corresponds with the width of the slide so as to prevent rotation thereof. This is desirable because any rotation of the slide along with the spindle 3 would give rise to a retarded reaction (backlast) of the transport movement. Moreover, two ledges 29 are formed in the manner shown in FIG. 2 in the sidewalls of the trough 1 so that the slide 21 is prevented from disengaging itself from the spindle 3 and from touching the resistance track 25 in case the force of the spring ends 27 would be insufficient to prevent such disengagement.
As is known the use of wire as a contact spring has the advantage of a cheap manufacture and a further important advantage is that the rounded, burr-free contact surfaces, here at 17 and 23, required for avoiding damage of the countercontact members 3 and 25 are found in wire by nature. The difficulty is, however, that a wire spring is comparatively tender and can thus be easily damaged and deformed, and that it tends to jump out of the assembly concerned. In the construction described this is avoided by the slide 21, in which the spring 15 is so to say imprisoned. In this connection it is advantageous to provide the lower side of the slide 21 with two longitudinal grooves 31 (see FIGS. 3, 4 and 5) of slightly larger width than the diameter of the contact spring 15, which grooves join the transverse groove 19, said longitudinal grooves receiving the two limbs of the U-shaped contact spring with a small lateral clearance (see particularly FIG. 4, in which the contact spring 15 and the spindle 3 are indicated by broken lines). Thus deformation or undesirable lateral displacement of the contact spring is almost completely avoided, even when the spindle 3 is turned too far. In the latter case the slide 21 contacts the front wall of the trough 1 or the bracket 7 so that the contact spring portion 17 temporarily jumps out of the screw thread of the spindle 3 but returns thereto immediately when the spindle is turned in the other direction.