GB2308506A - Motor/manually driven sliding variable resistor - Google Patents

Abstract

An annular groove 20a is formed in the support shaft 20 of a belt driven pulley 10. The support shaft is inserted into opposing slots/holes 17a, 16a in opposing plates 17, 16 such that the annular groove 20a grips the plate 17. A motor/manually driven sliding variable resistor uses this pulley support to simplify assembly, the pulley 10 being held on a support frame 6 by the tension of a belt 11. The resistor includes means to prevent the occurrence of noisy sounds in motor-driven operation. The motor 7 drives a driving pulley and toothed belt 11 guided around the driven pulley 10 to move the slider pieces 25, 26 of the resistor. The pieces are held on a slider receiver 22 mounted to a manually movable belt operating member 21, the belt being tightly held between the operating member and slider receiver. A protruding lug 22c formed on the slider receiver 22 is inserted into an L-shaped portion 21e formed on the operating member 21, tightly holding the belt 11 between the lug and L portion. At the same time, a latch pawl [21h, figure 9] is snap-fitted into a latch groove [22e, figure 11] for joining the operating member 21 and the slider receiver 22 together into one unit. A guide member 9 is disposed around the driving pulley 8 to leave a gap smaller than the tooth height of the belt teeth 11a between the guide member 9 and the belt 11. This stops the motor when the slider pieces have reached their end positions, thus reducing noise.

Description

MOTOR-DRIVEN SLIDING VARIABLE RESISTOR This invention relates to a rnotor-driven sliding variable resistor.

In motor-driven sliding variable resistors, a resistance value is adjustable by any of motor-driven operation and manual operation. The motor rotation is transmitted to a movable member of the variable resistor by a belt wound over a pair of pulleys. One prior art example of such motor-driven sliding variable resistors is disclosed in Japanese Utility Model Laid-Open No. 4-77204.

The disclosed motor-driven sliding variable resistor includes a driving pulley rotated by a motor and having a gear portion formed on its outer peripheral surface, a driven pulley disposed in oppositely spaced relation to the driving pulley with a predetermined distance therebetween and having a gear portion ormed on its outer peripheral Eur=ace, and a belt stretched between the driving pulley and the driven pulley and having teeth formed on its inner peripheral surface to be meshed with the gear portions of the pulleys. A movable member having a knob and a slider piece s fixed to the belt, the spider piece being held in side contact it a resistance material ilm on a resIstance board. the driven pulley is rotatably supported by liia n support start to a frame bv caulking the shat end, -nc ttina tre driven pulley over the support snafu.

When an operator manually operates the knob on the novacle member, a position or the slider piece relative to the resistance material film is changed and the resistance value is adjusted by the manual operation. Also, when the motor is rotated by an external signal, the driving pulley is rotated with the motor rotation, causing the belt to run about the driving pulley and the driven pulley through the meshing between the gear portions and the teeth. Therefore, the movable member fixed to the belt is moved and the resistance value is adjusted by the motor-driven operation.

During adjustment of the resistance value by the motordriven operation, however, if the motor rotation is continued after the movable member has reached the end of its movable range, only the driving pulley continues rotating with the movable member and the belt both held stopped. This raises the problem that the gear portion of the driving pulley rides over the teeth of the belt and generates noisy sounds. If the belt is stretched under a very high tension, the gear portion of the driving pulley could not ride over the teeth of the belt and the motor rotation could be locked forcibly. But this solution would give rise to another problem that because the torque necessary for rotating both the pulleys increases with an increase in the belt tension, the force required for the operator to manually adjust the resistance value becomes too large and operating efficiency is degraded.

The support structure of the driven pulley is also problematic. Specifically, since the support shaft of the driven pulley is fixed to the frame by caulking, the support shaft must be plastically deformed by using a caulking jig and hence assembling efficiency is very poor. Furthermore, since large pressing force is required .z carry out the caulking, the equipment needs a large capacity and the total cost is increased.

Another example of the prior art is disclosed in Japanese Utility Model Laid-Open No. 4-52705 (corresponding to GB t agrW or 9119203). This example comprises a driver and a variable resistor. The driver includes a driving pulley rotated by a motor, a plurality of driven pulleys, and a belt stretched between the driving pulley and the driven pulleys. The pulleys have gear portions formed thereon and the belt has teeth formed thereon to be meshed with the gear portions. On the other hand, the variable resistor includes a resistance board with a resistance material film printed on its surface, and a movable member provided with a slider held in slide contact with the resistance material film. The movable member comprises a slider receiver and a lever. A groove consisted of concave and convex portions is formed in an inner wall surface of the slider receiver. The movable member is fixed to the belt by inserting the belt into the groove and meshing the teeth of the belt with the concave and convex portions of the groove. Thus, when fixing the movable member to the belt, the belt must be inserted into the groove formed in the slider receiver. However, because the belt is made of a highly flexible material such as rubber, there is a problem that assembling efficiency is poor. Also, because the holding force between the movable member and the belt depends upon only the meshing between the concave and convex portions of the groove and the teeth of the belt, the number of the concave portions must be increased to strengthen the holding force. This results in the problem of needing the large groove length and impeding a reduction in the size.

The present invention has been made in view of the state of prior art as set forth above.

An object of the present invention is to provide a motor-driven sliding variable resistor with which a driven pulley can be simply supported on a frame in a rotatable manner.

According to the present invention there is provided a motor-driven sliding variable resistor comprising a driving pulley rotated by a motor, a driven pulley rotatably supported on a frame and disposed in oppositely spaced relation to the driving pulley with a predetermined distance therebetween, a belt stretched between the driving pulley and the driven pulley, a movable member fixed to part of the belt, a slider piece held on the movable member, and a resistance board held in slide contact with the slider piece, a resistance value of the variable resistor being adjusted depending on change in the slide contact position between the slider piece and the resistance board upon reciprocal movement of the movable member, wherein the frame includes a first receiver plate having an elongate hole and a second receiver plate having an engagement hole, these plates being disposed in opposed relation, an annular groove formed in one end of a support shaft of the driven pulley is fitted to an edge of the elongate hole to grip the first receiver plate, and the other end of the support shaft if inserted to the engagement hole.

In the foregoing arrangement, the engagement hole of the second receiver plate and the elongate hole of the first receiver plate may be extended in the same direction and disposed in opposed relation.

In the motor-driven sliding variable resistor according to the invention wherein an annular groove formed in one end of a support shaft of the driven pulley is fitted to an edge of an elongate hole of the first receiver plate, the other end of the support shaft is inserted to the engagement hole of the second receiver plate, and thereafter the belt is stretched between the driving pulley and the driven pulley, the driven pulley is pulled toward the driving pulley by the tension of the belt so that the support shaft of the driven pulley is locked to innermost edges of the elongate hole and the engagement hole. In this condition, since the annular groove of the support shaft grips the first receiver plate around the elongate hole and is urged by the tension of the belt toward the innermost edge of the elongate hole, the support shaft is surely prevented from slipping from the elongate hole.

Also, with the arrangement that the engagement hole of the second receiver plate and the elongate hole of the first receiver plate are extended in the same direction and disposed in opposed relation, the support shaft can be fitted in place by sliding it to the innermost edges of the elongate hole and the engagement hole from laterally of the first and second receiver plates, and the assembling work is further simplified.

An embodiment of the present invention will be described below with reference to the accompanying drawings, in which; Figure 1 is an exploded perspective view showing the overall construction of a motor-driven sliding variable resistor according to one embodiment of the present invention;- Figure 2 is a perspective view of a motor-driven unit; Figure 3 is an exploded perspective view showing the support structure of a driving pulley; Figure 4 is an explanatory view showing the positional relationship between the driving pulley and a guide member; Figure 5 is a sectional view showing a mounted condition of the guide member; Figure 6 is an exploded perspective view showing the support structure of a driven pulley; Figure 7 is a sectional view showing a mounted condition of the driven pulley; Figure 8 is an exploded perspective view of a movable member; Figure 9 is a side view of a lever; Figure 10 is a plan view of a slider receiver; Figure 11 is a rear view of the slider receiver; Figure 12 is a plan view showing a condition halfway the step of attaching the movable member to the belt; Figure 13 is a sectional view taken along line XIII - XIII in Figure 12; Figure 14 is a plan view showing a condition after the step of attaching the movable member to the belt; and Figure 15 is a sectional view taken along line XV - XV in Figure 14.

The motor driven sliding variable resistor of this embodiment mainly comprises a motor-driven unit 1 shown in Figure 2, a pair of covers 2 for covering front, rear and bottom surfaces of the motor-driven unit 1, a pair of Page 8 follows holders 3 for covering left and right side surfaces of the motor-driven unit 1, a face plate 4 for covering an upper surface of the motor-driven unit 1, and a resistance board 5 disposed in opposed relation to the motor-driven unit 1. On the surface of the resistance board 5, there are formed a resistance material film and a current collector (both not shown) which are held in slide contact with slider pieces described later. Also, positioning holes 5a are bored in both side edges of the resistance board 5. On the other hand, a pair of positioning grooves 2a extending in the lengthwise direction are formed by ledge on an inner surface of the cover 2. The resistance board 5 is inserted and held between the pair of positioning grooves 2a.

As shown in Fig. 2, the motor-driven unit 1 comprises a metal-made frame 6, a motor 7 fixed to one end of the frame 6, a driving pulley 8 fixed to a rotary shaft of the motor 7, a guide member 9 surrounding the driving pulley 8, a driven pulley 10 rotatably supported on the other end of the frame 6, a belt 11 stretched between the driving pulley 8 and the driven pulley 10 in a relatively slack state, a pair of guide shafts 12, 13 bridging between both the ends of the frame 6, and a movable member 14 capable of reciprocally moving along the pair of guide shafts 12, 13. These members are built up into one unit through the assembling steps described later.

The frame 6 is formed with a pair of legs 15 having positioning projections 15a, a second receiver plate 16 (Fig. 6) bent at a right angle from an upper end of the leg 15 dn the left side in Fig. 2, a first receiver plate 17 extending in opposed relation to the second receiver plate 16 with a predetermined spacing therebetween, a support plate 18 (Fig. 3) bent substantially at a right angle from an upper end of the leg 15 on the right side in Fig. 2, a pair of erect lugs 19 bent at a right angle respectively from the first receiver plate 17 and the support plate 18, etc. The frame 6 including the above bent portions is formed of a single metal plate by pressing. Slots 15b, 19a are formed respectively in the legs 15 and the erect lugs 19, and the guide shafts 12, 13 are fixed to the frame 6 by caulking open ends of the slots 15b, 19a. Also, the positioning projections 15a are engaged in the positioning holes Sa of the resistance board 5. By so restricting the resistance board 5 three-dimensionally in cooperation of the positioning grooves 2a of the cover 2 and the positioning projections 15a of the legs 15, the resistance board 5 is fixed to the frame 6 with no need of screws, caulking or other fixing means.

As shown in Figs. 3 to 5, a circular through hole 18a and a rectangular restricting hole 18b are formed in the support plate 18 of the frame 6, and a latch hole 19b is formed in the erect lug 19 on the right side in Fig. 2. The motor 7 is fixedly screwed to a lower surface of the support plate 18 such that a rotary shaft 7a of the motor 7 penetrates the through hole 18e and projects above the support plate 18. Here, the support plate 18 is bent at an angle a little smaller than the right angle relative to the leg 15. As a result, the rotary shaft 7a of the motor 7 attached to the support plate 18 is inclined outward a predetermined angle relative to the line perpendicular to the guide shafts 12, 13 (the angle of inclination is 1 30' in this embodiment). On the other hand, a gear-portion 8a is formed on an outer peripheral surface of the driving pulley 8 and a flange 8b is fcrmed at a lower end of the gear portion 8a. The driving pulley 8 is fixedly pressfitted over the rotary shaft 7a of the motor 7. A shaft hole of the driving pulley 8 and the rotary shaft 7a are each formed to have a D-shaped cross-section. This D-shaped cross-section ensures that the rotation of the rotary shaft 7a is surely transmitted to the driving pulley 8. As mentioned above, the belt 11 is wound over the driving pulley 8 and the teeth 11a formed on the inner peripheral surface of the belt 11 are meshed with the gear portion 8a of the driving pulley 8. Also, the rotary shaft 7a of the motor 7 is slightly inclined outward relative to the line perpendicular to the guide shafts 12, 13. Therefore, when the motor 7 is operated to rotate the driving pulley 8, the belt 11 wound over the driving pulley 8 is always subject to the force tending to bias the belt 11 down toward the flange 8b. Thus, the belt 11 can be surely prevented from slipping off from the driving pulley 8 just by forming the flange 8b at only the lower end of the driving pulley 8.

The guide member 9 is in the form of a box being open at the bottom and has a hole 9a bored in its upper surface.

The guide member 9 also has a first snap pawl 9b formed in one of opposite sides and extending downward from the upper surface, and a second snap pawl 9c formed in the other side and extending upward from the upper surface. Inside the first snap pawl 9b, there is formed a suspending lug 9d which is a little shorter than the first snap pawl 9b.

Here, the width of the first snap pawl 9b is set to be a little smaller than that of the restricting hole 18b in the support plate 18 and, similarly, the width of the second snap pawl 9c is set to be a little smaller than that of the latch hole l9b in the erect lug 19. Furthermore, the guide member 9 includes a pair of restricting portions 9e formed therein to face each other with the first snap pawl 9b therebetween, and opposing inner surfaces of the restricting portions 9e are curved as shown.

The guide member 9 thus constructed is put onto the driving pulley 8 and the belt 11 wound over the driving pulley 8, and is rotatably mounted on the support plate 18 of the frame 6 with the rotary shaft 7a projecting upward through the hole 9a, as shown in Fig. 5, by snap-fitting the first snap pawl 9b in the restricting hole 18b and the second snap pawl 2c in the latch hole l9b, respectively. In this condition, since the first and second snap pawls 9b, 9c are movable respectively in the restricting hole 18b and the latch hole 19b to some extent in the widthwise direction, the guide member 9 is rotatable through a small angle about the hole 9a. Also, in the condition where the belt 11 is stretched in a somewhat slack state from the driving pulley 8 toward the driven pulley 10 while extending through the gaps between the first snap pawl 9b and both the restricting portions 9e, the belt contacts both the restricting portions 9e near the exit of the guide member 9. Thus, tots the restricting portions 9e push the belt 11 inward so that the teeth 11a of the belt 11 are surely held in mesh with the gear portion 8a of the driving pulley 8. In this respect, because of the guide member 9 being rotatably mounted on the support plate 18, if the belt 11 is forced to strongly press against one of the restricting portions 9e due to eccentric rotation of the driving pulley 8 or deformation of the belt 11, for example, the guide member 9 is rotated following such a movement of the belt 11 to keep the contact between the belt 11 and both the restricting portions 9e in a satisfactory condition. Further, as shown in Fig. 4, a gap is maintained between a portion of the belt 11 going round the driving pulley 8 while meshing with it and the inner surface of the guide member 9 so as to keep them away from each other. A gap size a in the position where the spacing between the inner jells of the guide member has the minimum width is set to be smaller than the tooth height b of the teeth ila of the belt 11 (i.e., a < b).

As shown in Fig. 6, the first receiver plate 17 of the frame 6 is formed with an elongate hole 17a extending toward the support plate 18, and the second receiver plate 16 is formed with an elongate engagement hole 16a in opposed relation to the elongate hole 17a. On the other hand, a gear portion 10a is formed on an outer peripheral surface of the driven pulley 10, and a support shaft 20 having an annular groove 20a formed in its upper peripheral surface is inserted through a shaft hole lOb of the driven pulley 10.

As shown in Fig. 7, the support shaft 20 is inserted deeply into the elongate hole 17a to such an extent that the annular groove 20 fits to an innermost edge of the elongate hole 17a and grips the first receiver plate 17, and also that a portion of the support shaft 20 projecting downward below the driven pulley 10 abuts against an innermost edge of the engagement hole 16a. The driven pulley 10 is rotatably supported on the support shaft 20. At the time of forcibly sliding the support shaft 20 to the innermost edge of the elongate hole 17a and the engagement hole 16a, the support shaft 20 can be easily moved and locked to the frame 6 by winding the belt 11 over the driven pulley 10 in advance and then utilizing the tension of the belt 11.

As shown in Fig. 8, the movable member 14 comprises a lever 21 made of metal and a slider receiver 22 made of synthetic resin, these lever 21 and slider receiver 22 being joined together into one unit as described later. As also shown in Fig. 9, the lever 21 is provided with a knob 21a, a guide hole 21b through which the face plate 4 is inserted, a tubular portion 21c through which the guide shaft 12 is inserted, and a suspending leg 21d vertically extending downward from the tubular portion 21c. Sliding tubular members 23 made of synthetic resin are snap-fitted into both ends of the tubular portion 21c. The suspending leg 21d has an L-shaped bent portion 21e, a guide projection 21f and a support projection 21g all formed on its front surface, and a latch pawl 21h and a stepped stopper 21i both-formed on its rear surface. A first slider piece 24 is press-fitted to the support projection 21g.

As shown in Figs. 10 and 11, the slider receiver 22 is provided with a frame portion 22b having an accommodation hole 22a formed therein, a protrudent lug 22c extending upward from the frame portion 22b, and an annular portion 22d through which the guide shaft 13 is inserted. A stepped latch groove 22e is formed in an inner wall of the frame portion 22b. The protrudent lug 22c includes a toothed portion 22f formed in its upper end and consisted of concavities and convexities having the same configuration as the teeth lia of the belt 11, and a guide groove 22g formed in its rear surface. Further, a plurality of bosses 22h are formed on a front surface of the frame portion 22b, and the second and third slider pieces 25, 26 are fixed to the bosses 22h by caulking.

The movable member 14 thus constructed is fixed to the belt 11 as mentioned before. To this end, as shown in Figs.

12 and 13, the suspending leg 21d of the lever 21 is first inserted into the accommodation hole 22a of the slider receiver 22 while the protrudent lug 22c of the slider receiver 22 is moved relatively upward. At this time, the support projection 21g of the lever 21 abuts against one side surface of the protrudent lug 22c, and the guide projection 21f engagesin the guide groove 22g. With the aid of the support projection 21g and the guide projection 21f cooperating with the counterparts, the protrudent lug 22c can be smoothly moved with respect to the lever 21.

Then, when the upper end of the protrudent lug 22c reaches a position where it slightly enters a space embraced by the bent portion 21e, an upper end of the latch groove 22e abuts against a lower end of the stepped stopper 21i. At this position, the force necessary for further moving the protrudent lug 22c is increased and the operation of moving the protrudent lug 22c upward is once stopped there. In this condition, the belt 11 is laterally placed over the upper end of the protrudent lug 22c such that the teeth ila of the belt 11 are meshed with the toothed portion 22f of the protrudent lug 22c. After that, by further moving the protrudent lug 22c upward, the upper end of the protrudent lug 22c entirely enters the space embraced by the bent portion 21e and, therefore, the belt 11 is tightly held between the protrudent lug 22c and the bent portion 21e.

Simultaneously, the latch pawl 21h rides over a raised portion of the latch groove 22e to be snap-fitted in place, whereby the lever 21 and the slider receiver 22 are joined together into one unit.

The assembling steps of the motor-driven sliding variable resistor thus constructed will be described below.

First, the motor 7 is fixedly screwed to the support plate 18 of the frame 6, and the driving pulley 8 is fixedly press-fitted over the rotary shaft 7a of the motor 7. Then, in the condition where the support shaft 20 is inserted through the shaft hole lOb of the driven pulley 10 and the belt 11 is wound over the driven pulley 10, the annular groove 20a of the support shaft 20 is fitted to the edge of the elongate hole 17a and the lower end of the support shaft 20 is inserted to the engagement hole 16a. After that, by winding the belt 11 over the driving pulley 8, the support shaft 20 is forcibly moved to the innermost edges of the elongate hole 17a and the engagement hole 16a by the tension of the belt 11. The driven pulley 10 rotatably supported on the support shaft 20 is thus locked to the elongate hole 17a and the engagement hole 16a. Next, the guide member 9 is fitted over the driving pulley 8. Before or after so fitting the guide member 9, one guide shaft 12 is inserted through the sliding tubular members 23 snap-fitted into the tubular portion 21c of the lever 21, and both ends of the guide shaft 12 are fixed to the slots 19a of the frame 6 by caulking. Subsequently, the slider pieces 25, 26 are fixed to the slider receiver 22 by caulking. The protrudent lug 22c of the slider receiver 22 is then slightly inserted into the space embraced by the bent portion 21e of the lever 21.

In this condition, part of the belt 11 is laterally placed over the upper end of the protrudent lug 22c and the teeth lla of the belt 11 are meshed with the toothed portion 22f of the protrudent lug 22c. After that, the protrudent lug 22c is further moved upward, thereby tightly holding the belt 11 between the protrudent lug 22c and the bent portion 21e, as shown in Figs. 14 and 15. Simultaneously, the latch pawl 21h is snap-fitted beyond the raised portion of the latch groove 22e, whereby the lever 21 and the slider receiver 22 are joined together into one unit and the movable member 14 is fixed to the belt 11. Next, the first slider piece 24 is press-fitted to the support projection 21g for electrical connection between the first slider piece 24 and the lever 21. Further, the other guide shaft 13 is inserted through the annular portion 22d of the slider receiver 22, and both ends of the guide shaft 13 are fixed to the slots 15b of the frame 6 by caulking. As a result, the motor-driven unit 1 shown in Fig. 2 is obtained.

Then, the resistance board 5 is inserted between the positioning grooves 2a of one cover 2, and the motor-driven unit 1 is put on the one cover 2 such that the positioning projections 15a of the frame 6 fit respectively to the positioning holes 5a of the resistance board 5. At the same time, the other cover 2 is put on the opposite. side of the motcr-driven unit 1, following which both the covers 2 are fixedly screwed to each other. In this condition, while the resistance board 5 and most parts of the motor-driven unit 1 are housed within both the covers 2, the knob 21a and the guide hole 21b of the lever 21 are projecting from an upper gap-between both the covers 2. Thereafter, the sace plate 4 is inserted through the guide hole 21b to cover the upper gap between both the covers 2. Finally, the holders 3 are fixedly screwed to the left and right side surfaces of both the covers 2 for simultaneously securing both ends of the face plate 4 to the holders 3. The assembly of the motordriven sliding variable resistor is thereby completed.

When disassembling the motor-driven sliding variable resistor for replacement of the resistance board 5, the disassembly is performed by following steps which are a reversal of the assembling steps explained above. At this time, because the resistance board 5 is positioned and held in place by the positioning grooves 2a of the cover 2 and the positioning projections 15a of the frame 6, the resistance board 5 can be removed by simple work of just sliding it along the positioning grooves 2a of the cover 2, with no need of such troublesome operation as prying open the caulked portions or loosening the screws.

The operation of the motor-driven sliding variable resistor thus constructed wil be described below.

When the motor 7 is rotated forward or backward by an external signal, the driving pulley 8 is rotated clockwise or counterclockwise depending on the direction of the motor rotation, and the rotating force of the driving pulley 8 is surely transmitted to the belt 11 through the meshing between the gear portion 8a and the teeth gila. Therefore, the belt 11 travels round the driving pulley 8 and the driven pulley 10. With the belt 11 traveling round the pulleys, the movable member 14 fixed to the belt 11 reciprocally moves along both the guide shafts 12, 13 in the direction of arrow P or Q in Fig. 1. The slider pieces 24 to 26 are held on the movable member 14 and the slide contact position of the second and third slider-pieces 25, 26 with the resistance material film (not shown) on the resistance board 5 is changed upon the movement of the movable member 14. Accordingly, a resistance value corresponding to the slide contact position is output. In that process, when the movable member 14 moves to one of the opposite ends of the guide shafts 12, 13, the movable member 14 and the belt 11 can no longer move and hence are stopped there. At this time, if the motor 7 is continuedly energized to rotate, the gear portion 8a of the rotating driving pulley 8 and the teeth 11a of the stopped belt 11 are displaced out of phase, whereupon the belt 11 is pushed and spread outward by the driving pulley 8 into abutment against the inner wall of the guide member 9. Here, because the gap size a between the belt 11 and the guide member 9 in the position where the spacing between the inner walls of the guide member has the minimum width is set to be smaller than the tooth height b of the teeth Ila of the belt 11, the outward spreading of the belt 11 is stopped by the guide member 9 when the belt 11 is pushed and spread outwardly in an amount corresponding to the size a. Therefore, the gear portion 8a of the driving pulley 8 cannot ride over the teeth 11a of the belt 11, and the rotation of both the driving pulley 8 and the motor 7 is forcibly locked by the belt 11. As a result, noisy sounds otherwise caused upon repeated engagement and disengagement between the gear portion 8a and the teeth 11a are eliminated.

Meanwhile, when the operator touches the k-nob 21a of the lever 21 with the finger, the static electricity charged on the operator is output through the first slider piece 24 and the current collector (not shown) formed on the resistance board 5 and held in slide contact with the first slider piece 24. This output signal stops the rotation of the motor 7. Then, when the operator picks the knob 21a and reciprocally moves the movable member 14 in the direction of arrow P or Q in Fig. 1, the belt 11 runs with the rotation of the driving pulley 8 and the driven pulley 10, whereby a resistance value depending on the movement of the movable member 14 is output as with the motor-driven operation described above. In that process, the belt 11 is held in contact with both the restricting portions 9e of the guide member 9, and the position of the belt 11 with respect to the guide member 9 is displaced in a small amount depending on the direction of movement of the movable member 14. But this displacement is absorbed by the rotation of the guide member 9. More specifically, when the movable member 14 moves in the direction coming closer to the driving pulley 8 (i.e., in the direction of arrow P), the belt 11 between the movable member 14 and the driving pulley 8 is subject to compressive force. On the contrary, when the movable member 14 moves in the direction going away from the driving pulley 8 (i.e., in the direction of arrow Q), the belt 11 between the movable member 14 and the driving pulley 8 is subject to tensile force. However, since the guide member 9 can rotate following such a deformation of the belt 11, the belt 11 always contacts both the restricting portions 9e in a well balanced condition and hence the guide member 9 will not impede the smooth running of the belt 11.

In the foregoing embodiment, the elongate hole 17a and the elongate engagement hole 16a opposing to each other are formed respectively in the first and second receiver plates 17, 16 of the frame 6. But because the engagement hole 16a of the second receiver plate 16 serves just to restrict the movement of the support shaft 20 in the direction toward the driving pulley 8, it may be formed into a circular, rectangular or other desired shape.

Further, in the foregoing embodiment, the belt 11 is held in place by inserting the upper end of the protrudent lug 22c, i.e., a projection, formed on the slider receiver 22 into the space embraced by the bent portion 21e, i.e., a recess, formed on the lever 21 and then tightly fitting the projection and the recess to each other. But the recess and the projection may be reversed in positional relationship.

Thus, a projection formed on the lever 21 may be inserted into a recess formed in the slider receiver 22.

As described hereinabove, according to the invention defined in Claim 1, the guide member is disposed around the belt wound over the driving pulley and the gap smaller than the tooth height of the teeth of the belt is maintained between the guide member and the belt. Therefore, even when the motor is continuedly energized to rotate after the belt has stopped running, the gear portion of the driving pulley cannot ride over the teeth of the belt. As a result, noisy sounds otherwise caused upon repeated engagement and disengagement between the gear portion and the teeth are eliminated. Further, since outward spreading of the belt is restricted by the guide member, it is possible to prevent a reduction in smoothness during the manual operation with no need of increasing the tension of the belt excessively.

Also, since the restricting portion for pressing the belt against the driving pulley is provided in part of the guide member, the teeth of the belt can be more surely meshed with the gear portion of the driving pulley.

Further, since the guide member is provided to be rotatable about the shart parallel to the rotary shaft of the driving pulley, the guide member is rotated depending on displacement of the belt during its running and hence the guide member will not impede the smooth running of the belt during both the manual operation and the motor-driven operation.

According to the invention defined in Claim 4, the driven pulley is rotatably supported on the frame with the structure that the frame includes the first receiver plate havingthe.elongate hole and the second receiver plate having the engagement hole, these plates being disposed in opposed relation, the annular groove is formed in one end of the support shaft of the driven pulley, and the support shaft is forcibly slid to the innermost edges of the elongate hole and the engagement hole by utilizing the tension of the belt. Therefore, as compared with the prior art wherein the support shaft of the driven pulley is fixed to the frame by caulking, assembling efficiency is increased remarkably and the production cost can be cut down.

Also, since the engagement hole of the second receiver plate and the elongate hole of the first receiver plate are extended in the same direction and disposed in opposed relation, the support shaft can be easily inserted into the elongate hole and the engagement hole from laterally of the first and second receiver plates, and the assembling work can be further simplified.

According to the invention defined in Claim 6, since the movable member comprises the lever and the slider receiver which are joined together into one unit, and the belt is tightly held between the lever and the slider receiver, the movable member can be simply fixed to the beit. In this respect, since the recess is formed in one of the lever and the slider receiver, the projection fitted to the recess is provided on the other, and the belt is tightly held between the recess and the projection, the belt can be surely held between the inner wall of the recess and the projection. Further, since the toothed portion consisted of concavities and convexities meshing with the teeth of the belt is provided on the projection, the movable member can be surely fixed to the belt with a relatively small size of the recess and the projection and hence the size of the motor-driven sliding variable resistor can be reduced.

Since the holding portions for tightly holding the belt in sandwiching relation and the fixing portions for joining the lever and the slider receiver together into one unit are provided on the lever and the slider receiver at different positions from each other, the belt holding portions are less affected by the force applied to join the lever and the slider receiver together and hence assembling efficiency can be increased. Additionally, since the fixing portions are structured such that the leg provided on one of the lever and the slider receiver is slid into the guide hole formed on the other and then snap-fitted in the guide hole, the leg is guided by the guide hole when joining the lever and the slider receiver together, and hence assembling efficiency can be further increased.

Claims (3)

1. A motor-driven sliding variable resistor comprising: a driving pulley rotated by a motor, a driven pulley rotatably supported on a frame and disposed in oppositely spaced relation to said driving pulley with a predetermined distance therebetween, a belt stretched between said driving pulley and said driven pulley, a movable member fixed to part of said belt, a slider piece held on said movable member, and a resistance board held in slide contact with said slider piece, a resistance value of said variable resistor being adjusted depending on changein the slide contact position between said slider piece and said resistance board upon reciprocal movement of said movable member, wherein: said frame includes a first receiver plate having an elongate hole and a second receiver plate having an engagement hole, these plates being disposed in opposed relation, an annular groove formed in one end of a support shaft of said driven pulley is fitted to an edge of said elongate hole to grip said first receiver plate, and the other end of said support shaft is inserted to said engagement hole.

2. A motor-driven sliding variable resistor according to Claim 1, wherein said engagement hole and said elongate hole are extended in the same direction and disposed in opposed relation.

3. A motor-driven sliding variable resistor as claimed in Claim 1 and substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.