DE7709970U1 - Plastic gearwheel for the transmission of small torques, especially for quartz watches - Google Patents

Abstract

In many precision engineering applications, silent running of gear mechanisms is striven for. Difficulties are encountered in particular in stepwise driving due to the play which is required when the gearwheels are engaged in one another. The invention provides a gearwheel which is suitable for transmitting small torques and makes the production of a silent gear mechanism possible. This plastic gearwheel is composed of an inner pinion (10) and an outer gear rim (12) connected thereto by spiralling arms (16). By this means, a very soft spring action of the outer gear rim (12) is achieved, this permitting play-free and hence silent engagement in the said gear rim. Despite the long, thin arms, production of this gearwheel in one piece by injection moulding is possible in that in the centre of the said arms a thickened portion (24) is provided, in the region of which the injection mould is vented. <IMAGE>

Description

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Plastic gearwheel for the transmission of small torques, especially for quartz watches

The invention relates to a gear made of plastic for the transmission of small torques, in particular for quartz watches, the one-piece from an outer ring gear, from an inner pinion or a socket and from the ring gear and resilient arms connecting the pinion.

In numerous applications of gear drives, it is desirable for the gears to run as quietly as possible or necessary. For example, with quartz watches, especially when used as an alarm clock, a nearly Noiseless running required. This can be achieved with DC or synchronous motors, but what a means a relatively large effort. If, on the other hand, the cheaper stepper motors are used, this generally results Considerable noises in the gear train.

It is already known to reduce this noise by using loose disks as damping rings on the rotor of the stepping motor put on. It is also known that the teeth of the drive gear connected to the rotor and of the in to oil this engaging wheel very heavily. Both measures lead to a reduction in the resulting Sounds. The additional damping rings make the work more complex while the oiling is not stable to aging, so that after a certain

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Runtime the annoying noise occurs again.

In the DT-AS 24 19 189 a gear is described in which noise attenuation is achieved in that radial slots are provided between the individual teeth of the gearwheel. The one from a hard elastic Plastic existing teeth are resilient to a certain extent due to these slots in itself, so that they can give in to the noise-producing shocks during the gradual movement of the drive motor. Disadvantageous is with this solution that only a limited resilient resilience of the teeth can be achieved and in particular that this training of the teeth very easily leads to dividing springs and the shape of the teeth restrictions is subject.

Finally, in this DT-AS 24 19 189 a gear of the type mentioned is also described. With this one The gear is the outer ring gear via three radially extending, resilient arms with the central bushing tied together. These elastically resilient arms allow a slight elastic yield of the outer ring gear in the circumferential direction to avoid the shocks in the step-by-step movement of the drive motor.

However, this known gear cannot sufficiently dampen noise cause, since the hard-elastic plastics usually used for gears from manufacturing

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For technical reasons, a thickness of the resilient arms of 0.2 to 0.3 mm cannot be fallen below. The usage other, softer plastics, which would lead to better noise damping, is not possible because this Plastics worsen the sliding properties of the gear.

The invention has for its object to improve a gear of the type mentioned in such a way that in terms of production engineering a simple way a gear can be produced, whose spring properties between the inner pinion and outer ring gear regardless of the plastic material used can be selected according to the requirements for torque transmission and noise damping.

This object is achieved according to the invention in that in a gear of the type mentioned at least two arms substantially spiral from the pinion to Run the gear rim and have a thickening approximately in the middle of their length.

The arms expediently have thickened injection points at their attachment points on the pinion and the ring gear on.

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Due to their spiral-shaped training, they have elastically resilient arms a much greater length than was the case with the known gear. These long length leads to a significantly softer spring force between the inner pinion and the outer ring gear. The shocks generated by a stepper motor when driven can thus be completely absorbed, so that optimum noise attenuation is obtained.

The considerable extension of the resilient arms compared to the known gears is made possible by that the arms have a thickening in their middle. The plastic material is used in the manufacture of the gear from the two attachment points of the arms to the. outer ring gear or inner pinion injected, and is thus pressed into the thin spring arms from both sides at the same time. In the area of the thickening is while a vent for the injection mold is provided so that the plasticized plastic material also with very thin design of the spring arms can penetrate unhindered from both sides up to this thickening.

The thickening also causes that of both Sides flowing-in plastic material in the area of the thickening flow together completely without air inclusions so that no fractional

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Fahr exists.

Finally, the thickening allows the finished injection-molded part to be easily ejected from the mold without that there is a risk of damage to the thin spring arms. The ejectors cannot only work on the ring gear and attack the inner pinion, but a point ejector can also attack the thickening of the spring arms, so that the spring arms even with great length and low strength can be reliably and easily ejected from the casting tool.

The manufacturing method according to the invention thus makes it possible to provide any number of spring arms and the length and strength of the spring arms can be freely selected.

In addition, according to the invention by the production method no restriction imposed on the choice of plastics used for the gear. The choice of material can therefore only be used with regard to the properties that are favorable for the gear, such as sliding properties and wear resistance. In particular, the usual hard-elastic thermoplastics such as polyacetal, polyoxymethylene or polyamide.

The number, length and strength of the spring arms are not restricted by the manufacturing process.

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These sizes can be selected taking into account the plastic material used so that the optimal Spring properties in relation to the torque to be transmitted and the desired noise damping result.

Since in the gear wheel according to the invention, the entire noise attenuation is achieved by the elastically resilient arms can, the noise attenuation does not have to be taken into account when shaping the teeth. In particular, the teeth be designed to interlock exactly and free of play.

The spirally extending arms are expediently designed in the shape of an arc of a circle. The circular arc shape is particularly suitable because the required injection molding tools in a simple manner by spark erosion with easily manufacturable arc-shaped electrodes can be generated.

The arms expediently have the shape of a semicircle, the radius of which is the mean value between the inner radius of the Corresponds to the ring gear and the outer radius of the pinion. The arms are then each at diametrically opposite one another Points molded onto the pinion and the ring gear and run eccentrically to the gear.

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Of course, the gear wheel according to the invention can not only be used for the gear train of quartz watches, but is suitable for all applications in which small torques are transmitted at low speeds and a low noise level is required. Another example is the drive of film cameras called.

In the following, the invention will be explained using an exemplary embodiment with reference to the accompanying drawings explained in more detail. Show it:

Fig. 1 - a plan view of a gear according to the invention and

Fig. 2 - the gear of Fig. 1 in axial section.

The gear shown in the drawing consists of a pinion 1o, which sits on an axis, not shown. Of course, instead of the pinion 1o, a bushing can also be provided with which the gearwheel is non-rotatable sitting on a wave.

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The pinion 1o is surrounded by a concentric ring gear 12, the teeth 14 shaped in any way with any: Has division.

The pinion 1o is with the ring gear 12 preferably by two or three, in the example shown by three Arms 16 connected. The arms 16 have a thickness of 0.15 mm, for example. The arms 16 have the shape of a semicircular arc, whose radius is the mean value between the inner radius of the ring gear 12 and the outer radius of the Pinion corresponds to 1o. The arms 16 are arranged at equal angular intervals and are each opposite at diametx "al Molded points on the pinion and the ring gear.

The points 18 at which the arms 16 attach to the pinion 1o, and the points 2o, at which the arms 16 attach to the ring gear 12, are thickened and correspond to the Injection channels of the injection mold. Further thickened injection points 22 are around the circumference if necessary of the ring gear provided distributed.

A thickening 24 is provided in the middle of the arms 16.

In the following, the manufacturing method for the gear of Figures 1 and 2 will be explained.

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The gear is made in one piece in a plastic injection molding process manufactured. The depressions of the injection molding tool corresponding to the thin arms 16 are made with the aid of a Arc-shaped electrode made by spark erosion.

The sprues of the injection mold are located at the attachment points 18 and 2o of the arms ί 6 and at the points 22 of the ring gear 12. In the area of the thickenings 24, ventilation channels are provided in the injection mold.

The plasticized plastic material is at the gate points 18, 2o and 22 injected into the mold. First, the pinion 1o and the ring gear 12 are filled. Once this Moldings are filled, the plastic material is under high pressure pressed simultaneously from both sides from points 18 and 2o into the thin spring arms 16. Through the Ventilation channels in the area of the thickened areas 24 allow the air to escape from the spring arms 16. The plastic material can therefore flow unhindered and without air inclusions from both sides up to the thickening 24, where it flows together and unites completely.

The finished injection molded part is ejected from the tool by means of ring ejectors that are attached to the pinion 1o and act on the ring gear 12, as well as by point ejectors that act on the thickenings 24.

Claims (6)

Protection claims Gear is made of plastic for the transmission of small torques, in particular for quartz watches ^ integrally composed of an outer ring gear, an inner gear or a socket and from the ring gear and the pinion • ^r genetic Denden resilient arms, characterized in that at least two arms ( 16) run essentially spirally from the pinion (1o) to the ring gear (12) and have a thickening (24) approximately in the middle of their length. 2. Gear according to claim 1, characterized in that the arms (16) at their attachment points on the pinion (1o) and the toothed ring (12) have thickened injection points (18 or 2o). 3. Gear according to claim 1 or 2, characterized in that that the arms (16) are formed in the shape of a circular arc. Postal check account: Karlsruhe 76979 * 754 Bank account: Deutsche Bank AQ Vllllngen (BLZ 69470039) 146332 -2 4. Gear according to claim 3, characterized in that the arms (16) have the shape of a semicircle, whose radius is the mean value between the inner radius of the ring gear (12) and the outer radius of the pinion (1o) corresponds. 5- gear wheel according to one of the preceding claims, characterized characterized in that it consists of a thermoplastic customary for gear manufacturing, in particular Polyacetal or polyamide. 6. Gear according to one of the preceding claims, characterized in that the arms (16) have a thickness of less than 0.2 mm.