Patent Description:
The technical problem is to configure a belt pulley of a plastic material with as low weight as possible and with mechanical properties that will prevent deformation of the belt pulley in both radial and axial directions.

Laundry washing in a washing machine is carried out in a drum rotatably arranged within a washing tub of the washing machine. The drum rotates by means of a drive assembly comprising a shaft fixed to the drum and mounted in bearings, a belt pulley mounted on the shaft, an electric motor with a drive shaft, and a drive belt which transmits the revolutions of the electric motor via the belt pulley to the drum.

Belt pulleys, especially in high-spin washing machines, e.g. <NUM> rpm or more, and a large drum volume, e.g. <NUM> and more, are usually made of metallic materials due to high mechanical loads. A drawback of these belt pulleys is mainly their weight and also a problematic manufacturing. Therefore, attempts are being made to replace metal belt pulleys with belt pulleys made of a plastic material, which generally have poorer mechanical properties. Belt pulleys made of plastic can get deformed both during operation and at rest when they are subjected to a belt preload force and forces associated with a sufficiently strong connection of the belt pulley to the drum shaft. It is attempted to compensate for poorer strength properties of the material in particular by a more suitable shape of the connecting ribs between the hub and the tread surface of the belt pulley, on which the drive belt rests.

Thus, patent application <CIT> describes a belt pulley made of a plastic material and having a similar geometry characteristic of metal belt pulleys. The belt pulley has a central hub, an outer circumference for receiving the belt, and a plurality of radially connecting ribs. To ensure the mechanical properties that transmit forces during standstill and operation, the ribs have large cross sections, which consequently means a large weight of the belt pulley or a large amount of material used and thus higher manufacturing costs.

Patent application <CIT> describes a belt pulley made of a plastic material, which is made of a plurality of radial ribs that are circularly connected from the central hub to the outer circumference for receiving the belt. Even with such a technical solution, the sufficient stiffness of the belt pulley is provided by the ribs of larger cross-section. As a result, such a belt pulley has a large weight and thus a huge requirement regarding the material used.

Patent application <CIT> Aldescribes a belt pulley of a plastic material comprising a central hub, an outer circumference for receiving the belt, an inner section connecting several radial ribs, and an outer section containing several circumferential ribs inclined with respect to the radial ribs. Such shape of the belt pulley requires the use of special plastic materials having an elastic modulus exceeding <NUM>,<NUM> MPa. For this purpose, a considerable proportion of mineral or synthetic filler is added to the base material, which is precipitated at the front of the casting flow when the melt solidifies, creating weaker belt pulley spots in locations where the two flows meet, especially on the outer circumference. The large number of ribs that support the belt pulley circumference reduces the impact of weaker spots and at the same time ensures small radial deformations of the circumference and thus stable operation of the belt without periodic excitation. A disadvantage of such a belt pulley is that the ribs in the first and outer section are designed with a small cross section, which causes the belt pulley to bend in a direction outside the plane of action of radial forces. This problem can be solved with a larger number of ribs or a larger dimension of same, which increases the weight / material consumption in the manufacture.

<CIT> discloses a plastic belt pulley being constructed as a plastic plate which is reinforced with radial ribs in an inner intermediate section as well as in the outer intermediate section.

To ensure a stable position and proper operation of the belt both during operation and at rest, the belt is mounted on the belt pulley with a certain preload force. As a result, the belt pulley is loaded with a constant surface pressure on the circumferential surface on which the belt rests, which causes radial forces on the belt pulley. Static loading of the belt pulley can cause permanent deformations and thus creep of the material at elevated temperatures, especially during the storage and transport phase, due to high stresses. During operation, the belt pulley can bend outside the plane of action of radial forces due to the belt preload forces. This type of bending can cause the shift of the belt on the tread of the belt pulley and in extreme cases even falling-out of the belt.

The problem of belt pulley bending is most often solved with a larger number of ribs or with ribs of larger cross-section. Such a solution brings about an increased weight of the belt pulley due to the larger amount of material used, which in turn results in a higher weight and also higher manufacturing costs.

The common disadvantage of all these solutions is that they do not allow for an optimal distribution of internal stresses in the belt pulley and thus the deformations resulting from the manufacture of the belt pulley by injection moulding.

The technical problem is solved by a belt pulley made of a plastic material and having the characteristics defined in the independent claim <NUM>.

A belt pulley of a washing machine drum, particularly of a household washing machine, made of a plastic material, comprises a central part of the belt pulley formed as a hub, with which the belt pulley is fastened to a drum shaft; a belt pulley circumference arranged concentrically with respect to the hub and defining the belt pulley and receiving the belt on its outer surface; a substantially circular dividing rib arranged concentrically with respect to the circumference of the belt pulley so as to divide the belt pulley into two concentric sections, that is a first, inner section closer to the hub, and a second, outer section delimited by the circumference of the belt pulley, wherein a respective plurality of reinforcing ribs are arranged in the inner and outer sections, wherein the dividing rib is arranged on a line of a polygon having the centre in the belt pulley axis and exhibiting corners, into which, from a side facing the hub, a portion of the outermost reinforcing ribs lead, said ribs being part of a plurality of internal reinforcing ribs, wherein at least a portion of these reinforcing ribs leading into the corners of the dividing rib run, extend along a line of polygons, which are in the shape of hexagons, which are radially arranged in the inner section and in contact with each other along one of their sides, wherein, from at least one said corner of the dividing rib on the side facing the circumference, at least one arc-shaped reinforcing rib, preferably two arc-shaped reinforcing ribs, of the outer section originate, forming a pair of mirrored arc-shaped reinforcing ribs and their free ends contacting the belt pulley circumference.

The reinforcing ribs of the inner section, which run along the line of the polygons, form honeycombs, thus achieving greater radial strength of the inner section.

A pair of arc-shaped reinforcing ribs of the outer section may have an axial mirror-assigned second pair of arc-shaped ribs.

The reinforcing ribs of the outer intermediate section are arc-shaped, the size of the radius of the arc of the reinforcing rib being substantially equal to or larger than the radius of the belt pulley.

The reinforcing ribs in the inner section are arranged radially so as to form honeycombs. In this case, the honeycomb cells, which are symmetrical with respect to the belt pulley axis, are of the same dimensions. The size of the cells increases with their distance from said axis.

The high stiffness and thus the small deformations of the honeycomb under loads are also reflected in the smaller deformations of the intermediate dividing rib, the second intermediate section and also the circumference of the belt pulley itself. Radial deformations are more uniform. The result of minor radial deformations is minimized periodic excitation of the belt, which results in a more even running of the belt along the circumference of the belt pulley or the tread, which prevents the possibility of the belt falling out of the belt pulley.

Stresses and consequently deformations also occur during the cooling of the plastic material in the production of the belt pulley. The arc-shaped configuration of the reinforcing ribs contributes to the reduction in deformations, especially of the circumference of the belt pulley. When cooling, the material shrinks. Due to the arc-shaped configuration of the reinforcing ribs in the outer intermediate section, these ribs shrink or deform in such a way as that their curvature is reduced or their radius is increased. The stresses resulting from the shrinkage of the arc-shaped reinforcing ribs are transmitted to a much lesser extent to the circumference of the belt pulley and consequently the local deformations of the circumference of the belt pulley in the radial direction are greatly reduced in comparison to those that would be present in the case of straight reinforcing ribs. Due to a relatively large number of reinforcing ribs, the relative deformations of the circumference of the belt pulley are further reduced.

A further advantage of the belt pulley according to the invention is demonstrated in the injection moulding production process. In the process of injection moulding of products of circular shapes similar to that of a belt pulley, where it is necessary to maintain a centric or round shape on the circumference, the liquid material is usually fed into the mould in one place in the axis or in its immediate vicinity, wherein a unified flow of the liquid material into all parts of the mould should be provided for. The configuration of the reinforcing ribs and thus the entire belt pulley according to the invention is such that the liquid material fed in the axial direction flows throughout the flow step towards the circumference substantially in the radial direction all the way to the outer circumference of the belt pulley. Individual strands of the melt join together in the outer circumference of the belt pulley. Fillers that are present in the material and that represent the weakening spots are eliminated at the junction spots. The smaller the number of junctions, the more fillers are eliminated at such a spot. The aim is therefore to increase the number of contact points. This is achieved with the belt pulley according to the invention, because there is a large number of reinforcing ribs.

The belt pulley of the invention will be described in more detail below by way of an embodiment and drawings representing in.

A washing machine comprises a tub assembly <NUM> comprising a washing tub <NUM> and a drum <NUM> rotatably arranged therein. The drum <NUM> is rotated by a drive assembly <NUM> comprising a shaft <NUM> fastened to the drum <NUM> and mounted on bearings, which allows for a free rotation of the drum <NUM> around its axis, a belt pulley <NUM> arranged on the shaft <NUM>, an electric motor <NUM> with a drive shaft and a drive belt <NUM> that connects the drive shaft of the electric motor <NUM> and the belt pulley <NUM>, thus transferring the rotation from the electric motor <NUM> to the washing machine drum <NUM>.

The belt pulley <NUM> of the washing machine drum comprises.

The dividing rib <NUM> is arranged along the line of the polygon <NUM> with the centre in the axis of the belt pulley <NUM> and exhibits corners <NUM>. A plurality of reinforcing ribs <NUM> are arranged in the inner section <NUM> of the belt pulley <NUM>. A part of the reinforcing ribs <NUM> which are closer to the dividing rib <NUM> or furthest from the hub <NUM> converges into the corners <NUM> of the dividing rib <NUM> on the side of the corner facing the hub <NUM>. In the embodiment, two ribs <NUM> of the inner section converge in each corner <NUM>, which is not a mandatory feature. The convergence of the reinforcing ribs <NUM> can also be carried out in only a certain limited number of corners <NUM>, as well as it is not mandatory for the two reinforcing ribs <NUM> most distant from the hub to converge into a single corner <NUM>.

At least part of the reinforcing ribs <NUM> converging into the corners <NUM>, in the embodiment all the ribs <NUM> converging into the corners <NUM>, run along a line of polygons <NUM>, which are in the shape of hexagons. The polygons <NUM> are radially arranged in the inner section <NUM> and in contact with each other along one of their sides. The ribs <NUM> of the inner section <NUM> running along the line of the polygons <NUM> form a honeycomb <NUM>. The size of the cells of the honeycomb <NUM> increases with their distance from said axis.

A corner <NUM> of the dividing rib <NUM> forms an originating point <NUM> from which at least one arc-shaped rib <NUM> of the outer section <NUM> emerges, preferably two arc-shaped ribs <NUM>, <NUM>' so as to form a pair <NUM> of mirror arc-shaped reinforcing ribs <NUM>, <NUM>' and their free ends contact the outer circumference <NUM> of belt pulley <NUM>.

In the embodiment, a second pair <NUM>' of arc-shaped reinforcing ribs <NUM>, <NUM>' is axially mirrored to each pair <NUM> of arc-shaped reinforcing ribs <NUM>, <NUM>' of the outer section <NUM>, which is not a mandatory feature.

The number of pairs <NUM> of arc-shaped reinforcing ribs <NUM>, <NUM>' in the outer section <NUM> is equal to the number of corners <NUM> of the dividing rib <NUM>. The number of pairs <NUM> of reinforcing ribs <NUM>, <NUM>' depends on the required strength of the belt pulley <NUM> and increases with an increasing requirement for strength.

The ribs <NUM>, <NUM>' of the outer intermediate section <NUM> are arc-shaped, the size of the radius of the arc of the rib <NUM>, <NUM>' being substantially equal to or larger than the radius of the belt pulley <NUM>.

Due to the honeycomb shape, the system of ribs formed by the ribs <NUM> have a high axial and radial stiffness, which ensures high stiffness of the belt pulley <NUM> in the inner section <NUM> under axial and bending loads, thereby reducing the bending deformations of the belt pulley <NUM>. The high stiffness and thus the small deformations of the honeycomb <NUM> are also reflected in the smaller deformations of the dividing rib <NUM>.

The height of the reinforcing ribs <NUM>, that is the dimension of the rib in the direction of the axis of the belt pulley <NUM>, increases towards the central part <NUM> of the belt pulley <NUM>. As the height of the reinforcing ribs <NUM> or the height of the honeycomb <NUM> cell walls increases, a higher strength or stiffness of the belt pulley <NUM> in the central part is achieved, which is required due to the large compressive forces in this area.

Claim 1:
A belt pulley (<NUM>) of a washing machine drum (<NUM>), particularly of a household washing machine, made of a plastic material, which comprises a central part (<NUM>) of the belt pulley formed as a hub, with which the belt pulley (<NUM>) is fastened to a drum shaft (<NUM>); a belt pulley circumference (<NUM>) arranged concentrically with respect to the hub (<NUM>) and receiving a belt (<NUM>) on its outer surface; a substantially circular dividing rib (<NUM>) arranged concentrically with respect to the circumference (<NUM>) of the belt pulley so as to divide the belt pulley (<NUM>) into two concentric sections, a first, inner section (<NUM>) closer to the hub (<NUM>), and a second, outer section (<NUM>) delimited by the circumference (<NUM>) of the belt pulley, wherein a respective plurality of reinforcing ribs (<NUM>, <NUM>, <NUM>') are arranged in the inner (<NUM>) and outer sections (<NUM>), characterized in
that the dividing rib (<NUM>) is arranged on a line of a polygon (<NUM>) having the centre in the belt pulley (<NUM>) axis and exhibiting corners (<NUM>), into which, from a side facing the hub (<NUM>), a portion of the reinforcing ribs (<NUM>) lying the furthest away from the hub (<NUM>) lead, said ribs being part of a plurality of reinforcing ribs (<NUM>) of the inner section (<NUM>), wherein at least a portion of the reinforcing ribs (<NUM>) leading into the corners (<NUM>) of the dividing rib (<NUM>) extend along a line of polygons (<NUM>) which are in the shape of hexagons and radially arranged in the inner section (<NUM>) and in contact with each other along one of their sides,
that from at least one said corner (<NUM>) of the dividing rib (<NUM>) on the side facing the circumference (<NUM>), at least one arc-shaped reinforcing rib (<NUM>) originates, the free end of which contacts the circumference (<NUM>) of the belt pulley (<NUM>).