Patent Description:
As an example of a drive system for an aircraft, a gear has been known in which: a hub is provided on an inner peripheral side and is configured to be coupled to a rotation shaft; a rim is provided on an outer peripheral side and is formed with external teeth; and the hub and the rim are connected by a gear web extending obliquely toward both sides in an axial direction (for example, Non-Patent Document <NUM>). The gear web is configured to support engagement load on the external teeth and transmit the load from the external teeth to the rotation shaft. Suppression of deformation of the gear web when supporting the load makes it possible to provide large tooth contact of the external teeth. This is advantageous in ensuring durability of the external teeth. In Non-Patent Document <NUM>, the gear web is constituted of two divided bodies in the axial direction, and the two divided bodies are coupled with each other by a fastening member, such as a bolt. According to this configuration, the coupling between the two divided bodies forms the gear web extending obliquely toward both sides in the axial direction, thereby achieving high rigidity. As a result, it is possible to suppress deformation of the gear web.

Such a gear as in Non-Patent Document <NUM>, however, has a large number of parts. Thus, such a gear is complicated to assemble, resulting in high manufacturing cost. Additionally, in order to couple the two divided bodies, it is necessary to secure a part where the two members are overlapped, which makes it difficult to reduce the weight.

On the other hand, it is known to couple the upper and lower divided bodies by use of electron beam welding, instead of using a fastening member. This configuration makes it possible to omit such a fastening member to simplify the structure as well as to reduce the weight. However, even where electron beam welding is employed, the structure includes many parts because the structure is still constituted of the two upper and lower divided bodies, and also, the structure requires post-welding processes. Thus, the manufacturing cost is increased due to the increased number of processes.

<CIT> describes a rotor. A boss and rim of a gear are integrated by an arm structure constituted of a plane rib which continuously extends in a circumferential direction. The plane rib is constituted of main ribs arranged intermittently in the circumferential direction and alternately to the inside and outside in the axle direction and a connection rib connecting adjacent main ribs. <CIT> describes a spoke type helical gear. A device is provided with a hub, a plurality of spokes which extend radially from the hub, and a rim joined with the outer end parts of the spokes. Teeth are formed on the outer peripheral surface of the rim. Spokes are inclined between the hub and the rim. <CIT> describes a gear wheel which is indented or corrugated in a circumferential direction. The gear wheel has a gear rim and a gear wheel hub with an axial, concentric gear wheel hub channel, arranged to the axis of rotation of the gear wheel, with an opening. A circular connecting wall extends in the circumferential direction and in a radial direction, which interconnects two gear wheel sections at a distance in the radial direction. The connecting wall is formed wavy or serrated with mountains and valleys arranged next to each other in the circumferential direction. <NPL>, describes a web and spoke design for a wheel. Also illustrated is a corrugated web. <CIT> describes an injection-molded resin bevel gear. The injection-molded resin bevel gear includes a boss, a web, and a teeth section. The boss has an axis hole. The web is roughly disk-shaped spreads from an outer circumference side of the boss in an outward radial direction. The teeth section is positioned on an outward-radial-direction side of the web. The web is configured such that the thicknesses of a base end section, a horizontal beam section, a vertical beam section, and a teeth section connecting section are almost the same (thickness difference of about ±<NUM>%, taking into consideration manufacturing errors and the like).

An object of the present invention is to provide a light-weight and highly-rigid gear that has a reduced number of parts and can be manufactured at low cost.

In accordance with the invention, there is provided a gear as claimed in claim <NUM>.

According to this configuration, the first web and the second web that obliquely extend from the rim to the hub in mutually different directions make it possible to ensure rigidity of the gear. Moreover, the first web pieces and the second web pieces are alternately arranged in the circumferential direction and are located at mutually different circumferential positions. Thus, the gear can be formed in an integrated structure, thereby making it possible to reduce the number of parts as well as to reduce manufacturing cost.

According to the above configuration, spaces are defined at the other side in the axial direction with respect to the first web and at the one side in the axial direction with respect to the second web. This configuration can suppress weight increase in the entire web including the first web and the second web. As a result, it is possible to reduce the weight of the gear. Thus, the above configuration makes it possible to reduce the number of parts as well as to manufacture a light-weight and highly-rigid gear at low cost.

Each of the first web pieces and the second web pieces may have a circumferentially elongated cross section along the circumferential direction. This configuration makes it possible to ensure rigidity against radial load and circumferential load on the first web pieces and the second web pieces.

An entirety of the first web pieces excluding connection portions to the rim may be separated toward the one side in the axial direction with respect to an entirety of the second web pieces excluding connection portions to the rim. According to this configuration, since the first and second web pieces extend obliquely so as to be separated from each other in the axial direction, rigidity of the gear can be ensured.

Where the entirety of the first web pieces is separated toward the one side in the axial direction with respect to the entirety of the second web pieces, the connection portions of the first web pieces to the rim may be axially separated from the connection portions of the second web pieces to the rim. This configuration allows the first web pieces and the second web pieces to be shorter, which makes it possible to suppress weight increase in the first and second webs. As a result, the weight of the gear can be reduced.

The connection portions of the first web pieces to the rim and the connection portions of the second web pieces to the rim may be located at the same position in the axial direction. The expression "the same position" used herein means that their axial positions are exactly the same or are offset by <NUM>% or less of an axial width of the rim. According to this configuration, when viewed from a radial direction, the first web pieces and the second web pieces define triangles each having a vertex at the connection portions to the rim, thereby enhancing the rigidity of the gear.

A maximum value of an axial interval between each of the first web pieces and each of the second web pieces may be <NUM>% or more of the axial width of the rim. According to this configuration, an axial height of a base of a triangle or a trapezoid, which is viewed from the radial direction, is made larger, thereby enhancing the rigidity of the gear.

The first web pieces and the second web pieces that are adjacent in the circumferential direction may be connected by connection walls extending in the axial direction. According to this configuration, the rigidity of the gear can be further enhanced thanks to the connection walls.

The rim may have a flange protruding radially inward, and the flange may be connected with radially outer end portions of the first web pieces and the second web pieces. According to this configuration, since the first web pieces and the second web pieces have radial lengths that are shorter by a length of the flange, an amount of machine processing to these web pieces is reduced. As a result, the manufacturing cost of the gear can be reduced.

The external teeth are in the form of a helical gear or a bevel gear. Although a helical gear or a bevel gear is subjected to a load in an axial direction, a highly rigid gear can sufficiently bear such a load. In the case of a helical gear or a bevel gear, the first web pieces and the second web pieces have different thicknesses from each other in accordance with a magnitude of axial load to be applied. According to this configuration, the thicknesses of the first web pieces and the second web pieces can be varied in accordance with the axial load to be applied from the helical gear or the bevel gear. Thus, it is possible to suppress weight increase while ensuring necessary rigidity.

The present invention will be more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views.

Hereinafter, reference is made to the drawings to describe embodiments. <FIG> shows an example of a gear according to a first embodiment, the gear being applied to a speed reduction device that forms a part of a drive system of a helicopter. A gear <NUM> of this embodiment is rotatably supported by a speed reduction device casing <NUM> via bearings <NUM>. The gear <NUM> is configured to reduce a speed of rotation of an engine (not illustrated) inputted through an input gear <NUM> so as to transmit the rotation to a main shaft <NUM> that is a rotation shaft of a main rotor. The main shaft <NUM> constitutes a rotation shaft coupled to the gear <NUM> in a mutually non-rotatable manner. The rotation shaft rotates about an axial direction C. Hereinafter, the expressions "radial direction" as well as "circumferential direction" refer to a radial direction and a circumferential direction with respect to the axial direction C, respectively, unless specifically indicated otherwise. Similarly, the expressions "inner periphery" as well as "outer periphery" refer to an inner periphery and an outer periphery with respect to the axial direction C, respectively, unless specifically indicated otherwise.

The gear <NUM> shown in <FIG> includes: a hub <NUM> provided on an inner peripheral side and coupled to the main shaft <NUM>; a rim <NUM> provided on an outer peripheral side and formed with external teeth <NUM> to be gear-coupled to the input gear <NUM>; and a gear web <NUM> connecting the hub <NUM> and the rim <NUM>. The rim <NUM> has an annular shape that is concentric to the hub <NUM>.

The hub <NUM> is formed in a cylindrical shape expending in the axial direction C and has a hollow hole 10a. The hub <NUM> is provided so as to be coaxial with the main shaft <NUM> and is formed on the inner peripheral side of the gear <NUM>. The hub <NUM> is rotatably supported on an upper portion (i.e., on one side) and on a lower portion (i.e., on the other side) thereof in the axial direction C, by the speed reduction device casing <NUM> via the bearings <NUM>. In this embodiment, the hollow hole 10a of the hub <NUM> has a larger diameter than an outer diameter of the main shaft <NUM>. The main shaft <NUM> is inserted through the hollow hole 10a of the hub <NUM> and is coupled with an inner peripheral surface of the hub <NUM> through e.g. spline coupling. It should be noted that the hub <NUM> may have a smaller diameter than the outer diameter of the main shaft so that the hub <NUM> can be inserted into the main shaft <NUM> and is coupled with an inner peripheral surface of the main shaft <NUM> through e.g. spline coupling. Alternatively, the main shaft <NUM> and the hub <NUM> may be integrally coupled with each other by e.g. welding.

The rim <NUM> is formed in a cylindrical shape extending in the axial direction C. The rim <NUM> has a smaller axial width (height H) than an axial dimension of the hub <NUM>. The rim <NUM> is formed on the outer peripheral side of the gear <NUM>. That is, the rim <NUM> is formed so as to have a larger diameter than that of the hub <NUM>. In this embodiment, the outer diameter of the rim <NUM> is about <NUM> times as large as that of the hub <NUM>, and the height (axial dimension) of the rim <NUM> is about <NUM>/<NUM> of the height of the hub <NUM>. It should be noted that the size relationship between the rim <NUM> and the hub <NUM> is not limited to this. The rim <NUM> has an outer peripheral surface formed with external teeth <NUM>. The external teeth <NUM> are in the form of a helical gear. It should be noted that forms of the external teeth <NUM> are not limited to this configuration, but are solely limited by the appended claims.

The gear web <NUM> couples the hub <NUM> and the rim <NUM>. The gear web <NUM> supports engagement load on the external teeth <NUM> and transmits the load from the external teeth <NUM> to the hub <NUM>. The gear web <NUM> includes: a first web <NUM> on an upper side which obliquely extends upward (toward the one side in the axial direction C) from the rim <NUM> to the hub <NUM>; and a second web <NUM> on a lower side which obliquely extends downward (toward the other side in the axial direction C) from the rim <NUM> to the hub <NUM>. That is, the first web <NUM> extends upward and radially inward from the rim <NUM> to the hub <NUM>. The second web <NUM> extends downward and radially inward from the rim <NUM> to the hub <NUM>.

The gear web <NUM> has a general shape, i.e., a virtual shape connecting the first web <NUM> and the second web <NUM>, with two circular truncated cones being combined such that bases of the truncated cones are faced to each other. Specifically, each of the bases is a plane that is perpendicular to the axial direction C, and two respective virtual vertexes are located on opposite sides of the axial direction C with the bases being interposed therebetween.

In this embodiment, in the cross section shown in <FIG>, the first web <NUM>, the second web <NUM> and the hub <NUM> cooperate together to define a triangle having a base defined by a circumferential wall of the hub <NUM>. The angle defined by the first web <NUM> and the circumferential wall of the hub <NUM> (the axial direction C) is <NUM>° or smaller. Similarly, the angle defined by the second web <NUM> and the circumferential wall of the hub <NUM> (the axial direction C) is <NUM>° or smaller. In this embodiment, although the first web <NUM> and the second web <NUM> are linearly shaped in the cross section shown in <FIG>, they may be slightly curved in the axial direction C.

<FIG> is a perspective view of the gear <NUM>. <FIG> is a plan view of the gear <NUM> when viewed from the one side of the axial direction C (from above). As shown in <FIG>, the first web <NUM> includes a plurality of first web pieces <NUM> formed so as to be spaced apart in a circumferential direction. The second web <NUM> includes a plurality of second web pieces <NUM> formed so as to be spaced apart in the circumferential direction. There are the same number (<NUM>, in this embodiment) of the first web pieces <NUM> on the upper side and the second web pieces <NUM> on the lower side. The first web pieces <NUM> and the second web pieces <NUM> suppress deformation of the rim <NUM>.

As shown in <FIG>, the first web pieces <NUM> and the second web pieces <NUM> are separated from each other in the axial direction C, except for connection portions to the rim <NUM> on the outer side. Specifically, an entirety of the first web pieces <NUM> on the upper side is separated above with respect to an entirety of the second web pieces <NUM> on the lower side, except for the connection portions to the rim <NUM> on the outer side. The entirety of the second web pieces <NUM> on the lower side is separated below with respect to the entirety of the first web pieces <NUM> on the upper side, except for the connection portions to the rim <NUM> on the outer side.

As shown in <FIG>, the first web pieces <NUM> and the second web pieces <NUM> are alternately arranged in the circumferential direction and are located at mutually different circumferential positions or circumferential portions different to each other. Specifically, the respective first web pieces <NUM> are arranged so as to be separated from each other at intervals S1 in the circumferential direction. The respective second web pieces <NUM> are also arranged so as to be separated from each other at intervals S2 in the circumferential direction. When viewed from the axial direction C, the second web pieces <NUM> are arranged between the adjacent first web pieces <NUM>, <NUM>, and the first web pieces <NUM> are arranged between the adjacent second web pieces <NUM>, <NUM>. In this embodiment, the respective first web pieces <NUM> are arranged at equal intervals in the circumferential direction. Similarly, the respective second web pieces <NUM> are also arranged at equal intervals in the circumferential direction.

Therefore, the gear web <NUM> has a repeated alternate arrangement, along the circumferential direction, of the first web piece <NUM> on the upper side, the second web piece <NUM> on the lower side, the first web piece <NUM> on the upper side, and the second web piece <NUM> on the lower side. In other words, the first web pieces <NUM> and the second web pieces <NUM> are arranged so as to be separated from each other in the axial direction C, except for the connection portions to the rim <NUM> on the outer side, and are provided so as not to overlap with each other when viewed from the axial direction C.

In the gear <NUM> of this embodiment which has such a configuration, as shown in <FIG>, open spaces SP1 opened in the axial direction C are defined below (i.e., on the other side in the axial direction C with respect to) the first web pieces <NUM> on the upper side. Also, open spaces SP2 opened in the axial direction C are defined above (i.e., on the one side in the axial direction C with respect to) the second web pieces <NUM> on the lower side. In other words, the first web pieces <NUM> on the upper side as a whole are exposed through the intervals S2 when viewed from below (i.e., the other side in the axial direction C). The second web pieces <NUM> on the lower side as a whole are exposed through the intervals S1 when viewed from above (i.e., the one side in the axial direction C).

Each of the first web pieces <NUM> and the second web pieces <NUM> has a plate-like flat shape. As shown in <FIG>, each of the first web pieces <NUM> has a curved shape that is elongated in the circumferential direction and slightly projects upward in the cross section along the circumferential direction. Each of the second web pieces <NUM> has a curved shape that slightly projects downward in the cross section along the circumferential direction. The respective web pieces <NUM>, <NUM> have such curved shapes because these web pieces <NUM>, <NUM> form conical surfaces around the hub <NUM>, as can be seen from <FIG>. As long as the first web pieces <NUM> and the second web pieces <NUM> have circumferentially elongated shapes in the cross section along the circumferential direction, their cross-sectional shapes are not limited to the above-mentioned curved shapes and may be, for example, a rectangular shape elongated in the circumferential direction.

Alternatively, the first web pieces <NUM> may have, in the cross section along the circumferential direction, a linearly-shaped lower edge 22d at the other side (the lower side) in the axial direction C and a curved upper edge 22u that slightly projects upward, on the one side (the upper side) in the axial direction C. Similarly, the second web pieces <NUM> may have, in the cross section along the circumferential direction, a linearly-shaped upper edge 24u on the one side (the upper side) in the axial direction C and a curved lower edge 24d that slightly projects downward, on the other side (the lower side) in the axial direction C. It should be noted that the respective first web pieces <NUM> have the same shape, and the respective second web pieces <NUM> also have the same shape.

The first web pieces <NUM> and the second web pieces <NUM> have a thickness t that is about a maximum value at which deformation due to the engagement of the external teeth <NUM> of the rim <NUM> can be suppressed to an acceptable degree. Where the thickness t of the web pieces <NUM>, <NUM> is too large, that leads to increased weight. In contrast, where the thickness t of the first and second web pieces <NUM>, <NUM> is too small, deformation of the rim <NUM> cannot be suppressed due to insufficient rigidity. In this embodiment, the first web pieces <NUM> and the second web pieces <NUM> have mutually different thicknesses t in accordance with axial load applied from the external teeth (helical gear) <NUM>. The external teeth <NUM> of this embodiment are in the form of a helical gear in which load is concentrated to the other side of the axial direction C (to the lower side, in this embodiment). Thus, in this embodiment, the thickness t of the second web pieces <NUM> on the lower side is larger than the thickness t of the first web pieces <NUM> on the upper side.

As shown in <FIG>, the respective first web pieces <NUM> have the same shape, when viewed from the axial direction C. Similarly, the respective second web pieces <NUM> also have the same shape. On the other hand, the first web pieces <NUM> and the second web pieces <NUM> have mutually different shapes, when viewed from the axial direction C. That is, their shapes differ in the intervals S1, S2. In the circumferential direction, the first web pieces <NUM> are formed so as to be wider than the second web pieces <NUM>. In addition, the first web pieces <NUM> and the second web pieces <NUM> have an increasing circumferential width dimension from the hub <NUM> to the rim <NUM> (radially outward), when viewed from the axial direction C. In this embodiment, the adjacent first web pieces <NUM> and second web pieces <NUM> are arranged with substantially no space in the circumferential direction, when viewed from the axial direction C. However, there may be spaces in the circumferential direction between the adjacent first web pieces <NUM> and the second web pieces <NUM>.

As shown in <FIG>, the rim <NUM> includes an annular flange <NUM> that products radially inward. The flange <NUM> is formed in the vicinity of an intermediate part of the rim <NUM> in the axial direction C. It should be noted that the position where the flange <NUM> is formed is not limited to this position and may be suitably selected in accordance with load condition in the gear <NUM>. Although the flange <NUM> extends in the radial direction in this embodiment, the flange may obliquely extend radially inward toward the one side or the other side in the axial direction, or the inclination angle may be changed in the middle. The flange <NUM> has a radially inner end portion 26a to which radially outer end portions 22a of the first web pieces <NUM> and radially outer end portions 24a of the second web pieces <NUM> are connected as shown in <FIG>. The flange <NUM> enhances the rigidity of the rim <NUM>, whereby the flange <NUM> suppresses periodic change in deformation of the rim <NUM> due to difference in rigidity of the first web pieces <NUM> and the second web pieces <NUM>. Thus, stable tooth contact of the external teeth <NUM> can be achieved during rotation of the gear <NUM>.

Preferably, first connection portions <NUM> of the first web pieces <NUM> to the flange <NUM> of the rim <NUM> and second connection portions <NUM> of the second web pieces <NUM> to the flange <NUM> of the rim <NUM> are located at the same position in the axial direction C. The expression "the same position" used herein means that their axial positions are exactly the same or are offset by <NUM>% or less of an axial width (height H) of the rim <NUM>. The first web pieces <NUM> and the second web pieces <NUM>, however, are not necessarily connected to the flange <NUM> at the same position in the axial direction C.

The flange <NUM> may be omitted. In such a case, the radially outer end portions 22a of the first web pieces <NUM> and the radially outer end portions 24a of the second web pieces <NUM> are connected to the inner peripheral surface of the rim <NUM>. Even in this case, it is preferred that the connection portions <NUM> of the first web pieces <NUM> to the rim <NUM> and the connection portions of the second web pieces <NUM> to the rim <NUM> are located at the same position in the axial direction.

The third connection portions <NUM> of the first web pieces <NUM> to the hub <NUM> and the fourth connection portions <NUM> of the second web pieces <NUM> to the hub <NUM> are separated in the axial direction C. Specifically, in the axial direction C, the third connection portions <NUM> and the fourth connection portions <NUM> are located on opposite sides with the first connection portions <NUM> and the second connection portions <NUM> being interposed therebetween. An axial length L1 between the third connection portions <NUM> and the fourth connection portions <NUM> is preferably <NUM>% or more of the height H of the rim <NUM>. In this embodiment, the axial length L1 is substantially the same as the height H of the rim <NUM>. The axial length L1 is the maximum value for the axial intervals between the first web pieces <NUM> and the second web pieces <NUM>.

A method of manufacturing a gear <NUM> of this embodiment will be described. The gear <NUM> of this embodiment is obtained by forging to form a rough shape of the gear and then finishing through machine processing. In this process, the first web pieces <NUM> and the second web pieces <NUM> are processed by, for example, turning or rotary cutting using an endmill (milling).

Milling is performed by use of a tool T such as an endmill from above and below, as shown in <FIG>. As mentioned above, the first web pieces <NUM> and the second web pieces <NUM> are arranged so as to be separated from each other in the axial direction C, except for the connection portions to the rim <NUM> on the outer side, and are provided so as not to overlap with each other when viewed from the axial direction C. That is, as shown in <FIG>, the spaces SP1, SP2 are defined below the first web pieces <NUM> on the upper side and above the second web pieces <NUM> on the lower side, respectively. Further, the first web pieces <NUM> on the upper side as a whole are exposed through the intervals S2 when viewed from below, and the second web pieces <NUM> on the lower side as a whole are exposed through the intervals S1 when viewed from above.

Thus, when the underside of the first web pieces <NUM> is processed, the tool T can access the underside from below. When the topside of the second web pieces <NUM> is processed, the tool T can access the topside from above. That is, the first web pieces <NUM> and the second web pieces <NUM> can be easily formed by milling by use of the tool T. In this way, the gear <NUM> can be formed in an integrated structure that can be machined processed. As a result, it is possible to reduce the number of parts and thereby to reduce the manufacturing cost of the gear <NUM>.

The above configuration makes it possible to ensure rigidity of the gear <NUM> thanks to the first web <NUM> and the second web <NUM> that obliquely extend in mutually different directions from the rim <NUM> to the hub <NUM>, as shown in <FIG>. In addition, the external teeth <NUM> are supported by the rim <NUM>; periodic change in deformation of the rim <NUM> during rotation of the gear is suppressed by the flange <NUM> of the rim <NUM>; and the degree of deformation of the rim <NUM> is suppressed by the first web pieces <NUM> and the second web pieces <NUM>. This makes it possible to provide wide tooth contact of the external teeth <NUM>, thereby improving durability of the external teeth <NUM>.

The first web pieces <NUM> and the second web pieces <NUM> are alternately provided in the circumferential direction. That is, the first web pieces <NUM> are arranged at the intervals S1 in the circumferential direction, and the second web pieces <NUM> are arranged at the intervals S2 in the circumferential direction. Thus, weight increase of the gear web <NUM> is suppressed, whereby the weight of the gear <NUM> can be reduced. Moreover, since the first web pieces <NUM> and the second web pieces <NUM> can be separately formed through machine processing in this embodiment, they can be easily formed in a suitable shape in accordance with a necessary degree of deformation and/or a direction of load. As a result, it is possible to reduce the weight while ensuring necessary rigidity. In addition, since it is not necessary to use a fastening member or to perform welding as in the conventional example in which the web is divided, the number of parts or steps is reduced. Thus, it is possible to manufacture a highly rigid gear at low cost.

As shown in <FIG>, the first web pieces <NUM> and the second web pieces <NUM> have a circumferentially elongated shape in the cross section along the circumferential direction. Thus, it is possible to ensure rigidity against radial load and circumferential load on the first web pieces <NUM> and the second web pieces <NUM>.

As shown in <FIG>, the entirety of the first web pieces <NUM> on the upper side excluding the connection portions 22a to the rim <NUM> is separated above with respect to the entirety of the second web pieces on the lower side excluding the connection portions 24a to the rim <NUM>. Thus, the tool T (<FIG>) can approach from above or below to easily perform milling to the first web pieces <NUM> and the second web pieces <NUM>. In addition, since the first and second web pieces <NUM>, <NUM> obliquely extend so as to be separated from each other in the axial direction, the rigidity of the gear <NUM> can be ensured.

The rim <NUM> has the flange <NUM> that protrude radially inward, and the radially outer end portions 22a, 24a of the first web pieces <NUM> and the second web pieces <NUM> are connected to the flange <NUM>. Thus, the rigidity of the rim <NUM> is enhanced, whereby periodic change in deformation is suppressed. Moreover, since the first web pieces <NUM> and the second web pieces <NUM> have radial lengths that are shorter by a length of the flange <NUM>, a processing amount of milling is reduced. As a result, the manufacturing cost of the gear can be reduced.

Furthermore, the first connection portions <NUM> of the first web pieces <NUM> to the rim <NUM> and the second connection portions <NUM> of the second web pieces <NUM> to the rim <NUM> are located at the same position in the axial direction. Thus, the first web pieces <NUM> and the second web pieces <NUM> cooperate together to define triangles each having a vertex at the connection portions <NUM>, <NUM> to the rim <NUM> in the cross section shown in <FIG>, thereby enhancing the rigidity of the gear <NUM>. The maximum value L1 of the axial intervals between the first web pieces <NUM> and the second web pieces <NUM> is substantially the same as the axial width (height) H of the rim <NUM>. Thus, the dimension (maximum value L1) of the bases of the triangles is made larger, thereby further enhancing the rigidity of the gear <NUM>.

The external teeth <NUM> of this embodiment are in the form of a helical gear. Although a helical gear is subjected to a load in an axial direction (downward in this embodiment), a highly rigid gear according to the above configuration can sufficiently bear such a load. Further, the first web pieces <NUM> and the second web pieces <NUM> have different thicknesses from each other in accordance with a magnitude of axial load to be applied from the helical gear. Specifically, the second web pieces <NUM> on the lower side have a larger thickness than that of the first web pieces <NUM> on the upper side. Thus, it is possible to suppress weight increase while ensuring necessary rigidity.

The external teeth <NUM> may be in the form of a bevel gear, instead of a helical gear. Although a bevel gear is also subjected to a concentrated load on one side in an axial direction, a highly rigid gear according to the above configuration can sufficiently bear such a load. The gear according to the above configuration has the first web pieces <NUM> and the second web pieces <NUM> with mutually different thicknesses in accordance with axial load to be applied from the bevel gear. Thus, it is possible to suppress weight increase while ensuring necessary rigidity.

<FIG> shows a gear 1A according to a second embodiment of the present invention. In the second embodiment, the first connection portions <NUM> of the first web pieces <NUM> to the rim <NUM> and the second connection portions <NUM> of the second web pieces <NUM> to the rim <NUM> are separated from each other in the axial direction. Accordingly, the entirety of the first web pieces <NUM> including the first connection portions <NUM> and the entirety of the second web pieces <NUM> including the second connection portions <NUM> are separated from each other in the axial direction. In the second embodiment, an axial length L2 between the first connection portions <NUM> and the second connection portions <NUM> is smaller than the axial length L1 between the third connection portions <NUM> and the fourth connection portions <NUM>. That is, in the cross section shown in <FIG>, trapezoids are formed, each trapezoid having vertexes at the first connection portion <NUM>, the second connection portion <NUM>, the third connection portion <NUM> and the fourth connection portion <NUM>. Other features of the second embodiment are the same as those of the first embodiment.

According to the second embodiment, since the first connection portions <NUM> and second connection portions <NUM> are separated in the axial direction C, the first web pieces <NUM> and the second web pieces <NUM> provide an enhanced effect of suppressing deformation of the rim <NUM>. In addition, this configuration allows the first web pieces <NUM> and the second web pieces <NUM> to be made shorter thanks to smaller inclination with respect to the radial direction, which makes it possible to suppress weight increase in the first and second webs <NUM>, <NUM>. As a result, the weight of the gear can be reduced.

<FIG> shows a gear 1B according to a third embodiment of the present invention. In the third embodiment, side edge portions of the first web pieces <NUM> and side edge portions of the second web pieces <NUM> that are adjacent in the circumferential direction are connected by connection walls <NUM> extending in the axial direction C. The connection walls <NUM> are also shown in <FIG> by double dotted lines. In this embodiment, the second web pieces <NUM> are formed with lightening holes <NUM> in order to reduce the weight. Other features of the third embodiment are the same as those of the first embodiment. According to the third embodiment, the connection walls <NUM> further enhances the rigidity of the gear 1A. This makes it possible to achieve wide tooth contact of the external teeth <NUM>, thereby enhancing durability of the external teeth <NUM>.

The gear web structure of the gear is suitable for a large gear, in particular, a gear in which load is applied in an axial direction, such as a helical gear or a bevel gear. In such a gear, a gear web is important to enhance rigidity. The gear is, in particular, suitable for aircrafts, such as helicopter, in which weight reduction is demanded. In the above embodiments, since the description is made for the cases where a gear is applied to a helicopter, the axial direction C of the rotation shaft (main shaft) <NUM> coincides with the vertical direction. However, the axial direction of the rotation shaft is not limited to the vertical direction.

The present invention is not limited to the embodiments described above, and various additions, modifications, or deletions may be made without departing from the scope of the invention defined by the appended claims. For example, although the description is made for the cases where the gear is applied to a drive system of a helicopter in the above embodiments, a gear may be applied to other aircrafts besides the helicopter, as well as to other applications besides aircrafts. Application of a gear is not limited to the use in the speed reduction device. For example, it can be applied to a speed increase device.

The shapes of the first web pieces <NUM> and the second web pieces <NUM> are not limited to those described in the above embodiments. Although, in the above embodiments, the first and second web pieces <NUM>, <NUM> have a trapezoid shape when viewed from the axial direction C, they may have a rectangular shape.

Claim 1:
A gear (<NUM>, 1A, 1B) comprising:
a hub (<NUM>) provided on an inner peripheral side and configured to be connected to a rotation shaft (<NUM>);
a rim (<NUM>) provided on an outer peripheral side and formed with external teeth (<NUM>) in the form of one of a helical gear and a bevel gear;
a first web (<NUM>) connecting the hub and the rim, the first web extending obliquely toward one side of the rotation shaft in an axial direction from the rim to the hub; and
a second web (<NUM>) connecting the hub and the rim, the second web extending obliquely toward the other side of the rotation shaft in the axial direction from the rim to the hub, wherein
the first web includes a plurality of first web pieces (<NUM>) formed so as to be spaced apart in a circumferential direction,
the second web includes a plurality of second web pieces (<NUM>) formed so as to be spaced apart in the circumferential direction, and
the first web pieces and the second web pieces are alternately arranged in the circumferential direction and are located at mutually different circumferential positions,
characterised in that
the first web pieces and the second web pieces have different thicknesses from each other in accordance with axial load to be applied from the one of the helical gear and the bevel gear.