Patent Description:
For example, a mechanism for transmitting a rotational force (a driving force or a braking force) by means of a chain and a sprocket is widely used in motorcycles and the like. It is desirable for the sprocket to satisfy contradictory requirements: to be strong and lightweight. To satisfy the requirements, Patent Document <NUM> discloses that openings for weight reduction are formed in a sprocket to such an extent that the strength of the sprocket is not impaired. Document <CIT> relates to a sprocket for high-end motorcycle cycle racing.

According to Patent Document <NUM>, the openings have a circular shape. In this respect, it is necessary to devise shapes and arrangement of the openings in order to achieve a higher strength and further weight reduction. It is an object of the present invention to provide a sprocket having a high strength and a light weight.

The invention is directed to a sprocket including: mounting holes arranged at equal intervals in a circumferential direction; a plurality of sprocket teeth provided on an outer periphery; first openings each having a substantially triangular shape two corners of which respectively face the mounting holes and a remaining one corner of which is located toward the outer periphery; and second openings each having a substantially triangular shape one corner of which faces the mounting hole and remaining two corners of which are located toward the outer periphery. The first and second openings define therebetween first crossbars where a driving force and a load reaction force act toward each other and second crossbars where a driving force and a load reaction force act away from each other.

In the sprocket according to the above aspect, an inclination angle at which a centerline of a minimum-width site of each first crossbar is inclined with respect to a radial direction may be smaller than an inclination angle at which a centerline of a minimum-width site of each second crossbar is inclined with respect to a radial direction.

According to the invention, a first reference line passes through a center of one of the mounting holes and is parallel to a centerline of a minimum-width site of each first crossbar, and a second reference line passes through a center of one of the mounting holes and is parallel to a centerline of a minimum-width site of each second crossbar. An intersection point of the first reference line with the second reference line is positioned in a vicinity of an addendum circle of the sprocket teeth.

In the sprocket according to the above aspect, with respect to a center of rotation as a reference, an angle is formed by the intersection point and one of the mounting holes that is adjacent to the first crossbar, and an angle is formed between adjacent two of the mounting holes. A ratio of the former angle to the latter angle may be <NUM> or more and <NUM> or less.

In the sprocket according to the above aspect, a minimum width of each first crossbar may be larger than a minimum width of each second crossbar.

In the sprocket according to the above aspect, for a minimum-width site of each first crossbar, a tangent line to a side edge adjacent to the first opening may pass through a location toward a center of rotation relative to one of the mounting holes, and a tangent line to a side edge adjacent to the second opening may pass through a location toward the outer periphery relative to the one of the mounting holes. For a minimum-width site of each second crossbar, a tangent line to a side edge adjacent to the first opening may pass through one of the mounting holes, and a tangent line to a side edge adjacent to the second opening may pass through a location toward the outer periphery relative to the one of the mounting holes.

The sprocket of the present invention has a high strength and a light weight.

<FIG> is a planar view (seen in the direction of a rotational axis) that illustrates a sprocket <NUM> according to an embodiment of the present invention. <FIG> is a partial planer view illustrating the details of the shape of the sprocket <NUM>, on an enlarged scale.

The sprocket <NUM> illustrated in <FIG> is a driven sprocket that is driven by a chain (not shown) to thereby rotationally drive a rotary shaft (not shown) fixed to the sprocket <NUM>. The sprocket <NUM> rotates counterclockwise. Specifically, the sprocket <NUM> is driven at its outer periphery by the chain, and transmits torque acting in the counterclockwise direction.

The sprocket <NUM> has a substantial disk shape, and includes a central opening <NUM>, a plurality of mounting holes <NUM>, a plurality of sprocket teeth <NUM>, a plurality of first openings <NUM>, and a plurality of second openings <NUM>. When viewed from the center of rotation Cs of the sprocket <NUM>, the first openings <NUM> and the second openings <NUM> are alternately arranged in the circumferential direction.

The sprocket <NUM> includes a connecting frame <NUM> that is defined between the central opening <NUM> and the first openings <NUM> and connects sites where the mounting holes <NUM> are formed together. First crossbars <NUM> and second crossbars <NUM> are defined between the second openings <NUM> and the first openings <NUM>. In each first crossbar <NUM>, a driving force and a load reaction force act toward each other, whereas in each second crossbar <NUM>, a driving force and a load reaction force act away from each other. An outer peripheral frame <NUM> that supports the sprocket teeth <NUM> is defined radially outward of the mounting holes <NUM>. Note that "a driving force and a load reaction force act toward each other" means that the driving force and the load reaction force applied to a crossbar together act in a compression direction, and "a driving force and a load reaction force act away from each other" means that the driving force and the load reaction force applied to a crossbar together act in a tensile direction.

In the sprocket <NUM>, the first crossbars <NUM> mainly transmit torque, and the second crossbars <NUM> retain an inclination angle of the first crossbars <NUM> to suppress stress concentration that can be caused by deformation of the first crossbars <NUM>. In a driven sprocket, the first crossbar <NUM> extends with an inclination from one mounting hole <NUM> toward the outer periphery and backward in the rotation direction, and the second crossbar <NUM> extends with an inclination from one mounting hole <NUM> toward the outer periphery and forward in the rotation direction. In the case of a drive sprocket, the rotation direction is opposite.

The central opening <NUM> is formed at the center of the sprocket <NUM>, and defines a space in which the rotary shaft is disposed. The central opening <NUM> also contributes to weight reduction of the sprocket <NUM>. The size of the central opening <NUM> may be determined depending on required specifications.

The mounting holes <NUM> are formed in a vicinity of the central opening <NUM> and arranged at equal intervals in the circumferential direction. The mounting holes <NUM> allow insertion of bolts for mounting the sprocket <NUM> to a hub (not shown) fixed to the rotary shaft. The distance between each mounting hole <NUM> and the central opening <NUM> may be set to a distance with which a minimum area required for fastening the bolts is ensured. An auxiliary structure such as a counterbore <NUM> may be formed outside the mounting hole <NUM> for fastening the bolt. The size, number, radial position, and the like of the mounting holes <NUM> can be determined depending on required specifications.

The sprocket teeth <NUM> have a predetermined shape and are formed on the outer periphery of the sprocket <NUM> at a predetermined pitch so as to mesh with the chain. The shape and number of sprocket teeth <NUM> may be determined depending on required specifications. Therefore, the outer diameter of the sprocket <NUM> can be determined depending on the specifications required for the sprocket teeth <NUM>.

Each first opening <NUM> has a substantially triangular shape, specifically, a triangular shape having chamfered corners in the present embodiment. Two of the corners of the first opening <NUM> respectively face the mounting holes <NUM>, and the remaining one corner of the first opening <NUM> is located toward the outer periphery and faces the sprocket teeth <NUM>. The "substantially triangular shape" refers to a shape having three sides with a relatively small curvature and three corners with a relatively large curvature, and more specifically, a shape having three corner portions each defined as such a region that when points are set on the outline of the region so that adjacent ones thereof form an angle of <NUM>° with the center of gravity, the tangent lines to the outline at the points successively have an angular difference of <NUM>° or more. In other words, each of the sides of the first opening <NUM> may be a curve having a smaller curvature than the corner portions, and each of the corners of the first opening <NUM> may be chamfered into a shape that is bent stepwise to locally have a large curvature. In particular, for each first opening <NUM>, it is preferable that the side connecting the two corners facing the mounting holes <NUM> has an arc shape concentric with the central opening <NUM> so that the connecting frame <NUM> has a substantially constant width. On the other hand, it is preferable that the remaining two sides of the first opening <NUM> are at least partially linear in order to ensure a strength of the first crossbar <NUM> and a strength of the second crossbar <NUM> while allowing the first opening <NUM> to have a large area. A description that a corner "faces" a mounting hole means that the corner portion is closest to the mounting hole.

Each second openings <NUM> have a substantially triangular shape. One of the corners of the second opening <NUM> faces the mounting hole <NUM>, and the remaining two corners of the second opening <NUM> are located toward the outer periphery and face the sprocket teeth <NUM>. Each of the sides of the second opening <NUM> may also be a curve, and in particular, the side connecting the two corners located toward the outer periphery preferably has an arc shape concentric with the central opening <NUM> so that the outer peripheral frame <NUM> supporting the sprocket teeth <NUM> and having a constant width is defined. On the other hand, it is preferable that the remaining two sides of the second opening <NUM> are at least partially straight and parallel to the side edges of the first crossbar <NUM> and the second crossbar <NUM> that are adjacent to the first openings <NUM> in order to ensure the strength of the first crossbar <NUM> and that of the second crossbar <NUM> while allowing the second opening <NUM> to have a large area.

Preferably, a minimum width of the first crossbar <NUM> is preferably larger than a minimum width of the second crossbar <NUM>. In other words, it is preferable that the first crossbars <NUM> that mainly transmit torque have a relatively large width to ensure a suitable strength, and the second crossbars <NUM> that assist the first crossbars <NUM> have a relatively small width in order to reduce the weight.

It is preferable that for the minimum-width site of each first crossbar <NUM>, a tangent line L71 to a side edge adjacent to the first opening <NUM> is positioned toward the center of rotation Cs relative to the mounting hole <NUM>. It is also preferable that for the minimum-width site of each first crossbar <NUM>, a tangent line L72 to a side edge adjacent to the second opening <NUM> is positioned toward the outer periphery relative to the mounting hole <NUM>. In other words, it is preferable that the minimum width of the first crossbar <NUM> is larger than the diameter of the mounting hole <NUM> having a size allowing insertion of the bolt with a cross-sectional area required for torque to be transmitted.

Furthermore, it is more preferable that a centerline Lc1 of the minimum-width site of each first crossbar <NUM> passes through the mounting hole <NUM>. The centerline Lc1 is a perpendicular bisector of a line segment indicating the shortest distance between the first opening <NUM> and the second opening <NUM> (a line segment connecting the closest points). Due to this configuration, the torque that is transmitted from the chain via the first crossbars <NUM> can be efficiently transmitted to the hub to which the sprocket <NUM> is mounted, by means of the mounting holes <NUM>.

It is preferable that for the minimum-width site of each second crossbar <NUM>, a tangent line L81 to a side edge adjacent to the first opening <NUM> passes through the mounting hole <NUM>, and more preferably passes through the center Ch of the mounting hole <NUM>. It is also preferable that for the minimum-width site of each second crossbar <NUM>, a tangent line L82 to a side edge adjacent to the second opening <NUM> is positioned toward the outer periphery relative to the mounting hole <NUM>. Furthermore, it is more preferable that a centerline Lc2 of the minimum-width site of each second crossbar <NUM> passes through the mounting hole <NUM>. This configuration makes it possible to increase the strength of the sprocket <NUM> and suppress deformation of the first crossbars <NUM> while reducing the weight of the sprocket <NUM> as much as possible. In particular, the design in which the tangent line L81 to the side edge of the minimum-width site of the second crossbar <NUM> adjacent to the first opening <NUM> passes through the mounting hole <NUM> leads to an increase in the size of the first opening <NUM>, thereby further effectively contributing to reduction of weight of the sprocket <NUM>.

An inclination angle α at which the centerline Lc1 of the first crossbar <NUM> is inclined with respect to a radial direction of the sprocket <NUM> is preferably smaller than an inclination angle β at which the centerline Lc2 of the second crossbar <NUM> is inclined with respect to a radial direction of the sprocket <NUM>. This configuration in which the inclination angle α of the first crossbar <NUM> is smaller than the inclination angle β of the second crossbar <NUM> results in that the first crossbar <NUM> is smaller in length than the second crossbar <NUM>, and accordingly, leads to a relatively small increase in the area of the sprocket <NUM> and hence the mass of the sprocket <NUM> in a case where the width of the first crossbar <NUM> is increased for improvement of the strength.

Specifically, the lower limit of the inclination angle α of the centerline Lc1 of the first crossbar <NUM> is preferably <NUM>°, and more preferably <NUM>°. On the other hand, the upper limit of the inclination angle α of the centerline Lc1 of the first crossbar <NUM> is preferably <NUM>°, and more preferably <NUM>°. Setting the inclination angle α of the first crossbar <NUM> within such a range makes it possible to reduce a bending stress and a shear stress that act on the first crossbar <NUM> to relatively low levels in a typical motorcycle, thereby enabling relatively efficient transmission of the torque acting on the sprocket <NUM>.

A first reference line Lr1 that passes through the center Ch of the mounting hole <NUM> and is parallel to the centerline Lc1 of the first crossbar <NUM> intersects at an intersection point P with a second reference line Lr2 that passes through the center Ch of another mounting hole <NUM> and is parallel to the centerline Lc2 of the second crossbar <NUM>, and the intersection point P is preferably positioned in a vicinity of an addendum circle of the sprocket teeth <NUM>. Note that "vicinity of an addendum circle" refers to an area radially extending from the addendum circle by a distance equal to or less than a tooth depth, preferably by a distance equal to or less than an addendum. This configuration makes it possible to accurately divide a force transmitted from the chain into a component that transmits torque along the first crossbar <NUM> and a component that is orthogonal to the first crossbar <NUM> and to be absorbed by the second crossbar <NUM>, thereby enabling efficient transmission of the torque.

With respect to the center of rotation Cs of the sprocket <NUM> as a reference, an angle γ is formed by the intersection point P of the first reference line Lr1 and the second reference line Lr2 and the center Ch of one mounting hole <NUM> adjacent to the first crossbar <NUM>, and an angle θ is formed by two mounting holes <NUM> adjacent to each other. A ratio of the angle γ to the angle θ (i.e., γ/θ: hereinafter, referred to as the crossbar angle ratio) is preferably <NUM>, and more preferably <NUM>. On the other hand, the upper limit of the crossbar angle ratio is preferably <NUM>, and more preferably <NUM>. Setting the crossbar angle ratio equal to or greater than the lower limit described above makes it possible to reduce the maximum stress acting on the first crossbar <NUM> to a relatively low level. Setting the crossbar angle ratio equal to or lower than the upper limit described above makes it possible to prevent or reduce an increase in the total length of the first crossbar <NUM> and the second crossbar <NUM>, and to make the first crossbar <NUM> and the second crossbar <NUM> have small areas, thereby enabling reduction of the weight of the sprocket <NUM>.

As described above, the sprocket <NUM> is provided with the first openings <NUM> and the second openings <NUM> that define the first crossbars <NUM> and the second crossbars <NUM>, and each first crossbar <NUM> extends from one mounting hole <NUM> while being inclined so that the driving force and the load reaction force act toward each other whereas each second crossbar <NUM> extends from one mounting hole <NUM> while being inclined so that the driving force and the load reaction force act away from each other. Due to this configuration, the sprocket <NUM> is relatively lightweight while being highly strong and capable of transmitting a larger torque.

The design of the sprocket <NUM> can be produced by a method, which is a non-limiting example, including a step of arranging the central opening <NUM>, the mounting holes <NUM>, and the sprocket teeth <NUM> based on required specifications, a step of drawing a circle defining an outer edge of the connecting portion <NUM> (the side of each first opening <NUM> connecting the two corners facing the mounting holes <NUM>) and a circle defining an inner edge of the outer peripheral frame <NUM> (the side of each second opening <NUM> connecting the two corners adjacent to the outer periphery), a step of setting the first reference line Lr1 and the second reference line Lr2, a step of determining, as straight lines respectively parallel to the first reference line Lr1 and the second reference line Lr2, two sides of each first opening <NUM> and two sides of each second opening <NUM> that define the side edges of the first crossbar <NUM> and the side edges of the second crossbar <NUM>, and a step of chamfering the corners of the first opening <NUM> and the second opening <NUM>.

The step of setting the first reference line Lr1 and the second reference line Lr2 may be performed in the following manner. The first reference line Lr1 passing through the center Ch of the mounting hole <NUM> is drawn, the intersection point P is set on the first reference line Lr1 in the vicinity of the addendum circle of the sprocket teeth <NUM>, and then the second reference line Lr2 is drawn to connect the intersection point P and the adjacent mounting hole <NUM>. At this time, the position of the intersection point P may be the intersection point of the first reference line Lr1 with the addendum circle, or alternatively, an intersection point on a virtual circle having an easy-to-understand diameter, instead of the addendum circle the diameter of which is unlikely to have a value without fractions.

While the sprocket <NUM> according to one embodiment of the present invention has been described in the foregoing, the configuration and effects of the sprocket according to the present invention are not limited to those described above. For example, the first opening and the second opening of the sprocket according to the present invention may be formed into the shape of a substantial isosceles triangle to define the first crossbar and the second crossbar that are symmetrical in the rotation direction. Furthermore, the sides and chamfers of the first opening and the second opening of the sprocket according to the present invention may each have a shape varying continuously or stepwise.

Hereinafter, the present invention will be described in more detail with reference to examples. Note that the present invention is not limited to the following examples.

As Model No. <NUM>, the sprocket shown in <FIG> was modeled according to the above embodiment. In the sprocket of Model No. <NUM>, the radius of the central opening was <NUM>, the diameter of the mounting hole was <NUM>, the radius of the pitch circle of the mounting holes was <NUM>, the number of the mounting holes was <NUM>, the diameter of the addendum circle was <NUM>, and the radius of the dedendum circle was <NUM>. The radius of the outer edge of the connecting frame <NUM> was <NUM>, and the radius of the inner edge of the outer peripheral frame <NUM> was <NUM>. The first reference line inclined at an angle of <NUM>° with respect to a radial direction was drawn from the center of one mounting hole, and a second reference line was drawn to connect an intersection point (with a crossbar angle ratio γ/θ of <NUM>) of the first reference line with a virtual circle having a radius of <NUM> and the center of the adjacent mounting hole. In Model No. <NUM>, the width of the first crossbar was set to <NUM>, the width of the second crossbar was set to <NUM>, a straight line offset by <NUM> from the first reference line toward the center of rotation was set as the side edge of the first crossbar adjacent to the first opening, and the second reference line (i.e., a straight line with an offset amount of <NUM> toward the center of rotation) was set as a side edge of the second crossbar adjacent to the first opening.

<NUM> to <NUM> are different from Model No. <NUM> in the crossbar angle ratio. <NUM> to <NUM> are different from Model No. <NUM> in the width W1 of the first crossbar (an offset amount A by which the side edge adjacent to the first opening is offset from the first reference line toward the center of rotation). <NUM> to <NUM> are different from Model No. <NUM> in the width W2 of the second crossbar (an offset amount B by which the side edge adjacent to the second opening is offset from the second reference line). <NUM> to <NUM> are different from Model No. <NUM> in the offset amount B by which the side edge of the second crossbar adjacent to the second opening is offset from the second reference line (but are the same as Model No. <NUM> in the width of the second crossbar).

For each of Model Nos. <NUM> to <NUM>, the maximum stress was calculated by computer simulation, and the weight (volume) of the respective sprocket was calculated. The results are shown in Table <NUM> below, in which the values of Model No. <NUM> are defined as <NUM>%.

Claim 1:
A sprocket (<NUM>) comprising:
mounting holes (<NUM>) arranged at equal intervals in a circumferential direction;
a plurality of sprocket teeth (<NUM>) provided on an outer periphery;
first openings (<NUM>) each having a substantially triangular shape two corners of which respectively face the mounting holes (<NUM>) and a remaining one corner of which is located toward the outer periphery; and
second openings (<NUM>) each having a substantially triangular shape one corner of which faces the mounting hole (<NUM>) and remaining two corners of which are located toward the outer periphery,
the first and second openings (<NUM>, <NUM>) defining therebetween first crossbars (<NUM>) where a driving force and a load reaction force act toward each other and second crossbars (<NUM>) where a driving force and a load reaction force act away from each other,
characterized in that a first reference line (Lr1) passes through a center of one of the mounting holes (<NUM>) and is parallel to a centerline (Lc1) of a minimum-width site of each first crossbar (<NUM>), in that a second reference line (Lr2) passes through a center of one of the mounting holes (<NUM>) and is parallel to a centerline (Lc2) of a minimum-width site of each second crossbar (<NUM>),
and in that an intersection point of the first reference line (Lr1) with the second reference line (Lr2) is positioned in a vicinity of an addendum circle of the sprocket teeth (<NUM>).