Ball screw design method and ball screw

Abstract

To provide a ball screw design method and a ball screw that can secure smooth ball movement and cope with high speed movement.SOLUTION: In a ball screw design method, when a ball diameter is represented as Da, the length of a straight line between two curved lines as L, a ball pitch angle on the curved line as θb, and a central angle of the curved line as θr in the case where there exists a straight line between the two curved lines whose radiuses are the same as a central line of a circuit, an angle ratio of the curved lines is defined as θr / θb, and the θr / θb is determined as (A) θr / θb≒n+0.25 (n being an optional integer), or (B) when θr / θb≒n+0.25 is not satisfied, the value of L is set so as to satisfy formulas (1) and (2): (1) L / Da+θr / θb≒n+0.5 (n being an integer), (2) L≒(n+0.5-(θr / θb))×Da.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a ball screw design method and a ball screw. [Background technology]

[0002] A ball screw is a device that includes a nut with a female screw groove formed on its inner peripheral surface, a screw shaft with a male screw groove formed on its outer peripheral surface, balls arranged in a rolling path formed by the female screw groove of the nut and the male screw groove of the screw shaft, and a circulation path that returns the balls from the end point of the raceway to the start point, and the nut moves relative to the screw shaft as the balls roll in the rolling path. A return tube type is sometimes used as the circulation path for ball screws because of its advantages such as ease of assembly.

[0003] An example of a return tube type ball screw is disclosed in Patent Document 1. In this ball screw, the tip of the return tube, which serves as a circulation component, consists of an end face along a baseline perpendicular to the tube axis and a tongue that protrudes like a tongue from the baseline, and the tongue protrudes into the raceway and functions to scoop up balls from the raceway into the return tube. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-329099 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, when spheres are lined up and moved along a curved path made of a groove, cylinder, or the like, such as a return tube, a phenomenon occurs in which the total length of the sphere train changes depending on the position of the spheres along the curved path. For example, even if the same spheres SP are loaded along the same curved path CP, as shown in Figures 1 and 2, if the position of the spheres SP relative to the curved path CP is different, the total length L0 of the sphere train (the distance between the leading and trailing ends of the sphere train in the direction of travel of multiple spheres SP) will vary slightly. This is because when the straight line N connecting the centers of adjacent spheres SP deviates from the center line C of the curved path CP and takes a shortcut, the amount of shortcut changes depending on the position of the spheres SP.

[0006] In other words, as the train of spheres passes through the curved path CP, its total length L0 expands and contracts. If the train of spheres is relatively short, there is no particular problem even if this phenomenon occurs. However, if an infinite train of spheres passes through the curved path CP, the spheres SP in front of and behind the sphere SP in the direction of travel will act as resistance, so the total length L0 will remain almost unchanged, and instead the spheres SP themselves will elastically deform to cancel out the expansion. In this case, the spheres SP will push against each other, which will cause the train to be unable to pass through the curved path CP smoothly. However, because the amount of expansion and contraction of the total length of the train of spheres varies depending on the radius of curvature of the curved path CP and the length of the curve, if the shape of the curve that reduces the amount of expansion and contraction can be determined, it will be possible to achieve smooth passage of the spheres SP by taking into account the geometric dimensions of each part.

[0007] When the row of spheres expands or contracts, the amount of spheres SP that enter the entrance and the amount that come out at the exit of the curved path CP will not be the same, but will differ by the amount of expansion or contraction. Here, this phenomenon is called "entrance and exit fluctuation," and the amount of expansion or contraction is called "entrance and exit fluctuation amount." If this entrance and exit fluctuation is large, it could cause the ball screws to compete with each other, hindering the speed of the ball screws.

[0008] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a ball screw design method and a ball screw that can ensure smooth ball movement and accommodate higher speeds. [Means for solving the problem]

[0009] The ball screw of the present invention is a screw shaft having an outer peripheral spiral groove formed thereon; a nut having an inner peripheral spiral groove; a plurality of balls accommodated in a rolling path formed by the outer peripheral spiral groove and the inner peripheral spiral groove facing each other; a circulation part that returns the balls from one end of the rolling path to the other end, A design method for a ball screw, wherein a center line of a circulation path in the circulation part is configured by a curve with a single radius and straight lines connected before and after the curve, When the ball diameter is Da, the length of the straight line between the two curves is L, the ball pitch angle on the curve is θb, and the central angle of the curve is θr, the angle ratio of the curve is defined as θr / θb, (A) θr / θb ≒ n + 0.25 (n is any integer), or (B) When θr / θb≈n+0.25 is not satisfied, the value of L is set so as to satisfy the following expressions (1) and (2). L / Da + θr / θb ≒ n + 0.5 (n is an integer) (1) L≒(n+0.5-(θr / θb))×Da (2) [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a ball screw design method and a ball screw that can ensure smooth ball movement and accommodate higher speeds. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing a schematic diagram of a sphere passing through a curved path. [Figure 2] FIG. 2 is a diagram showing a schematic diagram of a sphere passing through a curved path. [Figure 3] FIG. 3 is a plan view showing the configuration of a ball screw according to an embodiment of the present invention. [Figure 4]FIG. 4 is a diagram showing the configuration of a ball screw, where (a) is a partial cross-sectional view as seen from the axial direction, and (b) is a partial cross-sectional view as seen from the side. [Figure 5] FIG. 5 is a graph showing the amount of change in the overall length of the sphere train passing through a portion of the circulating component. [Figure 6] FIG. 6 is a schematic diagram showing the center line of a route that connects a curved portion and a straight portion. [Figure 7] FIG. 7 is a schematic diagram showing balls passing side by side along a curved section. [Figure 8] FIG. 8 is a graph showing the amount of inward / outward fluctuation on the vertical axis and (central angle θr / pitch angle θb) on the horizontal axis under the condition that the central angle θr is constant. [Figure 9] FIG. 9 is a graph showing the amount of inward / outward fluctuation on the vertical axis and (central angle θr / pitch angle θb) on the horizontal axis under the condition that the pitch angle θb is constant. [Figure 10] FIG. 10 is a graph showing the amount of change in the overall length of the ball train, with the amount of change in overall length taken on the vertical axis and (ball movement amount / ball diameter) taken on the horizontal axis. [Figure 11] FIG. 11 is a schematic diagram showing balls passing through a circulation path in a circulation component. [Figure 12] FIG. 12 is a graph showing the amount of change in the overall length of the ball train, with the amount of change in overall length taken on the vertical axis and (ball movement amount / ball diameter) taken on the horizontal axis. [Figure 13] FIG. 13 is a diagram schematically showing a bent portion of two straight lines sandwiching a curve. [Figure 14] FIG. 14 is a graph in which the vertical axis represents the bending angle and the horizontal axis represents the ball diameter. [Figure 15] FIG. 15 is a graph showing the amount of change in the overall length of the ball train, with the amount of change in overall length taken on the vertical axis and (ball movement amount / ball diameter) taken on the horizontal axis. [Figure 16] FIG. 16 is a schematic diagram showing an example of a circulation path having two curved sections and three straight sections connected to the curved sections. [Figure 17]FIG. 17 is a schematic diagram showing an example of a circulation path having two bends, two curved portions, and three straight portions connected to the curved portions. DETAILED DESCRIPTION OF THE INVENTION

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of a ball screw according to the present invention will be described with reference to the drawings. Fig. 3 is a plan view showing the configuration of a certain embodiment of the ball screw, and Fig. 4 is a view showing the configuration of a certain embodiment of the ball screw, where (a) is a partial cross-sectional view as seen from the axial direction, and (b) is a partial cross-sectional view as seen from the side.

[0013] 3 and 4, the ball screw 1 of this embodiment has a screw shaft 10 with a spiral groove (outer peripheral spiral groove) 11 formed on its outer peripheral surface, a nut 20 with a spiral groove (inner peripheral spiral groove) 24 formed on its inner peripheral surface, balls SP as rolling elements, and a circulation part 30 which is a U-shaped return tube. The circulation part 30 is fixed to the nut 20 by an attachment part 40. The balls are arranged to be able to roll in a raceway formed by the spiral groove 11 of the screw shaft 10 and the spiral groove 24 of the nut 20, and in a circulation path formed by the circulation part 30.

[0014] 3 and 4, a part of the outer periphery of the nut 20 is formed as a recess 21, the bottom surface of which serves as an outer flat surface 22 to which the circulation component 30 is attached. A pair of through holes (circulation holes) 23, 23 are formed in the outer flat surface 22 of the nut 20.

[0015] The circulation component 30 is composed of a main body 31 machined from a metal (stainless steel, brass, carbon, etc.) and a pair of legs 32 attached to both ends of the main body 31. The circulation component 30 is composed of a main body 31 formed by bending a long cylindrical body into a U-shape, and the ends of two legs 32, 32 formed by machining a short cylindrical body are connected to both ends of the main body 31.

[0016] When installing the circulation part 30 in the through hole 23 provided in the outer flat surface 22 of the nut 20, the leg part 32 inserted into the through hole 23 is inserted in a direction intersecting the axial direction of the nut 20 relative to the through hole 23, and in this state the circulation part 30 is fixed to the nut 20 by the mounting part 40.

[0017] Furthermore, a tongue portion 32c is formed at the tip 32b of the leg portion 32 for scooping up a ball SP, which is a spherical body, in the tangential direction.

[0018] 3 and 4, when the nut 20 and the screw shaft 10 move relative to each other, the ball SP moves within the rolling path, rotates around the screw shaft 10 multiple times, and reaches the end point of the rolling path (the intersection of the rolling path and the circulation path), where it is scooped up into the circulation part 30 from one end (opening) of the circulation part 30 via the tongue part 32c. Furthermore, the ball SP that has passed through the circulation part 30 is returned into the rolling path by the tongue part 32c on the opposite side.

[0019] (Ball screw design method) FIG. 5 is a diagram showing the change in the overall length of a train of balls passing through a portion of the circulating component 30, with the vertical axis representing the change in overall length and the horizontal axis representing (ball movement distance / ball diameter). When the overall length changes, the ball moves its diameter and then returns to its original length. This process repeats periodically, so we will only consider the period during which the ball moves the diameter. By calculating the change in overall length as shown in FIG. 5, the difference Δ between its maximum and minimum values ​​can be calculated. If this difference Δ is large, the overall length of the train of balls will change significantly. Here, this difference Δ is referred to as the "incoming / outgoing fluctuation amount." By suppressing the ingoing / outgoing fluctuation amount, it is possible to suppress balls fighting over each other, etc.

[0020] Below, we will consider a method for suppressing inflow and outflow fluctuations in the circulation path, which can be used as an example of a ball screw design method. The circulation path of the circulation part 30, which is the return tube, is often made up of single-radius curves, straight lines, and bent sections. For example, the circulation path of the circulation part 30 is made up of two bent sections, a curved section, and three straight sections. Also, a ball screw with a tangent scooping method has no bent points and is made up only of multiple curved sections and straight sections.

[0021] Such fluctuations in the flow of circulating parts 30 occur at curved sections and bent sections. As a result of extensive research, the inventors have derived a method for reducing the path fluctuations throughout the entire circulation path by calculating the path fluctuations that occur at these two sections. This method is described below.

[0022] First, we model a path consisting of a single-radius curved section and straight sections connected to both ends of the curved section. Here, we assume that the center line of the circulation path within the circulating part is composed of a single-radius curved section and straight lines connected before and after the curved section. Figure 6 shows the center line CL of the path connecting the curved section and the straight section. We then determine the characteristics of the inflow and outflow fluctuations that occur in the train of balls passing through this path.

[0023] In Figure 6, the radius of the curved portion of the center line CL is R, the central angle is θr, and the length of the straight portion is Da, which is equal to the ball diameter. Also in Figure 7, the pitch angle of adjacent balls SP lined up on the curved portion is θb. Figure 8 shows the amount of in-out fluctuation when the radius R is changed while the central angle θr is constant, and Figure 9 shows the amount of in-out fluctuation when the central angle θr is changed while the radius R is constant. Here, θr / θb is called the angle ratio.

[0024] A comparison of Figures 8 and 9 clearly shows that in both cases the minimum value is reached when θr / θb=n+0.25. From this result, it can be inferred that no matter what the value of the curve radius R or central angle θr, the amount of inflow and outflow fluctuations will be small if θr / θb=n+0.25 (n is any integer).

[0025] However, there are cases where θr / θb=n+0.25 cannot be achieved due to limitations on the dimensions of each part of the ball screw, etc. In such cases, a method can be applied to cancel out the two in-and-out fluctuations. This method will be explained below. If the change in the total length of the row of balls passing through the path shown in Figure 6 is not near θr / θb=n+0.25, it will form a waveform as shown in Figure 10, with one cycle being the movement of the diameter of the ball.

[0026] Normally, the circulation path within the circulating element 30 has a symmetrical shape, so there are two curves CV of the same shape on the center line of the circulation path, as shown in Figure 11. Therefore, if the phases of the changes in the total length of the ball rows of the two curves CV are opposite, the changes will cancel each other out, and the overall amount of inflow and outflow fluctuation can be reduced. To achieve this, as shown in Figure 11, the positions of the balls SP at the inlets E of the two curves CV must be offset by half the ball diameter. Here, the inlet E of the curve CV is the tip of the curve on the side where the balls SP enter the circulating element 30.

[0027] For the above condition to be met, the length of the straight line LN between the two curves CV is L, the ball pitch angle on the curve CV (the angle formed by the line from the center of the curve CV to the center of the two balls SP when adjacent balls SP are in contact on the curve with no gaps) is θb, and the angle of the curve CV is θr.The number of balls between the first curve CV and the straight line LN must be n+0.5.From the above, the value of L is set so that the following equations (1) and (2) are met.Note that "≒" means that the difference between the value on the left side and the value on the right side is within ±10%. L / Da + θr / θb ≒ n + 0.5 (n is an integer) (1) L≒(n+0.5-(θr / θb))×Da (2)

[0028] If there is another free curve (including a straight line with an infinite radius of curvature) between two curves CV, the free curve between the curves CV is approximated by multiple infinitesimal curves, the total length of the infinitesimal curves is defined as L, the angle ratio of each infinitesimal curve is defined as θri / θbi, and equation (2) is replaced by equation (3) below. L≒(n+0.5-(θr / θb)-Σ(θri / θbi))×Da(i=1,2,3··) (3)

[0029] On the other hand, for the curve near θr / θb=n+0.25, the waveform has a period of 0.5 equal to the ball diameter, as shown in Figure 12. Therefore, if the number of balls between the first curve CV and the straight line LN is n±0.25, the change in the total length can be canceled out. In other words, it is clear that it is sufficient to satisfy equation (4). L≒(n+0.25-(θr / θb)-Σ(θri / θbi))×Da(i=1,2,3··) (4)

[0030] Next, consider the inward / outward fluctuation at the bend of the two straight lines LN that sandwich the curve. If the bend angle is the crossing angle θ of the two straight lines LN as shown in Figure 13, the inward / outward fluctuation amount changes depending on the crossing angle θ and the ball diameter Da. The smaller the ball diameter Da and the crossing angle θ, the smaller the fluctuation amount.

[0031] 14 is a graph showing the results of calculating the combination of the crossing angle θ and the ball diameter Da when the amount of in-and-out fluctuation is 0.05 mm and 0.1 mm. From FIG. 14, it can be seen that to keep the amount of in-and-out fluctuation at 0.05 mm or less, the following formula (5) should be satisfied. θ≦36.55Da -0.51 (5)

[0032] The change in the total length of the ball train at the bend also forms a waveform with one cycle being the movement of the diameter of a ball, as shown in Figure 15. In order to cancel out the change in the total length of the ball train in a path with two bends, the number of balls between the bends must be set to (n+0.5).

[0033] That is, the length of the straight line between the two bent portions is defined as L, and the curves are defined as θri and θbi as described above, so that the following equation (6) is satisfied. L≒(n+0.5-Σ(θri / θbi))×Da(i=1,2,3··) (6)

[0034] Furthermore, because the change in the overall length of the path at the bends is mountain-shaped as shown in Figure 15, even if they are in opposite phase, the amount of in-out fluctuation is only 1 / 2. Therefore, in order to keep the amount of in-out fluctuation at two bends to a guideline of 0.05 mm or less, the amount of in-out fluctuation at one bend must be 0.1 mm or less. In that case, the crossing angle θ from Figure 10 must satisfy the following equation (7). θ≦50.25 × Da 0.49 (7)

[0035] The present invention can be applied to ball screws having a circulation path (for example, return tube type, end deflector type, end cap type, etc.) that is made up of single-radius curves, straight lines, and bent sections.

[0036] In particular, ball screws with return tube-type circulation paths shown in Figures 3 and 4 have two curved sections CV and three straight sections LN connected to the curved sections CV on the center line of the circulation path as shown in Figure 16, or have two bent sections FL, two curved sections CV, and a straight section LN connecting the bent sections FL and the curved sections CV, as well as a straight section LN connecting the curved sections CV together on the center line of the circulation path as shown in Figure 17. In the example of Figure 17, the circulation path is set so that the bent section FL and the two curved sections CV satisfy equations (2) and (6), respectively.

[0037] The method of reducing the in-and-out fluctuation of the present invention is even more effective when retainer pieces are inserted between the balls. In a normal ball screw, when the balls are fully loaded, several balls are removed to ensure a gap. In contrast, when retainer pieces are inserted, the balls are adjusted to a nearly fully loaded state so that no large gaps are formed between them, in order to prevent the pieces from falling out.

[0038] In such cases, if there is a movement fluctuation when the ball row is tightly packed with no gaps, the balls will push against each other when the ball row tries to extend, causing a deterioration in the operation of the ball screw. Therefore, the present invention is particularly effective when a retainer piece is used or when the balls are nearly fully loaded (when a maximum of m balls can be loaded in one circuit, a state in which (m-1) balls are loaded in that circuit).

[0039] Furthermore, in ball screws, where balls are packed tightly together in localized areas with no gaps and compete with each other, a condition known as clogging, is likely to occur, and balls are also packed tightly in the circulation path, so they are similarly greatly affected by fluctuations in ball movement.

[0040] For example, ball screws with 90 or more but 100 or less balls in one circuit consisting of a rolling path and a circulation path, or double-start ball screws in which the lead is equal to twice the pitch, are known to be prone to clogging. Generally, the cross section of the nut is uneven where the circulation path is formed, making it prone to deformation due to heat treatment, etc. Double-start ball screws have two circulation paths, which means they are more susceptible to deformation, which often leads to ball clogging.

[0041] If the circulating part is made of metal, it is difficult for the deformation of the circulating part to absorb the change in the ball row due to the fluctuation in the ball movement, so it can be said that it is also susceptible to the influence of the fluctuation in the ball movement.Also, even if the circulating part is made of resin, if the entire surface is covered with metal, it is similarly susceptible to the influence of the fluctuation in the ball movement.

[0042] Furthermore, when the friction torque due to the preload is small, the torque fluctuation due to the fluctuation in the movement in and out can be more pronounced. Therefore, the effect of the present invention can be more effectively exhibited in the case of a ball screw with a small lead or a small preload.

[0043] Furthermore, in the case of a ball screw with a tangential scooping system, the amount of movement fluctuation can be minimized because there are no bending points in the tangential scooping system.

[0044] The present invention is not limited to the above-described embodiments. Any of the components of the above-described embodiments can be modified within the scope of the present invention. Furthermore, any of the components can be added or omitted from the above-described embodiments. [Explanation of symbols]

[0045] 1 ball screw 10 Screw shaft 11 Spiral groove of screw shaft (peripheral spiral groove) 20 nuts 21 Recess 22 Outer flat surface 23 Through hole (circulation hole) 24 Nut spiral groove (inner spiral groove) 30 Rotable Parts 31 Main body 32 Legs 40 Mounting parts

Claims

1. a screw shaft having an outer peripheral spiral groove formed thereon; a nut having an inner peripheral spiral groove; a plurality of balls accommodated in a rolling path formed by the outer peripheral spiral groove and the inner peripheral spiral groove facing each other; a circulation part that returns the balls from one end of the rolling path to the other end, A design method for a ball screw, wherein a center line of a circulation path in the circulation part is configured by a curve with a single radius and straight lines connected before and after the curve, When the ball diameter is Da, the length of the straight line between the two curves is L, the ball pitch angle on the curve is θb, and the central angle of the curve is θr, the angle ratio of the curve is defined as θr / θb, (A) θr / θb≈n+0.25 (n is any integer), or (B) If θr / θb≈n+0.25 is not satisfied, the value of L is set so as to satisfy the following equations (1) and (2): L / Da+θr / θb≈n+0.5 (n is an integer) (1) L≒(n+0.5-(θr / θb))×Da (2) A ball screw design method characterized by the above.

2. When a free curve exists between the two curves having the same radius, the free curve is approximated by a plurality of infinitesimal curves, the total length of the infinitesimal curves is defined as L, the angle ratio of each infinitesimal curve is defined as θri / θbi, and the value of L is set using equation (3) instead of equation (2). L≒(n+0.5-(θr / θb)-Σ(θri / θbi))×Da(i=1,2,3...) (3) 2. The method for designing a ball screw according to claim 1.

3. When θr / θb≈n+0.25 and a free curve exists between the two curves having the same radius, the free curve is approximated by a plurality of infinitesimal curves, the total length of the infinitesimal curves is defined as L, and the angle ratio of each infinitesimal curve is defined as θri / θbi, and the value of L is set so as to satisfy formula (4). L≒(n+0.25-(θr / θb)-Σ(θri / θbi))×Da(i=1,2,3...) (4) 2. The method for designing a ball screw according to claim 1.

4. A ball screw designed by the ball screw design method according to any one of claims 1 to 3, In the case where a maximum of m balls can be packed in one circuit consisting of the rolling path and the circulation path, (m-1) balls are packed in the circuit. A ball screw characterized by:

5. A ball screw designed by the ball screw design method according to any one of claims 1 to 3, The number of the balls filled in one circuit consisting of the circulation path and the rolling path is 90 or more. A ball screw characterized by:

6. A ball screw designed by the ball screw design method according to any one of claims 1 to 3, It is a double-start ball screw. A ball screw characterized by:

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