JP4253994B2 - Worm gear device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a worm gear device including a worm wheel and a worm meshing with the worm wheel.
[0002]
[Prior art]
Some worms used in the worm gear device as described above have a multi-lead worm in order to adjust the backlash between the worm wheel and the worm wheel engaged therewith. For example, as shown in FIG. 2, the multiple lead worm is a cylindrical worm 20 in which the leads and pressure angles of one tooth surface 22 and the other tooth surface 23 of the worm tooth portion 21 are different from each other. It is usually used in combination with an ordinary worm wheel 30 machined with a standard hob. The worm 20 of FIG. 2 consisting of a multi-lead worm has a reduced tooth thickness and an increased width of the tooth gap 24 as it advances to the right, so that backlash between the worm 20 and the worm wheel 30 If 20 is moved to the right, it decreases, and if it moves to the left, it increases, so that the backlash can be adjusted.
[0003]
In a normal worm gear device, the main power transmission direction is fixed. Therefore, in a conventional worm gear device using a multi-lead worm, one tooth surface of the worm 20 composed of a multi-lead worm that comes into contact with the main power transmission. The pitch of 22 (the value obtained by dividing the lead by the number of threads of the worm tooth portion 21) and the pressure angle are the circular pitch on the reference pitch circle of the worm wheel and the corresponding pressure angle, and the other worm tooth surface on the opposite side The pitch and the pressure angle of 23 are larger than that of the worm tooth surface 22. Here, the pressure angle is a value in a cross section including the central axis of the multiple lead worm.
[0004]
[Problems to be solved by the invention]
When adjusting the backlash with a multiple lead worm, it is preferable to reduce the amount of movement of the worm to widen the adjustment range of the backlash, and for this purpose, it is necessary to increase the difference between the leads of both tooth surfaces of the multiple lead worm as much as possible. . However, when used in an electric power steering apparatus, such a problem causes a difference in steering torque characteristics depending on the direction of steering, resulting in a feeling of strangeness in steering. The reason is described below.
[0005]
First, the case of a worm gear device using an ordinary worm that is not a multiple lead will be described with reference to FIG. The worm wheel 30 (same as that shown in FIG. 2) has a hob arranged so as to intersect with the worm 20A meshing with the worm wheel 30A so as to have a larger diameter than the lead angle of the worm tooth portion 21A. The wheel tooth portion 31 is cut by feeding from the radial direction. Alternatively, the wheel tooth portion 31 may be cut by sending a hob parallel to the axial direction of the worm wheel 30. In any case, crowning is formed on the wheel tooth surfaces 32 and 33 of the wheel tooth portion 31, and cross sections (pitches) of the tooth surfaces 32 and 33 on both sides of the wheel tooth portion 31 along the pitch cylindrical surface of the worm 20 </ b> A. As shown in FIG. 5, the cylindrical surface developed in a flat plane is the curved line protruding loosely, and the center line Q of the wheel tooth portion 31 is in the same direction as the lead wire of the worm 20A.
[0006]
Since the worm 20A meshing with the worm wheel 30 is a normal worm and the tooth thickness of the worm tooth portion 21A is constant, the directions of the lead wires of the worm tooth surfaces 22A and 23A on both sides of the worm tooth portion 21A are both. The direction of the lead wire of the worm tooth portion 21 </ b> A is the same, and the pitch thereof is the same as the circle pitch T 0 on the reference pitch circle Cp 0 of the worm wheel 30.
[0007]
Therefore, the contact position between the one worm tooth surface 22A and the wheel tooth surface 32 and the contact position between the other worm tooth surface 23A and the wheel tooth surface 33 are indicated by reference numerals C3 and C4, respectively. In any case, the center position of the wheel tooth portion 31 of the worm wheel 30 is in the tooth trace direction. Further, the center line P of the tooth groove 24 (see FIG. 2) between the worm tooth portions 21A and the center line Q of the wheel tooth portion 31 are both the center portion M of the meshing range of the worm tooth portion 21 and the wheel tooth portion 31. Both center lines P and Q coincide with each other.
[0008]
Next, referring to FIG. 6 (a), a case of a worm gear device using a multi-lead worm 20B will be described. The worm tooth portion 21B of the worm 20B is a multi-lead worm in which one worm tooth surface 22A is the same as that shown in FIG. 5 and the other worm tooth surface 23B is larger in pitch than the one worm tooth surface 22A. The worm wheel 30 is the normal one described above. In this case, since one worm tooth surface 22A is the same as that in FIG. 5, the contact position between the one worm tooth surface 22A and the wheel tooth surface 32 is indicated by the same symbol C3 as in FIG. Thus, although the wheel tooth portion 31 is in the center position in the tooth line direction, the lead angle of the other worm tooth surface 23B is larger than the other worm tooth surface 23A of FIG. The abutting position between is a position shifted from the center of the tooth tooth direction of the wheel tooth portion 31 as indicated by reference numeral C5. Further, the direction of the center line Pa of the tooth groove 24 between each worm tooth portion 21B is deviated from the direction of the center line P in FIG. 5 by half of the difference in lead angle between the worm tooth surfaces 22A and 23B. 5, the crossing angle corresponding to half the difference between the lead angles of the worm tooth surfaces 22A and 23B is generated between the center line Pa and the center line Q.
[0009]
6 (b) is the lead angle of one of the worm tooth surface 22A is the same as in FIGS. 6 (a), the lead angle of the other of the worm tooth surface 23C than the worm tooth surface 2 3B shown in FIG. 6 (a) A multiple lead worm 20C having a worm tooth portion 21C in which the difference in lead angle between both tooth surfaces 22A and 23C is increased by increasing the size is used. In this case, the contact position between the one worm tooth surface 22A and the wheel tooth surface 32 remains at the center position in the tooth line direction of the wheel tooth portion 31 as indicated by reference numeral C3, but the other worm tooth surface 23C. As shown by reference numeral C6, the other worm tooth surface 23C hits the other wheel tooth surface 33 at the corner of the end of the tooth line direction. In addition, the crossing angle between the center line Pb of the tooth gap 24 between each worm tooth portion 21C and the center line Q of the wheel tooth portion is also increased. In the state where the warm tooth surface 23C approaches state or to hit the corners of the wheel tooth surface 33, per the part will wheel tooth surfaces of the hand in the tooth trace direction center as shown at C3 One worm tooth surface 22A that hits 32 has a different contact condition, and therefore, friction characteristics differ between the other tooth surface 23C and tooth surface 33 and between one tooth surface 22A and tooth surface 32. Thus, the power transmission characteristics with slipping are different between the forward direction and the reverse direction.
[0010]
In the electric power steering apparatus having a power assist unit as shown in FIG. 2, the assist force is transmitted between the worm tooth surface 22A and the wheel tooth surface 32 when the handle is cut in one direction. Assist force transmission when cutting in the opposite direction is performed between the other worm tooth surface 23C and the other wheel tooth surface 33. However, as described above, when the power transmission characteristic accompanied by the slip between the worm 20 and the worm wheel 30 is different between the forward direction and the reverse direction, the steering torque characteristic is different depending on whether it is turned rightward or leftward. Therefore, there is a problem that the steering feels strange.
[0011]
As described above, in the electric power steering apparatus using the multiple lead worm, on the one hand, it is necessary to increase the difference between the leads of both tooth surfaces of the multiple lead worm in order to widen the adjustment range of the backlash. In order to prevent the occurrence of a sense of incongruity due to the difference in steering direction, there is a limit to the increase in the difference between the leads on both tooth surfaces. Thus, the problem caused by the difference in the power transmission characteristics of the worm gear device between the forward direction and the reverse direction is not limited to the case of the electric power steering device, and the worm gear that needs to transmit power in both the forward and reverse directions in the same way. Always present in the device. An object of the present invention is to widen the adjustment range of backlash without causing the power transmission characteristics of the worm gear device to differ between the forward direction and the reverse direction.
[0012]
[Means for Solving the Problems]
For this, the worm gear device according to the invention of claim 1, and the worm wheel consists of a worm having a worm toothing which wheel teeth mesh with the worm wheel, the worm in a worm gear unit consisting a double lead worm, The pitch of one tooth surface of the worm tooth portion is smaller by a predetermined amount than the circular pitch on the reference pitch circle of the wheel tooth portion, and the pitch of the other tooth surface of the worm tooth portion is smaller than the circular pitch on the reference pitch circle. The center of the tooth gap between the worm tooth portions at the center portion on the cylindrical surface including the center portion of the meshing range of the worm tooth portion and the wheel tooth portion around the center axis of the worm by increasing by a predetermined amount. The worm teeth and wheel teeth should be arranged so that the line matches the center line of the wheel teeth at the center on the cylindrical surface. The one in which the features. By arranging the worm tooth portion and the wheel tooth portion in this way, the contact position between one worm tooth surface and the wheel tooth surface and the contact position between the other worm tooth surface and the wheel tooth surface are as follows. In any case, the wheel teeth are shifted from the central position in the tooth trace direction by the same distance. As the lead angle difference between the worm tooth surfaces increases, the deviation of one and the other abutment position with respect to the center position in the tooth line direction of the wheel tooth portion increases in a similar manner. At the same time, each wheel tooth surface comes into contact with each corner at the end of the tooth trace direction.
[0013]
According to a second aspect of the present invention, there is provided a worm gear device comprising a worm wheel and a worm having a worm tooth portion meshing with a wheel tooth portion of the worm wheel, wherein the worm is a double lead worm. By adjusting the crossing angle between the axis of the worm wheel and the center axis of the worm wheel, the worm tooth portion on the cylindrical surface including the central portion of the meshing range of the worm tooth portion and the wheel tooth portion around the central axis of the worm. The worm tooth portion and the wheel tooth portion are arranged so that the center line of the tooth gap between the two and the center line of the wheel tooth portion at the center portion on the cylindrical surface coincide with each other. By arranging in this manner, the contact position between one worm tooth surface and the wheel tooth surface and the contact position between the other worm tooth surface and the wheel tooth surface are the same as in the invention of claim 1. In any case, the wheel teeth are shifted from the central position in the tooth trace direction by the same distance. As the lead angle difference between the worm tooth surfaces increases, the deviation of one and the other abutment position with respect to the center position in the tooth line direction of the wheel tooth portion increases in a similar manner. At the same time, each wheel tooth surface comes into contact with each corner at the end of the tooth trace direction.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment in which the worm gear device according to the present invention is applied to an electric power steering device will be described below with reference to FIGS. As shown in FIG. 1, the power assist unit of the electric power steering apparatus includes a worm wheel 30 connected to a steering shaft 15 rotated by a steering handle, and a wheel tooth portion 31 of the worm wheel 30 (see FIG. 2). ) Having a worm tooth portion 21 (see FIG. 2) and an electric motor 13 for rotationally driving the worm 20, and these are supported by a housing 10 fixed to the steering column. . The worm 20 and the worm wheel 30 constitute the worm gear device of the present invention. The control device for the electric power steering apparatus operates the electric motor 13 based on the steering torque detected by the torque sensor provided on the steering shaft 15, and applies assist force to the steering shaft 15 via the worm 20 and the worm wheel 30. give.
[0016]
Both ends of the worm 20 are supported by the housing 10 via ball bearings 11, and the tightening nuts 12a and 12b are tightened by loosening the tightening nuts 12a and 12b screwed to the both ends and adjusting the axial position. The tip 20a of the worm 20 is splined to the output shaft of the electric motor 13 in a slidable manner. As shown in FIG. 2, the worm 20 is a double-lead worm having two worm tooth surfaces 22 and 23 having different pitches T1 and T2 on both sides of the worm tooth portion 21, and the tooth thickness of the worm tooth portion 21 increases toward the right. As a result, the width of the tooth gap 24 is increased. Accordingly, the backlash between the worm 20 and the worm wheel 30 decreases when the worm 20 is moved to the right by adjusting the tightening nuts 12a and 12b, and increases when the worm 20 is moved to the left, thereby adjusting the backlash. Can do. Since the adjustment structure of the axial position of the worm 20 is as described above, it is difficult to increase the adjustment amount. Therefore, in order to widen the adjustment range of the backlash, the difference between the pitches T1 and T2 of the worm tooth surfaces 22 and 23 is different. Need to be larger.
[0017]
The worm wheel 30 is the same as that of the prior art described above, and is obtained by cutting a standard hob having a diameter larger than that of the worm 20 meshing with the worm wheel from the radial direction. The pressure angle (value in the cross section of FIG. 2) of the wheel tooth portion 31 cut in this way is α0, the circle pitch along the reference pitch circle Cp0 is T0, and the helical angle on the reference pitch circle Cp0. Is tan-1 (πd / nT0). Here, d is the diameter of the reference pitch circle of the worm 20 in contact with the reference pitch circle Cp0, and n is the number of threads of the worm tooth portion 21. The circular pitch T0 is the same value as the pitch of the worm tooth portion 21A of the ordinary worm 20A meshing with the worm wheel 30 described in the prior art shown in FIG.
[0018]
The worm 20 of this embodiment is a double lead worm having two worm teeth 21 as shown in FIG. 2, and the pitches T1 and T2 of the worm tooth surfaces 22 and 23 are used for backlash adjustment. When the difference between them is Td, T1 = T0-Td / 2
T2 = T0 + Td / 2
And Accordingly, when the lead angle difference corresponding to the difference Td between the pitches T1 and T2 of the worm tooth surfaces 22 and 23 is βd, the lead angle of one worm tooth surface 22 is the normal worm 20A shown in FIG. The lead angle of the other worm tooth surface 23 is larger by βd / 2 than the lead angle of the worm 20A. Between the pitches T0, T1 and T2 and the pressure angles α0, α1 and α2, T0 · cosα0 = T1 · cosα1 = T2 · cosα2
There is a relationship.
[0019]
In the state in which the worm 20 and the worm wheel 30 are engaged with each other, as shown in FIG. 2, with respect to one worm tooth surface 22 and the wheel tooth surface 32, their contact points (the actual contact points are those shown in FIG. 2). The first pitch line Lp1 on the worm 20 side and the worm wheel move along the line of action La1 when they are displaced from the cross section in the streak direction (see FIG. 3) and there is a slight gap in the cross section of FIG. The first pitch circle Cp1 on the 30 side makes rolling contact with slip, and the worm tooth portion 21 and the wheel tooth portion 31 mesh with each other before and after the position where this rolling contact is made. Similarly, with respect to the other worm tooth surface 23 and the other wheel tooth surface 33, their contact points move along the action line La2, and the second pitch line Lp2 on the worm 20 side and the second pitch line Lp2 on the worm wheel 30 side. The pitch circle Cp2 makes rolling contact with slip, and the worm tooth portion 21 and the wheel tooth portion 31 mesh with each other before and after the position where the rolling contact is made. Reference numeral M indicates the central portion of the meshing range between the worm tooth portion 21 and the wheel tooth portion 31.
[0020]
FIG. 3 is a cross section taken along the line AA passing through the contact point of each of the tooth surfaces 22, 32, 23, 33 shown in FIG. 2, and the worm tooth portion 21 is centered on the central axis O 1 of the worm 20. The cross section along the cylindrical surface containing the center part M of the meshing range of the wheel tooth part 31 is shown. Although there are a plurality of cylindrical surfaces in this case, FIG. 3 shows a cross section along each cylindrical surface expanded and connected to a plane. Since the worm wheel 30 is the same as that of the prior art described above, the cross-sectional shape of the wheel tooth portion 31 in FIG. 3 is the same as that of the prior art shown in FIG. 5 and FIG. The cross sections 32 and 33 are gently protruding curved lines, and the direction of the center line Q of the wheel tooth portion 31 at the center M of the meshing range is the same as in the prior art shown in FIGS.
[0021]
As described above, the lead angle of one worm tooth surface 22 is smaller by βd / 2 than the lead angle of the ordinary worm 20A shown in FIG. 5, and the lead angle of the other worm tooth surface 23 is that of the worm 20A. It is larger than the lead angle by βd / 2. Accordingly, the contact position between the one worm tooth surface 22 and the wheel tooth surface 32 and the contact position between the other worm tooth surface 23 and the wheel tooth surface 33 are as indicated by reference numerals C1 and C2, respectively. The wheel tooth portion 31 is shifted from the center position in the tooth trace direction by approximately the same distance. Also by offsetting them, the center line P of the tooth groove 24 between the worm teeth 21 as in the prior art shown in FIG. 5, coincides with the center line Q of the wheel teeth 31.
[0022]
As the difference between the lead angles of the worm tooth surfaces 22 and 23 increases, the displacement of each contact position C1 and contact position C2 with respect to the center position in the tooth line direction of the wheel tooth portion 31 increases in a similar manner. At the same time, it reaches the corner of the wheel tooth portion 31 at the end of the tooth line direction. The difference between the lead angles of the worm tooth surfaces 22 and 23 at this time is that one worm tooth surface 22A is in contact with one wheel tooth surface 32 at the center of the tooth direction of the wheel tooth portion 31 and the other worm tooth surface. Only 23 is approximately twice that of the prior art in which the other wheel tooth surface 33 comes into contact with the other end of the tooth trace direction. During this time, the contact condition at each of the contact positions C1, C2 is the same, and therefore the power transmission characteristics accompanied by the slip are the same in the forward direction and the reverse direction. In the electric power steering apparatus as shown in FIG. There is no difference in steering torque characteristics between when the steering wheel is turned to the right and when it is turned to the left.
[0023]
Thus, according to this embodiment, the left and right steering characteristics of the electric power steering device do not differ until the difference in the lead angle between the worm tooth surfaces 22 and 23 is about twice that of the prior art. The range of backlash adjustment can be expanded without causing a sense of incongruity due to the difference in steering direction. Note that after the contact positions of the worm tooth surfaces 22 and 23 and the wheel tooth surfaces 32 and 33 reach the corners at the ends of the wheel tooth surfaces 32 and 33, the surface pressure at the contact positions increases. Unstable frictional characteristics and increased wear are not suitable for practical use.
[0024]
Next, a second embodiment will be described with reference to FIG. In the first embodiment described above, the lead angle of one worm tooth surface 22 is smaller than the lead angle of the ordinary worm 20A, and the lead angle of the other worm tooth surface 23 is larger than the ordinary worm tooth portion 21A. However, in the second embodiment, the worm 20 uses an ordinary multi-lead worm 20C in which the difference between the lead angles of both tooth surfaces 22A and 23C is large as described in FIG. 6B. . In FIG. 6 (b), the central axis O1 of the worm 20C and the central axis O2 of the worm wheel 30 are orthogonal to each other, and the contact position between one worm tooth surface 22A and the wheel tooth surface 32 is denoted by reference numeral C3. Thus, the wheel tooth portion 31 is at the center position in the tooth line direction, and the contact position between the other worm tooth surface 23C and the wheel tooth surface 33 is the other wheel tooth surface at the corner of the tooth line direction end as indicated by reference numeral C6. 33 hits.
[0025]
In the second embodiment, as shown in FIG. 4, the central axis O1 of the worm 20C is from the position N orthogonal to the central axis O2 of the worm wheel 30 to determine the lead angle of each worm tooth surface 22A, 23C. The angle of intersection with the central axis O2 of the worm wheel 30 is adjusted by rotating in the counterclockwise direction by half of the difference βd. Thus is rotated in only half counterclockwise rotation direction of the difference βd of the worm tooth portion 21C also lead angle of FIG. 6 (b), the direction of the center line P b in between the tooth groove 24 centerline of the wheel teeth 31 Since the worm tooth surfaces 22A and 23C of the worm tooth portion 21C are in contact with the tooth surfaces 32 and 33 of the wheel tooth portion 31, the same as in the case of the first embodiment. Similarly, the center line Pb of the tooth gap 24 coincides with the center line Q of the wheel tooth portion 31. Accordingly, the contact position between one worm tooth surface 22 </ b> A and the wheel tooth surface 32 is separated from the center position in the tooth line direction of the wheel tooth portion 31, and the contact between the other worm tooth surface 23 </ b> C and the wheel tooth surface 33. The position approaches the center position in the tooth trace direction of the wheel tooth portion 31, and the respective contact positions are shifted by approximately the same distance from the center position in the tooth trace direction of the wheel tooth portion 31 as indicated by reference numerals C1 and C2 in FIG. It becomes the position.
[0026]
Also in the second embodiment, as the lead angle difference between the worm tooth surfaces 22 and 23 increases, the contact positions of the worm tooth surfaces 22 and 23 are substantially the same as the tooth line direction of the wheel tooth portion 31. It approaches the corner of the end and reaches this corner almost simultaneously. The difference between the lead angles of the worm tooth surfaces 22 and 23 when reaching this corner is about twice that of the prior art shown in FIG. Therefore, the friction characteristics of one contact position between the worm tooth portion 21 and the wheel tooth portion 31 are different from each other, and the power transmission direction is different between the forward direction and the reverse direction. The difference between the lead angles of the worm tooth surfaces 22 and 23 up to the same level also increases, and if this is used in an electric power steering device, the back without causing a sense of incongruity due to the difference in steering direction. Rush adjustment range is expanded.
[0027]
In each of the above-described embodiments, the center line P of the tooth gap 24 between the worm tooth portions 21 and the center line Q of the wheel tooth portion 31 are exactly coincident with each other. The backlash adjustment range can be expanded without causing a sense of incongruity due to a difference in steering direction. In each of the above-described embodiments, two worms have been described. However, the present invention can also be applied to one or three or more multiple worms.
[0028]
In the present invention, the center line P and the center line Q are first brought close to each other in the manner of the first embodiment, and then the center line P and the center line Q are made to coincide with each other in the manner of the second embodiment. It is also possible to implement.
[0029]
【The invention's effect】
As described above, according to the present invention, when the difference between the lead angles of both worm tooth surfaces becomes large, one of the worm tooth surfaces and the other worm tooth surface are at the same time the corner of the wheel tooth portion at the end of the tooth line direction. At this point, the wheel hits the corresponding wheel tooth surface. Until then, the contact state at each contact position is the same, and the power transmission characteristics accompanied by the slip are the same in the forward direction and the reverse direction. In addition, when each worm tooth surface comes into contact with the corresponding wheel tooth surface at the end of the tooth direction of the wheel tooth portion, the difference in the lead angle between both worm tooth surfaces is that only the other worm tooth surface has a tooth line. This is about twice as much as in the case of the prior art that comes into contact with the other wheel tooth surface at the corner of the direction end. Therefore, until the difference between the lead angles of the worm tooth surfaces is about twice that of the prior art, the power transmission characteristics do not differ between the forward direction and the reverse direction, so the power transmission characteristics differ between the forward direction and the reverse direction. The range of backlash adjustment can be expanded without causing problems.
[0030]
In addition, according to the power assist portion of the electric power steering configured by such a worm gear device, the left and right sides of the electric power steering device are left and right until the difference in the lead angle between both worm tooth surfaces is about twice that of the prior art. Since there is no difference in steering characteristics, the adjustment range of backlash can be expanded without causing a sense of incongruity due to a difference in steering direction.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a main part of a power assist portion of an electric power steering apparatus employing a first embodiment of a worm gear device according to the present invention.
FIG. 2 is a view showing a main part of the worm gear device shown in FIG. 1;
FIG. 3 is a partial cross-sectional view taken along line AA in FIG.
FIG. 4 is a view showing a second embodiment of the worm gear device according to the present invention.
FIG. 5 is a partial cross-sectional view corresponding to FIG. 2 of the prior art using an ordinary worm that does not have multiple leads.
FIG. 6 is a partial cross-sectional view corresponding to FIG. 2 of the prior art using a multi-lead worm.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 13 ... Electric motor, 15 ... Steering shaft, 20, 20C ... Worm, 21, 21C ... Worm tooth part, 22, 22A ... One worm tooth surface, 23, 23C ... Other worm tooth surface, 24 ... Tooth gap, 30 DESCRIPTION OF SYMBOLS ... Worm wheel, 31 ... Wheel tooth part, M ... Center part of meshing range, O1 ... Center axis line, O2 ... Center axis line, P ... Center line of tooth space , Q ... Center line of wheel tooth part .

Claims (2)

And the worm wheel consists of a worm having a worm toothing which wheel teeth mesh with the worm wheel, the worm in a worm gear unit consisting a double lead worm pitch of one tooth face of the worm teeth the By making a predetermined amount smaller than the circular pitch on the reference pitch circle of the wheel tooth portion, and making the pitch of the other tooth surface of the worm tooth portion larger than the circular pitch on the reference pitch circle by the predetermined amount , and the tooth space center line between the worm teeth of said central portion wherein a on the cylindrical surface including the central portion of the engagement range of the worm teeth and the wheel teeth around the center axis of the worm, the The worm tooth portion and the wheel tooth portion are arranged on a cylindrical surface so that the center line of the wheel tooth portion in the center portion coincides with the center line. Worm gear and wherein the. A worm gear device comprising a worm wheel and a worm having a worm tooth portion meshing with a wheel tooth portion of the worm wheel, wherein the worm is a double lead worm. By adjusting the angle, the tooth gap between the worm tooth portions on the cylindrical surface including the center portion of the meshing range of the worm tooth portion and the wheel tooth portion around the central axis of the worm The worm gear device and the wheel tooth portion are arranged so that the center line of the worm gear portion and the center line of the wheel tooth portion at the center portion coincide with each other on the cylindrical surface .

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