Patent Publication Number: US-2022213947-A1

Title: Planetary reduction gear

Description:
TECHNICAL FIELD 
     The present invention is related to a planetary reduction gear using helical gears, and more particularly, to a planetary reduction gear provided with a mechanism to suppress the displacement of the sun gear caused by thrust force generated due to engagement with the planetary gear. 
     BACKGROUND ART 
     A known planetary reduction gear is the helical gear-type planetary reduction gear using helical gears as the sun gear, planetary gear and internal gear. In this type of planetary reduction gear, a thrust force is generated along the center axis due to the engagement of the helical gears. If the sun gear is displaced relative to the planetary gear in the axial direction due to the thrust force generated between the sun gear and planetary gear, the rotational motion is not transmitted accurately, and the angle error between input and output rotation increases. 
     For example, bearings can be used to fix the sun gear to suppress the displacement of the sun gear in the axial direction. In this case, it is difficult to eliminate the angle error because the internal gap of the bearing appears as the displacement in the axial direction of the sun gear. 
     To eliminate the displacement of the sun gear in the axial direction, the helical planetary gear reducer described in patent document 1 has a front-stage planetary carrier sandwiched by thrust bearings on both sides in the direction of its center axis, and the thrust bearings are preloaded by spring members. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP 2005-291426 A 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In the case of a structure with two thrust bearings sandwiching the front-stage planetary carrier from both sides in the axial direction, the dimension in the axial direction increases as the front-stage planetary carrier and the two thrust bearings are arranged in the axial direction. This structure is not suitable if there is no space for installation in the axial direction. 
     The purpose of the present invention is to provide a planetary reduction gear of the helical gear-type incorporated with a simple and compact structure to suppress the movement of the sun gear in the axial direction. 
     Means of Solving the Problem 
     The planetary reduction gear of the present invention includes a cylindrical device housing, a front-stage planetary gear reduction mechanism and a rear-stage planetary gear reduction mechanism, these mechanisms being built into the device housing. The rear-stage planetary gear reduction mechanism includes: a rear-stage internal gear fixed to the device housing or integrally formed with the device housing; a rear-stage sun gear that is arranged coaxially inside the rear-stage internal gear and is connected and fixed coaxially to a front-stage planetary carrier, the front-stage planetary carrier being an output element of the front-stage planetary gear reduction mechanism; a rear-stage planetary gear engaging with each of the rear-stage internal gear and the rear-stage sun gear; a rear-stage planetary carrier that rotatably supports the rear-stage planetary gear and is rotatably supported by the device housing through a rear-stage carrier bearing; a first bearing supporting the front-stage planetary carrier; a second bearing supporting the rear-stage sun gear; and a preload mechanism that applies preload to the second bearing in a direction along the center axis towards the first bearing. The rear-stage internal gear, the rear-stage sun gear and the rear-stage planetary gear are helical gears, respectively. The first bearing is a radial bearing or a thrust bearing that rotatably supports the front-stage planetary carrier with respect to the device housing. The second bearing is a radial or thrust bearing that rotatably supports the rear-stage sun gear with respect to the rear-stage planetary carrier. The preload mechanism includes a fixed-position preload member that is fixed to the rear-stage planetary carrier and preloads the second bearing to a fixed position in the direction of the center axis. 
     In the planetary reduction gear of the present invention, the sun gear connected to the front-stage planetary carrier is supported from both sides in the direction of the center axis by the first bearing, which is a radial bearing or thrust bearing attached to the device housing, and the second bearing attached to the rear-stage planetary carrier. The sun gear is sandwiched between the first bearing and the second bearing with a predetermined preload force in a state in which a fixed-position preload is applied from the side of the second bearing to eliminate the bearing internal gap. 
     The increase in dimensions in the axial direction can be suppressed compared to the conventional structure where the front-stage planetary carrier is sandwiched by two pairs of thrust bearings on both sides in the direction of the center axis. By applying the fixed-position preload, the displacement of the sun gear in the direction of the center axis can be reliably prevented without being affected by any increase or decrease in the thrust force acting on the sun gear under load. 
     In the present invention, a thrust bearing which rotatably supports the rear-stage sun gear in the direction along the center axis can be used as the second bearing. A set screw that is fixed by screw-in to the rear-stage planetary carrier and applies a preload on the second bearing at a fixed position in the direction of the center axis can be used as the fixed-position preload member. When a set screw is used, it is desirable that a locking mechanism to prevent the set screw from loosening is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic longitudinal sectional view showing an example of a planetary reduction gear to which the present invention is applied; 
         FIG. 1B  is a partial longitudinal sectional view showing an example of the planetary reduction gear of  FIG. 1A  when a locking mechanism is provided to prevent the loosening of the set screw; and 
         FIG. 2  is a schematic longitudinal sectional view showing an example of a planetary reduction gear in a case in which the thrust bearing in the planetary reduction gear of  FIG. 1A  is replaced by a radial bearing. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of a planetary reduction gear to which the present invention has been applied is explained below. The following explanation is not intended to limit the present invention to this embodiment. 
       FIG. 1A  is a schematic longitudinal sectional view of an embodiment of a helical-gear-type planetary reduction gear according to the present invention. Helical gears are used for each component gear of the planetary reduction gear  1 . The planetary reduction gear  1  is a two-stage speed reducer provided with a front-stage planetary speed reduction mechanism  10  and a rear-stage planetary speed reduction mechanism  20 . The front-stage planetary speed reduction mechanism  10  is provided with a front-stage sun gear  11 , front-stage planetary gears  12 , a front-stage planetary carrier  13  and a front-stage internal gear  14 . The rear-stage planetary speed reduction mechanism  20  is provided with a rear-stage sun gear  21 , rear-stage planetary gears  22 , a rear-stage planetary carrier  23  and a rear-stage internal gear  24 . 
     For example, the front-stage internal gear  14  and the rear-stage internal gear  24  are fixed-side gears. The front-stage sun gear  11  is a rotation input element to which a rotation from the outside is input. The front-stage planetary carrier  13  transmits a reduced-speed rotation through the front-stage planetary speed reduction mechanism  10  to the rear-stage sun gear  21  of the rear-stage planetary speed reduction mechanism  20 . The rear-stage planetary carrier  23  is a reduced-speed-rotation output element that outputs a reduced-speed rotation through the rear-stage planetary speed reduction mechanism  20  to the outside. 
     The planetary reduction gear  1  is provided with a cylindrical device housing  2 . The device housing  2  is fixed to a fixed-side member (not shown) through the mounting flange  2   a  formed on its outer peripheral surface. The front-stage planetary speed reduction mechanism  10  and the rear-stage planetary speed reduction mechanism  20  are assembled coaxially inside the device housing  2  and are arranged in the direction of the center axis la. In the device housing  2 , one end thereof on the side of the front-stage planetary speed reduction mechanism  10  is referred to as an input end IN, and the other end thereof on the side of the rear-stage planetary speed reduction mechanism  20  is referred to as an output end OUT. 
     To explain in detail, the front-stage sun gear  11  is arranged concentrically on the side of the input end IN inside the device housing  2 . The front-stage internal gear  14  is formed on the inner peripheral surface of the device housing  2  concentrically surrounding the front-stage sun gear  11 . The front-stage planetary carrier  13  is provided with a disc part  13   a  located on the side of the output end OUT with respect to the front-stage sun gear  11 , a cylindrical part  13   b  extending from the outer periphery of the disc part  13   a  toward the input end IN, and one or a plurality of front-stage planetary shafts  13   c  fixed to disc  13   a.  The front-stage planetary shafts  13   c  extend in a direction parallel to the center axis la from the disc  13   a  towards the input end IN. The front-stage planetary gear  12  is rotatably supported on the front-stage planetary shaft  13   c.  The front-stage planetary gear  12  engages with both the front-stage sun gear  11  and front-stage internal gear  14 . The portion corresponding to each front-stage planetary gear  12  in the cylindrical part  13   b  of the front-stage planetary carrier  13  is an opening part. 
     The front-stage planetary carrier  13  is rotatably supported via a radial bearing  15  on the device housing  2 , the radial bearing being the first bearing. The leading edge of the cylinder  13   b  of the front-stage planetary carrier  13  is a circular portion  13   d.  A radial bearing  15 , for example, a deep groove ball bearing is mounted between the circular outer peripheral surface of the circular portion  13   d  and the inner peripheral surface  2   b  of the device housing  2  facing the circular outer peripheral surface. The radial bearing  15  is sandwiched in the direction of the center axis la by a circular step  13   e  formed on the circular portion  13   d  and a circular step  2   c  formed on the inner peripheral surface  2   b  of the device housing  2 . The radial bearing  15  is not limited to a deep groove ball bearing, and an angular contact ball bearing, a cylindrical roller bearing, a tapered roller bearing, a thrust ball bearing or a thrust roller bearing can also be used. 
     The rear-stage sun gear  21  of the rear-stage planetary reduction mechanism  20  is fixed coaxially to the disc part  13   a  of the front-stage planetary carrier  13  and extends from here towards the output end OUT. The rear-stage internal gear  24  is formed on the inner peripheral surface of the device housing  2  concentrically surrounding the rear-stage sun gear  21 . The rear-stage planetary carrier  23  is arranged coaxially at the side of the output end OUT with respect to the rear-stage sun gear  21 . The rear-stage planetary carrier  23  is provided with a disc-shaped main part  23   a  and a single or plurality of rear-stage planetary shafts  23   b  that are fixed to the disc-shaped main part  23   a.  The rear-stage planetary shaft  23   b  extends from the disc-shaped main part  23   a  towards the input end IN. The rear-stage planetary gear  22  is rotatably supported on the rear-stage planetary shaft  23   b.  The rear-stage planetary gear  22  engages with the rear-stage sun gear  21  and the rear-stage internal gear  24 . 
     The rear-stage planetary carrier  23  is rotatably supported via the rear-stage carrier bearing  25  on the device housing  2 . An inner ring mounting groove  23   c  is formed on the circular outer peripheral surface of the disc-shaped main part  23   a  of the rear-stage planetary carrier  23 , and an outer ring mounting groove  2   d  is formed on the inner peripheral surface of the device housing  2  where the inner ring mounting groove faces. The rear-stage carrier bearing  25  is mounted between these grooves. A deep groove ball bearing is used as the rear-stage carrier bearing  25  in this example. The rear-stage carrier bearing  25  is also not limited to a deep groove ball bearing, and an angular contact ball bearing, a cylindrical roller bearing, or a tapered roller bearing can be used. 
     The rear-stage sun gear  21  is supported by a thrust bearing  30 , which is the second bearing. The thrust bearing  30  is mounted in the direction of the center axis la between the rear-stage sun gear  21  and the rear-stage planetary carrier  23 . The rear-stage sun gear  21  is rotatably supported by the rear-stage planetary carrier  23  through the thrust bearing  30  from the direction (thrust direction) along the center axis  1   a.  A cylindrical roller bearing is used as the thrust bearing  30  in this example. The thrust bearing  30  is not limited to a cylindrical roller bearing, and a thrust ball bearing, a cylindrical roller bearing, or a rolling element such as a ball or a sliding bearing that receives the load in the thrust direction can be used. 
     A preload mechanism  40  is mounted on the rear-stage planetary carrier  23 . A preload is applied to the thrust bearing  30  by the preload mechanism  40  along the center axis la in the direction towards the rear-stage sun gear  21 . In this example, the preload mechanism  40  is provided with a set screw  41  and a pressure plate  42 . The set screw  41  is a fixed-position preload member that applies a preload to the thrust bearing  30  through the pressure plate  42  at a fixed position in the direction of the center axis la. 
     To describe in detail, the rear-stage sun gear  21  is provided with a gear end face  21   a  facing a carrier end face  23   d  on one side of the disc-shaped main part  23   a  of the rear-stage planetary carrier  23 . A small-diameter shaft end  21   b  part is formed at the center portion of the gear end face  21   a.  The shaft end part  21   b  protrudes coaxially from the gear end face  21   a  towards the carrier end face  23   d.  The carrier end face  23   d  of the disc-shaped main part  23   a  of the rear-state planetary carrier  23  is formed at its center portion with a circular recess part  23   e  that has a fixed depth in the direction of the center axis  1   a  and opens towards the rear-stage sun gear  21 . The shaft end part  21   b  of the rear-stage sun gear  21  is inserted coaxially into the circular recess part  23   e  of the rear-stage planetary carrier  23 . 
     A threaded hole  23   f  is formed in the disc-shaped main part  23   a  of the rear-stage planetary carrier  23 , the threaded hole passing through the center portion of the disc-shaped main part  23   a  in the direction of the center axis  1   a.  One end of the threaded hole  23   f  is open to the bottom surface of the circular recess part  23   e,  and the other end thereof is open to the other end face  23   g  of the disc-shaped main part  23   a.    
     The disc-shaped pressure plate  42  is mounted in the circular recess part  23   e,  such that it can slide in the direction of the center axis  1   a.  A thrust bearing  30  mounted on the small-diameter shaft end part  21   b  is sandwiched between the gear end face  21   a  and pressure plate  42 . A set screw  41  is fixed by screwing into the threaded hole  23   f  from the end face  23   g.  The tip end face of the set screw  41  is in contact with the pressure plate  42 . The set screw  41  presses the pressure plate  42  against the thrust bearing  30 , and the prescribed preload is applied to the thrust bearing  30 . 
     In this example, the single set screw  41  is arranged on the center axis  1   a , a plurality of set screws  41  can be arranged circumferentially with the center axis  1   a  as the center. It is preferred that a locking mechanism to prevent loosening be provided for the set screw  41 . For example, as shown in  FIG. 1B , a separate threaded hole  231  is provided at an angle to the threaded hole  23   f  in the disc-shaped main part  23   a  of the rear-stage planetary carrier  23 . A screw  232  is screwed into the threaded hole  231  that engages with the set screw  41 , preventing loosening. Instead of the set screw  41 , a fixed-position preload member such as a pin can be fixed to the disc-shaped main part  23   a  of the rear-stage planetary carrier  23  by press-fitting, bonding, welding or a combination of these methods. 
     As explained above, in the planetary reduction gear  1 , the rear-stage sun gear  21 , which is connected to the front-stage planetary carrier  13 , is supported by the device housing  2  through the radial bearing  15  (the first bearing) at the input end IN side and is also supported by the rear-stage planetary carrier  23  in the thrust direction along the center axis  1   a  through the thrust bearing  30  (the second bearing) at the output end OUT side. The rear-stage planetary carrier  23  is supported by the device housing  2  through the rear-stage carrier bearing  25 . The prescribed preload is applied to the thrust bearing  30  by the set screw  41  along the direction of the center axis  1   a.  With this configuration, the rear-stage sun gear  21  engages with the rear-stage planetary gear  22  in a state where the movement of the rear-stage sun gear is constrained in the direction of the center axis  1   a  (thrust direction). Therefore, the angle error between the input and output rotations can be suppressed. 
     The thrust bearing  30  and the preload mechanism  40  are assembled in the circular recess part  23   e  formed on the rear-stage planetary carrier  23 . The rear-stage sun gear  21  can be supported from the thrust direction without increasing the dimension along the center axis  1   a  of the planetary reduction gear  1 . The displacement of the rear-stage sun gear  21  is suppressed with a fixed-position preload mechanism having the set screw  41 . The support rigidity is high, and the positional stability of the rear-stage sun gear  21  in the direction (thrust direction) of the center axis  1   a  is excellent compared to when a constant-pressure preload mechanism having a spring member such as a washer is used. 
     The rear-stage sun gear  21  is supported by using the thrust bearing  30  as the first bearing in the planetary reduction gear  1 . A radial bearing can be used instead of the thrust bearing  30  as the first bearing. 
     For example, a planetary reduction gear  1 A shown in  FIG. 2  is provided with a radial ball bearing  30 A mounted between a shaft end part  21   b  of a rear-stage sun gear  21  and a circular inner peripheral surface of a circular recess part  23   e  of a rear-stage planetary carrier  23 . The radial ball bearing  30 A can slide in the direction of the center axis la along the circular inner peripheral surface of the circular recess part  23   e.  A preload is applied to the outer ring of the radial ball bearing  30 A by a preload mechanism  40 A that is provided with a set screw  41  and a pressure plate  42 A. The preload eliminates the internal gap of the radial ball bearing  30 A, and the rear-stage sun gear  21  and the rear-stage planetary gear  22  engage with each other in a state where the relative displacement along the center axis la is constrained. Since the configuration of the other parts in the planetary reduction gear  1 A is the same as the planetary reduction gear  1  shown in  FIG. 1 , the corresponding parts have been assigned the same symbols, and the explanation has been omitted.