Abstract:
A mechanism for converting rotary motion into linear motion includes a rod having a thread or threaded part at its outer peripheral surface, and a holder member on the outer peripheral surface of the rod, rotatable relative to the rod, and axially movable relative to the rod. Rollers are supported rotatably by the holder member and have annular grooves at their outer peripheral surfaces. Each roller is torsionally disposed at an axial angle greater than or equal to a lead angle of the thread with respect to a central axis of the rod. Both ends of the roller shaft are secured to the holder member, and the mechanism has a structure in which a hole having a diameter greater than that of the roller shaft is provided at one end of the holder member and the roller shaft is secured by a roller mounting member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotary-to-linear motion conversion mechanism for converting the direction of motion between rotary motion and linear motion. 
     2. Description of the Related Art 
     As a rotary-to-linear motion conversion mechanism, there has been known one wherein a rod having a thread at its outer peripheral surface, a holder member provided on the outer peripheral surface of the rod and provided so as to be rotatable relative to the rod and axially movable relative thereto, and a plurality of rollers supported rotatably by the holder member and having annular grooves which mesh with the thread of the rod at their outer peripheral surfaces, are disposed aslant with respect to the central axis of a holder, and the rotation of each roller is converted into linear motion of the rod by meshing engagement therebetween. 
     The rotary-to-linear motion conversion mechanism is, however, accompanied by problems that upon assembly thereof, since the rollers are different from one another in the tilt direction with respect to the holder, a difficulty occurs in fixing one end sides thereof to the holder and thereafter making fixation for the other end sides. On the other hand, there has been proposed a method such as turning only the ends of the rollers into tilt-free forms (refer to U.S. Pat. No. 3,101,623). 
     SUMMARY OF THE INVENTION 
     The above system was accompanied by the problems that it became complicated in shape and high in cost. An object of the present invention is therefore to provide a rotary-to-linear motion mechanism which enables an easiable-to-assemble structure at a low cost. 
     In order to achieve the above object, the present invention provides a mechanism for converting rotary motion into linear motion, comprising: a rod having a thread at an outer peripheral surface thereof; a holder member provided on the outer peripheral surface of the rod, and provided so as to be rotatable relative to the rod and axially movable relative thereto; and rollers supported rotatably by the holder member and having annular grooves brought into meshing engagement with the thread of the rod at outer peripheral surfaces thereof, and each torsionally disposed at an axial angle in the neighborhood of a lead angle of the thread with respect to the central axis of the rod, wherein each of the roller comprises a hollow roller annular groove section and a roller shaft disposed in a hollow portion of the roller so as to be rotatable relative to the annular groove section, wherein the roller shaft has a structure in which both ends thereof are secured to the holder member, wherein a hole having a diameter greater than that of the roller shaft at its end face is provided at one end of the holder member, and wherein the roller shaft is secured by a roller mounting member. 
     According to the present invention, there can be provided a rotary-to-linear motion conversion mechanism which is easy to assemble and is low in cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross-sectional view of a mechanism for converting rotary motion into linear motion, according to an embodiment of the present invention; 
         FIG. 2  is a side view of a sub-assembly of rollers and a holder member, which is employed in the embodiment of the present invention and is a view partly showing cross-sections thereof; 
         FIG. 3  is a side view of the roller employed in the embodiment of the present invention and is a view partly showing cross-sections thereof; 
         FIG. 4  is a view showing one end of the roller and a mounting portion of a holder both employed in the embodiment of the present invention; 
         FIG. 5  is a view illustrating one end of the roller and the mounting portion of the holder both employed in the embodiment of the present invention; 
         FIG. 6  is a view showing one end of the roller and the mounting portion of the holder both employed in the embodiment of the present invention; 
         FIG. 7  is a view depicting one end of the roller and the mounting portion of the holder both employed in the embodiment of the present invention; and 
         FIG. 8  is a view for explaining the operation of the mechanism according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of a mechanism for converting rotary motion into linear motion according to the present invention will be explained using  FIGS. 1 through 8 . 
       FIG. 1  is a longitudinal cross-sectional view of the first embodiment of the present invention,  FIG. 2  is a side view of a sub-assembly of rollers and a holder member,  FIG. 3  is a side view of the roller,  FIGS. 4 through 7  are respectively enlarged views of a portion for mounting the roller and its corresponding holder, and  FIG. 8  is a view for explaining the operation of the rotary-to-linear motion conversion mechanism according to the present embodiment. 
     A configuration of the rotary-to-linear motion conversion mechanism according to the present embodiment will first be explained. As shown in  FIG. 1 , the present rotary-to-linear motion conversion mechanism  1  is comprised principally of a motor  5 , a casing  6  consisting of a left casing  6   a  and a right casing  6   b , a plurality of rollers  21  through  23 , a rod  10 , etc. The rollers  21  through  23  rotate relative to the casing by force produced by the motor  5 , and the rod is linearly moved by meshing engagement of annular grooves respectively provided at the rollers  21  through  23  with a thread or threaded part of the rod  10 . 
     A concrete configuration of the rotary-to-linear motion conversion mechanism will next be described. The sub-assembly  30  comprised of the rollers  21  through  23  and a holder member  3  has a left holder end plate  3   b  which is inserted into the left casing  6   a  equipped with a left holder radial bearing  3   f  and a left holder thrust bearing  3   e  and is fixed or secured in a state of being rotatable relative to the left casing  6   a . Similarly, a right holder radial bearing  3   g  and a right holder thrust bearing  3   h  are inserted into the right casing  6   b . The sub-assembly  30  is mounted in a state of being rotatable relative to the right casing  6   b . The left and right casings are connected by screws and the like to form the casing  6 . 
     Further, a stator  5   b  is press-fitted into the right casing  6   b . In the holder member  3 , a rotor  5   a  is placed in a position opposite to the stator  5   b , and the motor  5  is formed by the stator  5   b  and the rotor  5   a.    
     The sub-assembly  30  comprised of the rollers  21  through  23  and the holder member  3  will next be explained using  FIGS. 2 and 3 . Incidentally, the rollers  21  through  23  are similar in configuration to the roller  21 . The roller  21  will therefore be described below. The roller  21  comprises a roller annular portion or section  21   a  having a through hole at its central axis portion and annular grooves at its outer peripheral surface, and a roller shaft  21   b  disposed in the through hole and disposed fixedly to the holder member  3 . A radial bearing  21   e  and a thrust bearing  21   f  are disposed on the right side of the roller annular portion  21   a . The roller annular portion  21   a  is rotatably connected to the roller shaft  21   b . A right holder end plate  3   c  is secured to the right side of the roller shaft  21   b  by a roller lock nut  36 . At this time, the roller  21  is placed aslant with respect to the central axis of the holder member  3  at an axial angle in the neighborhood of the twisted angle of the rod  10 . 
     Thus, the roller annular portion  21   a  is rotatably secured to the holder member  3 . A through hole  35  having a diameter appropriately larger than that of the roller shaft  21   b  at its end face is provided parallel to the roller shaft  21   b  in such a manner that the roller shaft  21   b  can be inserted into the left holder end plate  3   b . Then, the left roller radial bearing  21   g  is fitted in the through hole  35  and thereafter fills in the through hole  35 . A roller mounting member  33  inserted into the left side of the roller shaft  21   b  and secured to the left holder member  3   b  is embedded in the through hole  35 . Further, the left holder member  3   b  and the roller shaft  21   b  are fixedly disposed by a bolt  34  and the like. 
     Such an assembly method makes it easy to allow the rollers  21  through  23  inclined in different directions relative to the right holder end plate  3   c  to pass through the left holder end plate  3   b.    
     At this time, the left side of the roller shaft  21   b  may preferably be shaped in the form of a taper in such a manner that the diameter of its leftmost end becomes small. 
     By doing so, the diameter of the through hole  35  can be made small, thus making it possible to reduce the entire holder in size. Since it is possible to make the diameter of the through hole small, the strength of the holder can also be enhanced. 
       FIGS. 4 through 6  respectively show procedures for insertion of the roller shaft  21   b  into the left holder end plate  3   b  and necessary diameters. As shown in  FIG. 4 , the axial angle is assumed to be θ, the angle which the roller shaft forms with the central axis of the taper provided at one end side of the roller shaft  21   b  is assumed to be α, and the diameter of the roller shaft  21   b  at its end face is assumed to be D. 
     Consideration will first be given to the case where θ≧α. Consider where the right side of the roller shaft  21   b  as seen in each figure is inclined upward. The through hole is assumed to be provided parallel to the roller shaft  21   b . When a lower point as viewed in the drawing is assumed to be A at the left end face of the roller shaft  21   b  as shown in  FIG. 4 , the roller shaft  21   b  is inserted in such a manner that the point A overlaps with the right lowermost part of the through hole. Next, as shown in  FIG. 5 , the central axis of the roller shaft  21   b  is inserted into the left holder end plate  3   b  by a depth X of insertion therein. At this time, the diameter H of the through hole may be determined to such a size that the upper end of the roller shaft  21   b  and a point B of the through hole come into contact with each other. Thereafter, as shown in  FIG. 6 , the left holder end plate  3   b  is rotated in such a manner that the central axis of the roller shaft  21   b  and the central axis of the through hole  35  coincide with each other. 
     The size of H will now be explained. 
     Consider the through hole  35  whose upper and lower sides are separated into H 2  and H 1  with the central axis of the roller shaft  21   b  taken as the boundary in a state of  FIG. 5 . 
     At this time, H 1  is expressed in the following equation:
 
 H 1=( D/ 2)+ X 1×sin(θ)  (1)
 
where X 1  denotes the depth of insertion of the left lower end of the roller shaft  21   b  into the left holder end plate  3   b.  
 
     Also H 2  is expressed in the following equation:
 
 H 2=( D/ 2)+ X 2×sin(α)/cos(θ+α)  (2)
 
where X 2  indicates the depth of insertion of the left upper end of the roller shaft  21   b  into the left holder end plate  3   b.  
 
     Since X 1  and X 2  are represented by the following equations:
 
 X 1 =X−D× sin(θ)/2  (3)
 
 X 2 =X+D× sin(θ)/2  (4),
 
H (=H 1 +H 2 ) is as follows:
 
 H=D+ ( X−D/ 2×sin(θ))×sin(θ)+( X+D/ 2×sin(θ))×sin(α)/cos(θ+α)  (5)
 
     Next, consider where θ&lt;α. 
     Consider the through hole  35  whose upper and lower sides are separated into H 2  and H 1  with the central axis of the roller shaft  21   b  taken as the boundary in a state of  FIG. 7  as in  FIG. 5 . 
     At this time, H 1  is expressed in the following equation:
 
 H 1=( D/ 2)+ X 1×sin(α)/cos(α−θ)  (6)
 
where X 1  denotes the depth of insertion of the lower end of the roller shaft  21   b  at its left end face into the left holder end plate  3   b.  
 
     H 2  is expressed in the following equation:
 
 H 2=( D/ 2)+ X 2×sin(α)/cos(θ+α)  (7)
 
where X 2  indicates the depth of insertion of the upper end of the roller shaft  21   b  at its left end face into the left holder end plate  3   b.  
 
     Since X 1  and X 2  are represented by the following equations:
 
 X 1 =X−D× sin(θ)/2  (8)
 
 X 2 =X+D× sin(θ)/2  (9),
 
H (=H 1 +H 2 ) is as follows:
 
 H=D+ ( X−D ×sin(θ)/2)×sin(α)/cos(α−θ)+( X+D/ 2×sin(θ))×sin(α)/cos(θ+α)  (10)
 
It is thus possible to minimize the diameter of the through hole  35 .
 
     Finally, the roller shaft  21   b  is secured by its corresponding roller mounting member  33 . Although the hole extending through the left holder end plate  3   b  is taken as the through hole, it is not necessarily required to penetrate the left holder end plate  3   b . Alternatively, the hole may be provided as a penetration-free insertion hole. When the hole is taken as the through hole, the mounting member can be brought to a structure that it can be inserted from the non-roller annular groove section side of the through hole, thus resulting in easy assembly. 
     The operation of the present embodiment will next be explained using  FIG. 8 .  FIG. 8  is a view showing the outer peripheral surface of the rod  10  in a developed form in order to explain the principle of its operation. Now consider where the motor is limited to the rotation thereof down from above in  FIG. 8 . The holder member  3  is rotated about the axis of the rod  10  and the three rollers held by the holder member  3  also perform the same rotation as the motor. Thus, the roller is moved vertically from above (position A) to below (position B) in  FIG. 8 . A heavy line shown in  FIG. 8  indicates a right rod thread flank where the roller is placed in the position A. Consider where the roller is rotated by δ radians (rad) from the position A and moved by δ· (radius of rod shaft) on the circumference of the rod  10  to reach the position B. 
     At this time, the position of the face of each right roller annular groove is not moved in the direction of the rod shaft but moved only in the vertical direction on the developed view. Thus, when the rod is moved in its axial direction (horizontal direction on the developed view) by δ· (radius of rod shaft) tan(rod screw lead angle), the right rod thread flank is moved to the left to reach a position indicated by a broken line, where the meshing engagement between the roller annular groove and the rod thread is held. The rotary-to-linear motion conversion occurs in this way. 
     The amount of motion M of the rod per rotation of the motor becomes M=2π· (radius of rod shaft)·tan (rod screw lead angle) with δ taken as 2π. It is understood that as is apparent from this equation, a speed-reduction ratio can be increased by reducing the rod screw lead angle. Mesh points correspond to the annular groove faces on the roller side and the thread flanks on the rod side. Meshing engagement is made between faces small in curvature. Thus, since contact occurs in a wide range due to elastic deformation at the meshing engagement, the maximum value (hertz stress) of produced stress is suppressed. Therefore, the rotary-to-linear motion conversion mechanism according to the present embodiment brings about advantageous effects in that the load applied per meshing engagement increases and large thrust can be produced for its compact.