Small-sized linear motion guide assembly

An endless rolling contact linear motion guide assembly includes a rail extending straight over a predetermined length, a slider movably mounted on the rail and a plurality of rolling members for providing a rolling contact between the rail and the slider. The slider is provided with an endless circulating path having a load path section, a return path section and a pair of curved connecting path sections, each connecting the corresponding ends of the load and return path sections. The rail is formed with an inner guide groove and the slider is formed with an outer guide groove located opposite to the inner guide groove so that a combination of the inner and outer guide grooves defines the load path section. The outer guide groove has a Gothic arch shape having a pair of upper and lower curved surfaces, each of which is in contact with the rolling member at one contact point. A contact angle defined between a vertical line passing through the center of the rolling member and a straight line passing through the center of the rolling member and a contact point between the rolling member and each of the upper and lower curved surfaces of the outer guide groove is set at an angle larger than 45.degree. and preferably approximately at 50.degree..

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention generally relates to a linear motion guide assembly and in 
particular to a small-sized rolling contact type linear motion guide 
assembly using a plurality of rolling members. More specifically, the 
present invention relates to an endless linear motion guide assembly. 
2. Description of the Prior Art 
A linear motion guide assembly is well known in the art and it generally 
includes a rail extending over a length, a slider mounted on the rail for 
movement along the rail and a plurality of rolling members for providing a 
rolling contact between the rail and the slider. In such a linear motion 
guide assembly, a guide rail is provided with a pair of inner guide 
grooves on opposite sides and a slider is provided with a pair of outer 
guide grooves each of which is located opposite to the corresponding one 
of the pair of inner guide grooves of the rail. And, a plurality of 
rolling members, typically balls, are interposed between the rail and the 
slider to provide a sliding contact between them. In an endless linear 
motion rolling contact guide assembly, a slider is typically provided with 
a pair of endless circulation path which includes a load path section, a 
return path section and a pair of curved connecting sections, each 
connecting the corresponding ends of the load and return path sections. In 
this case, the load path section is defined by the outer guide groove of 
the slider and a corresponding portion of the inner guide groove of the 
rail. The rolling members are provided in the endless circulation path so 
that the rolling members roll along the endless circulation path as the 
slider moves along the rail, whereby those rolling members located in the 
load path section provide a rolling contact between the slider and the 
rail. 
In such a linear motion guide assembly, the balls are maintained in 
position only when the assembly is maintained in an assembled state. For 
example, when the slider is separated from the rail, the rolling members 
will fall off. In order to prevent this from occurring, a stopper is 
provided at each end of the rail to prevent the slider from slipping away 
from the rail. However, such a linear motion guide assembly is rather 
difficult to assemble because the rolling members tend to fall off. Under 
the circumstances, there has been proposed to provide a retainer plate in 
the slider to prevent the rolling members from falling off through an 
opening defined at the load path section when disassembled. However, the 
provision of such a retainer plate is not always advantageous because it 
tends to increase the sliding resistance of the rolling members and create 
a hindrance to make the entire structure smaller. Moreover, difficulty is 
normally encountered in fabricating and assembling such a linear motion 
guide assembly having a retainer plate. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a linear motion 
guide assembly comprising a rail extending over a length, a slider movably 
mounted on said rail and a plurality of rolling members. The rail is 
preferably rectangular or square in shape having a top surface and a pair 
of side surfaces An inner guide groove is formed in each of the side 
surfaces of the rail extending in parallel with the longitudinal axis 
thereof. The slider has a main section and a pair of leg sections 
depending from the main section to thereby define a generally U-shaped 
cross section. Each of the leg sections has a inner surface which faces a 
corresponding one of the side surfaces of the rail and is formed with an 
outer guide groove. When assembled, the outer guide groove of the slider 
is located opposite to a corresponding inner guide groove of the rail. 
The slider is, typically, provided with a pair of endless circulation paths 
each of which includes a load path section, a return path section and a 
pair of curved connecting sections, each of which connects the 
corresponding ends of the load and return path sections. When assembled, 
the load path section of the endless circulation path is defined by the 
outer guide groove of the slider and a corresponding portion of the inner 
guide groove of the rail which is located opposite to the outer guide 
groove of the slider. The rolling members, typically balls, are provided 
in each of the endless circulation paths, so that a rolling contact is 
provided between the rail and the slider through those rolling members 
located in each of the load path sections. 
In accordance with the distinctive features of the present invention, the 
outer guide groove of the slider has a particular cross sectional shape. 
Typically, the outer guide groove is Gothic arch groove. The depth of the 
outer guide groove is larger than the characteristic size, i.e., diameter 
in the case of balls, of the rolling members so that the center of each of 
the balls is located inside the a Gothic arch groove. An inwardly 
projecting ridge is formed at the top and bottom edge of the Gothic arch 
groove and the shortest distance between the tip ends of the oppositely 
located inwardly projecting ridges is smaller than the diameter of the 
balls. As a result, the balls are maintained in position in the outer 
guide groove and they are prevented from falling off through a window 
defined at the mouth of the outer guide groove of the slider. 
More specifically, the outer guide groove is so shaped that the balls are 
in contact therewith at two points. That is, since the outer guide groove 
is of the Gothic arch shape, it includes two curved surfaces, each of 
which is preferably defined by a predetermined radius of curvature. Thus, 
the balls are in contact with the outer guide groove at two points, i.e., 
one for each of the two curved surfaces of the Gothic arch groove. 
Preferably, the inner guide groove is also of the Gothic arch shape so 
that each of the balls is also in contact with the inner guide groove at 
two points. In total, each of the balls are in contact with the inner and 
outer guide grooves at four points when located in the load path section 
of the endless circulation path. Let a contact angle be defined as an 
angle between a vertical line passing through the center of a ball located 
in the load path section and a line extending straight between the center 
of the ball and a contact point between the ball and the upper curved 
surface of the outer guide groove of the slider. This contact angle should 
be set at more than 45.degree. and preferably approximately at 50.degree.. 
With this structure, the contact points between the ball and the outer 
guide groove may be located closer together, which then allows to increase 
a load bearing capacity in the horizontal direction. Accordingly, the 
overall integrity of the assembly may be increased and this structure 
allows to provide an increased ease of manufacture, so that the resulting 
structure may have high precision. Moreover, when the contact angle is set 
at more than 45.degree., even if the inner guide groove must be made 
relatively shallow because of the provision of the deeper outer guide 
groove, the inner guide groove can provide a sufficient support to the 
balls in the load path section since the contact points between the balls 
and the inner guide groove is set closer together. 
It is therefore a primary object of the present invention to obviate the 
disadvantages of the prior art as described above and to provide an 
improved linear motion guide assembly. 
Another object of the present invention is to provide an improved linear 
motion guide assembly small in size, but rigid in structure. 
A further object of the present invention is to provide an improved linear 
motion guide assembly easy to manufacture and with high precision. 
A still further object of the present invention is to provide an improved 
rolling contact type endless linear motion guide assembly which allows to 
prevent its rolling members from falling off unintentionally even when 
disassembled. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now particularly to FIGS. 1 and 2, there is schematically shown a 
linear motion guide assembly constructed in accordance with one embodiment 
of the present invention. As shown, the present linear motion guide 
assembly is of the rolling contact type endless type and it generally 
includes a rail 3 extending straight over a predetermined length, a slider 
mounted on the rail 3 movably therealong, and a plurality of balls 12 to 
provide a rolling contact between the rail 3 and the slider. The rail 3 is 
elongated in shape and it is generally rectangular in shape so that it 
includes a flat top surface, a flat bottom surface and a pair of opposite 
side surfaces 1, 1. Each of the side surfaces 1 is formed with an inner 
guide groove 2 which extends straight in parallel with the center line of 
the rail 3. In the illustrated embodiment, the inner guide groove 2 has a 
Gothic arch shape as will become more clear later and thus it includes an 
upper curved surface portion and a lower curved surface portion. The rail 
3 is also formed with a mounting hole 28 at each end so that the rail 3 
may be fixedly attached to a desired object using bolts or the like. 
Slidably mounted on the rail 3 is the slider which generally includes a 
casing or main body 7 comprised of a flat base section and a pair of leg 
sections each depending from a corresponding side of the base section, 
thereby defining an inverted U-shaped cross section. Each of the leg 
sections of the main body 7 is located adjacent to a corresponding side 
surface 1 of the rail 3 so that the main body 7 of the slider appears to 
straddle the rail 3. The slider also includes a spacer plate 16, which 
will be described more in detail later with reference to FIG. 4, located 
at each end of the main body 7. An end plate 11 is fixedly attached to 
each end of the main body 7 with the spacer plate 16 sandwiched 
therebetween. A seal 27 is fixedly attached to the end plate 11 to seal 
holes formed in the end plate 11. The main body 7 has a bottom surface 4 
which faces and is located adjacent to the top surface of the rail 3. 
The slider is provided with a pair of endless circulating paths each of 
which includes a load path section 9, a return path section 6 and a pair 
of curved connecting path sections 10 each of which connects the 
corresponding ends of the load and return path sections 9 and 6. The 
slider or main body 7 is formed with an outer guide groove 5 at an inner 
side surface of its leg section and the outer guide groove 5 defines the 
load path section 9 together with a corresponding portion of the inner 
guide groove 2 of the rail 3. As will be described more in detail later, 
the outer guide groove 5 of the slider has also a Gothic arch shape and 
thus it is generally defined by a pair of upper and lower curved surfaces. 
A plurality of balls 12 is provided in each of the endless circulating 
paths so that those balls located in each of the load path sections 9 
provide a rolling contact between the rail 3 and the slider. In the 
illustrated embodiment, the outer guide groove 5 and the return path 
section 6 are formed in each of the leg sections of the main body 7 and 
the curved connecting path section 10 is formed in each of the end plates 
11. The main body 7 is also formed with a plurality of mounting threaded 
holes 29 at its top surface so that the slider or main body 7 may be 
fixedly attached to any desired object. 
As best shown in FIG. 2, the balls 12 roll along the load path section 9 
between the rail 3 and the slider and the balls 12 move into the return 
path section 12 through one of the curved connecting path section 10. 
Then, the balls 12 again move into the load path section 9 from the return 
path section 6 through the other curved connecting path section 10. In 
this manner, balls 12 will roll along the endless circulation paths 
indefinitely so that the slider may move along the rail 3 over its entire 
length in either direction. The curved connecting path section 10 is, in 
fact, defined between a portion of the spacer plate 16 and a recess formed 
in the end plate 11. Described more in detail in this respect with 
particular reference to FIG. 4, the spacer plate 16 includes an 
inverted-U-shaped section 19 and a pair of inclined projections 18 each of 
which projects downward section 19. The included projection 18 is 
generally columnar in structure and it is narrowed at its center to 
thereby define a pair of top and bottom fat portions 20. Although not 
shown specifically, the end plate 11 is formed with a pair of upper and 
lower notches above and below the recess for defining the curved 
connecting path section 10, respectively, to thereby receive the upper and 
lower fat portions 20 of the projection 18 when assembled. Thus, the 
surface of the recess 10 and the surface of the narrowed section 17 of the 
inclined projection define the curved connecting path section 10 when 
assembled. The spacer plate 16 is also formed with a pair of positioning 
holes 22 and a pair of mounting holes 25. The spacer plate 16 may be set 
in position by bolting the end plate 11 against one end of the main body 7 
using a bolt 26 passing through the mounting hole 25. The spacer plate 16 
is described in the Japanese Utility Model Laid-open Publication No. 
63-133636, which has been assigned to the assignee of this application. 
As shown in FIG. 2, the return path section 6 is located lower in position 
than the load path section 9. Thus, the mounting hole 29 may be formed 
relatively deeper. Moreover, the curved connecting path section 10 is also 
defined to be somewhat curved when viewed in the longitudinal direction as 
shown in FIG. 2. Such a structure allows to provide a smoother rolling 
movement of the balls 12 along the endless circulation path. 
FIG. 3 schematically illustrates the basic feature of the present invention 
incorporated in the present linear motion guide assembly shown in FIGS. 1 
and 2. As shown in FIG. 3, the basic feature of the present invention 
resides in a particular structure of the load path section defined between 
the slider or main body 7 and the rail 3. The load path section is defined 
by the inner guide groove 2 of the rail 3 and the outer guide groove 5 of 
the slider or main body 7. Each of the inner and outer guide grooves 2 and 
5 has a Gothic arch shape defined by a pair of upper and lower curved 
surfaces. In the illustrated embodiment, each of the upper and lower 
curved surfaces is defined by a radius of curvature which is larger than 
the diameter of the ball 12 so that the ball 12 makes a contact with each 
of the upper and lower curved surfaces at one point. As a result, the ball 
12 is in rolling contact with the inner and outer guide grooves 2 and 5 at 
four points in total. 
It is to be noted that the Gothic arch shape of each of the inner and outer 
guide grooves 2 and 5 is so defined that a contact angle alpha is larger 
than 45.degree. and preferably set approximately at 50.degree.. The 
contact angle alpha is defined as an angle between a vertical line passing 
through the center of the ball 12 and a straight line passing through the 
center of the ball 12 and a contact point between the ball 12 and the 
upper curved surface of either of the inner and outer guide grooves 2 and 
5. With this structure, the contact points between the ball 12 and the 
upper and lower curved surfaces of the inner or outer guide grooves 2 or 5 
are set closer together so that there is provided an increased load 
bearing capability in the horizontal direction with a corresponding 
reduction in the load bearing capability in the vertical direction. As a 
result of an extensive study made by the present inventor, it has been 
found that the contact angle should be set larger than 45.degree.and 
preferably approximately at 50.degree. in many instances. 
The outer guide groove 5 has a larger depth so that the ball 12 is 
substantially received in the outer guide groove 5 with the center of the 
ball 12 located inside the space defined by the outer guide groove 5. The 
outer guide groove 5 is also formed with a pair of upper and lower 
inwardly projecting ridges each having a tip end A. The shortest distance 
between the tip ends A of the pair of inwardly projecting ridges is set to 
be smaller than the diameter of the ball 12. As a result, the ball 12 is 
prevented from falling off the outer guide groove 5 and maintained 
properly in position. Such a structure is particularly advantageous when 
the slider is disassembled from the rail 3 for some reason because the 
balls 12 do not fall off the slider. In addition once the balls 12 are set 
in the endless circulating paths of the slider, they are maintained in the 
endless circulating paths so that assemblage of the present linear motion 
guide assembly is greatly facilitated. 
In the structure shown in FIG. 3, since the outer guide groove 5 is made 
deeper to substantially receive therein the ball 12, the inner guide 
groove 2 must be made correspondingly shallow. However, in accordance with 
the present invention, since the contact angle alpha is set to be larger 
than 45.degree. and preferably at 50.degree. or its vicinity, the ball 12 
is still supported by the shallow inner guide groove 2 sufficiently. If 
the contact angle alpha is relatively small, i.e., 45.degree. or smaller, 
then the contact point between the ball 12 and the inner guide groove 2 
may be located sufficiently close to the side surface of the rail 3, so 
that the ball 12 may be derailed or roll out of the inner guide groove 2. 
Moreover, since the contact points between the ball 12 and the outer guide 
groove 5 are set closer together toward the horizontal line passing 
through the center of the ball 12, it provides for increased ease in 
processing the upper and lower surfaces of the outer guide groove 5. That 
is, even if the outer guide groove 5 is defined to be somewhat deeper so 
as to enclose the ball 12 sufficiently, the critical sections of the upper 
and lower guide surfaces may be finished at high precision because of 
increased accessibility from outside. In manufacture, the casing or main 
body 7 is first processed to a near finish dimension through drawing and 
forging. Then, broaching is applied to form the outer guide groove 5 
having a desired shape as shown in FIG. 3, followed by a step of finishing 
the upper and lower surfaces of the outer guide groove 5 through grinding. 
Such a process can be easily applied in the structure of the present 
invention because the critical portions, where the ball 12 comes into 
rolling contact, of the outer guide groove 5 are located closer to the 
center of the groove 5. 
While the above provides a full and complete disclosure of the preferred 
embodiments of the present invention, various modifications, alternate 
constructions and equivalents may be employed without departing from the 
true spirit and scope of the invention. Therefore, the above description 
and illustration should not be construed as limiting the scope of the 
invention, which is defined by the appended claims.