Description:
BACKGROUND OF THE INVENTION
This invention relates to a method and a device for grinding the flat end faces of cylindrical bodies and, more particularly, of rollers for rolling bearings in which said bodies move along a substantially circular path, between two rotary grinding wheels disposed one in front of the other.
Cylindrical bodies are utilized in several applications, particularly in roller bearings wherein they constitute the rolling bodies. For a correct assembling of such rollers between the rings or races of a bearing, as well as for a good operation of the latter during use, it is necessary that each roller should have a high surface finish and a perfect perpendicularly between its end faces and its axis. Both these favourable properties, which can be obtained solely by means of grinding operations of the rollers, contribute in increasing the efficiency of the bearing, by decreasing the friction forces and improving the axial guide of the same rollers.
In order to grind the end faces of the rollers, the latter are fed between two rotating grinding wheels disposed one in front of the other and, simultaneously, the rollers are allowed to rotate about their own axes. Said rollers are fed by a disk disposed between the two aforementioned grinding wheels and entrained in rotation by an independent motor, usually provided by a variator.
The rollers are disposed in housings constituted by perforations provided in the periphery of said disk, whose axes are parallel to the axis of the latter. In a second known device, the housings of the rollers are constituted by grooves; in order to hold the rollers in position, a flexible metallic member is wound thereon, whose ends are fixed; this member is adapted to apply to the rollers substantially radial pressures, directed towards the axis of the disk. On account of the contact between the rollers and said flexible member, during the rotation of the disk a rolling movement of the rollers on said member also takes place and, therefore, a rotational movement of the same rollers about their axes as well.
In a third known grinding device, the rollers are caused to roll on both the cylindrical surface of a disk entrained in rotation and on the cylindrical surface of a corresponding stationary and somewhat resilient shoe, so as to form a channel for the circulation of said rollers, which is defined by two coaxial rolling tracks.
The known methods and devices of the kind described above suffer from several drawbacks. In the third device just described, the rotational speed of the roller about its own axis is not independent of the speed of rotation of said disk. As a consequence, during processing, it is impossible to choose a ratio, between these two speeds, adapted to attain grinding of the end faces of the rollers with high productivity and quality of work.
In the first device described above, the rotational speed of the roller about its axis depends solely on the tangential actions generated by the friction of the grinding wheels during the cutting action; this action is variable depending on many factors, such as the materials to be removed, the physical characteristics of the grinding wheels and the coolant, etc., and is generally different from a roller to another, thereby causing variations of the quality of the surface. Moreover, the positioning of each roller inside the housing of the disk is scarcely accurate, thereby frequently resulting in errors of perpendicularity between the end faces of the ground rollers and the corresponding axes.
The second device described above has the same defects as the third, but to these the disadvantage must be added that, due to the friction of the cylindrical surface of the roller in the housing, sliding movements with respect to the flexible member can take place, with variation of the speed and considerable wear of the components of said device.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method and a device for grinding the flat end faces of cylindrical bodies, wherein said bodies move along a substantially circular path between two rotary grinding wheels disposed one in front of the other, which enable to avoid the disadvantages enumerated above.
According to the present invention it is provided a method for grinding the flat end faces of cylindrical bodies, comprising:
carrying said bodies along a substantially circular path between two rotary grinding wheels disposed one in front of the other, by a rotating transport member actuated by first actuating means;
rotating each body about its own axis, during said movement, by second actuating means.
Furthermore it is provided a device for grinding the flat end faces of cylindrical bodies comprising:
a transport member apt to carry said bodies along a substantially circular path between two rotary grinding wheels disposed one in front of the other, said transport member allowing rotation of each body about its own axis;
first actuating means to drive said transport member;
second actuating means to drive the rotation of each body about its axis.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, both the various steps of the method and a particular embodiment of the device utilized in said method will now be described, merely by way of non-limiting example, with reference to the accompanying drawings, wherein:
FIG. 1 shows a perspective view, partly in section and partly exploded, of a portion of the device of the invention;
FIG. 2 shows a diagrammatic side view of the device of FIG. 1;
FIG. 3 shows a side view of a portion of the device of FIG. 1;
FIG. 4 shows a section of the portion of the device of FIG. 3, made along the lines IV--IV;
FIG. 5 shows a section of the portion of the device of FIG. 3, made along the lines V--V;
FIGS. 6 and 7 show, respectively, a diagrammatic section of the portion of the device shown in FIG. 4 and a diagram of an epicyclic train kinematically equivalent to said portion, and
FIG. 8 is a diagrammatic side view of said epicyclic train.
DETAILED DESCRIPTION OF THE INVENTION
The embodiment of the device in accordance with the invention illustrated in the drawings, by means of which it is possible to carry out the method of the invention, substantially comprises a feeder of the rollers to be ground generally indicated at 1 in FIG. 1, a pair of grinding wheels 2 disposed one in front of the other so as to define, with their active faces, a space 3 inside which the aforementioned feeder is partially housed, and means for actuating the feeder 1, generally indicated at 4.
The grinding wheels 2 (FIGS. 1 and 2), according to a known arrangement, have their axes substantially coincident (or forming a small angle therebetween) and rotate in the same direction and with the same angular velocity. These wheels are driven by means not shown in the drawings comprising, for instance, two electric motors each of which will cause one corresponding grinding wheel to rotate.
The feeder 1 substantially comprises a disk 5 (FIGS. 1 through 5) on whose cylindrical side surface a rolling track 6 (FIGS. 1 and 4) is provided, for a crown of rollers 7 whose end faces 7' must be ground. Said disk is caused to rotate about its own axis by means not shown, for instance by a drive in which a speed variator is inserted, which is actuated, in turn, by an electric motor, so as to obtain any desired angular velocity of said disk, comprised in a predetermined range.
Coaxially with the disk 5 and substantially in the same plane of the latter, there is disposed a roller cage or retainer 8 (FIGS. 1, 2 and 4) of substantially annular shape, whose cylindrical inner surface 11 (FIG. 5) coacts with the aforementioned rolling track 6 of the disk 5, by sliding on the same, in order to allow rotation of the cage relative to said disk. It is apparent that the cage 8, during its rotation, could be supported in a manner different from that described above, for instance with support means known per se and independent of the disk 5, which would eliminate any skimming between the surface 11 and the track 6.
In the radially innermost part of the cage 8 a number of housings 12 for the rollers 7 is provided (FIGS. 1 and 3), each of which is defined by a cylindrical surface 13 (FIG. 13) and a pair of opposite planar faces 14. The distance between these faces is slightly greater than the diameter of a roller 7, so that the latter may roll inside the corresponding housing, although being guided by said faces. Further, when the cylindrical surface of each roller is in contact with the rolling track 6 of the disk 5, it will have a certain clearance with respect to the cylindrical surface 13 of the corresponding housing 12, as clearly seen in FIG. 3.
In the central part of the cage 8 an annular groove 15 is provided (FIGS. 1, 4 and 5), whose depth in radial direction, indicated at a in FIGS. 4 and 5, is such as to put each housing 12 in communication with the same groove. The radially outermost part of said groove is provided with a pair of conical surfaces 16 (FIG. 4), which coact with a V-belt 17; this belt forms part of the actuating means 4 of the feeder 1 or is wound about a pulley 18 (FIG. 2).
In the outer peripheral part of the disk 5 an annular groove 19 is also provided (FIGS. 4 and 5), which is disposed substantially in the same plane of the groove 15 and inside which a number of thin plates 20 coact. Each of these plates is provided with a pair of projections 21 (FIGS. 1 and 3) housed in corresponding recesses 22 (FIG. 5) provided in the radially innermost part of the cage 8, between a pair of adjacent housings 12 of said cage. The plates 20 are secured to the cage 8 by means of pins 23, each of which passes through the cage 8 and a projection 21 of one of said plates; the latter serve the purpose of preventing any lateral displacement of the cage 8 relative to the disk 5 when the former rotates with respect to the latter, in a manner to be described below.
Inside the annular groove 15 of the cage 8 there is disposed a small belt 24 (FIGS. 1, 2 and 4) which is wound about a part of the rollers 7 and around a pulley 25 forming part of the aforementioned actuating means 4 of the feeder 1. Each of the pulleys 18 and 25 (FIG. 2) is driven by a corresponding drive (not shown) in which a speed variator is inserted; this variator is, in turn, driven by an electric motor, so as to cause each pulley to rotate with any angular velocity comprised in a predetermined range.
The operation of the device described above, for carrying out the method of the invention, is as follows.
The rollers 7 to be ground are loaded on the feeder 1 either manually or automatically, by inserting each roller inside a corresponding housing 12 of the cage 8, while the latter rotates in a manner to be described below. This loading is effected in one or more housings 12 comprised in the zone of the cage 8 where no contact between the belt 24 and the rollers 7 takes place (i.e., the zone comprised between the straight lines a and b, FIG. 2). The rollers situated in the housings comprised in the remaining part of the cage 8 are held in contact with the rolling track 6 of the disk 5 by the action of the belt 24, which applies radial pressures to the cylindrical side surface of each of them.
The operation of the feeder 1 is as follows.
The disk 5 (FIG. 2) is caused to rotate by the corresponding actuating means, for instance in a counterclockwise direction, whereas the cage 8 is caused to rotate in the opposite direction by menas of the V-belt 17 driven by the pulley 18. Simultaneously, the rollers 7 are caused to roll on the rolling track 6 of the disk 5 by the action of the small belt 24 actuated by the pulley 25. As a consequence, the rollers 7 are fed between the grinding wheels 2, thereby following a substantially circular path therebetween; during this feed movement, the grinding wheels 2 will remove materiale from the end faces 7' of the rollers 7 situated between them. The feed speed of the rollers along said path corresponds to the tangential velocity of the axes of the same rollers, which depends on the angular velocity of the cage 8. During this feed movement, the rollers will also rotate about their own axes on account of their rolling movement on both the track 6 of the disk 5 and the belt 24 which is wound about them. The rotational speed of each roller about its own axis, once a predetermined feed speed has been established, can have a value whatever, inasmuch it depends on the speeds of the other components of the feeder 1. For a more complete examination of the feeder 1, from the kinematic point of view, it will be suitable to expound the following theoretical results.
Assuming that there is no sliding or skimming between the belt 24 and the rollers 7, as well as between the latter and the rolling track 6, the feeder 1 will then constitute an epicyclic train, wherein the sun wheel or gear is formed by the disk 5, the planetary gears by the rollers 7, the crown by the section of the belt 24 which is wound about said rollers and, finally, the train carrier by the cage 8. In FIGS. 6 and 7 can be seen, respectively, a section of the feeder 1 and a diagram of an equivalent epicyclic train, represented with the modalities usually utilized for the study of such mechanisms. FIG. 8 shows a side view, rather diagrammatic, of said epicyclic train. In these equivalent diagrams, the various components of the train are designated by the same reference numerals as utilized for the description of the corresponding parts of the feeder 1; in particular, from the equivalent diagram of FIG. 7 it can be seen that the sun wheel 5 corresponds to the disk 5, the crown 24 to the belt 24, the planetary gears 7 to the rollers 7 and the train carrier 8 to the cage 8.
Since, as is well known, in an epicyclic train the angular velocities of the sun wheel 5, of the crown 24 and the train carrier 8 are not independent, once two of the aforementioned velocities have been fixed, the third will depend upon the first and the second. In fact, if we indicate with ω , ω' and Ω (FIG. 8), respectively, the angular velocities of the sun wheel 5, of the crown 24 and the train carrier 8, and with r and r' the radii of the sun wheel 5 (radius of the rolling track 6 of the disk 5) and of the crown 24 (radius of the circumference externally tangent to the surfaces of the rollers 7), by applying the well known formula of Willis, we will obtain the following equation:
ω' - Ω = (Ω - ω) (r/r') (1)
from which it can be noted that the angular velocity of each of the three aforementioned elements of the epicyclic train depends on that of the other two.
For the purposes of the conclusions to be expounded below, it is important to know the angular velocity ω s of the planetary gears 7 (of the rollers 7) about their own axes of rotation. This velocity, as is known, can be calculated by assuming to apply to the epicyclic train a fictitious rotation -Ω about the axis of rotation of the sun wheel 5, so as to make this train an ordinary one (that is to say, so as to obtain an epicyclic train with fixed train carrier). With such argument, with the symbols introduced above and still indicating with r s the radius of the planetary gear 7, we will obtain the following relation for the aforementioned angular velocity ω s of the planetary gear:
ω s = (ω - Ω) (r/r s ) (2)
from which it can be seen that the angular velocity ω s of each planetary gear depends on the angular velocity of two other elements of the train (sun wheel or crown).
Therefore, from the theoretical arguments expounded above, it will be apparent that the grinding operation of the opposite end faces 7' of each roller can be accomplished -- as contrasted with the known devices of the prior art -- with any feed speed of the rollers between the grinding wheels about their own axes. In fact, the first of the two aforementioned speeds is the velocity Ω of the train carrier, whilst the second is the velocity ω s of the planetary gears; as seen from the relation (2), the ratio between these velocities can have whatever value, inasmuch it depends solely on the choice of the velocity ω of the sun wheel.
Therefore, with the device of the invention, after choosing a suitable feed speed of the rollers 7 between the grinding wheels 2 (velocity Ω ), on which the quantity of the rollers ground in the unit of time will depend, it is possible to select a suitable speed of rotation (ω s ) of the rollers 7 about their own axes, so as to obtain a high quality of processing of the faces 7' ground by the grinding wheels.
Furthermore, it is apparent that with the device described above, as contrasted with the known devices of the prior art, a direct drive can be attained of all the rotary components of the feeder 1 as well as of the rollers 7, with the advantage of obtaining speeds of said parts and of the rollers exactly corresponding to the theoretical velocities, and of avoiding sudden accelerations or jammings of the same parts.
As stated above, the velocities ω , ω' and Ω are interdependent according to the formula (1) and, therefore, since the speeds of the disk 5 and the belt 24 are driven by two independent motors, respectively, the system is kinematically complete even without the belt 17 and the corresponding pulley 18. Nevertheless, the use of this means has been recognized to be necessary in order to avoid that any sudden variation of the forces transmitted by the grinding wheels should cause a skimming of the belt 24 on the rollers 7, with a precession of the cage 8. Of course, the pulley 18, which is driven through a variator, must be perfectly synchronized -- to be in accordance with the formula (1) -- either manually or automatically, with known mechanical or electric devices.
Obviously, many modifications and variations can be introduced in the embodiment of the present invention described above, concerning both the shape and the arrangement of the various parts and components, without departing from the scope of the invention.