PROCESS FOR THE MANUFACTURE OF A ROTOR FOR SHAFTS OF ELECTRIC MOTORS, AND APPARATUS FOR THE EXECUTION OF THE PROCESS

Abstract: A process and an apparatus are described for the manufacture of a rotor, and in particular of a fan intended to be keyed to a shaft. An axial insertion elastic ring is included in the hub of the rotor, obtained by molding, and means are provided in the mold which are adapted to fix the elastic retaining ring during molding in the position it will assume on the shaft, keeping the ring free to exert a radial locking pressure on said shaft at the end of the molding.

Description: This invention relates to a process and to an apparatus for the manufacture of a rotor, and in particular of a cooling fan in plastic material, intended to be keyed to the steel shaft of an electric motor. Various manners are known in the prior art to lock on a shaft a molded rotor, and in particular a fan, for example by means of knurlings provided respectively on the inner cylindrical surface of the hole of the fan and in the outer surface of the shaft, by forcing the fan on the shaft, by means of radial locking screws, by prismatic tab-and-recess coupling, and the like. Such keying methods according to the prior art are not fully satisfactory due to several disadvantages, among which the cost of machining, the cost of the assembly work, the uncertainty of the locking or the introduction of undesired strains and stress concentrations in the fan. Therefore, the object of the present invention is to provide a process and an apparatus for the manufacture of a molded rotor intended to be keyed on a shaft, which is simple and reliable, which has modest machining and assembly costs, and which introduces no undue concentrations of stresses in the rotor. More in particular, an object of the present invention is to provide a process and an apparatus for the manufacture of a fan in plastic material, to be keyed to the shaft of an electric motor, having the above described advantages. The above and other objects are achieved, according to the present invention, by a process characterized in that the rotor is manufactured in a mold comprising a first and a second part by performing the following steps: - an axial insertion elastic retaining ring is located at the center of the inner face of the first part of the mold, the ends of the ring being inserted on a first axially offset pin which extends protrudingly, parallel to the axis of the mold, towards its interior, and having such dimensions and location as to keep the elastic ring locked in an elastically divaricated state, so that its inner diameter assumes the value of the nominal diameter of the driving shaft on which the rotor is to be locked, and so that it is coaxial thereto; - the mold is closed by placing the second part of the mold in contact with the first, the second part being provided with a second centered axial pin, having a diameter equal to the nominal diameter of the rotor shaft, said second pin being protrudingly raised towards the inside of the mold and having such an axial length as to pass, upon the closure of the mold, through the elastic ring locked on the first pin rigidly associated with the first part of said mold; - a fluid plastic material is injected into the mold; - when the molding is complete, the mold is opened, this simultaneously causing the extraction of the molded rotor from the mold and the removal from the pins of the elastic ring, which remains trapped and torsionally associated with the hub of the molded rotor and assumes a radially narrower rest position, such as to cause it to protrude radially from the inner surface of the hole of the rotor hub produced by the second pin. The apparatus for the embodiment of the process according to the present invention comprises a mold for the manufacture of a rotor by means of a per se known molding process, the mold being constituted by a first part and by a second part, composable with one another in the position of closure of the mold by axial translatory motion, characterized in that the first part of the mold is provided with a first axially offset pin, protrudingly extending parallel to the axis of the mold from the inner surface thereof, and having such dimensions and location as to keep the elastic ring locked in an elastically spread state, so that its inner diameter assumes the value of the nominal diameter of the driving shaft on which the rotor is to be locked, and is coaxial thereto, the second part of the mold being provided with a second centered axial pin, protrudingly raised from said second part in such a position as to occupy, with the mold closed by placing the first and the second part in contact, the hole for the shaft in the rotor hub, the diameter of the second pin being nominally equal to that of the driving shaft on which the rotor is to be mounted. The present invention and its peculiarities and advantages will become apparent from the following description of a preferred, but non-limitative, embodiment thereof, illustrated in the accompanying drawings, wherein: figure 1 is a front view of a fan manufactured by means of the process and the apparatus according to the present invention; figure 2 is an axial sectional detail view thereof; figure 3 is a schematic front view of the inner side of the first part of a mold for the manufacture of the fan shown in figures 1 and 2; figure 4 is a schematic lateral view thereof in axial cross section; figure 5 is a schematic front view of the inner side of the second part of the mold; figure 6 is a schematic lateral view thereof in axial cross section; figure 7 is a schematic lateral view, in partial axial cross section, of the closed mold, and figure 8 is a view of an enlarged detail of figure 3. Figures 1 and 2 show, respectively in front view and in axially sectional detail view, an injection-molded plastic fan 1 according to the invention, the driving shaft whereof is the shaft of an electric motor, not shown. In figures 1 and 2 an axial insertion elastic retaining ring 2 (also called Seeger ring) is partially visible, and constitutes the element for locking by friction the fan to its driving shaft. In figure 1 only a radially inner perimetral part of the elastic ring 2 can be seen, the outer perimeter of the latter, eccentric with respect to the inner one and represented by a broken line 2′, being accommodated in an annular recess 3 of the hub of the fan, which can be seen in cross section in figure 2. The elastic ring is illustrated in the condition of radial expansion which it assumes when it is keyed, together with the fan which includes it, on the driving shaft, not shown, which passes through the hole 4 of the hub of the fan. The state of elastic expansion of the ring 2 is due to the interference between its inner diameter and the diameter of the shaft. In this state the ring 2, which is free in a radial direction in the annular recess 3 of the fan but is rigidly torsionally associated therewith, locks by friction the fan on the shaft. According to the present invention, the elastic ring 2, generally of hardened steel, is included in the annular recess 3 of the hub of the fan during the molding, preferably by injection, of said fan. In figures 3 to 7, the mold for the manufacture of the fan is illustrated, comprising a first part respectively shown in front view from inside (figure 3) and in axial cross section (figure 4), and a second part also shown in front view from the interior of the mold (figure 5) and in axial cross section (figure 6). Figure 7 is an axial cross section view of the closed mold. Figure 8 is an enlarged detail of figure 3. The process for the manufacture of the fan, which allows its locking to the driving shaft as described above, is as follows: - the elastic retaining ring 2 is located on a conveniently shaped pin 5 (figures 3 and 8) located in the first part of the open mold, - the position assumed by the retaining ring in this step is that which it would have if it operated on the driving shaft of the fan 1, - during the molding step the mold is closed and the ring 2 remains in the previous position, allowing the injection of the plastic around its outer circumeference through the channel 7. The pin 6 of the second part of the mold, which has the same diameter as the driving shaft, assumes the position thereof in the hole of the hub, - upon the opening of the mold, simultaneously with the extraction of the plastic fan, the elastic retaining ring 2 is forced to exit from the pins 5 and 6, again assuming, by virtue of its elasticity, its normal rest position. In this manner an empty chamber is formed around the elastic ring which allows the same to return to its working position when the fan is to be mounted on its shaft. The material preferably used for the fan is polypropylene or a polyamide resin loaded with glass. It has been observed that after its extraction from the mold the elastic ring maintains, in the annular recess 3, the freedom of radial contraction required to exert the locking pressure on the shaft, without requiring the application of a mold lubricating agent. The latter is however naturally applicable if the adopted types of material and molding process require it. The pin 5, besides having the function of positioning the elastic retaining ring 2, also has the function of keeping free from molding material the space between the two ends of said ring to allow its elastic return when it is extracted from the mold. The fan thus manufactured is mounted by forcing on the shaft on which it is to be keyed, and is locked thereon by interference friction against the elastic ring, the shaft being beveled at one end so as to allow the elastic ring to be introduced by virtue of the elastic expansion thereof caused by the forcing. Naturally, the locking process and the apparatus for its embodiment according to the present invention are susceptible to numerous modifications without thereby abandoning the scope of the invention. Thus, for example, they are applicable with obvious modifications to rotors having any shape and of any appropriate plastic or metallic material, and to any appropriate molding or sintering or die-casting process and the like.

Claims: 1. Process for the manufacture of a rotor for shafts of electric motors by plastic transformation in a mold constituted by at least one first part and one second part, characterized by the following steps: - an axial insertion elastic retaining ring (2) is located at the center of the inner face of the first part of the mold, the ends (8, 8′) of the ring being inserted on a first axially offset pin (5) which extends protrudingly, parallel to the axis of the mold (9), towards its interior, and having such dimensions and location as to keep the elastic ring (2) locked in an elastically divaricated state, so that its inner diameter assumes the value of the nominal diameter of the driving shaft on which the rotor (1) is to be locked, and so that it is coaxial thereto: - the mold is closed by placing the second part of the mold in contact with the first, the second part being provided with a second centered axial pin (6), having a diameter equal to the nominal diameter of the rotor shaft, said second pin being protrudingly raised towards the inside of the mold and having such an axial length as to pass, upon the closure of the mold, through the elastic retaining ring (2) locked on the first pin (5) rigidly associated with the first part of said mold; - a fluid plastic material is injected into the mold; - when the molding is complete, the mold is opened, this simultaneously causing the extraction of the molded rotor (1) from the mold and the removal from the pins (5, 6) of the ring (2), which remains trapped and torsionally associated with the hub of the molded rotor and assumes a radially narrower rest position, such as to cause it to protrude radially from the inner surface (4) of the hole of the rotor hub produced by the second pin (6). 2. Apparatus for the execution of the process according to claim 1, comprising a mold for the manufacture of a rotor by plastic deformation, said mold being constituted by a first part and by a second part, composable with one another in the position of closure of the mold by axial translatory motion, characterized in that the first part of the mold is provided with a first axially offset pin (5) which extends protrudingly, parallel to the axis of the mold (9), from the inner surface thereof, and has such dimensions and location as to keep the elastic retaining ring (2) locked in an elastically spread state, so that its inner diameter assumes the value of the nominal diameter of the driving shaft on which the rotor (1) is to be locked, and so that it is coaxial thereto, and in that the second part of the mold is provided with a second centered axial pin (6), protrudingly raised from said second part in such a position as to occupy, with the mold closed by contact between the first part and the second part, the hole (4) for the shaft in the rotor hub, the diameter of the second pin (6) being nominally equal to that of the driving shaft on which the rotor (1) is to be mounted, the shaft being beveled at one end. 3. Process according to claim 1, characterized in that the rotor is a fan for an electric motor. 4. Process according to claim 1, characterized in that the 2 molding material is polypropylene with glass fiber load. 5. Process according to claim 1, characterized in that the molding material is a polyamide resin with glass fiber load.

Inventors: VEZZOLI, VITTORE
Applicants: GRUPPO INDUSTRIALE ERCOLE MARELLI S.P.A.
Application year: 1987
Main classification: B29C67
IPC codes: B29C33, B29C45, B29C70