GB2114685A - Supporting the rotor of a stepper motor - Google Patents

Abstract

In a stepper motor, two bearings (44, 45) for rotatably supporting the rotor are assembled with the rotor shaft (40) prior to attachment of the bearings to attachment plates (28, 33) carried by the stator. Thereafter, each bearing is bonded to one of the attachment plates. The bonding material (55) is injected through a hole (52, 54) in each attachment plate into the region between that plate and its respective bearing. Each bearing may be formed with an annular channel (51, 53) for accommodating the bonding material. <IMAGE>

Description

SPECIFICATION Method of supporting the rotor of a stepper motor, and stepper motor including such support This invention relates to stepper motors, such as those illustrated and described in U.S. Patents Nos. Re. 28,705 (particularly Figs. 6 and 8) and 3,633,055. More specifically, the invention relates to the rotatable support for the rotors of such stepper motors.

In such motors, the rotor shaft is often supported within a sleeve bearing which extends for only a portion of the length of the shaft.

However, for certain applications of such motors, it is desirable to support the rotor shaft at two points spaced as far as possible from each other along the length of the shaft. Such support increases the life of the bearings and shaft. In these cases, it is common to employ sintered bearings which are press fit or staked into holes in attachment plates secured to the two end faces of the stepper motor. The step of attaching the bearing in this way to its attachment plate usually distorts the bearing, necessitating a resizing of the hole in the bearing so that it properly accommodates the rotor shaft. This adds to the cost.of manufacture of the motor.

Moreover, it is important that the two bearings 'be perfectly aligned, so as to reduce friction between the rotor shaft and the bearings.

Reducing friction permits the motor to produce more torque, increases its efficiency, and increases stepping accuracy. Obtaining perfect alignment of the bearings is difficult without resorting to more expensive self-aligning bearings.

It is an object of the present invention to provide a method of assembling the rotor and two spaced-apart bearings of a stepper motor with the other parts of the motor in a way which avoids distortion of the bearings and hence eliminates the need to resize the holes in the bearings.

It is another object of the invention to provide such a method whereby the bearings become aligned with each other and with the shaft prior to securing the bearings to the stationary part of the stepper motor, thus avoiding the need for selfaligning bearings.

It is a further object of the invention to provide such a method wherein the entire stepper motor is assembled prior to securing the bearings to the stationary part of the stepper motor; after assembly, a bonding material is injected through holes in the stationary part of the stepper motor into recesses in the bearings to secure the bearings in place.

It is another object of the invention to provide a stepper motor having an attachment plate secured to each of its end faces, and a bearing bonded to the inner face of each plate, the bearings rotatably supporting the rotor shaft.

Additional objects and features of the invention will be apparent from the following description in which reference is made to the accompanying drawings.

In the drawings: Fig. 1 is an axial cross-sectional view of a stepper motor according to the invention; and Fig. 2 is an enlarged fragmentary view of the motor, showing one step in the assembly procedure.

The stepper motor chosen to illustrate the present invention is of the type shown and described in more detail in co-pending application Serial No. 285,657, filed July21, 1981. The stepper motor includes two stator plates 10 and 11, and two stator cups 12 and 13. Stator plates 10 and 11 are virtually identical, each being of annular shape. A plurality of tapered poles 1 5 surround the central opening of stator plate 10, the poles being circumferentially spaced apart. All the poles 1 5 project perpendicularly in the same direction from the plane of plate 1 0. The peripheral margin 1 6 of the plate is deformed out of the plane of the plate in the direction in which poles 1 5 project from the plate.

Stator plate 11 is substantially identical to plate 10, and the parts of plate 11 corresponding to those of plate 10 bear the same reference numerals employed above, followed by a prime. In order to form the inner stator member of the stepper motor, stator plates 10 and 11 are placed back-to-back and permanently fixed together, such as by welding or by a suitable adhesive.

Stator cup 12 is of annular shape, having a central opening in its back wall 24. A plurality of tapered poles 25 surround the central opening, the poles being circumferentially spaced apart. All the poles 25 project perpendicularly in the same direction from the plane of back wall 24 of the cup. A peripheral side wall 26 projects from back wall 24 in the direction in which poles 25 project from the back wall. The free edge 27 of side wall 26 is enlarged in diameter so that the internal diameter of free edge 27 is about equal to the external diameter of stator plate 10, whereby the peripheral edge 1 6 of plate 10 fits snugly within edge 27 of cup 12.

Stator cup 13 is substantially identical to cup 12, and the parts of cup 13 corresponding to those of cup 12 bear the same reference numerals employed above, followed by a prime. A bearing attachment plate 28, having a central opening 29, is fixed to the outer face of back wall 24 of stator cup 12, such as by welding or a suitable adhesive, and a bearing attachment plate 33, having a central opening 35, is similarly fixed to the outer face of back ball 24' of stator cup 13. Plate 33 is larger than plate 28, and has mounting holes 34 near its extremities, so that it can serve as a mounting plate for the stepper motor.

A coil of electrical wire 31 wound on a spool 32 surrounds poles 1 5 and 25 of plate 10 and cup 12, respectively, and a similar coil 31' wound on a spool 32' surrounds poles 15' and 25' of plate 11 and cup 13, respectively. The free edges 27 and 27' of stator cups 12 and 13 are bent into the gap 19 between the peripheral edges 27 and 27' of stator plates 10 and 11 at a number of circumferentially spaced-apart locations along the free edges, as indicated at 39. In this way, the edges 27 and 27' are staked into the gap 19 to thereby permanently mechanically join each stator cup 12 and 13 to its respective stator plate 10 and 11.

The rotor, which cooperates with the stator described above, includes a shaft 40, a hub 41 surrounding and fixed to the shaft, an annular support 42 surrounding and fixed to the hub, and a tubular permanent magnet 43 surrounding and fixed to the support. Shaft 40 is rotatably supported within two bearings 44 and 45, spaced apart along the length of the shaft. Each bearing is a disc-like member, made of a suitable material such as sintered bronze or plastic, having a larger diameter portion and a smaller diameter portion.

The smaller diameter portion 44a (Fig. 2) of bearing 44 fits snugly within opening 29 of attachment plate 28, the larger diameter portion 44b engaging the inner face of the plate. Similarly, the smaller diameter portion 45a of bearing 45 fits snugly within opening 35 of attachment plate 33, the larger diameter portion 45b engaging the inner face of the plate.

A compression spring 48 surrounds a portion of shaft 40 between the bearings, one end of the spring being seated around a collar 49, within which the shaft rotates, and the other end of the spring pressing against hub 41. Since hub 41 rotates and bearing 44 does not, a low-friction washer 50 is provided between the hub and the bearing.

Bearing 44 is formed with an annular channel 51 in the surface of larger diameter portion 44b which faces attachment plate 28, the channel surrounding smaller diameter portion 44a. Plate 28 has a small hole 52 extending from the exterior of the stepper motor to channel 51. Similarly, bearing 45 is formed with an annular channel 53 in the surface of the larger diameter portion 45b which faces attachment plate 33, the channel surrounding smaller diameter portion 45a. Plate 33 has a small hole 54 extending from the exterior of the stepper motor to channel 53. After the parts have been assembled, as shown in Fig. 1, bearings 44 and 45 are bonded to attachment plates 28 and 33, respectively, by injecting a flowable bonding material 55 (Fig. 2), such as a suitable adhesive, into channels 51 and 53 through holes 52 and 54, respectively.Injection may be accomplished using a fine-pointed nozzle 56. After injection, the bonding material hardens to permanently secure bearings 44 and 45 to their respective attachment plates.

In assembling the stepper motor, bearings 44 and 45 are slipped on to shaft 10 of the rotor, along with spring 48, collar 49, and washer 50.

The rotor is placed within the central opening of stator plates 10 and 11, coils 31 and 31' are placed around the poles 1 5 and 15', respectively, and stator cups 12 and 13 are placed over stator plates. As the cups are brought toward each other to the position shown in Fig. 1, spring 48 is compressed, pushing bearings 44 and 45 against the inner faces of attachment plates 28 and 33, respectively. The free edges 27 and 27' of the stator cups are then bent inwardly, as at 39, to mechanically lock the stator cups in place.

Since bearings 44 and 45 are not yet secured to attachment plates 28 and 33, respectively, they are free to move laterally, i.e., transverse to the axis of shaft 10, so that the shaft actually aligns the bearings with itself and with each other. The final step of securing the bearings in place then takes place, by injecting a bonding agent into channels 51 and 53 of the bearings through holes 52 and 54, respectively. The bonding material fills the channels, which are closed on their outwardly facing sides by the plates 28 and 33. Thus, when the bonding material sets, the bearings 44 and 45 are permanently fixed to the plates 28 and 33, respectively.

The invention has been shown and described in preferred form only, and by way of exampie, and many variations may be made in the invention which will still be comprised within its spirit. It is understood, therefore, that the invention is not limited to any specific form or embodiment except insofar as such limitations are included in the appended claims.

Claims (10)

1. A method of rotatably supporting the rotor of a stepper motor, comprising the steps of: providing an annular stepper motor stator having a bearing attachment plate secured to each of its ends, so that the plates are spaced apart along the axis of the stator, providing a rotor within the stator, the rotor having a shaft extending along the axis of the stator, assembling two bearings with the shaft prior to securing the bearings to the attachment plates, the shaft being rotatable in the bearings, and each bearing being adjacent to the inner surface of one of the plates, and thereafter bonding each bearing to its respective attachment plate.

2. A method as defined in Claim 1 wherein a portion of each bearing is spaced from the attachment plate to which that bearing is adjacent, and the bonding step includes injecting a bonding material into that space.

3. A method as defined in Claim 2 including a hole in each attachment plate communicating with the space between that attachment plate and its respective bearing, and wherein the bonding step includes injecting the bonding material through the hole into that space.

4. A method as defined in Claim 3 wherein the space into which the bonding material is injected is defined by an annular channel in the surface of the bearing which faces the attachment plate.

5. A method as defined in Claim 1 including a hole in each attachment plate, and wherein the bonding step includes injecting a bonding material, through the hole, between that attachment plate and its respective bearing.

6. A stepper motor comprising: a stator having a bearing attachment plate secured to each of its ends, the plates being spaced apart along the axis of the stator, a rotor within the stator, the rotor having a shaft extending along the axis of the stator, a bearing adjacent to the inner face of each attachment plate, the rotor shaft being rotatably supported by the bearings, and a bonding material between the opposed faces of each bearing and its adjacent plate for securing the bearing to the plate.

7. A stepper motor as defined in Claim 6 including a channel in the face of each bearing opposite its respective attachment plate, the bonding material occupying the channel.

8. A method as defined in Claim 7 including a hole in each attachment plate, axially aligned with the channel, through which the bonding material is injected into the channel.

9. The method of rotatably supporting the rotor of a stepper motor substantially as hereinbefore described with reference to and as illustrated in the various figures of the accompanying drawings.

10. A stepper motor substantially as hereinbefore described with reference to and as illustrated in the various figures of the accompanying drawings.