Rotating mirror optical scanner with grooved grease bearings

Abstract: An optical scanner (10) has grease bearings (26) between the rotor (16) and the housing ( 14). The grease bearings are grooved (32) which tends to pump the grease back into the bearing cup (28). The thixotropic property of the grease prevents its escape when the optical scanner is not operating. The grease bearings can be frusto-conical or spherical in shape with a complementary shaped bearing cup. The bearing cup (28), rather than the bearings, may be grooved. The bearing cup may also be stepped (Fig.3) to provide a pumping region and a reservoir region for the grease. Pressure means (88, Fig. 5) on the scanner housing may force the bearing cup onto the grease bearing.

Description: The present invention relates to optical scanners in general and more particularly to optical scanners employing grooved grease bearings. Laser printers use a rotating polygon mirror optical scanner to produce the latent image on the photoreceptor. The polygon mirror is typically supported by a rotor, which is driven at a high angular velocity by an electric motor. The motor/polygon mirror assembly used in high volume/high resolution printers must operate at speeds from 15,000 to 35,000 RPM for 15,000 hours, produce little audible noise, be contamination free and survive occasional shock loading. Conventional ball bearings can not survive at the high speeds of an optical scanner. They are extremely noisy, particularly in comparison to the quiet office environment that most optical scanners and laser printers operate in. Additionally, ball bearings at high speeds suffer from run-out, deviation from the desired spin axis, which is unacceptable for the delicate beam alignment needed for optical scanning. Gas bearings are normally used in high speed optical scanners. They rely on the pressure created between a pair of cylindrical surfaces in relative motion to support the rotor load. One solid cylindrical surface, typically part of the rotating polygon mirror, is within a complementary stationary hollow cylindrical surface, typically part of the motor and scanner housing. One of the two cylindrical surfaces is usually grooved. Since the gas in the gas bearing is typically air, which has a notoriously low viscosity, the clearance space between the rotating part of the bearing and the stationary part of the bearing is quite small, on the order of 3.5 microns (0.000 135 inches) with a latitude of +/- 0.25 microns (0.000 010 inches). This small manufacturing latitude makes gas bearings both expensive to make and sensitive to contamination and moisture. Furthermore, the lifetime of gas bearings is limited by the wear of the bearing surfaces rubbing together when the optical scanner is started and stopped. The cylindrical surfaces touch at rest and static friction must be overcome before the polygon mirror rotates. The friction and surface rubbing also limits the minimum rotational speed at which the gas bearings can be used. The wear, static friction and minimum speed problems are compounded when the rotor is horizontally oriented. It is an object of this invention to provide, in an optical scanner, rotor bearings that enable the problems of prior art bearings to be overcome. JP-A-56-101120 discloses a photoscanner device including a polygon mirror on a shaft rotated at high speed (10,000-20,000 rpm). The rotating shaft is rotated on bearings lubricated by oil. US-A-4,043,612 discloses the use of grooved bearings, and the proposal to use grease as a lubricant there for. According to this invention, there is provided an optical scanner as defined in claim 1. Grease bearings at the end of the rotor with opposing bearing cups in the scanner housing are the only points of contact between the rotating motor/polygon mirror assembly and the housing.The grease bearings are grooved which tends to pump the grease back into the bearing cup. The thixotropic property of the grease prevents its escape when the optical scanner is not operating. The grease bearings can be frusto-conical or spherical in shape with a complementary shaped bearing cup. The bearing cup, rather than the bearings themselves, may be grooved. The bearing cup steps provide a pumping region and a reservoir region for the grease. Pressure means within the scanner housing may force the bearing cup onto the grease bearing. By way of example only, embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a side elevation of an optical scanner with frusto-conical grooved grease bearings. Figure 2 is a schematic illustration of an alternative form of the frusto-conical grease bearings with a grooved bearing cup. Figure 3 is a schematic illustration of a frusto-conical grooved grease bearings with a stepped bearing cup, according to the invention. Figure 4 is a schematic illustration of a modification with spherical grooved grease bearings. Figure 5 is a schematic illustration of an optical scanner with grease bearings and a spring pressured bearing cup. Reference is now made to Figure 1 wherein there is shown an optical scanner 10 consisting of a motor/polygon mirror assembly 12 inside a scanner housing 14. The motor/polygon mirror assembly has a rotor 16 upon which is mounted a faceted polygon mirror 18 and a magnetic yoke 20. A stator coil 22 mounted in the scanner housing opposes the magnetic yoke to form the motor. Power supplied to the stator coil produces an electromagnetic field which rotates the magnetic yoke of the rotor and thus rotates the attached polygon mirror. A window 24 in the housing faces the polygon mirror so that light reflected from the rotating mirror passes through the window. A bearing 26 is provided at both ends of the rotor to allow the motor/polygon mirror assembly to rotate. Each bearing is frusto-conical in shape. Complementary frusto-conical shaped bearing cups 28 in the housing oppose each bearing. It is preferred that the bearings be formed integral with the rotor and the bearing cups be formed integral with the scanner housing to allow for more precise alignment and fitting. Alternatively, the bearings and bearing cups can be formed separately and attached to the rotor and scanner housing, respectively, by adhesive or other means. The small space or clearance 30 between the ends and sides of the bearing and the bearing cup is filled with grease. The grease bearing 26, the grease in the clearance 30 and the bearing cup 28 are the only points of contact between the motor/polygon mirror assembly 12 and the scanner housing 14. The conical surface of each grease bearing 26 has a series of grooves 32. Alternatively, a spiral-shaped groove may be used instead of the series of grooves. The grooves may have a variety of cross sections, rectangular, semi-circular or v-shaped for example. The grooves are angled forward toward the outer edge 34 of the bearing cup in the direction of rotation of the motor/polygon mirror assembly. The grooves of the bearing tend to pump the grease back into the bearing cup toward the base 36 of the cup. In the view shown in Figure 1 the rotor and polygon mirror rotate to the left and the grease is pumped to the right and toward the bearing cup base. Thus the grease is captured within the clearance between the bearing and the bearing cup and cannot escape during operation of the optical scanner. The thixotropic property of the grease prevents its escape when the optical scanner is not operating. The grease is commercially available. The pumping property of the grooved cone while in operation and the thixotropic property of the grease while the scanner is at rest permit the optical scanner to operate at any angular orientation, not just when the rotor is vertical. It is well known that the base oil contained within the grease acts as a lubricant. This lubricating action reduces the starting and stopping wear of the bearing surfaces to such a point that this wear is no longer of concern. The base oil lubrication also reduces the static friction to be overcome before the polygon mirror rotates. The minimum rotational speed of the grease bearing optical scanner is less than that of a comparable gas bearing optical scanner. The grease in the grease bearings also provide a cushion so that the optical scanner can survive substantial shock loading. The viscosity of grease is approximately four to five orders of magnitude greater than the viscosity of air. Well known hydrodynamic theory predicts that the radial clearance between the bearing and the bearing cup in grease lubricated bearings can be approximately ten times larger than the clearance between the bearing and the bearing cup in an equivalent gas bearing. The radial clearance is determined by the cone angle and the amount of axial play in the system when it is ungreased. For example, the axial play is about 150 microns (0.006 inches). Radial clearance for a 10° cone angle is 12.5 microns (0.0005 inches). Manufacturing latitudes of the grease bearings are also approximately an order of magnitude greater than those required by the gas bearing counterparts. Grease bearings are also cheaper by a tenfold factor than gas bearings and simpler to manufacture. The bearing and cups can be made from metals or polymers. Because the design tolerances are generous, injection molding process can be used to construct parts which otherwise would require ultra-precision machining. The surface area of the grease exposed to the atmosphere in the grease bearing is less than the surface area exposed to the atmosphere in a ball bearing. The hydrodynamic grease bearing contains approximately 0.1 to 0.01 times less the amount of grease contained in a ball bearing, thus lessening the possibility of contamination of the optical surfaces of the mirror and window by grease. An optical scanner with grease bearings can operate over a wide range of speeds and has a long lifetime of over 100,000 hours. In Figure 2, grease bearing 38 and bearing cup 40 are of identical structure to those of optical scanner 10 of Figure 1, except that the bearing cup has a series of grooves 42. The grooves are angled forward in the direction of rotation of the motor/polygon mirror assembly so that the grooves of the bearing cup tend to pump the grease back into the bearing cup. Alternatively, a spiral-shaped groove may be used instead of the series of grooves. The grooves may have a variety of cross sections, rectangular, semi-circular or v-shaped for example. Also in the alternative, both the bearing and the bearing cup may have grooves. Figure 3 shows a grooved grease bearing used, in accordance with the invention, in the optical scanner of Fig. 1. In Figure 3, grease bearing 44 with a series of grooves 46 and bearing cup 48 are of identical structure to those of optical scanner 10 of Figure 1, except that the bearing cup is stepped. The bearing cup 48 has two circumferrential steps 50 and 52 which divide the bearing cup into a lubricating region 54, a pumping region 56 and a reservoir region 58. The lubricating region 54 is between the base 60 of the bearing cup and the first circumferential step 50. The space 62 between the bearing and the bearing cup at the lubricating region is where the hydrodynamic lubrication of the grease bearing occurs. Radial clearance in this region between the bearing and the bearing cup is approximately 12.5 microns (0.0005 inches). The pumping region 56 is between the first circumferential step 50 of the bearing cup and the second circumferential step 52. The space 64 between the bearing and the bearing cup at the pumping region is where the grooves 46 on the bearing pump the grease back into the lubricating region 54 of the bearing cup. Radial clearance in this region between the bearing and the bearing cup is approximately 125 microns (0.005 inches). The reservoir region 58 is between the second circumferential step 52 and the outer edge of the bearing cup 66. The space 68 between the bearing and the bearing cup at the reservoir region acts as a reservoir for the grease and an overflow area. Radial clearance in this region between the bearing and the bearing cup is approximately 500 microns (0.020 inches). A circumferential groove 70 can be provided between the grease bearing 44 and the rotor 72 to prevent the grease from moving into the motor/polygon mirror assembly from the reservoir region. The pumping grooves 46 do not extend all the way through the bearing into the reservoir region and the groove 70 acts as a terminating point for the grease. One inherent problem in the use of grease bearings is the impact upon performance of the grease bearing if too much or too little grease is installed during assembly of the bearing. A stepped bearing cup in accordance with the invention reduces the sensitivity of the bearing to grease volume variations by providing a narrower lubricatating region with a pumping region to insure a constant supply of grease in the case of too little grease and an overflow reservoir region in the case of too much grease. This reduction in sensitivity to grease volume between the bearing and bearing cup will aid in the manufacture of the grease bearing. In Figure 4, grease bearing 74 with a series of grooves 76 and bearing cup 78 are of identical structure to those of Figure 3, except that the grease bearing and complementary bearing cup are spherical in shape, rather than frusto-conical. (In Fig. 4, the steps in the bearing cup are omitted.) The spherical grease bearing has certain advantages, even over the frusto-conical grease bearing. The spherical grease bearings are relatively insensitive to misalignment of the bearing cups with each other and still allow the rotor to rotate. Spherical grease bearings are even cheaper and simpler to manufacture than frusto-conical grease bearings. The previous alternative embodiments of grooves on the bearing cup, rather than the bearing,and stepping the bearing cup also apply to a spherical grease bearing. In the case of a spherical grease bearing, it is preferable to have the grooves on the bearing cup for ease of manufacture. A small ball 80 from a conventional ball bearing can be attached to the rotor end 82 by adhesive or other means. The grooved spherical bearing cup is relatively simple to fabricate. In an alternative to complementary bearing and bearing cup design, the frusto-conical grease bearing of Figure 1 can be matched to a conical bearing cup or a conical grease bearing can be used with the conical bearing cup. The use of a conical grease bearing and conical cup does require more precise alignment, and is more sensitive to physical shocks and damage than the frusto-conical bearing with either shape bearing cup. The spherical grease bearing of Figure 4 can also be matched with a frusto-conical or a conical bearing cup. Reference is now made to Figure 5 wherein there is disclosed an optical scanner 84 in which the grooved grease bearing of Fig. 3 may be used and which is identical in structure to optical scanner 10 of Figure 1, except that the scanner housing 86 has spring pressure means 88 to force the bearing cup 90 onto the grease bearing 92 and the rotor 94. (In Fig. 5, the steps in the bearing cup are omitted.) The spring pressure means aids in alignment of the rotor and attached polygon mirror 96 and helps avoid damage due to physical shocks to the optical scanner. The hydrodynamic effect betwen the bearing and the bearing cup maintains the radial clearance, prevents static friction and prevents the two surfaces from being forced together such that the rotor can not rotate or can not rotate easily. This embodiment requires that the bearing cup is not formed integral with the scanner housing and that bearing cup have free movement relative to the scanner housing along the axis of the rotor by lubrication or other means. It will be appreciated that, in the arrangement of Figs 3, 4 and 5, the grooves may be provided on the internal surface of the bearing cup, rather than on the bearing, in accordance with Fig. 2.

Claims: An optical scanner (10; 84) comprising: a scanner housing (14; 86) a mirror assembly (12; 18; 96) mounted on a rotor (16; 72; 94; 82), said rotor and said mirror assembly being inside said scanner housing, means (20, 22) for rotating said rotor and said mirror assembly, and a bearing (26, 38, 44, 74, 92) at each end of said rotor, characterised by including a bearing cup (28, 40, 48, 78, 90) in said scanner housing receiving each bearing, wherein the internal surface of each bearing cup is stepped (50, 52), and by including grease between each bearing and a respective bearing cup, and a groove (32, 42, 46, 76) on the surface of either each bearing or the respective bearing cup or both, such that said groove keeps said grease between each bearing and the respective bearing cup when said rotor is rotating and said grease remains between each bearing and the respective bearing cup when said rotor is not rotating. An optical scanner as claimed in claim 1, wherein each bearing and/or the respective bearing cup has a series of grooves on its surface. An optical scanner as claimed in claim 1 or claim 2, wherein each bearing and/or the respective bearing cup has a spiral-shaped groove on its surface. An optical scanner as claimed in any one of claims 1 to 3, wherein each bearing is frusto-conical in shape. An optical scanner as claimed in claim 4, wherein each bearing cup is frusto-conical in shape, complementary to the respective bearing. An optical scanner as claimed in any one of the preceding claims, wherein a circumferential groove (70) is formed between said bearing (44) and said rotor (72). An optical scanner as claimed in any one of claims 1 to 3, wherein each bearing (74) is spherical in shape. An optical scanner as claimed in claim 7, wherein each bearing cup (78) is spherical in shape, complementary to the respective bearing. An optical scanner as claimed in any of the preceding claims, wherein each bearing cup has two circumferential steps (50, 52) dividing the surface of the bearing cup into three regions (54, 56, 58), each of said regions having a different radial clearance between said bearing and said bearing cup. An optical scanner as claimed in claim 9, wherein said radial clearances of said regions are progressively larger for each region away from the base (60) of said bearing cup. An optical scanner as claimed in claim 9 or claim 10, wherein said first region (54), between said base of said bearing cup and a first one (50) of the circumferential steps, primarily lubricates with said grease between said bearing and said bearing cup, said second region (56), between said two circumferential steps, primarily pumps said grease keeping said grease between said bearing and said bearing cup when said rotor is rotating, and said third region (58), between the second of the circumferential steps (52) and the edge of said bearing cup primarily stores said grease. An optical scanner as claimed in any one of the preceding claims, wherein said scanner housing has pressure means (88) to force said bearing cup onto said bearing. An optical scanner as claimed in claim 13, wherein said pressure means is a spring.

Inventors: CASTELLI VITTORIO R, JONES KEVIN E, KLECKNER ROBERT J, MIRCHANDANI VINOD, ZEDEKAR STANLEY L, CASTELLI, VITTORIO R., JONES, KEVIN E., KLECKNER, ROBERT J., MIRCHANDANI, VINOD, ZEDEKAR, STANLEY L.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1991
Main classification: G02B26
IPC codes: F16C33, G02B26, H04N1