Generally, an image forming apparatus involves causing toner to adhere to a latent electrostatic image, formed by an optical device, in a fuser device, transferring the toner image to copier paper, and fusing the toner image to the paper. During this fusing process, the toner image passes between a fuser roller having a built-in heater and a pressure roller. A transfer image formed by the toner image is thereby heated, melt-cast, and fused onto the copier paper.
The fuser roller is made of soft metal with a linear or rodlike heater built into a core section thereof, and has a cylindrical shape with small-diameter shaft parts projecting from both ends thereof. The fuser roller is of a metal material having superior thermal conductivity, such as aluminum or an aluminum alloy (A 5056, A 6063). The surface of the fuser roller is finished via turning or grinding. The surface of the fuser roller is also covered with a highly non-adhesive resin, such as a fluororesin. The temperature of the surface of the fuser roller is heated to around 180-250° C. by the heater. The pressure roller is made of iron or a soft material coated with silicone rubber or the like, and presses the copier paper to the fuser roller while being rotatably driven. The pressure roller is heated to roughly 70-150° C. by heat transferred from the heater roller. Alternatively, a heater is provided within the pressure roller as in the case of the fuser roller, and this heater heats the pressure roller to roughly 150-250° C. Hereafter, a roller, like the fuser roller and pressure roller described above, that is heated by a built-in heater or by heat transfer from another member will be referred to as a “heated roller.”
The heated roller, which is heated to a high temperature, is rotatably supported at the shaft parts on both ends by deep-groove ball bearings within a housing, and an insulating sleeve of synthetic resin or the like is interposed between the ball bearings and the shaft parts of the heated roller. This arrangement is in order to prevent heat escaping through the ball bearings at both ends when the heated roller is being heated, which can lead to an uneven temperature distribution along the axial direction of the heated roller.
One instance of a supporting bearing for a heated roller utilizes a resin sliding bearing. The sliding bearing is formed, for example, from a synthetic resin such as polyphenylene sulfide (PPS) resin, polyamide (PA) resin, polyamide imide (PAI) resin, polyimide (PI) resin, polyether ether ketone (PEEK) resin, or the like.
If a resin sliding bearing is used, an insulating sleeve is generally not interposed between the resin sliding bearing and the shaft parts of the heated roller, as the resin sliding bearing itself has insulating properties. Normally, a roller bearing or a resin sliding bearing will be used according to the specifications of the fuser unit of the image forming apparatus. Generally, roller bearings are used in mid- or high-grade machines having high pressure/velocity (PV) specs, and resin sliding bearings are used in low-grade devices having comparatively low PV.
However, the deep-groove ball bearings serving as bearings for the heated roller of the fuser device in the image forming apparatus described above are complex in structure and expensive to manufacture. The necessity of the resin insulating sleeve in order to prevent disruptions in temperature distribution uniformity further increases costs. Moreover, bending of the support shaft due to errors in mounting precision in the support shaft of the heated roller or moment loads carries the risk of damaging the bearings.
By contrast, a resin sliding bearing of PPS resin or the like has the advantages that it can be used without the need to interpose an insulating sleeve, is simple in structure, and can be injection molded, allowing for low-cost production. However, such resin sliding bearings have low load capacity compared to ball bearings, and 10 times or more the level of friction torque. As a result, the capacity of the actuating motor must be increased when the bearings are used in an image forming apparatus such as a multifunction machine, printer, or FAX machine, leading to an overall increase in expense. Such bearings are also highly sensitive to the material and surface roughness of the rollers against which they slide. A rough roller sliding surface will increase friction torque and wear levels, and, if the roller is of a soft metal, the resin sliding bearing will cause friction damage in the roller, leading to specifications not being met. In general, a soft metal such as aluminum is used for the material of the heated roller. As surface roughness decreases, machining costs increase. Moreover, there is a limit to the extent to which the surface roughness of a soft metal such as aluminum can be reduced, since they cannot be ground. The low load capacity of a resin sliding bearing also depends upon the material and surface roughness of the counterpart roller.
Even if the sliding surface of the bearing is greased in order to reduce friction torque, there may be insufficient grease on parts subjected to heavy loads, with the result that specifications cannot be met. Moreover, an ordinary resin sliding bearing does not have an aligning function to compensate for moment loads. Thus, moment loads create uneven bearing contact, leading to partial high-pressure surface sections and increasing wear. In addition, the presence of sliding scratches in the contacting surfaces of the roller and the shaft when the bearing is replaced after a fixed period of extended usage means that both the roller and the shaft must be replaced, leading to the problem of increased cost of replacement.
Apart from bearings for heated rollers used in fuser devices, similar problems are present in bearings used in developer units, photosensitive units, paper ejector units, paper feeder units, and the like, which are used at room temperature, and in bearings used in belt-driving units for ink cartridge carriages of inkjet printers and the like.
A sliding bearing comprising an outer ring and an inner ring, in which an annular projection or annular groove formed on an inner circumference of the outer ring and an annular groove or annular projection formed on an outer circumference of the inner ring engage, is known in the art as a bearing intended to address these problems (see patent document 1). Also known is a sliding bearing comprising an outer ring and an inner ring, in which the inner ring is formed from a melt-cast and cured resin composition and a bearing gap between the inner ring and the outer ring is formed by the contraction of the resin when the inner ring cures (see patent document 2).
Also known in the art as a heated roller bearing capable of addressing these problems is a sliding bearing comprising an outer ring and an inner ring, in which one of the outer ring or the inner ring is made of a synthetic resin and the other is made of sintered metal, and the inner circumferential surface of the outer ring and the outer circumferential surface of the inner ring are in sliding contact relative to one another (see patent document 3). Also known as a similar bearing is a sliding bearing constituted by a combination of an inner ring and an outer ring, in which grease is enclosed between the inner ring and the outer ring, and the bearing supports a rotating body that rotates in only one direction, wherein grease-retaining grooves for retaining grease are formed in the outer circumferential surface of the inner ring, the grease-retaining grooves are two rows of pluralities of convergingly disposed pairs of rectangular grooves provided along the entirety of the outer circumferential surface of the inner ring, the more open ends of the pairs being formed so as to face the direction of rotation of the inner ring (see patent document 4).
Food product machines are machines used to mix, blend, heat, dry, chill, fill, wrap, store, or otherwise process raw food materials and finished (or semi-finished) food products. Food product machines, like other types of machinery, are equipped with bearings and other types of sliding parts, and it is necessary to prevent food products from being contaminated by toxic substances leaking from these parts. For this reason, the resins, metals, lubricants, greases, additives, and other materials making up these parts must be carefully selected according to statutory sanitary standards.
Well-known examples of statutory sanitary standards pertaining to materials used in food product applications include regulations for food products and additives (MHLW bulletins) provided for by the Food Sanitation Act and approval standards such as the H-1 designation (a rating for substances that are completely non-toxic to humans even after coming into direct contact with food products) of the U.S. Food and Drug Administration (FDA) and the U.S. Department of Agriculture (USDA). These standards designate materials that can be used in food product machines separate from general industrial materials.
A solid lubricant and a roller bearing for a food product machine in which a solid bearing lubricant is used so that there is no lubricant leakage even if water penetrates into the bearing, and rust does not readily form even if salt solutions or the like penetrates into the bearing, are known in the art as a bearing for food product machines (see patent document 5).
Also known are roller bearings for food product machine in which a solid lubricant for food product machinery that is not washed away by water and can withstand continuous use at high temperatures of 150° C. or more is used, the lubricant being enclosed within the bearings so that the sliding surface does not readily rust even in conditions involving contact with salt solutions (see patent documents 6 and 7). Also known are a bearing in which a pair of slingers are fitted in place onto an immobile shaft onto which a sealed roller bearing is fitted so as to sandwich the roller bearing in the axial direction in order to improve grease leakage resistance (see patent document 8) and a bearing provided with a plurality of seals (see patent document 9).