Bearings – Rotary bearing – Plain bearing
Reexamination Certificate
1999-11-09
2001-01-16
Hannon, Thomas R. (Department: 3682)
Bearings
Rotary bearing
Plain bearing
C384S297000, C384S902000, C384S907100
Reexamination Certificate
active
06174087
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for fabricating a friction bearing, in particular for ceramic shafts, made of metallic hardenable sintered material having an open pore volume of at least 15%, wherein the sintered material is brought into a predetermined shape and is subsequently sintered.
BACKGROUND OF THE INVENTION
Methods for fabricating friction bearings of the above-mentioned kind are known from industrial practice. The friction bearing is thereby made of sintered bronze or sintered iron, respectively, and the pore volume of this sintered bearing is maximally 30%. Such a pore volume is necessary for assuring good lubricating properties of the bearing with sufficient consistency. In general, the porous bearing is infiltrated with oil under negative pressure or vacuum, respectively, with the result that the oil settles in the interconnected pore volume. In order to allow the lubricant to also settle in the interior area of the porous bearing, open pores are required.
In the first place, the disadvantage of said known friction bearings consists in a bad running performance at low circumferential speeds of the shaft and/or with large lateral forces exerted on the shaft.
If the circumferential speed of the shaft drops below a certain value with a constant lateral force, or if the lateral force exerted on the shaft exceeds a certain value at a constant circumferential speed, insufficient pressure is built up in the gap of the bearing between shaft and bearing to effect a hydrodynamic lubrication process where the shaft floats on a lubricating film.
At sufficiently high circumferential speeds or sufficiently low lateral forces, the shaft floats on said lubricant and the bearing friction is solely determined by the friction of the fluid.
At low circumferential speeds, a mixed friction prevails between the shaft and the bearing, where, besides the fluid friction, there is also direct contact between the shaft and the bearing. This range of mixed friction involves an increase in wear and tear and an increased development of heat due to the increased friction in the bearing.
Through a high lateral force exerted on the shaft, as occurs in particular with engine drives, the region of mixed friction is shifted towards higher running speeds; a direct contact between the shaft and the bearing already occurs at high speeds.
The above-mentioned friction bearing technology common in industrial practice has the disadvantage that it fails in the region of mixed friction, where the heat development caused by the large amount of dry friction is so high and where such high local temperatures in the bearing occur, that the lubricant decomposes and the porous bearing is subject to wear and tear. The shaft seizes the bearing.
It is, therefore, known from the EP 0 428 539 B1 to substitute the sintered bronze bearing with a metal-ceramics sinter mixture as, on one hand, the combination of material ceramics-ceramics for bearings and shafts has very low sliding friction coefficients and a high temperature resistance in the region of mixed friction and on the other hand, the metal has a good thermal conductivity.
It turned out, however, that with high lateral forces being exerted on a bearing made of a metal-ceramics sinter mixture and a ceramic shaft, a local decomposition of the oil occured in the area of mixed friction due to coking.
SUMMARY OF THE INVENTION
For this reason, ball bearings have so far been exclusively used in the region of mixed friction or with non-stationarily driven shafts. In small-sized electric motors and the drive shafts thereof, however, the construction size and the prices of the available small-sized ball bearings are in the range of those of an electric motor.
It is, therefore, the object of the invention to improve the known friction bearings by avoiding, or at least considerably reducing, a local wear and tear of the shaft and the bearing in the region of mixed friction with high lateral forces.
According to the invention this object is achieved by hardening the friction bearing after the sintering.
The solution according to the invention has the surprising advantage that hardened porous bearings with a minimum pore volume of 15%, especially in correlation with ceramic shafts, are practically not subject to wear and tear in the region of mixed friction. The inventive friction bearings can, thus, be used over a wide range of speed and with high lateral forces up to the region of mixed friction. The temperature of the bearing even remains low enough in the area of mixed friction such that a decomposition of the lubricant does not take place. Also a local sparking in case of direct contact, which results in a partial combustion of bearing material and lubricant, as is known of ceramic bearings, no longer occurs with the bearings, according to the invention.
Although the hardness of the inventive bearings is reduced as compared to ceramic bearings, practically no wear and tear occurs with the inventive hardened bearings in the region of mixed friction despite the, in contrast to the known bearings, smaller difference in hardness to the shaft.
In a particularly advantageous embodiment, the friction bearing can be tempered after the hardening to reduce the brittleness of the friction bearing.
In an advantageous embodiment of the invention, the sintered material can be sintered steel. Sintered steel is especially low in price and can easily be heat-treated, i.e. hardened and tempered. With sintered steel, surface treatments such as nitriding, boronizing, carbonizing, case hardening and boundary layer hardening can also be performed.
In a further embodiment, the sintered material can also be a hardenable bronze, for example, an aluminium nickel alloy. As is known, bronze is low in price and has a sufficient surface hardness and rigidity.
It can, moreover, be that the sintered material is a hardenable nickel alloy. Nickel alloys have a particularly high rigidity, stability and thermal resistance.
In observation of the dimensional tolerances of the friction bearing, it is advantageous if the running surface of the friction bearing can be calibrated after the sintering. As the sintered material can be easily calibrated after the sintering and prior to the hardening it is, moreover, advantageous if the running surface can be calibrated prior to the hardening. It can, however, also be that the running surface is calibrated after the hardening. By means of the calibration after the hardening, the dimensional accuracy of the friction bearing is increased, as, in this way, the structural changes from the thermal treatment can no longer influence the dimensions of the friction bearing.
The invention relates further to a friction bearing, in particular for ceramic shafts, made of hardenable metallic sintered material having an open pore volume of at least 15%.
In view of the friction bearing, the object of the invention is achieved in that the friction bearing is hardened.
A shaft-bearing combination subject to particularly low wear and tear in the area of mixed friction is obtained, if the friction bearing system is constructed such that ,besides the inventive bearing, the shaft is equally made of sintered material.
Equally, very good wear and tear values were obtained if, in a further embodiment, the shaft was made of hard metal. In a further embodiment, zirconium oxide could also be used as material. Hard metal or zirconium oxide, respectively, have a high degree of hardness with the result that the shaft has a higher surface hardness than the bearing. In the interest of prevention from wear and tear, a harder shaft has shown to be the best solution in friction bearing technology.
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patent: 5120091 (1992-06-01), Na
Hannon Thomas R.
Maxon Motor GmbH
Tilton, Fallon, Lungmus & Chestnut
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