Bearing unit, drainage pump and hydraulic turbine each incorporating the bearing unit, and method of manufacturing the bearing unit

Abstract: Disclosed is a bearing unit which can be used in a drainage pump operated without supply of clean water or a hydraulic pump, which exhibits excellent wear resistance against water containing earth and sand and satisfactory assembling facility, and a method of manufacturing the bearing unit. The contact surface of a bearing (3) made of stainless steel and/or that of a sleeve (2) is applied with a sprayed coating (11,9), the main component of which is WC, and which contains one or more elements selected from a group consisting of nickel, chromium and cobalt as a binder material thereof, or a sprayed coating (11,9), the main component of which is Cr₃C₂, and which contains NiCr as a binder material, after the sprayed coating has been formed, heating at from 300°C to 550°C is performed for one hour or longer so that hardness and wear resistance equivalent to those of a WC - 12% sintered article is attained.

Description: The present invention relates to a bearing unit comprising a bearing, and a sleeve which is in slide-contact with said bearing, as well as to a method of its manufacturing and to its preferred uses. Since substantially all precipitation flows in the drainage ditches due to the recent urbanization taken place rapidly, the precipitation cannot be completely drained off and municipal floods causing roads to be under water tend to be increased. Hence, drainage facilities each provided with a drainage pump system have been established, causing another problem that the cost of operation and upkeep rises. Therefore, an automatic operation drainage pump system has been examined. The drainage pump system of this type must improve the performance and reliability thereof and non-water-supply operation technology. In order to achieve this, enlargement of the capacity and precedent-standby operation have been studied. The non-water-supply operation technology is to operate the drain pump while supplying no water to the space between a sleeve on a shaft and a bearing. Since no clear water supply apparatus is provided, a malfunction does not take place due to a failure of the clear water supply apparatus and that of the clear water supply sensor, resulting in satisfactory reliability. In order to enable the non-water-supply operation to be performed, a bearing unit must not be worn even if earth and sand contained in the drain invade thereto. A conventional bearing unit adapted to the non-water-supply technology has been constituted by a sleeve (a sintered article) made of tungsten carbide (hereinafter called "WC") exhibiting excellent wear resistance and corrosion resistance and by a ceramic bearing (a sintered article). The structures constituted by combining the sleeve made of WC and the ceramic bearing has been disclosed in JP-A-60-81517 and JP-A-60-88215. The enlargement of the pump capacity inevitably requires the diameter of the bearing thereof to be enlarged. However, the conventional bearing unit constituted by the WC sleeve and the ceramic bearing must employ a large-diameter sintered sleeve and a sintered bearing. However, there arise a problem in that the sintering technology cannot meet the requirements and the weight of the parts cannot be reduced, causing a difficulty to arise in assembling the bearing unit. Therefore, a hard film coating technology capable of coating a film having hardness equivalent to that of ceramics has been investigated energetically. It is noted that a non-water supply drainage pump using a sleeve applied with a sprayed WC coating and a ceramic bearing has been disclosed as an example of use of a sprayed coating as the hard film in "Trybologist", No. 2, vol. 36, pp. 144 to 147. Also the bearing for a hydraulic turbine must have wear resistance against water including sediment. Therefore, a conventional bearing for a hydraulic turbine has been made of ceramics such as silicon carbide (SiC). As a general rule, a sprayed coating is used as the hard film for the bearing portion of a drain pump. However, the fact that the sprayed coating is inferior to a sintered article having the same composition in terms of the hardness and the strength causes an investigation for improving the hardness and the strength of the sprayed coating to be made. For example, technology for improving the adhesive strength and the strength by plasma-spraying a material composed of, by weight, 50% chromium and 50% nickel and by holding the sprayed coating at from about 700°C to 800°C for from about 1 to 100 hours has been disclosed in JP-A-57-2872. Another technology has been disclosed in JP-A-60-149762 which is capable of strengthening the sprayed coating by forming a 20 to 80 wt% Ni-Cr sprayed coating, the main component of which is Ni-P on a ferrous material, and by subjecting them to a heat treatment set to from about 600°C to 1000°C. Since the hydraulic turbine has a large diameter, the conventional structure has an arrangement that sectioned ceramics are disposed. Therefore, there has been a desire of using the hard film similarly to the bearing for a drain pump. A bearing unit for a hydraulic turbine that uses a sprayed WC coating and chromium carbide (Cr 3 C 2 ) has been disclosed on pp. from 80 to 85, in resumes of the twenty-sixth lecture meeting titled "Turbo Machines", May 1991. The precedent standby operation is an idle operation executed before commencement of drainage in a state where the water level is lower than the level at which the drainage operation can be performed. By performing the precedent standby operation, full power operation can be performed when the water level rises to a level at which the drainage can be performed. Therefore, rapid precipitation increase can be quickly coped with the subject system. However, the fact that the bearing must be idly performed without drain to be introduced into the sliding or bearing portion thereof for a short time causes the friction coefficient between the bearing and the sleeve must be extremely low so as to be freed from being damaged during the foregoing dry operation. A bearing capable of performing both of a long time precedent standby operation, in which the idle operation is carried out for a long time, and the non-water-supply operation has not been developed yet. However, a method capable of supplying external water to the sliding portion of the bearing at the time of the idle operation has been examined, resulting in a disclosure filed in JP-A-55-90718. In order to drain water containing earth and sand, the bearing must have satisfactory wear resistance. In particular, the non-water supply bearing must have wear resistance against slurry because clear water for protecting the bearing and the sleeves is not supplied. The major portion of the earth and sand is composed of feldspar and quartz. Therefore, earth and sand have a maximum hardness of about Vickers (Hv)1000 (which is the hardness of quartz). Accordingly, the hardness required for the bearing and the sleeve is Hv 1000 or higher. A sintered article composed of anyone of WC, SiC and silicon nitride (Si 3 N 4 ) is excellent in the hardness while revealing satisfactory wear resistance because these materials have a hardness of Hv 1000 or higher, that is, WC has a hardness of about Hv 1400, SiC has a hardness of about Hv 2800 and Si3N4 has a hardness of about Hv 1600. However, the sintering technology, the assembling facility and the manufacturing cost cause a limit to present in the size of the sleeve and the bearing that are fully made of a sintered material composed of WC, SiC or Si 3 N 4 . Therefore, there has been investigated the use of a hard film in place of the aforesaid sintered article. Since the film thickness must be several hundreds micron if the quantity of wear is taken into consideration, there is a limit in the method of manufacturing the hard film. As the hard film for the bearing portion of a drainage pump, a thermally-sprayed coating mainly composed of WC or Cr 3 C 2 and having a thickness of from about 100 to 200 µm has been widely used because a relatively thick film and satisfactory hardness can be obtained. However, the sprayed coating mainly composed of WC or Cr 3 C 2 generally has unsatisfactory hardness as compared with a sintered article although depending upon the material, the forming method and conditions, the aforesaid sprayed coating having a hardness of from about 600 Hv to 1000 Hv. The relationship between a variety of conventional sprayed coatings and the wear resistance against water containing earth and sand will be described with reference to Fig. 21 which is a graph showing the wear rates measured by element experiments to be described later in the description about preferred embodiments. Each of sprayed coatings was formed on a stainless steel plate to serve as a rotary side specimen, while α - SiC was employed to serve as the stationary side specimen. The residual conditions were as follows: the surface pressure was 2 kg/cm 2 , the peripheral speed was 0.5 m/sec, the concentration of silica particles in earth and sand in water was 9 wt%. It is noted that the detailed shape of the specimen and the sliding method and the like were the same as those of the experimental conditions shown in Fig. 19 with regard to the preferred embodiments. The axis of abscissa of a graph shown in Fig. 21 stands for cross-sectional Vickers hardness of the sprayed coatings, and the axis of ordinate stands for relative wear ratio standardized while making the wear ratio of WC - 12% cobalt sintered article to be a reference. Sprayed coatings 1, 2 and 3 were conventional sprayed coatings formed by a high speed spraying method and a detonation spraying method and mainly composed of WC or Cr 3 C 2 . As shown in Fig. 21, the wear resistance against water containing earth and sand considerably depends upon the hardness of the sprayed coating. It is preferred that a satisfactory effect can be obtained if the hardness is Hv 1000 or higher as described above. As can be understood from Fig. 21, the conventional sprayed coating cannot enable satisfactory hardness and wear resistance against water containing earth and sand to be obtained. In general, spraying is an operation in which not only hard particles such as WC or Cr 3 C 2 are heated and sprayed but also metal particles such as nickel, chromium or cobalt particles are mixed and simultaneously heated and sprayed. As a result, WC particles or Cr 3 C 2 particles are bound by molten metal of nickel, chromium or cobalt to form a film. The reason why the sprayed coating is lower in hardness than that of the sintered article is not that the WC or Cr 3 C 2 particles are not of a lower hardness but that the binder metal for binding the WC or Cr 3 C 2 particles has defects such as blow holes or that the binding strength between the binder metal and the WC or Cr 3 C 2 particles is not fully high. In order to enlarge the binding strength between WC or Cr 3 C 2 particles, the spraying method has been improved variously. For example, there have been developed a high speed flame spraying method increasing the spraying velocity of the particles by utilizing the combustion energy of a combustible gas and a detonation spraying method utilizing a detonation of a combustible gas. As a result, the particle velocity at the time of spraying can be raised as compared with the speed realized when the conventional plasma sprayed coating is formed, resulting in a sprayed coating of higher hardness. However, the wear resistance against sediment water containing earth and sand is not fully high even if the sprayed coating is formed by anyone of the aforesaid spraying methods. It has been understood from the element test shown in Fig. 18 that the reason for this is that the hardness of the sprayed coating is lower. The foregoing conventional ceramic bearings and those disclosed in JP-A-60-81517 and JP-A-60-88215 are not technologies that have been established while taking into consideration the difficulty in the sintering technology, deterioration in the reliability and unsatisfactory assembling facility due to an increase in the weight caused from enlarging the diameter of the bearing and the sleeve. The non-water supply drainage pump using a combination of the WC sprayed coating and the ceramic bearing (sintered article) ("Trybologist", No. 2, vol. 36) and the bearing structure for a hydraulic turbine that uses a WC sprayed coating and Cr 3 C 2 sprayed coating (resumes of the twenty-sixth lecture meeting titled "Turbo Machines") were not established while thoroughly investigating the hardness, the wear resistance and the reliability of the sprayed coating, the main component of which is WC or Cr 3 C 2 . The improving methods for properties of the sprayed coating respectively according to the prior art, and disclosed in JP-A-57-2872 and JP-A-60-149762 encounter a limitation in coating materials for use therein. As the coating material, those having a thermal expansion coefficient which approximates that of ferrous alloys, which are primary base materials, or these which are not decomposed by the improving heat treatment, are used. The sprayed coating, the main component of which is WC or Cr 3 C 2 , and from which satisfactory hardness can be attained, is caused to have a large difference in the thermal expansion with respect to the base material. As a result, the coating is ruptured and it cannot be used as it is. Furthermore, an apparatus such as a pump that is used in water must be mainly made of stainless steel. Therefore, according to the conventional heat treatment of improving properties of the sprayed coating, the heat treatment temperature is higher than the annealing temperature of a stainless steel as a base material or the temperature at which the stainless steel is sensitized to intergranular corrosion. Hence, the hardness and the wear resistance of the base material deteriorate, causing a problem to arise in that the reliability deteriorates. That is, the prior art has not been subjected to a consideration taken about the improvement in properties of the sprayed coating, the main component of which is WC that has a large difference in the thermal expansion coefficient from ferrous alloys. What is worse, no consideration has been made about change in the hardness of the base material and that in the wear resistance. The prior art disclosed in JP-A-55-90718 and having the arrangement that external water is supplied to the sliding portion of the bearing arises a problem of reliability because the operation of the drainage facility is stopped due to a malfunction of an external water supply apparatus and the drainage operation can be stopped in an emergency. The US-A-4 912 835 discloses a tungsten carbide sprayed coating on a roll for use in rolling processes of non-ferrous metals. The tungsten carbide sprayed coating according to the US-A-4 912 835 contains nickel, chromium, or cobalt as binder material thereof. Another special feature of said sprayed coating is that the portion of the surface area of the coating covered by blow holes is less than 1.8 %. The use of customary tungsten carbide sprayed coatings on rolls for different rolling processes in metallurgy and paper industry respectively is furthermore disclosed in the JP-A-63 139 724, JP-A-03 053 055, and the US-A-4 989 306. The US-A-4 997 324 discloses the use of a thermally sprayed tungsten carbide base cemented carbide powder on a spindle structure for a machine tool. The sprayed coating according to the US-A-4 997 324 contains cobalt and iron as binder materials thereof. The use of customary tungsten carbide sprayed coatings on bearing units of different types is disclosed in the US-A-3 936 295, and in the JP-A-59 067 363. The DE-A-37 33 117 discloses a pressurized air bearing unit the two parts of which comprise sprayed tungsten carbide coatings which contact each other only under low speed conditions. The most relevant background prior art is described in US-A-4 317 850, which discloses a method for applying a tungsten carbide coating on a metal object, in particular a bearing bushing. According to the US-A-4 317 850 tungsten carbide together with a bonding material which can be nickel, cobalt, iron, or chromium is sprayed onto a metal surface. Thereafter, a gas-tight metal foil is applied over the sprayed coating and the metal foil together with the sprayed coating is isostatically compressed onto the metal object under a pressure in the range from about 500 to 2000 bar at a temperature within a range from about 500°C to about 2000°C for a period from about 1/2 hour up to about 8 hours. After the pressing process the metal foil is removed. Another method for producing a heat and wear resistant coating on a steel object is disclosed in the JP-A-55 058 360. According to this method, a nickel alloy or a cobalt alloy containing phosphorus and/or boron is deposited on the surface of the steel article by using an electroplating method or a chemical plating process. Thereafter, tungsten carbide mixed with cobalt, aluminum, or nickel as binder material thereof is sprayed onto the alloy layer. The entire structure is then heated up to 300 to 600°C. The object of the present invention is to provide a bearing unit with excellent wear resistance, assembling facility, and satisfactory reliability, to provide a method for manufacturing such a bearing unit, and preferable uses of such a bearing unit. According to the invention this object is achieved by providing a bearing unit comprising a bearing, and a sleeve which is in slide-contact with said bearing, wherein from said bearing and said sleeve one is selected to be made of a ferrous alloy having a contact surface applied with a sprayed coating comprising WC or Cr 3 C 2 as the main component and at least one element of Ni, Cr and Co as binder material, wherein the number ratio of blow holes contained in the surface of said sprayed coating and having a size of not smaller than 20 µm is 15 per square millimeter or less. Preferred embodiments of this bearing unit are subjects of claims 2 to 8. Further, the object is achieved by providing a method for manufacturing the bearing unit according to claim 9, and by using such a bearing unit in a drainage pump according to claims 10 to 12 or in a hydraulic turbine according to claims 13 and 14. According to the present invention, the bearing unit exhibits excellent wear resistance and assembling facility and satisfactory reliability, and is adaptable to a hydraulic turbine or to a drainage pump system, preferably to a pump of a type operated while being freed from clear water supply to the bearing portion thereof. According to the present invention, the wear resistance of a sprayed coating applied to the bearing surface of a bearing unit is improved by heating the sprayed coating, the main component of which is WC or Cr 3 C 2 , while determining the temperature conditions so as to make the sprayed coating to be freed from deterioration of the adhesive property and denaturing, and making the base material of stainless steel to be freed from excessive deterioration of the hardness and sensitization to intergranular corrosion due to thermal stress. The formed sprayed coating is preferably heated at a temperature from 300°C to 550°C for not shorter than one hour to have a hardness of not lower than Hv 1000 to have sufficient wear resistance against sediment water containing earth and sand. The material of the bearing and/or the sleeve applied with the sprayed coating is preferably stainless steel. If either of the bearing and the sleeve is applied with the sprayed coating, the residual is of SiC or Si 3 N 4 provided at the contact or bearing surface thereof or may be entirely made of SiC or Si 3 N 4 . The sleeve or the bearing may be divided into two or more sections. The drainage pump or the hydraulic turbine according to the present invention incorporates a bearing unit characterized by anyone of the arrangements stated above. In the method of the present invention the sprayed coating may be formed on the contact surface of the bearing and/or the sleeve machined to have a predetermined shape and made of stainless steel and it may be subjected to finish machining so as to have a predetermined size. There has been said that heating of a sprayed coating, the primary component of which is WC or Cr 3 C 2 , will release the strain which contributes to harden the coating, and therefore the hardness is lowered. It has been considered that, if the spraying coating mainly composed of WC or Cr 3 C 2 having a small thermal expansion coefficient is formed on a base made of a ferrous alloy (a stainless steel of JIS SUS403, for example, is used in the present invention), heating of the sprayed coating after the coating has been formed causes the following problems due to the difference in the thermal expansion coefficient between them: adhesion strength of the coating deteriorates due to the thermal stress, and WC particles or Cr 3 C 2 particles are oxidized, causing the properties of the sprayed coating to deteriorate. Therefore, there has been considered that it is not preferable to heat the sprayed coating mainly composed of WC or Cr 3 C 2 after the coating has been formed. However, the hardness can be improved while eliminating adverse influence on the sprayed coating if proper heating conditions are selected. For example, the hardness of the sprayed coating can be improved by heating it at from 300°C to 550°C for not shorter than one hour to have sufficient wear resistance against water containing earth and sand. The mechanism and the results of examinations will be described in the description of preferred embodiment later. If blow holes are present in the sprayed coating, the strength of the sprayed coating is decreased. Furthermore, the binding strength between particles of WC or Cr 3 C 2 is also decreased, causing the particles of WC or Cr 3 C 2 to be separated from the coating during operation. As a result, the wear resistance deteriorates. However, heat treatment of the formed coating performed for the purpose of eliminating the blow holes will cause particles of the binder metal, and particles of WC or Cr 3 C 2 to be oxidized, adhesion strength of the sprayed coating to deteriorate, and the hardness and the corrosion resistance of the base material to deteriorate. As a result, the reliability deteriorates. However, it has been found that the present invention enables the number of the blow holes in the sprayed coating to be decreased, the size of the blow hole to be reduced and the hardness of the sprayed coating to be improved by a heat treatment at a low temperature which does not cause the oxidation of the WC or Cr 3 C 2 sprayed coating, deterioration of adhesion strength of the coating due to the thermal stress and the deterioration of the hardness and the corrosion resistance of the base material. The above effects can be obtained by selecting the temperature and the time in which heating is performed. In particular, it has been found that heat treatment conditions according to the present invention enables the number of large blow holes to be decreased, herein, the large blow holes being defined as those having a size of not smaller than 20 µm that is larger than the maximum particle size of particles of WC or Cr 3 C 2 . The large blow holes separate particles of WC or Cr 3 C 2 from the coating. Therefore, the decrease in the number of the large blow holes may improve the strength of the sprayed coating. If the number of the large blow holes are decreased, the sprayed coating is attained ductility, causing resistance against impact to be improved. As a result, occurrence of rupture and separation of the coating can be prevented and the reliability of the coating therefore is improved. Since the blow hole serves as the trigger point of corrosion of the sprayed coating, the decrease in the number of the blow holes cause particles of WC or Cr 3 C 2 to be positioned more closely one another. Therefore, the corrosion generated among particles can be prevented and the corrosion resistance against drain can be improved. If the drainage pump of the present invention is used to serve as a precedent standby operation drainage pump, the arrangement that the bearing, which can be operated without lubrication, is employed will enable satisfactory reliability to be attained. By constituting the sleeve and/or the bearing to have a separated structure, the manufacturing process can be simplified. It is especially suitable to a large-diameter pump. Other and further objects, features and advantages of the invention will be appear more fully from the following description and the accompanying drawings in which Fig. 1 is a cross sectional view which illustrates the structure of a drainage pump according to an embodiment of the present invention; Fig. 2 is a perspective view partially including a cross sectional view which illustrates a bearing unit according to an embodiment of the present invention; Fig. 3 is a vertical cross sectional view which illustrates a bearing unit according to another embodiment of the present invention; Fig. 4 is a vertical cross sectional view which illustrates a bearing unit according to another embodiment of the present invention; Fig. 5 is a vertical cross sectional view which illustrates a bearing unit according to another embodiment of the present invention; Fig. 6 is a perspective view which illustrates the structure of a sleeve according to another embodiment of the present invention; Fig. 7 is a cross sectional view taken along line A-A of Fig. 5; Fig. 8 is a perspective view partially including a cross sectional view which illustrates a bearing according to another embodiment of the present invention; Fig. 9 is a graph which shows the relationship W/D 2 between the outer diameter "D" of a sleeve according to another embodiment of the present invention and the weight "W"; Fig. 10 is a cross sectional view which illustrates the structure of a hydraulic turbine according to another embodiment of the present invention; Fig. 11 is a graph which illustrates change in the hardness of a WC - 27 % NiCr sprayed coating taken place when it is heated at 400°C; Fig. 12 is a graph which illustrates change in the hardness of a WC - 27% NiCr sprayed coating taken place when it is heated at 500°C; Fig. 13 is a graph which illustrates change in the hardness of a WC - 12% cobalt sprayed coating taken place when it is heated at 400°C; Fig. 14 is a graph which illustrates change in the hardness of a Cr 3 C 2 - 25 % NiCr sprayed coating taken place when it is heated at 400°C; Figs. 15A, 15B and 15C are photographs which illustrate the change of the surface of the WC-27 % NiCr sprayed coating when it is heated at 400°C; Fig. 16 is a graph which illustrates the relationship between the number ratio of blow holes in the surface of the WC-27 % NiCr sprayed coating and the heating time when it is heated at 400°C; Fig. 17 is a graph which illustrates the relationship between the number ratio of blow holes in the surface of the WC-27% NiCr sprayed coating and the hardness when it is heated at 400°C; Figs. 18A, 18B, 18C and 18D illustrate the shape of the wear test specimen and the method of sliding it; Fig. 19 illustrates the relationship between the distance of sliding and the wear loss obtained by a wearing test subjected to a WC - 12% cobalt sintered article, the WC - 27% NiCr sprayed coating subjected to no heat treatment and the WC - 27% NiCr sprayed coating subjected to the heat treatment; Fig. 20 is a graph which illustrates the relative wear rates of combinations of materials including the WC -27% NiCr sprayed coating subjected to the heat treatment, Cr 3 C 2 - 25 % NiCr subjected to the heat treatment; and Fig. 21 is a graph which illustrates the hardness of conventional sprayed coatings and relative wear ratio against water containing earth and sand with respect to WC - 12% sintered article as a reference material. As shown in Fig. 1, a drainage pump has two bearings respectively disposed adjacent to an impeller and in an upper portion thereof. A bearing unit according to this embodiment can be adapted to either of the two bearings. Referring to Fig. 1, reference numeral 1 denotes a main shaft, 2 a sleeve fixedly attached on the main shaft 1, and 3 a bearing. The main shaft 1, the sleeves 2 and the bearings 3 were made of a stainless steel of JIS SUS403. The pump is provided with a plurality of pipes 30 each of which has a lower end opening in a drain suction port zone of a pump casing and an upper end opening in the atmosphere, and is vertically arranged along the outer surface of the casing, and which is supported by a rib of the casing at a lower end and by a floor of the drainage facility. In the pump, if the drainage water level is low, difference takes place between the internal pressure of the pump and the atmosphere, causing air to be sucked into the drain through the pipe. As a result, the quantity of the flow is reduced and generation of vortexes in the drain level is restricted. The generation of the vortexes must be prevented because it causes vibration force to be generated, thus resulting in the vibration of the pump. By sucking air through the pipe 30, the generation of the vortexes can be prevented even if the waver is a low level at which the vortexes are easily generated. Therefore, drainage can be performed stably. If the water level has been raised sufficiently, the pressure difference across the casing is reduced, causing the suction of air through the pipes 30 to be naturally interrupted. Since the pressure change in the casing taken place due to the drain level is utilized, any external control is not required and accordingly satisfactory reliability is attained. By providing a valve at the upper opening end portion of the pipes 30, the quantity to be sucked can be controlled and the above-mentioned function can be cancelled. Then, the bearing unit according to this embodiment will now be described with reference to Figs. 2 and 3. Fig. 2 is an enlarged view which illustrates the bearing unit disposed in the upper portion of the drainage pump shown in Fig. 1. Fig. 3 is an enlarged view which illustrates the bearing or contact portion shown in Fig. 2 while further enlarging the contact portion. The sleeves 2 and the bearing 3 are manufactured as follows: a sleeve base 8 and a bearing base 10 are manufactured by machining; and heat treatments of quenching and tempering are performed to improve the hardness of a stainless steel of JIS SUS403. The tempering temperature was so set to abut 700°C that the stainless steel of JIS SUS403 does not become brittle at the time of improving the property of the sprayed coating to be performed later. After the heat treatment was completed, sprayed coatings 11 and 9 were formed on a sliding portion of the bearing 3 and the sleeve base 8 by a high speed flame spraying method, the sprayed coatings 11 and 9 being mainly composed of WC and containing NiCr as a binder metal. After the sprayed coatings 9 and 11 were formed, a heat treatment was performed by holding the bearing 3 and the sleeve base 8 at 400°C for 20 hours to improve the hardness of the sprayed coatings 9 and 11. After the heat treatment was completed, the sleeve 2 and the bearing 3 were machined to respectively have predetermined inner and outer diameters and a predetermined surface roughness. The sleeve 2 is secured to the main shaft 1 by a rotation stopper 4. The bearing 3 was fastened to a metallic backing member 6 for the bearing 3 followed by being fixed by a fixing member 7 while interposing a buffer material 5 for the bearing 3. Furthermore, an invasion prevention member 12 for preventing invasion of sediment into the sliding portion was fastened to the bearing 3. Since this embodiment has the arrangement that stainless steel of JIS SUS403 is employed as the material for the sleeve base 8, its thermal expansion coefficient coincides with that of the material of the main shaft 1. As a result, generation of thermal stress can be prevented. Furthermore, the realized satisfactory toughness enables the sleeve to be obtained which is able to stably holding the sprayed coating formed on the surface thereof. Furthermore, the heat treatment applied to the WC sprayed coating enables the wear resistance to be improved, causing the durability and the reliability of the bearing unit to be improved. Although this embodiment has the arrangement that the sprayed coating to be formed on the sleeve and the bearing is mainly composed of WC and contains NiCr as the binder material, a similar effect was obtained from a sprayed coating, the main component of which was WC, and which used cobalt as the binder material, and a sprayed coating, the main component of which was Cr 3 C 2 , and which used NiCr as the binder material. The drainage pump exhibits stable bearing performance even if the bearing unit is under non-lubrication state, that is, non-water-supply state. If water is supplied, the drainage pump is able to further satisfactorily perform the bearing operation. The drainage pump has also an invasion prevention member to prevent invasion of sediment into the bearing or contact portion of the bearing unit. If sediment mixed with the drain, however, invades into the sliding portion, the operation can be performed stably because the bearing portion exhibits satisfactory wear resistance. Since excellent bearing performance is attained in a non-lubrication state, the drainage pump can be reliably operated in a state where no-lubrication operation and lubricated operation are repeated, that is, in the precedent standby operation. Fig. 4 is a vertical cross sectional view which illustrates a bearing unit for a pump according to another embodiment of the present invention. The contact surface of the sleeve base 8 made of a stainless steel of JIS SUS403 is applied with the sprayed coating 9, the main component of which was WC, and which contained cobalt as the binder material. Furthermore, a bearing 13 which supports the sleeve 2 was made of SiC. The sleeve 2 was subjected to a heat treatment set to 400°C for 20 hours after the sprayed coating 9 has been formed to improve the hardness of the sprayed coating 9. The residual structures are the same as those according to the foregoing embodiment shown in Figs. 1 to 3. Since this embodiment has an arrangement that the material of the bearing portion is uniform, hard and sintered ceramics, the durability and the reliability of the bearing unit can be improved. Although this embodiment has the arrangement that the sprayed coating to be formed on the sleeve is mainly composed of WC and contains NiCr as the binder metal, a similar effect is obtained from a sprayed coating, the main component of which is WC, and which uses cobalt as the binder material, and a sprayed coating, the main component of which was Cr 3 C 2 , and which used NiCr as the binder material. Fig. 5 is a vertical cross sectional view which illustrates a bearing unit for a pump according to another embodiment of the present invention. This embodiment has an arrangement about a bearing structure capable of performing an idling operation even if the drainage water level does not reach the height of the bearing at the time of a precedent standby operation. It is noted that the detailed cross sectional structure of the sleeve 2 is omitted from illustration in order to show the overall structure. A sprayed coating 11, the main component of which is WC, and which contained NiCr as the binder metal, is applied to the contact surface of the sleeve 2 made of stainless steel of JIS SUS403. After the sprayed coating 11 has been formed, a heat treatment set to 400°C for 20 hours is performed to improve the hardness of the sprayed coating. Furthermore, the bearing 13 which supports the sleeve 2 is made of SiC. The bearing 13 is fastened to the metallic backing member 6 and fixed to a fixing member 7' while interposing the buffer material 5. Furthermore, a backing plate 17 is secured to the fixing member 7'. In addition, a rotary water tank 18 is secured to the main shaft 1. Water is enclosed in the rotary water tank 18 so that lubricant water is present in the contact portion of the bearing if the idle operation is performed in which the drainage water level does not reach the height of the bearing. The structure according to this embodiment encounters a fact that earth and sand are sometimes invaded into the contact portion at the time of the operation because the rotary water tank 18 easily accumulates the sediment. However, the sprayed coating 11 on the contact surface of the sleeve 2 has sufficient wear resistance attained due to the heat treatment, resulting no problem to arise. Figs. 6 and 7 respectively are views which illustrate a state where a sleeve of a bearing unit for a pump according to another embodiment of the present invention is fixed, and a cross sectional view which illustrates a fixing portion. The sleeve is divided into four sections in the circumference. Four sleeves 2a, 2b, 2c and 2d formed by the foregoing division were made of stainless steel of JIS SUS403. A sprayed coating, the main component of which is Cr 3 C 2 , and which contained NiCr as the binder metal, is formed on the contact surface side of each of the sleeves 2a, 2b, 2c and 2d with which the bearing is brought into contact. The sleeves 2a, 2b, 2c and 2d is subjected to a heat treatment set to 400°C for 20 hours. Each of the sleeve sections 2a, 2b, 2c and 2d is fixed to the main shaft 1 by two bolts 14 at the axial directional two portions thereof. The residual structures are the same as those according to the foregoing embodiments shown in Figs. 1 to 3. The methods of manufacturing and assembling the bearing portion according to another manner will now be described. The sleeves sections 2a, 2b, 2c and 2d are manufactured by using stainless steel of JIS SUS403, and they are subjected to the heat treatment arranged similarly to that applied to the method of manufacturing the bearing. After the heat treatment has been completed, sprayed coating WC, the main component of which is WC, and which contains NiCr or cobalt as the binder material, is formed on the contact surface of each sleeve by high speed flame spraying. After the sprayed coating has been formed, the sleeve is heated to 400°C and held at the temperature for 20 hours. After the heat treatment has been completed, the sleeve is fixed to the main shaft 1, and the sleeve is machined to have a predetermined size. Although the bolts are sufficient to fix the sleeve to the main shaft 1, use of an adhesive agent will improve the reliability. Since this embodiment has an arrangement that the sleeve is divided into a plurality of sections, the operation for forming the sprayed coating can be facilitated. Furthermore, the width of each of the divided sleeves can be reduced to a quarter of the corresponding integrated sleeve, causing an effect to be obtained in that the sleeve can be handled easily. Fig. 8 is a schematic view partially including a cross sectional view which illustrates a bearing of a bearing unit for a pump according to another embodiment of the present invention. This embodiment has an arrangement that bearing sections 19 formed by dividing the bearing portion of a bearing are arranged. Each of the bearing sections 19 is produced by forming the sprayed coating 11 thereon, the main component of which is WC, and which contains NiCr or cobalt as the binder metal, by high speed flame spraying on the base 10 made of stainless steel of JIS SUS403. After the sprayed coating has been formed, a heat treatment set at 400°C for 20 hours is performed to improve the hardness of the sprayed coating. The bearing sections 19 are fastened to the metallic backing member 6 while interposing the buffer material 5, the bearing sections 19 being then disposed in a bearing housing 20. Although the bearing sections 19 are fastened to the bearing housing 20 by a fixing jig 21, use of an adhesive agent will improve the reliability. Since this embodiment has an arrangement that the bearing portion of the bearing is divided into a plurality of sections, the sprayed coating can easily be formed. Furthermore, the arrangement that each sliding section 19 is fixed while interposing the buffer material 5 will effectively prevent an unsymmetrical contact. The sleeve which has been made by sintered carbide mainly composed of WC can be made of a stainless steel according to the present invention. Therefore, the weight can be considerably reduced. Furthermore, the arrangement that the sleeve base is made of stainless steel will enable thread hold machining to be performed. Therefore, handling can be facilitated. In particular, a significant effect can be obtained from the present invention if a sleeve has a large diameter. The effects of the present invention attained to reduce the weights of sleeves are collectively shown in Fig. 9. Fig. 9 is a graph which illustrates the relationship between the outer diameters (hereinafter abbreviated to symbol "D") and the weights (hereinafter abbreviated to symbol "W") of sleeves. Since the sleeve weight "W" is varied when the outer diameter "D" of the sleeve is changed, the relationship is standardized to W/D 2 . According to the present invention, a W/D 2 of 0.05 or less is attained. Fig. 10 is a cross sectional view which illustrates a hydraulic turbine which incorporates the bearing unit according to the present invention as the bearing for the hydraulic turbine. Referring to Fig. 10, reference numeral 22 represents a main shaft for a hydraulic turbine, and 23 represents a sleeve rotation stopper. Reference numeral 24 represents a sleeve made of stainless steel of JIS SUS403 manufactured by forming a sprayed coating (hereinafter called a "WC - 27% NiCr sprayed coating"), the main component of which is WC, and to which NiCr is added by 27 weight % as a binder metal, followed by heat treatment applied thereto. Reference numeral 25 represents a bearing made of stainless steel of JIS SUS403 manufactured by forming a sprayed coating (hereinafter called a Cr 3 C 2 - 25 % NiCr sprayed coating), the main component of which is Cr 3 C 2 subjected to a heat treatment, and to which NiCr is added as a binder metal by 25% and by applying heat treatment. The sprayed coating was formed on the surface of the sleeve and the bearing by using a high speed flame spraying method. As can be understood from the aforesaid results of the element test, the sprayed coating subjected to the heat treatment has wear resistance similar to that attained from the sintered article. Furthermore, the aperture can easily be enlarged and therefore an effect of reducing the cost and the weight can be obtained if the arrangement of this embodiment is adapted to a hydraulic turbine. The wear resistance properties of the bearing according to the foregoing embodiment were evaluated by examining the change in the hardness of the sprayed coating taken place due to heat treatment applied. The change in the hardness of the sprayed coating was examined under conditions: heating temperature was ranged from 200°C to 600°C; and heating time was ranged from 1 to 30 hours. The hardness of the sprayed coating was measured by a Vickers hardness meter. In order to eliminate the influence of the base, the hardness was measured at the cross sectional of the sprayed coating and a load of 300 g is equally applied. Fig. 11 illustrates a portion of the results of the aforesaid measurements, wherein change in the hardness taken place when a WC - 27 % NiCr sprayed film is heated to 400°C is shown. The axis of abscissa stands for the heating time, while the axis of ordinate stands for the hardness of the sprayed coating. The sprayed coating was formed by high speed flame spraying to have a thickness of about 100 µm after the surface has been polished. The base is made of stainless steel of JIS SUS403 subjected to heat treatments of quenching and tempering at about 700°C. Since the values of the measured hardness disperse, ten points were measured to show the maximum value and the minimum value of the measured values are designated by a line while showing average values by a mark "○" As shown in Fig. 11, the hardness of the sprayed coating increases with heat treatment time and reaches its maximum hardness in 10 to 30 hours. Then, the hardness becomes substantially constant value although slight dispersion takes place. The maximum value of the average hardness is Hv 1014 (maximum value Hv 1065 and minimum value Hv 890). The thus attained hardness is inferior to an average value of Hv 1317 (maximum value Hv 1404 and minimum value Hv 1235) of the hardness of a sintered article (hereinafter called a "WC - 12% cobalt sintered article), the main component of which is WC, and to which cobalt is, as the binder, added by 12 weight %. However, an increase of an average value of about 34% is attained as compared with an average value of Hv 727 (maximum value Hv 869 and minimum value Hv 604) of the hardness realized before the heat treatment is performed, resulting in a significant improvement. Fig. 12 illustrates a part of the results of the measurements, wherein change in the hardness taken place when a WC - 27% NiCr sprayed coating is heated to 500°C is shown similarly to Fig. 11. The method of forming the sample, the shape of the same and the method of measuring the hardness are the same as those employed to obtain the results shown in Fig. 11. The hardness of the sprayed coating substantially reaches the maximum hardness when the sample is heated for about 1.5 hours. The maximum value of the average hardness is Hv 1115 (maximum value Hv 1139 and minimum value Hv 1084). The averaged value is increased by about 47% as compared with the hardness realized before the heat treatment is applied, resulting in similar hardness to that of the WC-12% cobalt sintered article. Also Fig. 13 shows a part of the results of the measurements, wherein change in the hardness taken place when a sprayed coating (hereinafter called a WC - 12% cobalt sprayed coating), the main component of which is WC, and to which cobalt is, as the binder material, added by 12 weight %, is heated to 400°C is shown. Also the WC - 12% cobalt sprayed coating is formed by the high flame spraying similarly to each sample shown in Figs. 11 and 12 to have a thickness of about 100 µm after its surface has been polished. The base is made of stainless steel of JIS SUS403 subjected to the heat treatment. Also the shape of the sample and the method of measuring the hardness are the same as those employed to obtain the results shown in Figs. 11 and 12. The hardness of the WC - 12% cobalt sprayed coating increases with heat treatment time and reaches the maximum hardness in 10 to 20 hours, followed by slight deterioration in 25 hours. The average value of the maximum hardness is Hv 946 (maximum value Hv 1149 and minimum value Hv 792). An increase in the average value of about 62% is attained as compared with an average hardness value of Hv 584 (maximum value Hv 1149 and minimum value Hv 490) realized before the heat treatment. A similar hardness change to that attained in the case of the WC - 27% NiCr sprayed coating was taken place in the case where heating to 500°C was performed. In this case, the maximum hardness was attained by performing heating for about one hour. Also Fig. 14 shows a portion of the results of the measurements, wherein change in the hardness is shown which takes place when a Cr 3 C 2 - 25% NiCr sprayed coating is heated at 400°C. Also the Cr 3 C 2 - 25% NiCr sprayed coating is formed by a similar high speed flame spraying employed to form each sample for use in the measurement shown in Figs. 11, 12 or 13 to have a thickness of about 100 µm after its surface has been polished. The base is made of stainless steel of JIS SUS403 subjected to the heat treatment. The shape of the sample and the method of measuring the hardness are the same as those employed to obtain the results shown in Figs. 11, 12 and 13. The hardness of the Cr 3 C 2 - 25% NiCr sprayed coating gradually increases with heat treatment time. The maximum value of the average hardness in 25 hours of heating is Hv 958 (maximum value Hv 1043 and minimum value Hv 905). The average value increased by about 21% as compared with an average value Hv 791 (maximum value Hv 817 and minimum value Hv 744) of the hardness before the heat treatment is performed. In this case, the attained increase is inferior to that realized in the case of the WC - 27% NiCr sprayed coating and the WC - 12% cobalt spray coating. Also the hardness increases with time like the case in which heating to 400°C is performed, resulting in a larger degree. The hardness of each of the foregoing WC-27% NiCr sprayed coating, the WC-12% cobalt sprayed coating and the Cr 3 C 2 - 25% NiCr sprayed coating is significantly improved due to the heating from 400°C to 500°C, resulting in hardness equivalent to the hardness of WC - 12% cobalt sintered article. It can be considered that the heating from 400°C to 500°C increases the binding strength for particles of WC or Cr 3 C 2 with the binder material in the sprayed coating to a level substantially equivalent to the sintered article. The results of examinations of the heating temperature range from 200°C to 600°C and a heating time range from 1 to 30 hours will now be described. Although the hardness of the sprayed coating increases if the heating temperature is set to 300°C or lower, the increase ratio is excessively low, resulting in a difficulty in industrial use. It is therefore preferable that the heat treatment temperature be 300°C or higher, more preferably 350°C or higher. However, thermal stress is generated, causing the adhesive strength of the film to deteriorate and resulting in a separation of the film from the base material in an excessive case due to the difference in the thermal expansion coefficient between the sprayed coating and the base because of the following fact: ferrous alloys for use to form the pump has a thermal expansion coefficient of about 12 to 17 x 10 -6 /°C and WC or Cr 3 C 2 type sprayed coating has a thermal expansion coefficient of about 5 to 7 x 10 -6 /°C. Furthermore, the heat treatment performed in the atmosphere will cause an oxidation of the sprayed coating to take place, resulting in a deterioration of the wear resistance. Therefore, if the WC or Cr 3 C 2 sprayed coating is used, the heat treatment temperature must be set to 550°C or lower, preferably 500°C or lower. In order to prevent the oxidation of the sprayed coating, it is preferred to perform heating in inert gas or in vacuum. Fig. 15 shows a microscopic photograph of a state where the WC - 27 % NiCr sprayed film formed to have a thickness of abut 200 µm was heated at 400°C followed by performing surface polishing until the thickness is reduced to about 100 µm, and heating is again performed followed by performing polishing to attain a mirror surface. The final thickness is about 90 µm. Fig. 15A is a photograph of the surface of non-treated sprayed coating, Fig. 15B is a photograph of the surface of the sprayed coating after it has been heated for 10 hours, Fig. 15C is a photograph of the surface of the sprayed coating after it has been heated for 20 hours. Each of the photographs are taken at a magnification of 100 times. Since the observation is observed with no etching performed, the difference between the WC particles and the binder material NiCr cannot be observed, resulting in being observed as a full white base. Referring to Figs. 15A, 15B and 15C, dotty black portions are blow holes. Comparing Figs. 15A, 15B and 15C, the number of the blow holes decreases and the size of the same is reduced with a lapse of the heat treatment time. Since the samples for use in the experiments are, by cutting, obtained from a sprayed coating formed on a plate (26 mm x 26 mm) made of stainless steel of JIS SUS403, the difference in the number of the blow holes has not taken place due to the spraying conditions but has taken place due to the fact that the number of the blow holes has decreased by the heat treatment. A similar phenomenon was observed in the WC - 12% cobalt sprayed coating or the Cr 3 C 2 - 25% NiCr sprayed coating. Fig. 16 illustrates the relationship between the heating time and density of blow holes having a size larger than 20 µm observed on the surface when the WC - 27% NiCr sprayed coating is heated at 400°C. It should be noted that the method of observing the blow holes formed in the surface is the same as the method employed to obtain the results shown in Fig. 15. When the structure of the WC-27% NiCr sprayed coating was observed by a scanning type electronic microscope, the average particle size of the WC particles and the particle of the NiCr binder material was about 10 µm and the maximum size was 20 µm or less. Therefore, the presence of blow holes having a size of several µm does not affect the separation of the WC particles. If the size of the blow holes is larger than 20 µm, the WC particles are separated, causing the strength of the sprayed coating to deteriorate. Therefore, the blow holes larger than the particle size of 20 µm or more were measured. The term "blow holes" is meant blow holes having a size of 20 µm hereinafter. As shown in Fig. 16, the number of blow holes rapidly increases if the sprayed coating is heated at 400°C for 15 hours or longer. The change in the number ratio of the blow holes coincides with the change in the hardness shown in Fig. 7. That is, it can be considered that the change in the hardness of the sprayed coating due to heating takes place depending upon the number ratio of the blow holes in the sprayed coating. The relationship between the number ratio of the blow holes and the hardness obtained from the change in the hardness of the heating time and the change in the number ratio of the blow holes respectively shown in Figs. 12 and 16 is shown in Fig. 17. If the number ratio of the blow holes is 30 per square milimeter (mm 2 ) or lower, the hardness commences to increase, and the hardness preferable for the bearing portion of the pump is realized when the number ratio is 15 per square milimeter (mm 2 ) or lower. Therefore, it is preferable that the sprayed coating to be applied to the bearing portion of the pump has a number ratio of the blow holes having a size larger than 20 µm and present in the surface of 15 per square milimeter (mm 2 ) or less, preferably 10 per square milimeter (mm 2 ) or less. It is noted that the WC -12 % cobalt sprayed coating and the Cr 3 C 2 - 25% NiCr sprayed coating as well as attained a similar phenomenon. In order to evaluate the wear resistance of the bearing and the sleeve having the sprayed coating subjected to the heat treatment against water containing earth and sand, the following element test was carried out. The method of the element test and a portion of results will now be described with reference to Figs. 18A - 18D and 19. A rotation-side specimen 15 shown in Fig. 15A and a stationary-side specimen shown in Fig. 15B are dipped in water containing quartz sand and assumed to be water containing earth and sand. Then, the rotary-side specimen is rotated as shown in Fig. 18D to be slid while being applied with a predetermined surface pressure. After they are slid for a predetermined time, the abrasion losses of the two specimens were measured. The wear losses were evaluated by measuring the thickness of the rotation-side specimen 15 before and the after the test. The surface shape of the stationary-side specimen 16 before and after the test was measured by a surface roughness meter. The examining conditions are as follows: the surface pressure is 1 to 10 kg/cm 2 , the peripheral speed is about 0.5 to 5 m/sec, and concentration of quartz sand is 0.1 to 10 wt%. The sliding test was repeated as follows: the water containing earth and sand was changed at each sliding operation covering a distance of 3.6 km in order to reduce the influence of the wear of the quartz sand; and the quartz sand was interposed between the two specimens whenever water containing earth and sand was changed in order to cause the quartz sand to be easily caught between the two specimens. Fig. 19 shows a portion of results of the experiments of the wear resistance of the sprayed coating subjected to the heat treatment. The axis of abscissa stands for the distance of sliding and the axis of ordinate stands for the average wear loss of the rotational side specimen. The stationary-side specimen is α - SiC, while the rotation-side specimen is WC - 27% NiCr sprayed coating having a thickness of about 100 µm. The base is made of stainless steel of JIS SUS403 subjected to heat treatments of quenching and tempering. The conditions of the heat treatment applied to the sprayed coating was determined while referring to the results shown in Fig. 12 to be 400°C for 20 hours. As a comparative material, a WC - 12% cobalt sintered article and non-treated WC - 27% NiCr sprayed coating were used as the rotation-side specimens under the similar conditions. The residual test conditions are as follows: the surface pressure is 2 kg/cm 2 , the peripheral speed is 0.5 m/s and the concentration of the quartz sand is 9 wt%. Referring to Fig. 19, a line (1) expressed by -▴- shows the change in the abrasion loss of the WC - 27 % NiCr sprayed coating subjected to the heat treatment and applied to the bearing member according to the present invention, a line (2) expressed by -o- shows the change in the wear loss of WC - 12% cobalt sintered article which is the comparative material applied to the conventional bearing member, and a line (3) expressed by -▵- shows the change in the wear loss of the WC - 27% NiCr sprayed coating which is the comparative material and to which no heat treatment is applied. The wear rate of the WC - 27% NiCr sprayed coating which is not subjected to the heat treatment is about three times the wear loss of the WC - 12% cobalt sintered article. On the other hand, the wear rate of the WC - 27% NiCr sprayed coating is substantially the same as the wear rate of the WC - 12% cobalt sintered article. That is, the heat treatment set to 400°C for 20 hours causes the wear loss of the WC - 27% NiCr sprayed coating against water containing earth and sand was reduced to one-third. It can be considered that the foregoing reduction in the abrasion loss was caused from the increase in the hardness due to the heat treatment shown in Fig. 12 to which the sprayed coating was subjected. The wear resistance of the sprayed coating subjected to the heat treatment is a reasonable value because of the relationship between the hardness of the sprayed coating and the abrasion rate shown in Fig. 21. Although omitted from Fig. 19, an excellent wear resistance is attained from a combination of the WC - 27% NiCr sprayed coating subjected to the heat treatment set to 400°C for 20 hours and Si3N4. The Si3N4 film employed by the fixed-side specimen exhibited excellent wear resistance superior to SiC. Also the WC - 12% cobalt sprayed coating enables a similar effect to be obtained from a similar heat treatment as a result of a similar experiment. However, the WC - 12% cobalt sprayed coating does not enable the hardness to be improved unlikely the WC - 27% NiCr sprayed coating. Therefore, the wear resistance of the WC - 12% cobalt sprayed coating is inferior to that of the WC - 12% cobalt sintered article and that of the WC - 27% NiCr sprayed coating subjected to the heat treatment. Furthermore, cobalt employed as the binder material is inferior to NiCr in terms of the corrosion resistance. Therefore, there is a possibility of generation of abrasion due to corrosion. Since the number ratio of the blow holes in the surface of the aforesaid WC - 12% cobalt subjected to the heat treatment decreases, use of the cobalt binder material enables the wear resistance against corrosive abrasion to be improved. However, the attained wear resistance is insufficient against drain containing salinity such as sea water. Therefore, a bearing which uses the WC - 12% cobalt sprayed coating cannot be used depending upon the conditions for use. As a result of a similar experiment, Cr 3 C 2 - 25% NiCr sprayed coating enables improved wear resistance to be realized. However, the realized improvement in the wear resistance is inferior to that realized in the WC - 27% NiCr sprayed coating and and WC-12% cobalt sprayed coating. The reason for this is that the fact that inherent hardness of the Cr 3 C 2 - 25% NiCr sprayed coating is inferior to the hardness of the WC - 27% NiCr sprayed coating and that of WC-12 % cobalt sprayed coating deteriorates the effect of the heat treatment. Fig. 20 collectively shows the wear rates of the results shown in Fig. 19, results obtained when WC - 27% NiCr sprayed coatings are used in both the stationary-side and the rotation-side specimens, and those obtained when the WC - 27% NiCr sprayed coating was used as the rotation-side specimen and the Cr 3 C 2 - 25% NiCr sprayed coating subjected to the heat treatment was used as the stationary-side specimen. Fig. 20 shows the wear rates of the stationary-side and the rotation-side specimens after they were subjected to the experiment for 60 hours. The test conditions are the same as those of the experiment shown in Fig. 19. The relative wear rates shown on the axis of ordinate are values of the wear rates of specimens expressed by relative values while making the wear rate of the rotation-side and the stationary-side specimens of (A) which is the combination of WC - 12% cobalt sintered article and α - SiC to be a reference value 1.0. In the bar graph, the bars each having diagonal lines entered therein stand for the wear rates of the rotation-side specimens and the bars having no diagonal lines stand for the wear rates of the fixed-side specimens. The combination (A) is a combination of the WC - 12% cobalt sintered article and α - SiC and is the results of a curve (2) shown in Fig. 19. Combination (B) is a result of a combination of WC - 27% NiCr sprayed coating subjected to no heat treatment designated by curve (3) shown in Fig. 19 and α - SiC. Combination (C) is a result of a combination of the WC - 27% NiCr sprayed coating subjected to the heat treatment which is the material of the bearing according to the present invention and designated by curve (1) shown in Fig 19 and α - SiC. Combination (D) is a result of a combination of the WC - 27% NiCr sprayed coating subjected to the heat treatment which is the material for the bearing according to the present invention and the WC - 27% NiCr sprayed coating subjected to the heat treatment. Combination (E) is a result of a combination of the WC - 27% NiCr sprayed coating subjected to the heat treatment and the Cr 3 C 2 - 25% NiCr sprayed coating subjected to the heat treatment. It should be noted that the conditions of the heat treatment to which the each sprayed coating is subjected are as follows: 400°C for 20 hours. As described with reference to Fig. 19, the combination (C) enables both of the stationary-side specimen and the rotation-side specimen to have the wear resistance substantially equivalent to that attained from combination (A), that is, the combination of the WC - 12% cobalt sintered article and α - SiC. Also the combination (D) having the arrangement that the WC - 27% NiCr sprayed coatings subjected to the heat treatment are used in both of the stationary-side specimen and the rotation-side specimen enabled the rotation-side specimen and the stationary-side specimen to have the wear resistance substantially equivalent to that of the WC - 12% cobalt sintered article. The WC - 27% NiCr sprayed coating subjected to the heat treatment for use in the stationary-side specimen exhibited excellent wear resistance superior to that of α - SiC. The hardness of α - SiC is an average value of Hv 2704 (maximum value Hv 2874 and minimum value Hv 2591) which is considerably superior to the hardness of the WC - 27% NiCr sprayed coating subjected to the heat treatment. The reason why the WC - 27% NiCr sprayed coating subjected to the heat treatment exhibited the superior wear resistance to that of a-SiC in spite of the aforesaid fact lies in the difference in the ductility between the two materials, that is, the difference in the fracture toughness. The fracture toughness of α - SiC is about 3.9 (MN/m. m) which is inferior to that of the WC - 27% NiCr sprayed coating, that is about 13 (MN/m. m). It can be considered that the excellent fracture toughness of the WC - 27% NiCr sprayed coating subjected to the heat treatment affects the wear resistance rather than the hardness does and the foregoing excellent wear resistance is obtained. It should be noted that the fracture toughness was measured by a Vickers impression method. The combination (E) in which the WC - 27% NiCr sprayed coating subjected to the heat treatment was used as the rotation-side specimen and the Cr 3 C 2 - 25% NiCr sprayed coating subjected to the heat treatment was used as the stationary-side specimen enabled the rotation-side specimen to have a similar wear rate to that realized by each of the combinations (C) and (D). However, the wear rate of the Cr 3 C 2 - 25% NiCr sprayed coating subjected to the heat treatment and used as the fixed-side specimen was higher than that of α - SiC and that of the WC - 27% NiCr sprayed coating subjected to the heat treatment. It can be considered that the reason for this lies in that the hardness of the Cr 3 C 2 - 25% NiCr sprayed coating cannot be significantly improved even if it is subjected to the heat treatment. However, the Cr 3 C 2 - 25% NiCr sprayed coating exhibited equivalent or superior wear resistance to that of the WC - 27% NiCr sprayed coating if the temperature of the drain is high. It can be considered that the reason for this lies in that the hardness of the Cr 3 C 2 - 25% NiCr sprayed coating does not deteriorate considerably at high temperature. Therefore, the combination of the WC - 27% NiCr sprayed coating subjected to the heat treatment and the Cr 3 C 2 - 25% NiCr sprayed coating subjected to the heat treatment causes a satisfactory effect to be obtained depending upon the condition for use. The aforesaid results will be summarized below: If the WC - 27% NiCr sprayed coating subjected to the heat treatment is used in the sleeve to be combined with a bearing made of SiC or Si 3 N 4 , excellent wear resistance and sliding characteristics can be obtained. In this case, it is preferable that SiC or Si3N4 be used in the bearing having a structure which is supported by the metallic backing member by shrinkage fit because the strength of SiC and Si3N4 is improved due to compressive stress applied thereto. If the WC - 27% NiCr sprayed coating subjected to the heat treatment is used in both of the sleeve and the bearing, totally excellent wear resistance and sliding characteristics can be attained. Although the WC sprayed coating containing cobalt as the binder material as well as exhibits excellent wear resistance, it is difficult to be used because of a problem of corrosion if drain contains salinity. If the WC - 27% NiCr sprayed coating and the Cr 3 C 2 - 25% NiCr sprayed coating subjected to the heat treatment are used as the sleeve and the bearing, the obtained wear resistance is inferior to that obtainable from the combinations of the WC - 27% NiCr sprayed coatings. However, further improved characteristics can be realized if the temperature of the drain is high. In this case, the WC - 27% NiCr sprayed coating may be employed in either of the sleeve or the bearing, resulting in the similar characteristics. Therefore, the bearing structure according to the present invention enables the contact surface of each of the sleeve and the bearing to have similar wear resistance to that realized by a structure in which WC - 12% cobalt sintered alloy is used as the bearing. Furthermore, assembling facility can be improved because the weight of the sleeve and bearing can be reduced. In addition, the fact that the sleeve and the bearing reveal excellent toughness will prevent fracture due to impact, causing the reliability to be improved. Therefore, a rotational machine such as a pump and a hydraulic turbine that incorporates the bearing structure according to the present invention is attained satisfactory reliability. Although each of the foregoing embodiments employs the high speed flame spraying method for forming the sprayed coating, an explosive spraying method, a pressure reduction plasma spraying method, a laser spraying method or a plasma spraying method may be employed while freed from particular limitation. It should be noted that it is preferable that the hardness before the heat treatment is performed is higher. It is preferable to employ the high speed flame spraying method or the explosion spraying method because a hard sprayed coating can be formed. Although each of the foregoing embodiments employs stainless steel of JIS SUS 403 as the material for the sleeve base and the bearing base, another material which can be used in water may be employed. Furthermore, it is preferred to employ stainless steel of JIS SUS403 in place of stainless steel of JIS SUS304 because of a larger thermal expansion coefficient if the thermal expansion coefficient of the sprayed coating, the main component of which is WC or Cr 3 C 2 . In addition, it is preferable to employ stainless steel of JIS SUS304L or JIS SUS316L containing low carbon if the deterioration of the corrosion resistance of the stainless steel due to the heat treatment is taken into consideration. Although each of the foregoing embodiments employs NiCr and cobalt as the binder material by contents of 12 wt%, 25 wt% and 27 wt%, the content is not limited if the characteristics required for the sprayed coating are met. The present invention has the arrangement that the sprayed coating, the main component of which is WC or Cr 3 C 2 , is heated under a predetermined condition. Therefore, the binding strength between the WC particles or Cr 3 C 2 particles with the binder material is enlarged. As a result, the blow holes in the sprayed coating can be reduced, causing the average of the hardness to be increased by about 50 to 60%. Therefore, the wear resistance against water containing earth and sand can be improved by three times the wear resistance before the heat treatment is performed. The thus realized characteristics are equivalent to those of the ceramics sintered article such as the WC - 12% cobalt. Therefore, the sleeve and bearing formed by applying sprayed coating on the surface of a metallic base can be used in place of the sleeve and the bearing made of WC - 12% sintered article. As a result, a bearing and a sleeve each having a large diameter can be manufactured which cannot be manufactured by using the sintered articles such as WC, SiC or Si 3 N 4 . Furthermore, the weight of the bearing and that of the sleeve can be reduced regardless of the diameter, causing the assembling facility to be improved significantly. Moreover, stainless steel can be used to form the base in place of the sintered articles such as WC, SiC or Si 3 N 4 suffering from unsatisfactory toughness. Therefore, the reliability of the bearing and the sleeve can be improved. Consequently, a pump or a hydraulic turbine incorporating the bearing unit according to the present invention can be improved.

Claims: A bearing unit comprising a bearing, and a sleeve which is in slide-contact with said bearing, wherein from said bearing and said sleeve one is selected to be made of a ferrous alloy having a contact surface applied with a sprayed coating (9, 11) comprising WC or Cr 3 C 2 as the main component and at least one element of Ni, Cr and Co as binder material, wherein the number ratio of blow holes contained in the surface of said sprayed coating (9, 11) and having a size of not smaller than 20 µm is 15 per square millimeter or less. A bearing unit according to claim 1, wherein the non-selected one of said bearing and sleeve is made of a ferrous alloy having a contact surface applied with a sprayed coating (9, 11) comprising WC or Cr 3 C 2 as the main component and at least one element of Ni, Cr and Co, and the number ratio of blow holes contained in the surface of said sprayed coating (9, 11) and having a size of not smaller than 20 µm is 15 per square millimeter or less. A bearing unit according to claim 1, wherein the non-selected one of said bearing and sleeve is made of silicon carbide or silicon nitride. A bearing unit according to claim 1, wherein the non-selected one of said bearing any sleeve is made of a ferrous alloy provided with silicon carbide or silicon nitride at the contact or bearing surface thereof. A bearing unit according to any of the claims 1 to 4, wherein said sprayed coating (9, 11) of the selected one of said bearing and sleeve is heated at a temperature of from 300°C to 550°C for at least one hour after said sprayed coating (9, 11) has been formed, said sprayed coating (9, 11) having a hardness of Hv 1000 or higher and the sliding contact surface of the non-selected one also having a hardness of Hv 1000 or higher. A bearing unit according to claim 2, wherein the sprayed coating (9, 11) of the non-selected one of said bearing and said sleeve is heated at a temperature of from 300°C to 550°C for at least one hour after said sprayed coating (9, 11) has been formed, said sprayed coating (9, 11) having a hardness of Hv 1000 or higher. A bearing unit according to any of the preceding claims, wherein the sleeve is the selected one of said bearing and sleeve, and the ratio W/D 2 of weight W (kg) of said sleeve and the outer diameter D (cm) of the same is 0.05 or less. A bearing unit according to any of the preceding claims, wherein the thickness of each single sprayed coating (9, 11) is from 100 µm to 200 µm, and a contact portion of said sleeve and/or said bearing is divided into two or more sections (2a, 2b, 2c, 2d, 19). A method of manufacturing the bearing unit according to claim 1 comprising the steps of applying sprayed coating (9, 11), comprising WC or Cr 3 C 2 as the main component and at least one element of Ni, Cr and Co as a binder, to a contact portion of a metallic bearing or a metallic sleeve respectively, and heating said metal bearing or said metal sleeve respectively having said contact portion applied with said sprayed coating (9, 11) at from 300°C to 550°C for one hour or longer. Use of a bearing unit (2, 3) according to any of the claims 1 to 8 in a drainage pump comprising a casing in which drain flows, and a main rotary shaft (1) supported in said casing by said bearing unit (2, 3) and having an impeller in a portion thereof and having an arrangement that at least a portion of said bearing unit (2, 3) is operated without external water supply. Use of a bearing unit (2, 3) according to claim 10, wherein said drainage pump comprises a water reservoir tank (18) having an opened portion in an upper portion thereof to surround the bottom surfaces and the outer surfaces of said bearing (3, 13) and a bearing fixing member (7') on the outside of said main shaft (1), said water reservoir tank (18) being fixed to said main shaft (1). Use of a bearing unit according to any of the claims 10 and 11, wherein said drainage pump serves as a precedent standby operation drainage pump. Use of a bearing unit (24, 25) according to any of the claims 1 to 8 in a hydraulic turbine comprising a casing in which fluid flows, and a main rotary shaft (22) supported in said casing by said bearing unit (24, 25) and having a runner in a portion thereof and having an arrangement that at least a portion of said bearing unit (24, 25) is operated while being supplied with said fluid. Use of a bearing unit (24, 25) according to claim 7 in a hydraulic turbine comprising a casing in which fluid flows, and a main rotary shaft (22) supported in said casing by said bearing unit (24, 25) and having a runner in a portion thereof.

Inventors: AIZAWA KOOJI, HANEDA MITSUAKI, OKADA RYOJI, OTANI KENJI, TAKADA KUNIO, YAMADA MASAYUKI, YAMADA TOSHIHIRO, AIZAWA, KOOJI, HANEDA, MITSUAKI, OKADA, RYOJI, OTANI, KENJI, TAKADA, KUNIO, YAMADA, MASAYUKI, YAMADA, TOSHIHIRO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: F16C33
IPC codes: F16C33, C23C4, F04D29