Fluid Dynamic Bearing Unit for the Home Ventilation Fan

Transcription

1 NTN TECHNICAL REVIEW No New Product Fluid Dynamic Bearing Unit for the Home Ventilation Fan Masaharu HORI As a rule, the installation of the ventilation equipment has come to be required for all buildings that have rooms where people live since the revision of the Building Standard Law. Therefore, the demand for home ventilation fans has increased for ordinary families and the demand for high reliability and low noise for home ventilation fans has also increased. NTN has developed a fluid dynamic with excellent reliability and quietness that is easily used to replace rolling bearings. This fluid dynamic can be applied not only for use in home ventilation fans, but also in other applications where rolling bearings are used and quietness is demanded. This report introduces this fluid dynamic for home ventilation fans. 1. Introduction With the aim of reducing the room concentration of chemical substances causing sick house syndrome, the revised Building Standard Law took effect on July 1, 23 that regulates building materials and ventilation facilities used in buildings. This has made it obligatory, as a rule, to install mechanical ventilation facilities in all buildings because of emissions from furniture even if building materials do not diffuse formaldehyde. Obligatory installation of ventilators in living rooms, bedrooms, and other rooms as a result of the law revision has increased the need for freedom from noise in particular, bringing about requests of replacing roller bearings with sliding bearings. In response to these requests, we have developed a fluid dynamic bearing that has the same dimension as that of a roller bearing and that can replace it easily; the new bearing is presented below. 2. Construction and features of the new bearing Currently, roller bearing 695 (a deep-grooved ball bearing with dimensions of mm) is used mainly, and the major dimensions of the newly developed product was adapted to those of bearing 695 as shown in Fig. 2. In addition, in response to the strong request of a ventilator manufacturer having production lines using roller bearings, we decided to develop a new product that can be handled in a similar way as current roller bearings. Fig. 1 shows how the rotor and the fluid dynamic bearing are assembled and Fig. 2 the bearing as a unit. With the aim of reducing costs, we adopted moldproduced products in all component parts of a fluid dynamic bearing. Press-machined products were adopted in both of two housings and sintered oilretaining bearings were used for the rotating bodies. Two housings are fastened by means of bonding as shown in Fig. 2; forming the cross-section of each of the two housings in an L shape enlarged the overlapping of the bonded parts of the housings, resulting in increased joint strength. Figs. 3 and 4 show the shapes of fluid dynamic grooves. The thrust grooves were provided on the inside end surface of both housings. The radial grooves were provided on the outside diameter surface of the rotating body. The oil membrane formed by these fluid dynamic grooves supports the rotating body in perfect noncontact state in both radial and thrust directions. Fluid Dynamic Bearing Unit Division Engineering Dept

2 NTN TECHNICAL REVIEW No Rotor Fig. 1 Schematic layout of rotor and fluid dynamic To fabricate groove processing molds, the mold design technology was used that had been cultivated in the production of fluid dynamic Bearphite, currently mass-produced for use in support bearings for hard disk drives (referred to as HDDs in the following). This allowed high-accuracy fluid dynamic grooves to be produced. Fig. 5 shows the picture of the appearance of the thrust groove section. 4 Housing (Outside) Housing (Inside) bearing section 13 5 Rotating body Fig. 5 Photo of thrust groove Bonded part Adhesive application position Fig. 2 Cross-section shape of fluid dynamic Fluid dynamic groove Side face of the housing inside Fig. 3 Thrust groove 2.2 Method of processing radial grooves Unlike fluid dynamic Bearphite currently in mass production, radial grooves must be provided on the outside diameter surface of a rotating body; therefore, a groove processing method based on strong sizing, which is a conventional means of processing on the Bearphite inside diameter surface, cannot be adopted. To solve this problem, the porosity of sintered oilretaining bearings was used to adopt the component rolling scheme; this allowed satisfactory bearing surfaces to be obtained. Fig. 6 shows the picture of the appearance. Provided on the outside face of the rotating body Fig. 6 Photo of radial groove Fig. 4 Radial groove 2.1 Method of processing thrust grooves The housing was a stamped product, and incorporating the fluid dynamic groove processing into the stamping process prevented an increase in the total number of processes. 2.3 Assembling method A method of assembling proven in mass-producing HDD-use fluid dynamic bearings was applied. Three members were brought into intimate contact with one another first, and then one housing was moved by the distance equal to the axial gap, and adhesive was applied to the housing to bond and fasten other two housings, and after this, the bearing was impregnated with lubricant

3 Fluid Dynamic Bearing Unit for the Home Ventilation Fan 2.4 Load capacity The rotating body weight of the actual device requires a radial load capacity in the horizontal condition of 1 N or more. To meet this requirement, we calculated the load capacity using the program for analyzing the characteristics of a porous oil-retaining developed by us to determine the bearing specification. Table 1 shows the conditions for analysis and Fig. 7 the results of analysis. As shown in Fig. 7, it was ascertained that a radial gap of 2 mm allows a load capacity of 1.44 N to be secured. Load capacity N Table 1 Analysis condition Rotational speed 2 min -1 Temperature of atmosphere 25 C 2 1 extends over 22 years when converted into the working condition of a ventilator (6 C), satisfying the service life requirement of 15 years shown in Table 2. The durability is being tested on an actual device described in subsection Oil membrane formation test To make sure that oil membrane is formed during operation, the NTN fluid dynamic bearing test unit shown in Fig. 8 was used to measure the oil membrane formation rate. 1) Evaluation conditions Test machine: NTN-made fluid dynamic test machine Measuring item: Oil membrane formation rate; torque Rotational speed: 2, min -1 (Used in mass production of HDD fluid dynamic bearings) The oil membrane formation rate is measured by means of the electric conduction between the main shaft and the housing. Axial load application jig Housing Main shaft Test bearing Fig. 7 Radial Analysis gap result μm Fig. 7 Analysis result Rotation 3. Evaluation of performance 3.1 Working conditions of a ventilator bearing Table 2 shows common functional requirements for ventilator bearings. Since a fluid dynamic bearing rotates on oil film without coming into contact, the service life of a bearing depends on the evaporation of lubricant. The lubricant used this time is identical to the one used in mass-produced HDD fluid dynamic bearings, with an extremely low evaporation rate of less than 1 wt% at 8 C after 5, hours in service. It is ascertained experimentally that the evaporation speed roughly doubles due to a temperature rise of 1 C. Accordingly, the service life of the bearing Table 2 Functional requirements Service life Ambient temperature 15 years -1 6 C Ambient humidity 1% Rotational speed 2 min -1 Magnet Radial load application jig Fig. 8 Schematic view of test rig Fig. 9 shows that the oil membrane formation rate reaches 1% at an early stage of rotation with oil membrane formed completely and that the bearing rotates without contact. Rotational speed min Torque mn m Oil membrane formation rate % Oil membrane formation rate Rotational speed Torque 2min Fig. 9 Evaluation result of fluid dynamic -119-

4 NTN TECHNICAL REVIEW No Temperature characteristic test The variation of rotational speed resulting from temperature variation was investigated. 1) Evaluation conditions Applied voltage: AC 1 V Measuring item: Rotational speed Temperature of atmosphere: -1 6 C In a fluid dynamic bearing, as shown in Fig. 1, the increase in lubricant viscosity in the low temperature range causes the rotational speed to lower; however, the customer requirement of 2, min -1 was achieved. Motor rotational speed min Temperature of atmosphere C Fig. 1 Evaluation result as electric motor 3.4 Sound measurement on the actual device Noise produced by the newly developed product was compared with that produced by roller bearings. Fig. 11 shows the method of measuring noise. The method of measuring noise from a ventilator is specified by JIS (Noise test: JIS C963); the measurement is conducted using the noise meter specified by JIS C152 (normal noise meter). For the reason of comparing noise between the roller bearing and the fluid dynamic, the measurement was conducted with the fan removed and hence without the effect of wind noise. In addition, since noise values were small, the measurement was conducted with the distance to the microphone shortened to 1 mm, one tenth of the specified value. Ventilator Fig. 11 Method of noise measurement 1) Measuring conditions (not equipped with a fan) Rotational speed: About 3, min -1 (Applied voltage: AC 1 V) Temperature of atmosphere: 25 C Motor position: Shaft horizontal Distance to the microphone: 1 mm Measuring environment: Anechoic room (Ground noise: 13.5 dba) As shown in Fig. 12, the fluid dynamic produces less noise than the current roller bearing. The audible frequency range is commonly said to be from 2 Hz to 2 khz, and the frequencies that are uncomfortable to human ears are said to be near 3 khz. The fluid dynamic bearing exhibited lower values in the entire range of interest. A difference in noise value of 14.5 dba shows that the fluid dynamic bearing produces about one fifth of the noise the current bearing does. Since the fluid dynamic bearing unit is driven without contact, the sound produced by the bearing is extremely small. Noise value dba Noise value overall: 23.9 dba Noise value overall: 38.4 dba 1 Frequency khz 15 Fig. 12 Result of noise measurement 3.5 Endurance test on the actual device An endurance test was conducted using the motor on the actual device. This test was conducted to make an evaluation at 8 C in contrast to 6 C, the upper limit of the required temperature of the atmosphere. Accordingly, this test is one with a four-time acceleration compared with the experimental result with a double evaporation rate for an increase in temperature of 1 C, described in subsection ) Endurance test conditions (not equipped with a fan) Rotational speed: About 3, min -1 (Applied voltage: AC 1 V) Temperature of atmosphere: 8 C 2-12-

5 Fluid Dynamic Bearing Unit for the Home Ventilation Fan Motor position: Shaft horizontal As shown in Figs. 13 and 14, both the rotational speed and the vibration value do not exhibit significant changes after 1, hours in operation, showing a satisfactory result. The test will continue for two years to check functional aspects and to determine the evaporation of the lubricant on the basis of the decrease in the weight. Rotational speed min -1 Vibration value m/s Operation time h Fig. 13 Result of rotational stability Operation time h Fig. 14 Result of vibration stability (Radial direction) 3.6 Hydrolysis test on lubricant Unlike HDDs, ventilators are expected to be used in high-humidity environments. In consideration of these circumstances, a comparative test on hydrolysis was conducted between the base oil of ester-based grease for roller bearings, used in ventilators, and lubricant for fluid dynamic s. The test method was based on ASTM D 2619: the test oil and water were mixed, and the mixture was stirred at a high temperature and in the presence of catalyst to observe the change in the acid value in the oil layer and that in the water layer. 1) Test conditions Mixing ratio: Test oil: 75 g; water: 25 g Test temperature: 93 C Catalyst material: Copper Stirring speed: 6 min -1 Test duration: 48 hours 2) Test results As shown in Table 3, the lubricant for the fluid dynamic exhibits a smaller acid value increment and a smaller water layer acid value than those the base oil for ester-based grease for roller bearings does, being resistant to hydrolysis and excelling in humidity resistance in terms of the characteristics of the lubricant itself. Before test After test 4. Conclusion We have developed a fluid dynamic for home-use ventilators that capitalizes on the freedom from noise and high reliability, features of a fluid dynamic bearing, and that has the same dimensions as those of the current roller bearings and can be used in the same manner as those current roller bearings. The newly developed product can be extended to other applications needing freedom from noise and high reliability than ventilators; we will extend it into various series to apply it to other product models and applications. Photo of author Masaharu HORI Fluid Dynamic Bearing Unit Division Engineering Dept. Table 3 Result of hydrolytic reactivity Item to be studied Dynamic viscosity 4 C mm 2 /s Acid value Dynamic viscosity 4 C mm 2 /s Acid value Acid value increment Water layer acid value Base oil for esterbased grease Lubricant for fluid dynamic