Download complete journal in PDF form - Academy Publish

More documents

Recommendations

Info

TRIBOLOGY OF HIGH-SPEED MOVING MECHANICAL SYSTEMS FOR SPACECRAFTS: LUBRICATION SYSTEMS OF BALL BEARINGS K. Sathyan Oozing Flow Lubricator The ooze flow lubricator was invented by Fukuo Hashimoto (Hashimoto, 2001). The lubricator is fitted to the outer spacer of the bearing unit, the ends of which are formed as the bearing outer race. A set of precision turned helical grooves are made at the interface of the inner and outer part of the lubricator. The width of the groove is at least 20 times the depth. The helical grooves run in the axial direction and deliver lubricant to each of the bearings. The rate of flow is controlled by the dimensions of the helical groove and the speed of rotation of the bearing shaft. The mathematically determined acceptable flow rate for the design was 5 – 50 µg/h. With the design goal of 20 µg/h, the expected life of the system was claimed as 15 years when operating at 12 000 rpm. The space cartridge bearing system presented by Kingsbury et al. (Kingsbury et al., 1999), the oozing flow lubricator presented by Jones et al. (Jones et al., 1997), and Singer et al. (Singer, Gelotte, 1994) resembles the one mentioned above. Figure 1 shows the space bearing cartridge with oozing flow lubricator (Kingsbury et al., 1999). through the bleed path. The oil coming out of the lubricator is guided to the bearings mounted on either side of the lubricator. Fig.2. A typical bearing unit assembly used in momentum wheels (Sathyan, 2003) Fig.1. Space cartridge bearing system with oozing flow lubricator (Kingsbury et al., 1999) Wick Feed Systems In wick feed lubrication system (Loewenthal et al., 1985) a cotton wick saturated with oil is continuously in contact with the bearings. The frictional contact causes small amount of oil to be deposited on to the contact surface. From this contact surface, oil migrates to the bearing. The other end of the wick is in contact with oil in a reservoir and it absorbs and maintains its saturation level. This system is used in early momentum wheels and with the advent of more robust systems, its use has been discontinued. The major disadvantage with this system is that the fibers in the wick may contaminate the bearing surfaces. Centrifugal Lubricator This is the most common type of lubricator currently used in momentum/reaction wheels and control moment gyros (CMG). In this lubricator, the lubricant (grease or oil) is filled in a cylindrical container and is assembled to the rotating part of the bearing unit (Figure 2). A lubricant bleed path is provided on the outer surface of the reservoir, through which the lubricant flows out. When the bearing unit is rotating, the lubricator attached to it also rotating at the same speed. The centrifugal force thus generated forces the lubricant to flow out The centrifugal lubricators need to be well characterized under the operating environments before it can be used in the actual system. The most promising advantage of this type of lubricator is that no external actuators are needed and it assures unattended long-term operation. It has the drawback of decreasing flow rate gradually, since the flow rate is proportional to the head of oil in the reservoir which is progressively diminishing with time. Some manufacturers use grease instead of oil to overcome the difficulty of flow control. But experience shows that the flow rate decreases drastically in this type of systems because of change in consistency of the remaining grease in addition to the decrease in the head of oil (Kingsbury et al., 1999). Figure 3 shows the centrifugal oil lubricator developed by Sathyan et al. (Sathyan et al., 2008, Sathyan et al., 2010, Sathyan et al., 2010, Sathyan, 2010). In this lubricator, the lubricant oil is filled in a metallic reservoir that contains an inner sleeve and an outer cup. The capacity of this reservoir is approximately 5000 cc. On the periphery of the outer cup, a small hole is drilled through which the lubricant flows out due to the centrifugal force. The diameter of this hole is about 100 µm. Since the pressure developed due to the rotation is sufficiently high, it is only a matter of hours to empty the reservoir through this hole. Therefore, to control the flow rate to the lowest possible, a restrictor mechanism is fitted on the reservoir directly above the hole. The flow is restricted by means of a micro orifice created on a metal foil of thickness 50 µm. The diameter of the micro orifice for the required flow rate can be obtained from the theoretical model of the lubricator. The flow rate from the lubricator (Sathyan, 2010) is given by: 2 2 4 2 2 r R3 R 1 q K 8 R3 R2 where K is the flow coefficient (0.326), ρ is the density of the lubricant (kg/m 3 ), η is the dynamic viscosity of the lubricant (kg/m-s), ω is the angular speed (rad/s), r is the radius of the orifice (m), R 1 is the (1) AcademyPublish.org – Journal of Engineering and Technology Vol.2, No.2 41
TRIBOLOGY OF HIGH-SPEED MOVING MECHANICAL SYSTEMS FOR SPACECRAFTS: LUBRICATION SYSTEMS OF BALL BEARINGS K. Sathyan instantaneous radius of oil inner layer in the reservoir (m), R 2 is the radius at which oil enters the orifice (m) and R 3 is the radius at which oil leaves the orifice (m). In this case, R 2 and R 3 are constants and the difference between the two gives thickness of the orifice plate. q is the mass flow rate (kg/s). The coefficient K is obtained from the experimental and CFD simulation results. Thus, if the flow rate required is finalized, the flow area can be calculated. It is understood that the lubricant flow rate required maintaining continuous EHD film is of the order of micrograms/hour. The diameter of the control orifice required to obtain a flow rate of 10 µg/h is obtained using Eq. (1) is near to 2.8 µm. The orifice is created on the copper foil using a pulsed laser system. The lubricator assembly consists of two lubricators one for each bearing as shown in Figure 4 (Sathyan, 2010) The lubricator assembly can be mounted in the free spaces available between the bearings in the bearing unit as shown in Figure 2. The lubricant coming out of the lubricator flows to the bearings mounted adjacent to it. The predicted performance of the centrifugal lubricator is shown in Figure 5 (Sathyan, 2010). The diameter of the orifice is selected as 2.5 µm and the operating speed and temperature are 5000 rpm and 23°C respectively. The temperature corresponds to the maximum that a momentum wheel experiences in a geostationary satellite. The lubricant selected for the calculation is KLUBER PDP-65; synthetic diester oil used in high speed MMS (Sathyan et al., 2008, Sathyan et al., 2010). It can be seen that the initial flow rate is about 6.5 µg/h and the flow rate at the 50 th year is about 4.36 µg/h. Also, the lubricator has consumed only 1920 mg oil, i.e. 40% of the total oil filled at the beginning, for lubricating the bearings. The interesting feature of this lubricator is that the flow rate can be varied by varying the quantity of lubricant filled in the reservoir. This lubricator is a strong candidate for future spacecraft requiring longer mission life. Fig.5. Predicted flow rate and total flow of the centrifugal lubricator (Sathyan, 2010) Fig.3. Centrifugal oil lubricator (Sathyan, 2010) ACTIVE LUBRICATION SYSTEMS Fig.4. Centrifugal oil lubricator assembly (Sathyan, 2010) Active lubrication systems, also known as positive lubrication systems, supply a controlled amount of lubricant to the bearings when it is actuated by external commands. The command to actuate the lubricator is executed when a demand for lubricant arises. The demand for lubricant is indicated either by an increase in power consumption or by increase in bearing temperature as a result of increased bearing friction torque. Lubricant film thickness sensors are also used to measures the film thickness at the designated point. When the film thickness is less than a predetermined value, the lubricator is actuated and supplies lubricant. This type of systems contains a lubricant reservoir in which the lubricant is stored mostly under pressure. These systems are static and are generally mounted external to the bearing assembly. The flow from the reservoir is controlled by some mechanism that is actuated by external commands. There are arrangements to deliver the lubricant directly to the bearings. Different versions of positive lubrication systems are available with different actuators such as solenoid valves, electric heaters etc. The commandable oiler (Glassow, 1976) developed by the Hughes Aircraft Company, the positive lubrication system (PLUS) developed by Smith and Hooper (Smith, Hooper, 1990), the positive–pressure feed system proposed by James (James, 1977) etc., are examples of solenoid operated systems. The in-situ on demand AcademyPublish.org – Journal of Engineering and Technology Vol.2, No.2 42
Page 1 and 2: AcademyPublish.org Volume 2 Issue 2
Page 3 and 4: PARAMETRIC STUDY OF SANDWICH PANEL
Page 17 and 18: A STUDY OF SIMPLIFIED SHALLOW WATER
Page 19 and 20: A STUDY OF SIMPLIFIED SHALLOW WATER
Page 21 and 22: ON BICRITERIA LARGE SCALE TRANSSHIP
Page 27 and 28: TRIBOLOGY OF HIGH SPEED MOVING MECH
Page 35 and 36: THE USE OF IRON IN PEAT WATER FOR F
Page 37 and 38: THE USE OF IRON IN PEAT WATER FOR F
Page 39: TRIBOLOGY OF HIGH-SPEED MOVING MECH
Page 43 and 44: TRIBOLOGY OF HIGH-SPEED MOVING MECH
Page 45: Other access free resources from Ac

Download complete journal in PDF form - Academy Publish

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?