Mechanical Enginner

Momentum/reaction wheels are used in a spacecraft for the attitude control. They are used for the real time control of spacecraft attitude towards the desired direction as required by the mission. These are one of the few systems of a spacecraft which need to work continuously from the beginning to end of the mission. Thus the life of a space mission depends largely on the performance of the attitude control wheels. The common mode of failure in momentum/reaction wheel is mechanical and mostly related to the bearing and lubrication. This paper presents the design and development of a bearing cartridge assembly including the lubrication system for long-term performance of spacecraft attitude control wheels. The results of both theoretical and experimental analysis are presented. The experimental study of the assembly shows that it can perform continuously for a period of 30 years with reduced frictional losses.

Momentum wheels are spacecraft actuators of large angular momentum used for the attitude control and stabilization of spacecrafts. The on-orbit performance of spacecrafts depends largely on the performance of the momentum wheels which in turn depends on its bearings and its lubrication. Currently, the life cycle of spacecrafts are aimed to be around 20-30 years. However, the increases in size, complexity and life expectancy of spacecrafts demand advanced technologies especially in tribology and in turn the development of more innovative lubrication systems for long-term operation. This article reviews the tribological requirements of momentum wheels and the various lubrication systems used in the past and raises a necessity to develop a new system to cope with the new requirements.

The success of any satellite mission largely depends upon the performance of the attitude control systems such as gyroscopes and momentum/reaction wheels. The required life and performance quality of these rotating mechanisms are ensured by the selection of bearings and its lubrication. The design and development of lubrication system to meet the long-term uninterrupted performance is a challenging task before the tribologists. This article describes the developmental study of a lubrication system for long-term requirements of momentum/reaction wheels. The developed system is compact and can be placed inside the bearing unit assembly. It works on centrifugal force and able to supply lubricant continuously at a very low rate of few micrograms per hour for many years. Further, the system can be tuned for any flow rate depending on the requirement.

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This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

The success of a spacecraft mission depends to a great extent on the performance of the moving mechanical systems. The most common mode of failure in these systems is tribological. Tribological failures occur mainly due to non availability of lubricant at the working zone of the bearings as a result of degradation, evaporation and creep. The life of these moving mechanical systems could be extended, if lubricant is replenished by some means. This paper describes the development of a positive lubrication system named as command lubrication system (CLS). This is an active lubrication system which when actuated by external commands delivers lubricant to the bearings. It is actuated only when there is a demand for lubricant indicated by increase in frictional torque or bearing temperature. The outstanding feature of this system is that the lubricant is stored under ambient pressure and hence less chance of leakage. The CLS can solve the lubrication problem of spacecraft systems which require very long mission life of more than 20 years. It is also suitable to tribosystems in terrestrial devices.

The requirement to make low profile ohmic contacts to a piezo-resistive MEMS pressure sensor has highlighted limitations of ultrasonic wire bonding technology. Wire bonding typically uses 25-50 m diameter gold or aluminium wire and ultrasonic welding to the contact pads of micro-electronic devices results in a contact wire proud of the pad surface. If the application involves the MEMS pressure sensor and contacts being encapsulated, then repetitive changes in pressure flexing the contact wires can lead to fracture. A possible solution is to scale down laser welding technology to fuse materials at the micron scale. For this purpose a precision ophthalmic surgical laser system has been modified to investigate optimum conditions for laser welding, both at the micron scale and for the typical geometries involved. Typical requirements involve a cylindrical contact wire to be bonded to a thin contact pad on the MEMS device. Since the pad size is of similar dimension to the wire, and the requirement for a low profile stable configuration, a keyhole welding strategy is required. The Nd:YAG based ophthalmic laser has been modified, the Q-switch removed and the output pulse width and energy controlled principally via control of the flashlamp.