高速下的外压式空气轴颈轴承研究

本文基于有限元方法,针对不同的速度参数和离心率,采用一种直接的数字方法来计算多孔外压空气轴颈轴承的性能指标,并把模拟结果同参考资料上一种空气心轴轴颈轴承的计算进行了比较。     

采用空气轴承的大行程快速刀具伺服系统

随着对结构化超精密部件需要的不断增加,出现了加工具有高的表面光洁度以及形状精度的非旋转对称镜片的新技术。根据现有的快速刀具伺服系统,研制出了最大行程为16mm的新型快速刀具伺服系统(图1)。以前的快速刀具伺服系统采用了弹性导向机构,而现在的快速刀具伺服系统则采用了空气轴承,并且用了一个高频直线电机来驱动这个系统。由于采用了一个新的功率放大器,使得这个系统能够最高达到100Hz的频率以及1mm的振动幅度。这篇文章给出了这种新型快速刀具伺服系统的技术细节。     

Book reviews

War, Culture and the Media: Representations of the Military in 20th Century Britain. Edited by Ian Stewart and Susan L. Carruthers, Trowbridge: Flicks Books, (1996), ISBN 0-948911-86-7 (pbk), £14.95.

The Future of Europe. By Peter Coffey, Aldershot: Edward Elgar, (1995), ISBN 1-85278-586-1 (hardback), £39.95, ISBN 1-85278-587-X (pbk), £12.95.

Global Dangers: Changing Dimensions of International Security. Edited by Sean M. Lynn-Jones, Steven E. Miller, London: the MIT Press, (1996), ISBN O-262-62097-9 (pbk), £13.50;.

New Studies in Post-Cold War Security. Edited by K.R. Dark, Aldershot: Dartmouth Publishing Company, (1996), ISBN 1-85521-728-7 (hardback), £42.50.

Enlarging NATO - The Russian Factor. By Richard L. Kugler with Marianna V. Kozintseva, Santa Monica, CA: National Defense Research Institute and Rand Corporation, (1996), ISBN 0-8330-2357-8, $20.00.     

Values for optimum reject allowances

Industrial situations exist where it is necessary to estimate the optimum number of parts to start through a manufacturing process in order to obtain a given number of completed good items. The solution to this problem is not straightforward when the expected number of rejects from the process is a random variable and when there are alternative penalties associated with producing too many or too few items. This paper discusses various aspects of this problem as well as some of the proposed solutions to it. In addition, tables of optimum reject allowances based on a comprehensive model are presented.     

On queueing systems with variable service capacities

In some queueing systems the total service capacity utilized at any given time is a variable under the control of a decision maker. Management doctrines are examined which prescribe the actual service capacity as a function of the queue length and the recent history of the system. Steady state probabilities, expected queue lengths and frequencies of change in capacity are evaluated for a wide class of possible control schemes. Optimization procedures are outlined.     

Allocation of resources to randomly occurring opportunities

An allocation problem is considered in lvhich different kinds of resources must be allocated to various activities, within a given time period. The opportunities for allo'cation appear randomly during this period. Certain assumptions about the values of possible allocations and the distribution of occurrences of opportunities lead to a dynamic programming formulation of the problem. This leads to a system of ordinary differential equations which are (in theory) solvable recursively, and can be solved numerically to any desired degree of precision. An example is given for the allocation of aircraft-carried weapons to targets of opportunity.     

Scheduling workers in a constricted area

A generalization of the equi-partitioning problem, termed the 2D-Partition Problem, is formulated. The motivation is an aircraft maintenance scheduling problem with the following characteristics. The complete maintenance overhaul of a single aircraft requires the completion of some 350 tasks. These tasks require a varying number of technicians working at the same time. For large subsets of these 350 tasks, the constraining resource is physical space—tasks must be completed in a physical space of limited size such as the cockpit. Furthermore, there is no precedence relationship among the tasks. For each subset, the problem is to schedule the tasks to minimize makespan. Let m denote the maximum number of technicians that can work at the same time in the physical area under consideration. We present optimization algorithms for m = 2 and 3. © 1996 John Wiley & Sons, Inc.