Silicon Nitride Rolling Bearings for Extreme Operating Conditions

The high hardness and excellent high-temperature performance of ceramics are attractive for critical rolling-bearing applications, such as in high-thermal-efficiency engines. In addition, the lower density of ceramics permits lower centrifugal loads, less skidding at high speeds, and, consequently, results in longer bearing component life than conventional bearing steels or alternative super-alloys or cermets. Silicon nitride has the lowest friction and elastic modulus of the available ceramics, which reduce bearing contact stresses. This paper reviews the merits and demerits of silicon nitride as a bearing material, its processing, and different mechanisms for solid-lubricant replenishment in severe environments. Test data and application design guidelines accumulated to dale are also provided.     

Design Criteria for Externally Pressurized Gas Bearings for Operation at High Temperatures and High Speeds

The paper describes an experimental program using externally pressurized gas bearings whose objective was to gain design information which would be applicable to operation at high speeds and high temperature. The objective was to maintain flow rates to some acceptable minimum while at the same time attaining requirements of load capacity, temperature, and stable operating conditions. The paper describes work that has been done on a variety of configurations in order to establish some nondimensional stability map. The bulk of the variations were observed at low temperatures as a function of nondimensional parameters which took into account temperature dependent variables. These results were later checked to temperatures of 1200 F and speeds to 60,000 rpm. In order to obtain results with regard to the parameters which were not influenced by certain geometry changes, care was taken to insure that isothermal conditions existed. Later work will be necessary to interject the variable of possible temperature gradients which will result in distortions and geometrical changes.     

The Inventory Approach to Urban Growth Boundaries

In this article, we argue that insights can be gained by analyzing the use of urban growth boundaries (UGBs) for urban growth management as an inventory problem. The analysis provides three useful insights. First, it shows that where UGBs are employed, too much emphasis is placed on whether UGBs contain sufficient land to accommodate 20 years of growth and too little is placed on how frequently, how much, or under what circumstances UGBs should be expanded. Second, it shows that where market factors are used, they have been used inappropriately to increase the size of UGBs immediately after expansion, when they should be used to influence the size of UGBs immediately before expansion. Third, it shows that UGBs are likely to work better if expansions occur not after an arbitrary period of time, but when the supply—or the price—of land reaches some critical threshold. We conclude by asserting that the use of UGBs should not be governed by arbitrary rules of thumb, because such rules do not stand up to critical examination, they preclude adaptation to local situations, and they prevent experimentation.     

The Complexity of Public Attitudes Toward Compact Development

Problem: The future of compact development depends in part on understanding and shaping the public's attitudes toward it. Previous studies have suggested life cycle, socioeconomic, attitudinal, and ideological dimensions to preferences regarding development patterns, but rarely have all of these factors been examined systematically across a broad, generalizable sample of respondents.

Purpose: To examine public attitudes toward compact development, we asked survey respondents to weigh four important tradeoffs between compact and sprawling growth. We assess the relative influence of a variety of individual characteristics on these attitudes.

Methods: We use results from two large-scale, randomized telephone surveys, one conducted in California in 2002 and the other in four other southwestern states in 2007. Using logistic regression, we assess which personal characteristics are associated with stated preferences regarding compact development, and illustrate their degree of influence.

Results and conclusions: Support for the compact development alternatives is significant, in some cases exceeding support for traditional, decentralized suburban patterns. However, question wording appears to matter considerably, and individuals’ beliefs about different facets of compact development are often inconsistent. Although race, income, age, and the presence of children in the household are strongly associated with some views on the four tradeoffs, only political ideology is consistently associated with opposition to compact development.

Takeaway for practice: The significant support evident for compact development may not translate into actual housing choices unless local governments and lenders do more to support the production of such housing and neighborhood environments. If, as our results suggest, a major constituency for transit-oriented and mixed-use projects is low income residents, renters, and minorities, then well crafted urban infill projects that take into account the needs of these groups will help fulfill the potential of smart growth. Advocates might also frame compact development to appeal more to political conservatives.

Research support: The 2002 survey was conducted by the Public Policy Institute of California, with financial support from the William and Flora Hewlett Foundation, James Irvine Foundation, and David and Lucille Packard Foundation. The 2007 survey was conducted and supported by the Institute for Social Science Research at Arizona State University. All views expressed are solely those of the authors, not these organizations.     

Replicating the cross-sectional distribution of house-dust mite allergen found in carpet

With a view to producing carpets that could be used to determine the ease of particulate aerosolisation during domestic activity, we measured the cross-sectional distribution of dust-mite allergen, Der p 1, produced using American Society for Testing and Materials method (ASTM F608–89) for embedding house dust in carpets with that produced by several alternative protocols. Allergen concentrations produced at different levels within the pile using the different techniques were also compared with those in carpets from actual houses – in which the majority of allergen is typically found towards the base of the pile. To obtain profiles of allergen, horizontal sections, 2-mm thick, were taken from new carpets after they had been seeded with dust and embedded using one of four following techniques: (1) dragging a fixed roller across the surface of the carpet four times, (2) using the same roller but following it up with 200 revolutions in a hexapod wear simulator, (3) dragging the fixed roller across the carpet surface 30 times (the ASTM method), and (4) 2 minutes under a commercial plate compactor. Fibre from each 2-mm-thick section was collected and the Der p 1 content determined using enzyme linked immunosorbent assay and results expressed as ng Der p 1 per area in each section. Embedding with a fixed roller alone was not found to be particularly effective, resulting in roughly equal amounts of dust being apportioned within each pile layer, irrespective of the number of embedding passes used. In contrast, a distribution biased towards the base of the pile was found after roller-embedding/hexapod wear, although still to a lesser extent than has been observed in used carpets. Plate compaction gave a similar allergen distribution profile to combined roller/hexapod treatment but was considerably easier to perform. Thus, both techniques offer promise for researchers seeking to replicate the cross-sectional distribution of dust mite allergen found in carpets after actual use (and conceivably other particulate pollutants also).