Effect of the preload in the general static load‐carrying capacity of four‐contact‐point slewing bearings for wind turbine generators: theoretical model and finite element calculations

This paper presents a generalization of previous works developed by the authors in the field of the calculation and selection of slewing bearings, where a theoretical model for the estimation of the static load‐carrying capacity of four‐contact‐point slewing bearings was obtained. Those previous works assumed that there was no preload in the balls; in the present work, the model has been improved in order to consider the effect of the preload, in such a way that it provides more realistic results because this type of bearings are preloaded in several applications to increase the stiffness and therefore the accuracy of the system. In parallel, and for comparison purposes, the finite element model built by the authors in previous works has been also adapted to include the preload in the balls. Both models, theoretical and FE, assess in complete agreement the increase of the stiffness with the preload level; the results show that the static load‐carrying capacity does not vary appreciably with the usual values adopted for the preload. Copyright © 2013 John Wiley & Sons, Ltd.     

Investigation of planetary milling for nano-silicon carbide reinforced aluminium metal matrix composites

High-energy planetary milling was used for mixing aluminium powders with 1 vol.% of silicon carbide (SiC) nanoparticles. A number of milling parameters were modified for constituting the relationship between the energy input from the balls and the hardness of the bulk nanocomposite materials. It was shown that mixing characteristics and reaction kinetics with stearic acid as process control agent can be estimated by normalised input energy from the milling bodies. For this, the additional parameter characterising the vial filling was determined experimentally. Depending on the ball size, a local minimum in filling parameter was found, laying at 25 or 42% filling of the vial volume for the balls with diameter of 10 and 20 mm, respectively. These regions should be avoided to achieve the highest milling efficiency.After a hot compaction, fourfold difference of hardness for different milling conditions was detected. Therewith the hardness of the Al–1 vol.% nanoSiC composite could be increased from 47 HV_0.5 of pure aluminium to 163 HV_0.5 when milling at the highest input energy levels.     

Experimental and theoretical investigation of diffusion processes in a membrane anaerobic reactor for bio-hydrogen production

This paper explores the potential for advanced membranes to act as a sink for hydrogen generated during anaerobic digestion thereby maintaining very low hydrogen concentrations and more favorable conditions for fermentative and possibly acetate oxidative pathways. This necessitates that the membranes function when submerged in water.     

Analysis of directional factors in milling: importance of multi-frequency calculation and of the inclusion of the effect of the helix angle

The concept of directional factor for chatter stability analysis has been used from a long time ago. The analysis of its evolution for different feed directions in milling processes provides a good way of selecting the best cutting conditions. For very stable cutting directions, corresponding to very low directional factors, the single frequency analysis gives unacceptable results and multi-frequency, or alternative solutions as semi-discretisation must be used. It is found also that the period doubling lobes extend at both sides of the tooth pass frequency equivalent to twice the natural frequency. In these cases, helix angle has a very important effect on the stability. For end milling processes, where the mill axial pitch can be of the order of the stable limit depth of cut, a very stable situation is found except in those areas corresponding to period doubling, where instability islands are found. Besides, a graphical construction for accurate estimation of the stable limit depth of cut and starting rotational speed of the period doubling lobes is proposed.     

Machine tools for large parts

This paper reviews the design, engineering principles and applications of machine tools specially developed for large parts. Large workshop machines are commonly used for manufacturing, where the impact of the general engineering principles differs substantially from those applied to conventional size machines. Portable machines are used during assembly and operation due to mobility, agility and energy constraints. Such large dimensions produce an amplification factor of any error source, so verification and calibration of such large or portable machines becomes even more critical than in conventional machines. The paper also includes future trends and unsolved challenges.     

Assessing the Integration of Landscape Connectivity into Comprehensive Spatial Planning in Spain

The integration of landscape connectivity criteria into spatial planning through ecological networks formed by coherent open space systems has received increasing attention in Europe in the last few decades. In Spain, such integration began recently, in parallel with the development of a supralocal comprehensive spatial planning policy, which was practically non-existent a decade ago. An assessment of such integration is reported here, specifically on ecological networks inclusion within the spatially explicit zoning regulations of 11 regional and 66 subregional plans which had been approved in Spain until the end of 2012. At the same time, a survey was conducted among 22 Spanish and 14 European practitioners in order to detect possible deficiencies and opportunities to optimise this integration in Spain through a comparison with other European countries. An increasing integration of ecological networks into supralocal plans was observed, especially in subregional plans approved in the last five years, coinciding with a period of greater development in supralocal spatial planning policy, the end of the Spanish speculative housing bubble and new national legislation which encourages the promotion of ecological networks. However, the integration process is very uneven because homogeneous guidelines are not applied in the different regions. Spanish practitioners discerned wide room for improvement. The integration of ecological networks into multiscale spatial planning should be optimised in Spain, including improvements in local planning, strategic environmental assessment and training of planners in this topic.     

New Control Techniques Based on State Space Observers for Improving the Precision and Dynamic Behaviour of Machine Tools

This paper describes control techniques based on state space observers that have been developed to improve the precision and dynamic behaviour of machine tools.To improve precision, an accelerometer located close to the tool centre point (TCP) is used, so that its position is estimated by an state space observer and used as position feedback instead of the position read by the linear scale. This way, the structural deformation from the slide to the tool is compensated.Dynamic behaviour is improved estimating the acceleration of the TCP by the aforementioned observer and feeding this noiseless signal into the control system, rather than the noisy signal of the accelerometer itself.These techniques provide a simple way to achieve considerable improvements without using complex and expensive TCP position measurement devices. Tests on actual machines have shown that errors produced by deflections between the slide and the TCP can be reduced by 70%.     

Natural variation of toxicity in encrusting spongeCrambe crambe (Schmidt) in relation to size and environment

The presence of intraspecific variation in toxicity and its relationship with biological or ecological factors were studied in the spongeCrambe crambe. Within-specimen (periphery and central part), between-size (<1000 mm² in area, between 1000 and 10,000 mm² and >10,000 mm²) and between-habitat (well-illuminated and dark communities) variations in toxicity were evaluated by the Microtox bioassay. Quantitative differences were detected that were not attributable to within-specimen variation but to size and habitat effects. Habitat comparisons showed that sponges in the shaded habitat were significantly more toxic than those of the well-illuminated community. Sponges of the smaller size classes displayed significantly less toxicity than the medium-sized specimens. Results are interpreted under the optimal defense theory and their ecological implications are considered.     

Performance assessment of membrane distillation for skim milk and whey processing

Membrane distillation is an emerging membrane process based on evaporation of a volatile solvent. One of its often stated advantages is the low flux sensitivity toward concentration of the processed fluid, in contrast to reverse osmosis. In the present paper, we looked at 2 high-solids applications of the dairy industry: skim milk and whey. Performance was assessed under various hydrodynamic conditions to investigate the feasibility of fouling mitigation by changing the operating parameters and to compare performance to widespread membrane filtration processes. Whereas filtration processes are hydraulic pressure driven, membrane distillation uses vapor pressure from heat to drive separation and, therefore, operating parameters have a different bearing on the process. Experimental and calculated results identified factors influencing heat and mass transfer under various operating conditions using polytetrafluoroethylene flat-sheet membranes. Linear velocity was found to influence performance during skim milk processing but not during whey processing. Lower feed and higher permeate temperature was found to reduce fouling in the processing of both dairy solutions. Concentration of skim milk and whey by membrane distillation has potential, as it showed high rejection (>99%) of all dairy components and can operate using low electrical energy and pressures (<10 kPa). At higher cross-flow velocities (around 0.141 m/s), fluxes were comparable to those found with reverse osmosis, achieving a sustainable flux of approximately 12 kg/h·m² for skim milk of 20% dry matter concentration and approximately 20 kg/h·m² after 18 h of operation with whey at 20% dry matter concentration.