Оптимизация конструкции захвата для детали «Барабан»

Грузозахватные приспособления обычно применяются при производстве работ по подъему и перемещению грузов с применением грузоподъемных машин. Использование приспособлений позволяет реализовать максимальное удобство и безопасность производственного процесса. Грузозахватные приспособления конструируются для определенного этапа технологического процесса, для конкретного изделия. При проектировании таких приспособлений необходимо учитывать основные показатели оптимальности конструкции: прочность, надежность, простота, удобство и безопасность при эксплуатации, эргономичность. Кроме того, нужно стремиться к наименьшей массе и, соответственно, металлоемкости захвата. Конструкция грузозахватного приспособления, в основном, будет зависеть от назначенных технологом поверхностей, за которые можно крепиться и от максимальной высоты подъема крюка крана. В статье описана задача по конструированию захвата для детали «Барабан¬ в новом технологическом процессе. Рассмотрена конструкция существующего захвата, взятого за прототип. Приведен анализ различных вариантов конструктивных решений, созданных в процессе проектирования. Выбран вариант конструкции захвата, который в наибольшей степени соответствует требованиям технического задания. Конструкция этого модернизированного приспособления представляет собой захват с тремя лапами, удерживающими деталь, и подвес в виде траверсы. Разработанная конструкторская документация утверждена производством и отделом промышленной безопасности

Monitoring the chloride concentration in the concrete pore solution by means of direct potentiometry

Determination of the chloride concentration at the depth of the rebar is important when studying the mechanisms of chloride-induced corrosion. It is generally assumed that only the chloride ions dissolved in the concrete pore solution can initiate corrosion, while those bound by the constituents of the cement paste are considered harmless. Attempts to use ion selective electrodes in concrete, to non-destructively monitor the free chloride concentration, have thus been made by several researchers. In the present work, results from laboratory experiments with such chloride sensors are presented. It was found that the presence of concentration gradients such as pH differences markedly influences the measurement of the sensor potential and thereby the accuracy of this chloride measurement technique. This has to be taken into account carefully when designing an experimental setup involving the use of ion selective electrodes

Chloride induced reinforcement corrosion: electrochemical monitoring of initiation stage and chloride threshold values

Reinforcement steel embedded in six different concrete mixes was exposed to chloride by wetting/drying cycles. Various parameters were continuously monitored during more than 1 year. Cement replacement with fly ash had beneficial long-term effects regarding chloride penetration resistance. Concerning corrosion performance, the by far most dominant influencing parameter was the steel/concrete interface since corrosion initiated on the lower side of the rebar (with respect to casting direction) regardless of binder type and w/b ratio. In many cases, after the first signs of depassivation, a marked increase in chloride content was required to prevent repassivation and to enable stable pit growth

Chloride induced reinforcement corrosion: rate limiting step of early pitting corrosion

Transition from passive state to stable localised corrosion of reinforcement steel in concrete owing to chloride ingress takes place over a period of time rather than being a one-step-occurrence. The depassivation process was characterised by frequent measurements of corrosion potential, polarisation resistance, and macro-cell currents when short-circuiting the working electrode with additional cathode surface. In addition, the concrete resistivity was continuously monitored and cathodic and anodic polarisation curves were measured. The results are consistent and imply that the localised corrosion process is in the initial phase of pit growth under mixed anodic/ohmic control. With time, the anodic reaction kinetics become more limited and the corrosion rate gets almost entirely determined by anodic control. The observed relationship between achieved maximum corrosion current and concrete resistivity indicates that the extent to which the anodic reaction kinetics are restricted is determined by the concrete microstructure and its ability to retain ionic movement

Critical Chloride Content in Reinforced Concrete - A Review

Chloride induced corrosion as the major cause for degradation of reinforced concrete has been the subject of great research efforts over the last fifty years. The present literature review summarises the state of the art by presenting the concept of the critical chloride content, discussing influencing factors, and assessing available measurement techniques. A large number of published chloride threshold values together with the respective experimental details are collected. While today's experience is mostly based on Portland cement, more modern studies with non-traditional binders often reported contradictory results. The present literature evaluation highlights the strong need for a practice-related test method, and, in this regard, focuses especially on experimental procedures by discussing advantages and drawbacks of methods and setups. It clearly emerges that many of the setups used to determine critical chloride contents are not suited to give realistic results

Diffusion potentials in porous mortar in a moisture state below saturation

Diffusion potentials (membrane potentials) were measured in mortar in a moisture state below saturation for the first time. In contrast to conventional diffusion cell setups, the experimental approach was based on embedded ion selective electrodes. A numerical model with coupled Nernst-Planck equations for the most abundant species in the pore solution allowed simulating ion transport processes and the correspondingly arising diffusion potentials. It was found that Portland cement mortar with a water/cement ratio of 0.6 does not exhibit any significant permselective behaviour – even not in an unsaturated state determined by self-desiccation. Apparently, the stable radius of water-filled capillaries is still large enough to allow ionic transport through those fractions of pore solution that are unaffected by double layer effects. In addition, the relative importance of liquid junction potentials at the reference electrode / sample interface and internal diffusion or membrane potentials are discussed with respect to application of direct potentiometry to cement based materials

Probabilistic considerations on the effect of specimen size on the critical chloride content in reinforced concrete

A probabilistic model was used to predict the critical chloride content (Ccrit) for reinforcement corrosion as a function of specimen size. The specimen size is likely to be a reason for the large scatter of Ccrit values as well as for the high threshold levels often observed in laboratory setups. Regarding test methods, it is discussed that the common combination of small specimens and low numbers of parallel samples results in poor reproducibility. If experimental data is to be used for service life modelling, the geometrical dimensions of the specimens on which Ccrit was measured have to be taken into account