Experiment and research on high strength bolts in specific environments

The crane beam is situated between the open furnace span and the ingot span on a 205-ton loader. Due to the tight steel production schedule, it was not feasible to fully suspend the reinforcement work, making the reinforcement of the crane beam particularly challenging. After multiple discussions with the production plant, design, and construction teams, a jacking scheme was ultimately adopted—replacing the upper and lower chords and the contact plate with a column joist. The connections were made using torsion-shear high-strength bolts. However, there remained a critical question: could these bolts be effectively used in the reinforcement and maintenance project, especially in connecting old and new steel structures under high-temperature conditions? How would their performance differ from that at normal temperatures? These issues needed to be resolved. The test specimens used bolts with a diameter of 26 mm, manufactured by Shanghai Pioneer Screw Factory, with a thread size of 2 mm. Given that the original gusset plates had larger rivet holes, there was a 4 mm difference between the bolt hole and bolt diameters. The bolts were tightened using a Japanese electric wrench. To accurately measure the pre-tension of each bolt, two resistive strain gauges were symmetrically attached to the polished portion of the bolt shank, ensuring the wires were protected from damage. Temperature control was managed using a thermocouple, an adjustable millivoltmeter (E-type), and a contactor. The loading process was stopped once the secondary slip load was achieved, at which point the testing machine recorded the load-deformation curve. To determine the slip load, several lines were drawn on the sides of the specimen to monitor any misalignment during loading. The tensile test of the hot-state specimen was conducted on an East German ZDML-400T horizontal tensile testing machine. The load-deformation curves for the test pieces at 350°C after 20 cycles, at 525°C, and at normal temperature (18 tons, 45 test piece 1) showed significant differences. At normal temperature, the test piece produced a loud sound when slipping, while in the hot state, the slip occurred more gradually, with only occasional weak sounds. To better understand the reduction in friction coefficient at different temperatures, the "coefficient of reduction of the friction coefficient" and the "reduction factor of the slip load" were defined. These factors helped eliminate variations caused by different pre-tensions of the friction surfaces. The average pre-tension of the heated specimens was 23.4 tons, and the tensile force in the hot state was about 2.7 tons. Using the slip load reduction coefficient might introduce errors, so the friction coefficient reduction factor was considered more accurate. This data, along with results from Tanaka’s experiments in Japan, showed a similar linear trend at 350°C and 450°C. Tanaka’s data fell slightly to the right of the line, possibly due to testing under normal conditions rather than large-hole scenarios. The reduction factor here was slightly higher, but the downward trend matched. Thus, 350°C can be considered a critical point where the slip load and friction coefficient of high-strength bolts begin to drop significantly, consistent with international findings. As the number of thermal cycles increased, both the slip load and friction coefficient decreased, but not indefinitely. The decrease was approximately 3% per cycle, yet the values stabilized after a certain point. The gradual decline in slip load and friction coefficient at high temperatures is attributed to several factors: deformation of the bolt and nut threads due to annealing, plastic deformation of the washer, oxidation of the friction surface reducing the coefficient, and a decrease in the elastic modulus of the bolt material. Therefore, stress relaxation in high-strength bolts appears to be the primary cause of the observed reductions. In conclusion, based on the experimental results, the friction coefficient of high-strength bolt joints decreases as temperature increases. At 350°C, the reduction becomes significant, aligning with previous international studies. This suggests that the use of torsion-shear high-strength bolts in high-temperature environments requires careful consideration of their performance characteristics and potential degradation over time.

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A typical version of an overhead garage door used in the past would have been built as a one-piece panel.The panel was mounted on each side with unequal parallelogram style hinge lifting mechanism. Newer versions of overhead garage doors are now generally built from several panels hinged together that roll along a system of tracks guided by rollers. The door is balanced by either a torsion spring system or a pair of extension springs.A remote controlled motorized mechanism for opening garage doors adds convenience, safety, and security

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