According to the principle of thermal expansion and contraction, after the bolt is cooled, the nut will be pressed tightly. Traditionally, the "carbon silicon tube bolt heater" has been used. This heater has the following drawbacks: First, the resistance values ​​are inconsistent. Actual measurement of the carbon-silicon tube with the same nominal power, the maximum resistance value is 85Ω, and the minimum is only 25Ω, which is very different. The meter readings are monitored at rated voltage, with a maximum of 10A and a minimum of 3A, which is also a large difference. Power and resistance and current Received date: 2000-08-18 Ren Guofeng, male, born in 1971, assistant engineer. The process is proportional to 150,040, so the power of the carbon-silicon tube is inconsistent after power-on, and the heat emitted is definitely different. Then the heated bolts (the same specification) receive different temperatures, even if the strength of each bolt is used to be uniform, but because the bolts are heated at different temperatures, the tightening force of the bolts will be inconsistent and uneven after cooling. Then there is a gap in the middle facet.
Secondly, the heater has low strength and high brittleness. After heating and heat-tightening a group of bolts, when the other group is replaced, during the process of pulling out and inserting, it will be slightly inadvertently broken.
Third, if the voltage is slightly increased, the carbon tube heating wire will be blown due to overcurrent. If the difference in heating time occurs in the same batch of heated bolts due to the replacement of individual heaters, the consistency of heat and thermal elongation of the bolts is also affected.
Then, as the first point mentioned above, there must be a slight gap in the middle facet, which eventually leads to leakage.
Practice has proved that the use of the resistance wire bolt heater with 1Cr18Ni9Ti casing is better, and the resistance value of the heater with the same identification power is basically the same, and the difference is small. Therefore, the heat is consistent, so that the bolts are equal in heat and the elongation is basically the same. Therefore, the tightening force after cooling tends to be uniform. However, it also has the disadvantage of over-voltage and over-current being burned. In the case where the resistance value is constant, the larger the current, the greater the power. Whether it is a carbon silicon tube bolt heater or a wire heater with a bushing, their resistance is constant. Therefore, once the grid voltage fluctuates, it will cause current fluctuation. When the current exceeds the rated value, the generated power will far exceed the rated power of the heater. (The power of the heater is fixed due to its resistance, so the power is also fixed. Value), so the heating tube is burned in an instant.
It was found that the voltage in the workshop during the day was relatively stable, while at night it was high, fluctuating around 240V. Therefore, it is recommended to carry out heat tightness during the day, and there should be no equipment such as welding machines on the power line.
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