Removing the screw using the pulling method
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Our unit often uses hydraulic stretchers to remove large screw nuts when repairing machinery. If it is disassembled frequently, the screw will be stretched repeatedly, which will reduce its strength? Will the tensile strength of such screws be lower after a certain period of time? Does it need to be replaced after a certain period of time?
The material of screws generally possesses high tensile strength and toughness (such as high-carbon alloy steel with a carbon content of over 0.45%). I’m not aware of the specific composition of the screw you have, but it should be fine in most cases. But it cannot withstand large impact loads and will break
Firstly, the tensile strength does not decrease as a result of repeated cold working and hardening; on the contrary, it increases. However, its yield strength decreases due to the stresses generated by cold working and hardening, which leads to a reduction in toughness. Secondly, in addition to damage caused by insufficient strength, the material can also suffer from fatigue damage, and it will eventually fail and break once its fatigue life is reached.
If the screws are frequently removed, it is recommended to use high-strength screws. It is best to have the screws and nuts finely processed to reduce wear and extend their service life.
We used a hydraulic wrench to remove the large screws; the vacuum cleaner costs 350,000.
The use of stretchers to install bolts is widely applied at home and abroad, and the manufacturing and usage technologies are already quite mature. The operating procedure can refer to HG21573. Domestic users generally use a small number of stretchers at the same time; if 6 stretchers are used for 12 screws, the operating procedures from abroad can be followed (the pressing procedure is the opposite of that in HG21573). Whether it is HG21573 or foreign operating procedures, the loading force during disassembly and assembly should not exceed the yield limit of the screw; the standard recommended abroad is 44%. The loading force, in Newtons, is obtained by multiplying the pressure of the pump (in Mpa) by the effective oil pressure area of the tensioner (in MM^2). By dividing this force by the stress area of the screw, the stress on the screw under loading conditions can be determined; this stress level should be around 44% (as there is also a working load, and when this is added to it, the stress level increases). The oil pressure load applied when removing the screw is about 5% higher. As long as the preload plus the working load does not exceed the yield limit of the screw, the screw will always stretch and contract within the elastic range, allowing it to be reused. Of course, for important critical bolts, considering the safety factor, they need to undergo flaw detection testing before being used. :) This post was last edited by Lao Jiu on 2009-3-27 09:24 ]
What is mentioned on floor 6 is already very professional; many of the equipment flanges and pipe flanges in our unit use hydraulic tension bolts. Some of these bolts were introduced along with imported equipment, while others were modified by us. They are extremely reliable and convenient to use, and the bolts that have been in use for the longest time are over 30 years old without ever needing to be replaced. However, non-destructive testing is carried out on these bolts whenever a major repair is performed.
Perform non-destructive magnetic particle testing regularly. There should be no problem; the bolts in our unit have been in use for 17 years.