In the absence of hydraulic wrench, the petrochemical industry, the machinery industry workers are to make every effort to use sledgehammer to lock the high pressure vessel main flange bolts, time-consuming physical strength. There is no way, only through the workers of the amount and feel, in fact not accurate control of torque, because the torque is not enough or uneven water pressure test sometimes need to be successful several times.

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 With the popularity of hydraulic wrenches, today, the lockout key connection large bolts are generally made with hydraulic wrenches or other power tools. Hydraulic wrench to high-pressure hydraulic oil as the driving force to promote the piston drive transmission mechanism, the linear motion to rotary motion, through the four-way drive shaft or hexagonal hollow ratchet, to apply torque to the nut, the bolt nut to achieve precise locking or loose release. Hydraulic wrench structure is relatively simple but has its own characteristics, in order to operate light, requiring light weight, effort.