The main principles of bolt preloading

At present, the main principle of bolt pre-tightening is that ordinary (not required) bolts are usually mechanically (pure human and mechanically tightened), and torque wrenches are required to use torque wrenches or labor-saving wrenches, which have strict requirements for torque or bolt extension. Long-term bolts are pre-tightened by electric heating or hydraulic stretching according to the specific conditions (in the use of Shenyang Pump Co., Ltd., the early use tends to be electric heating, and now tends to hydraulic stretching method) . In the production of pumps, the series of high-pressure boiler feed pumps are pre-tightened by electric heating, the nuclear power pumps (including nuclear main pumps) are produced, and the subcritical and supercritical high-pressure boiler feed pumps are hydraulically stretched. Preloaded.

Since the pre-tightening of the bolt by the electric heating method requires drilling on the bolt, it is suitable for the bolt having a larger diameter, and the hydraulic stretching method requires a longer thread length, and is suitable for a pump having a large pre-tightening space. There are many pumps that do not have such conditions. For example, early single-shell boiler feed pumps and petrochemical hydro-pumps have larger bolt diameters and more compact space. Only general-purpose wrenches can be used. Today's high-pressure boiler feed pumps for generator sets (Double-shell structure) bolts, the diameter of the pump cylinder is large, the space is relatively large, and the hydraulic drawing method is relatively accurate (the domestic pump and the foreign early adopting the electric heating method, and the foreign countries now mostly adopt the hydraulic stretching method) Pre-tightening). Hydraulic stretching method for bolt pre-tightening, usually by high-pressure hydraulic manual (plunger) pump (including high-pressure oil meter and transition joint), three-way (also five-way as required), high-pressure oil hose, quick connector Bolt (dedicated) hydraulic tensioner.