2024-09-18 16:59:00 Jinghong

High Strength Bolt Twisting Process

  The superstructure of the Yellow River Bridge is a 13-hole 64-meter single-track railway welded steel girder with simple support bolt. The main structure is connected with M22 high strength bolts on site. The high strength bolt material is 40B. The nuts and washers are 45 steel, manufactured by Wuhan Automobile Standard Parts Factory. Screw, nut, washer are phosphating. About 130,000 sets of high-strength bolts are used throughout the bridge. The design pretension of the bolt is 196KN, and the friction coefficient between the plate surfaces is 0.45. The friction coefficient shall be rechecked at the site before the beam is erected, and the beam shall be erected only when it meets the design requirements. Aluminum alloy is sprayed on the bolted surface of steel truss and beam joints in construction site.
  1. Spot check and management of high strength bolts
  The MATERIALS, SPECIFICATIONS, DIMENSIONS AND TECHNICAL conditions USED FOR high-strength bolts, nuts and WASHers must conform to the relevant provisions in GB1228~1231~84, and the factory should attach the factory certificate for bolts. The bolts shall be inspected randomly after entering the site. Those without factory certificate and unqualified in random inspection shall not be used.
  When the bolts are put into storage, they should be checked, stored in different specifications, and the inventory list and the distribution quantity table should be established to strengthen the management of bolts. Each bolt, nut and washer should be inspected in appearance. Those with defects and severe rust should not be used. Have SLIGHT RUSTING PERSON, WANT TO USE STEEL WIRE BRUSH TO REMOVE RUST, HAVE DIRT PERSON USABLE CLEAN COTTON YARN TO WIPE CLEAN, BUT MUST NOT USE ORGANIC SOLVENT SUCH AS GASOLINE, DIESEL OIL TO SCRUB.
  After cleaning, the bolts should be equipped with one screw, one nut and two washers for each set. When the washer is placed, attention should be paid to its direction, and the side with a 45 degree slope should be attached to the bolt head or nut support surface. The bolts must be obtained in strict accordance with the specifications and quantities actually required by the design drawing. The bolts shall not be long or short during installation.
Two, the general requirements before twisting
  Spraying aluminum plate surface has peeling phenomenon or in the process of transportation is scraped, should be in the assembly before spraying aluminum treatment. When assembling, wet cotton yarn will be used to wipe the surface clean on the plate surface and the flying puncture of the plate edge and hole. At the beginning of unscrewing the gap between plates, the depth between plates inserted by 0.3mm shall not be greater than 20mm.
The bolts used should be used on the day of unpacking. The bolts corroded, damaged or stuck with sand should not be used. During construction, in case of wind and sand flying, necessary measures should be taken to protect the bolts, so as not to bring sand on the bolts and affect the quality of bolt twisting. Before putting the screw, the dirt rust in the nail hole should be removed.
  When assembling in the pre-assembly field, the punching nail is 25%, and the temporary clamping bolt is 75%. After the nodal plate positioning (i.e., after 75% bolt tightening), the 25% punching nail should be replaced with a bolt. Leave four rows of empty holes near the center line, and put two temporary clamping bolts on the first row, remove them before assembling, and bolt on the four rows of empty holes after assembling.
Cantilever assembly part punching nails and bolts 50% each.
As for the bolt direction, the high-strength bolts and nuts of all the facades of the bridge shall be installed on the outside of the nodal plate.
  Three, twisting process
  The high strength bolts of this bridge are constructed by torque-axis force method. The initial twist, the final twist two stages.
  The first twist is applied with a sound fixed wrench. After test, the initial twisting torque coefficient K=0.113, the initial twisting torque M =294NM.
  The final screw is applied with a calibrated electric wrench. The shaft force method is aimed at the bolt final screw force reaching the standard shaft force N0 (construction shaft force). Its principle is to use the t-test in mathematical statistics. The specific method is: randomly select 5 sets of bolts from the bolts to be used on the day, set them on the shaft force meter or pressure ring, and finally screw them with electricity to measure the axial force N1 and N2 of the 5 sets of bolts. The average axial force N and standard deviation σN of the 5 sets of bolts are calculated and tested with the statistic T = (n-n0) /σN (=5). The significance level A =5%× T0.05 can be checked from the mathematical statistics table. When t < T0.05, it is considered that the mean value N of bolt axial force applied by the electric lever is equal to the standard axial force N when the significance level is 5%.
 The torque coefficient K directly reflects the coefficient between the torque and the axial force in the process of bolt tightening. It not only depends on the friction coefficient of the friction surface, but also depends on the geometric condition of the screw connection pair. For certain ideal threaded connection vice, when the friction coefficient is determined, will determine the torque coefficient, but the actual threaded connection inevitably exist manufacture error, and sometimes there are threads are bruised, rust and other defects, at this point, even if a batch of bolt connection vice friction factor constant of skyscraper, the torque coefficient will inevitably exist certain difference. It's not a constant corresponding to friction. In extreme cases, when interference occurs, the bolt shaft force may be small despite a sufficiently large tightening torque. Generally, the K value varies from 0.100----0.450 depending on thread connection mode, surface friction conditions, and thread manufacturing quality.